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Sample records for accreting young stars

  1. Accretion Models for Young Neutron Stars

    OpenAIRE

    Alpar, M. Ali

    2003-01-01

    Interaction with possible fallback material, along with the magnetic fields and rotation rates at birth should determine the fates and categories of young neutron stars. This paper addresses some issues related to pure or hybrid accretion models for explaining the properties of young neutron stars.

  2. Accretion Disks around Young Low Mass Stars

    Directory of Open Access Journals (Sweden)

    Paola D´Alessio

    2001-01-01

    Full Text Available In the past decade, it has become clear that almost half of the low mass pre-main sequence stars are surrounded by disks, which are responsible for the observed infrared and optical-UV excess emission. The characterization of the structure of circumstellar disks is a crucial step towards understanding the early stellar evolution and planet formation. The thesis summarized here presents physical models of the detailed structure of accretion disks surrounding T Tauri stars. The disks are assumed to be in steady state, in vertical hydrostatic equilibrium, and with a turbulent viscosity described by the alpha-prescription. We consider different heating mechanisms: viscous dissipation, heating by cosmic rays and radioactive decay, irradiation by the central star or irradiation by an infalling envelope. The energy is transported in the vertical direction by radiation, convection and the turbulent flux. Give n the disk structure, we calculate its emission by integrating the radiative transfer equation for an arbitrary orientation of the disk relative to the line of sight. Spectral energy distributions (SEDs and images are compared with observations, and disk properties can be inferred or constrained.

  3. Multi-dimensional structure of accreting young stars

    CERN Document Server

    Geroux, C; Viallet, M; Goffrey, T; Pratt, J; Constantino, T; Folini, D; Popov, M V; Walder, R

    2016-01-01

    This work is the first attempt to describe the multi-dimensional structure of accreting young stars based on fully compressible time implicit multi-dimensional hydrodynamics simulations. One major motivation is to analyse the validity of accretion treatment used in previous 1D stellar evolution studies. We analyse the effect of accretion on the structure of a realistic stellar model of the young Sun. Our work is inspired by the numerical work of Kley \\& Lin (1996, ApJ, 461, 933) devoted to the structure of the boundary layer in accretion disks. We analyse the redistribution of accreted material with a range of values of specific entropy relative to the bulk specific entropy of the material in the accreting object's convective envelope. A primary goal is to understand whether and how accreted energy deposited onto a stellar surface is redistributed in the interior. This study focusses on the high accretion rates characteristic of FU Ori systems. We find that the highest entropy cases produce a distinctive ...

  4. Cooling of young stars growing by disk accretion

    CERN Document Server

    Rafikov, Roman

    2007-01-01

    In the initial formation stages young stars must acquire a significant fraction of their mass by accretion from a circumstellar disk that forms in the center of a collapsing protostellar cloud. Throughout this period mass accretion rates through the disk can reach 10^{-6}-10^{-5} M_Sun/yr leading to substantial energy release in the vicinity of stellar surface. We study the impact of irradiation of the stellar surface produced by the hot inner disk on properties of accreting fully convective low-mass stars, and also look at objects such as young brown dwarfs and giant planets. At high accretion rates irradiation raises the surface temperature of the equatorial region above the photospheric temperature T_0 that a star would have in the absence of accretion. The high-latitude (polar) parts of the stellar surface, where disk irradiation is weak, preserve their temperature at the level of T_0. In strongly irradiated regions an almost isothermal outer radiative zone forms on top of the fully convective interior, l...

  5. SPIN EVOLUTION OF ACCRETING YOUNG STARS. II. EFFECT OF ACCRETION-POWERED STELLAR WINDS

    Energy Technology Data Exchange (ETDEWEB)

    Matt, Sean P. [Laboratoire AIM Paris-Saclay, CEA/Irfu Universite Paris-Diderot CNRS/INSU, 91191 Gif-sur-Yvette (France); Pinzon, Giovanni [Observatorio Astronomico Nacional, Facultad de Ciencias, Universidad Nacional de Colombia, Bogota (Colombia); Greene, Thomas P. [NASA Ames Research Center, M.S. 245-6, Moffett Field, CA 94035-1000 (United States); Pudritz, Ralph E., E-mail: sean.matt@cea.fr, E-mail: thomas.p.greene@nasa.gov, E-mail: gapinzone@unal.edu.co, E-mail: pudritz@physics.mcmaster.ca [Physics and Astronomy Department, McMaster University, Hamilton, ON L8S 4M1 (Canada)

    2012-01-20

    We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind (APSW). For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1-10 days in the age range of 1-3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to stars with the lowest accretion rates, strongest magnetic fields, and/or highest stellar wind mass outflow rates. We also make a direct, quantitative comparison between the APSW scenario and the two types of disk-locking models (namely, the X-wind and Ghosh and Lamb type models) and identify some remaining theoretical issues for understanding young star spins.

  6. Circumstellar Disks of the Most Vigorously Accreting Young Stars

    CERN Document Server

    Liu, Hauyu Baobab; Kudo, Tomoyuki; Hashimoto, Jun; Dong, Ruobing; Vorobyov, Eduard I; Pyo, Tae-Soo; Fukagawa, Misato; Tamura, Motohide; Henning, Thomas; Dunham, Michael M; Karr, Jennifer; Kusakabe, Nobuhiko; Tsuribe, Toru

    2016-01-01

    Young stellar objects (YSOs) may not accumulate their mass steadily, as was previously thought, but in a series of violent events manifesting themselves as sharp stellar brightening. These events can be caused by fragmentation due to gravitational instabilities in massive gaseous disks surrounding young stars, followed by migration of dense gaseous clumps onto the star. We report our high angular resolution, coronagraphic near-infrared polarization imaging observations using the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) of the Subaru 8.2 m Telescope, towards four YSOs which are undergoing luminous accretion outbursts. The obtained infrared images have verified the presence of several hundred AUs scale arms and arcs surrounding these YSOs. In addition, our hydrodynamics simulations and radiative transfer models further demonstrate that these observed structures can indeed be explained by strong gravitational instabilities occurring at the beginning of the disk formation p...

  7. Spin Evolution of Accreting Young Stars. II. Effect of Accretion-Powered Stellar Winds

    CERN Document Server

    Matt, Sean P; Greene, Thomas P; Pudritz, Ralph E

    2011-01-01

    We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind. For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1--10 days in the age range of 1--3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to s...

  8. Spin Evolution of Accreting Young Stars. I. Effect of Magnetic Star-Disk Coupling

    CERN Document Server

    Matt, Sean P; de la Reza, Ramiro; Greene, Thomas P

    2010-01-01

    We present a model for the rotational evolution of a young, solar mass star interacting with an accretion disk. The model incorporates a description of the angular momentum transfer between the star and disk due to a magnetic connection, and includes changes in the star's mass and radius and a decreasing accretion rate. The model also includes, for the first time in a spin evolution model, the opening of the stellar magnetic field lines, as expected to arise from twisting via star-disk differential rotation. In order to isolate the effect that this has on the star-disk interaction torques, we neglect the influence of torques that may arise from open field regions connected to the star or disk. For a range of magnetic field strengths, accretion rates, and initial spin rates, we compute the stellar spin rates of pre-main-sequence stars as they evolve on the Hayashi track to an age of 3~Myr. How much the field opening affects the spin depends on the strength of the coupling of the magnetic field to the disk. For...

  9. Vertical Structure of Magnetized Accretion Disks around Young Stars

    CERN Document Server

    Lizano, S; Boehler, Y; D'Alessio, P

    2015-01-01

    We model the vertical structure of magnetized accretion disks subject to viscous and resistive heating, and irradiation by the central star. We apply our formalism to the radial structure of magnetized accretion disks threaded by a poloidal magnetic field dragged during the process of star formation developed by Shu and coworkers. We consider disks around low mass protostars, T Tauri, and FU Orionis stars. We consider two levels of disk magnetization, $\\lambda_{sys} = 4$ (strongly magnetized disks), and $\\lambda_{sys} = 12$ (weakly magnetized disks). The rotation rates of strongly magnetized disks have large deviations from Keplerian rotation. In these models, resistive heating dominates the thermal structure for the FU Ori disk. The T Tauri disk is very thin and cold because it is strongly compressed by magnetic pressure; it may be too thin compared with observations. Instead, in the weakly magnetized disks, rotation velocities are close to Keplerian, and resistive heating is always less than 7\\% of the visc...

  10. A Continuum of Accretion Burst Behavior in Young Stars Observed by K2

    Science.gov (United States)

    Cody, Ann Marie; Hillenbrand, Lynne A.; David, Trevor J.; Carpenter, John M.; Everett, Mark E.; Howell, Steve B.

    2017-02-01

    We present 29 likely members of the young ρ Oph or Upper Sco regions of recent star formation that exhibit “accretion burst” type light curves in K2 time series photometry. The bursters were identified by visual examination of their ∼80-day light curves, though all satisfy the Mtheory of pre-main sequence accretion history.

  11. Circumstellar disks of the most vigorously accreting young stars.

    Science.gov (United States)

    Liu, Hauyu Baobab; Takami, Michihiro; Kudo, Tomoyuki; Hashimoto, Jun; Dong, Ruobing; Vorobyov, Eduard I; Pyo, Tae-Soo; Fukagawa, Misato; Tamura, Motohide; Henning, Thomas; Dunham, Michael M; Karr, Jennifer L; Kusakabe, Nobuhiko; Tsuribe, Toru

    2016-02-01

    Stars may not accumulate their mass steadily, as was previously thought, but in a series of violent events manifesting themselves as sharp stellar brightening. These events can be caused by fragmentation due to gravitational instabilities in massive gaseous disks surrounding young stars, followed by migration of dense gaseous clumps onto the star. Our high-resolution near-infrared imaging has verified the presence of the key associated features, large-scale arms and arcs surrounding four young stellar objects undergoing luminous outbursts. Our hydrodynamics simulations and radiative transfer models show that these observed structures can indeed be explained by strong gravitational instabilities occurring at the beginning of the disk formation phase. The effect of those tempestuous episodes of disk evolution on star and planet formation remains to be understood.

  12. ACCRETION RATES ON PRE-MAIN-SEQUENCE STARS IN THE YOUNG OPEN CLUSTER NGC 6530

    Energy Technology Data Exchange (ETDEWEB)

    Gallardo, Jose; Del Valle, Luciano; Ruiz, Maria Teresa, E-mail: gallardo@das.uchile.cl, E-mail: ldelvall@das.uchile.cl, E-mail: mtruiz@das.uchile.cl [Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago (Chile)

    2012-01-15

    It is well accepted that during the star formation process, material from a protoplanetary disk is accreted onto the central object during the first {approx}1-5 Myr. Different authors have published measurements of accretion rates for young low- and intermediate-mass stars in several nearby star-forming regions (SFRs). Due to its somewhat larger distance, the SFR M8 (the Lagoon Nebula) has not been studied to the same extent, despite its abundant population of young stellar objects. We have obtained optical band low-resolution spectra of a sample of pre-main-sequence stars in the open cluster NGC 6530 located in the aforementioned nebulae using the Gemini Multi Object Spectrograph at Gemini-South in multi-object mode. Spectra cover the H{sub {alpha}} emission line used to measure the accretion rate, following the method presented by Natta et al. The observed spectral characteristics are fully consistent with pre-main-sequence stars, showing lithium absorption lines, which are very common in young stellar objects, as well as prominent and broad H{sub {alpha}} emission lines, indicating a T Tauri evolutionary stage. This work presents the first determinations of mass accretion rates of young stellar objects in the open cluster NGC 6530, confirming that they are classical T Tauri stars going through the accretion phase. These observations contribute to a better understanding of the stellar content and evolutionary phase of the very active Lagoon Nebula SFR.

  13. `Tail-end' Bondi-Hoyle accretion in young star clusters: Implications for disks, planets, and stars

    CERN Document Server

    Throop, Henry B

    2008-01-01

    Young stars orbiting in the gravitational potential well of forming star clusters pass through the cluster's dense molecular gas and can experience Bondi-Hoyle accretion from reservoirs outside their individual protostellar cloud cores. Accretion can occur for several million years after the stars form, but before the cluster disperses. This accretion is predominantly onto the disk and not the star. N-body simulations of stars orbiting in three young model clusters containing 30, 300, and 3000 stars are presented. The simulations include the gravitational potential of the molecular gas which smoothly disperses over time. The clusters have a star formation efficiency of 33% and a radius of 0.22 pc. We find that the disks surrounding solar-mass stars in the N=30 cluster accretes ~0.01 M_sol (~1 minimum-mass solar nebula, MMSN) per Myr. The accretion rate scales as M^2.1 for stars of mass M. The accretion rate is ~5 times lower for N=3000 cluster, due to its higher stellar velocities and higher temperature. The ...

  14. Accretion Disks around Young Stars: An Observational Perspective

    Science.gov (United States)

    Ménard, F.; Bertout, C.

    Accretion disks are pivotal elements in the formation and early evolution of solar-like stars. On top of supplying the raw material, their internal conditions also regulate the formation of planets. Their study therefore holds the key to solve this long standing mystery: how did our Solar System form? This chapter focuses on observational studies of the circumstellar environment, and in particular of circumstellar disks, associated with pre-main sequence solar-like stars. The direct measurement of disk parameters poses an obvious challenge: at the distance of the typical star forming regions ( e.g. 140 pc for Taurus), a planetary system like ours (with diameter simeq50 AU out to Pluto, but excluding the Kuiper belt which could extend much farther out) subtends only 0.35''. Yet its surface brightness is low in comparison to the bright central star and high angular and high contrast imaging techniques are required if one hopes to resolve and measure these protoplanetary disks. Fortunately, capable instruments providing 0.1'' resolution or better and high contrast have been available for just about 10 years now. They are covering a large part of the electromagnetic spectrum, from the UV/Optical with HST and the near-infrared from ground-based adaptive optics systems, to the millimetric range with long-baseline radio interferometers. It is therefore not surprising that our knowledge of the structure of the disks surrounding low-mass stars has made a gigantic leap forward in the last decade. In the following pages we will attempt to describe, in a historical perpective, the road that led to the idea that most solar-like stars are surrounded by an accretion disk at one point in their early life and how, nowadays, their structural and physical parameters can be estimated from direct observations. We will follow by a short discussion of a few of the constraints available regarding the evolution and dissipation of these disks. This last topic is particularly relevant today

  15. Workshop on Physics of Accretion Disks Around Compact and Young Stars

    Science.gov (United States)

    Liang, E (Editor); Stepinski, T. F. (Editor)

    1995-01-01

    The purpose of the two-day Workshop on Physics of Accretion Disks Around Compact and Young Stars was to bring together workers on accretion disks in the western Gulf region (Texas and Louisiana). Part 2 presents the workshop program, a list of poster presentations, and a list of workshop participants. Accretion disks are believed to surround many stars. Some of these disks form around compact stars, such as white dwarfs, neutron stars, or black holes that are members of binary systems and reveal themselves as a power source, especially in the x-ray and gamma regions of the spectrum. On the other hand, protostellar disks are believed to be accretion disks associated with young, pre-main-sequence stars and manifest themselves mostly in infrared and radio observations. These disks are considered to be a natural outcome of the star formation process. The focus of this workshop included theory and observations relevant to accretion disks around compact objects and newly forming stars, with the primary purpose of bringing the two communities together for intellectual cross-fertilization. The nature of the workshop was exploratory, to see how much interaction is possible between distinct communities and to better realize the local potential in this subject. A critical workshop activity was identification and documentation of key issues that are of mutual interest to both communities.

  16. CSI 2264: characterizing accretion-burst dominated light curves for young stars in NGC 2264

    Energy Technology Data Exchange (ETDEWEB)

    Stauffer, John; Cody, Ann Marie; Rebull, Luisa; Carey, Sean [Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125 (United States); Baglin, Annie [LESIA, Observatoire de Paris-Meudon, 5 place Jules Janssen, F-92195, Meudon (France); Alencar, Silvia [Departamento de Física-ICEx-UFMG, Av. Antônio Carlos, 6627, 30270-901, Belo Horizonte, MG (Brazil); Hillenbrand, Lynne A.; Carpenter, John; Findeisen, Krzysztof [Astronomy Department, California Institute of Technology, Pasadena, CA 91125 (United States); Venuti, Laura; Bouvier, Jerome [UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d' Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, F-38041 (France); Turner, Neal J. [Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States); Plavchan, Peter [Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125 (United States); Terebey, Susan [Department of Physics and Astronomy, 5151 State University Drive, California State University at Los Angeles, Los Angeles, CA 90032 (United States); Morales-Calderón, María [Centro de Astrobiología, Dpto. de Astrofísica, INTA-CSIC, P.O. Box 78, E-28691, ESAC Campus, Villanueva de la Cañada, Madrid (Spain); Micela, Giusi; Flaccomio, Ettore [INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134, Palermo (Italy); Song, Inseok [Department of Physics and Astronomy, The University of Georgia, Athens, GA 30602-2451 (United States); Gutermuth, Rob [Five College Astronomy Department, Smith College, Northampton, MA 01063 (United States); Hartmann, Lee, E-mail: stauffer@ipac.caltech.edu [Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48105 (United States); and others

    2014-04-01

    Based on more than four weeks of continuous high-cadence photometric monitoring of several hundred members of the young cluster NGC 2264 with two space telescopes, NASA's Spitzer and the CNES CoRoT (Convection, Rotation, and planetary Transits), we provide high-quality, multi-wavelength light curves for young stellar objects whose optical variability is dominated by short-duration flux bursts, which we infer are due to enhanced mass accretion rates. These light curves show many brief—several hours to one day—brightenings at optical and near-infrared wavelengths with amplitudes generally in the range of 5%-50% of the quiescent value. Typically, a dozen or more of these bursts occur in a 30 day period. We demonstrate that stars exhibiting this type of variability have large ultraviolet (UV) excesses and dominate the portion of the u – g versus g – r color-color diagram with the largest UV excesses. These stars also have large Hα equivalent widths, and either centrally peaked, lumpy Hα emission profiles or profiles with blueshifted absorption dips associated with disk or stellar winds. Light curves of this type have been predicted for stars whose accretion is dominated by Rayleigh-Taylor instabilities at the boundary between their magnetosphere and inner circumstellar disk, or where magneto-rotational instabilities modulate the accretion rate from the inner disk. Among the stars with the largest UV excesses or largest Hα equivalent widths, light curves with this type of variability greatly outnumber light curves with relatively smooth sinusoidal variations associated with long-lived hot spots. We provide quantitative statistics for the average duration and strength of the accretion bursts and for the fraction of the accretion luminosity associated with these bursts.

  17. Formation of new stellar populations from gas accreted by massive young star clusters.

    Science.gov (United States)

    Li, Chengyuan; de Grijs, Richard; Deng, Licai; Geller, Aaron M; Xin, Yu; Hu, Yi; Faucher-Giguère, Claude-André

    2016-01-28

    Stars in clusters are thought to form in a single burst from a common progenitor cloud of molecular gas. However, massive, old 'globular' clusters--those with ages greater than ten billion years and masses several hundred thousand times that of the Sun--often harbour multiple stellar populations, indicating that more than one star-forming event occurred during their lifetimes. Colliding stellar winds from late-stage, asymptotic-giant-branch stars are often suggested to be triggers of second-generation star formation. For this to occur, the initial cluster masses need to be greater than a few million solar masses. Here we report observations of three massive relatively young star clusters (1-2 billion years old) in the Magellanic Clouds that show clear evidence of burst-like star formation that occurred a few hundred million years after their initial formation era. We show that such clusters could have accreted sufficient gas to form new stars if they had orbited in their host galaxies' gaseous disks throughout the period between their initial formation and the more recent bursts of star formation. This process may eventually give rise to the ubiquitous multiple stellar populations in globular clusters.

  18. Evolution of dynamo-generated magnetic fields in accretion disks around compact and young stars

    Science.gov (United States)

    Stepinski, Tomasz F.

    1994-01-01

    Geometrically thin, optically thick, turbulent accretion disks are believed to surround many stars. Some of them are the compact components of close binaries, while the others are throught to be T Tauri stars. These accretion disks must be magnetized objects because the accreted matter, whether it comes from the companion star (binaries) or from a collapsing molecular cloud core (single young stars), carries an embedded magnetic field. In addition, most accretion disks are hot and turbulent, thus meeting the condition for the MHD turbulent dynamo to maintain and amplify any seed field magnetic field. In fact, for a disk's magnetic field to persist long enough in comparison with the disk viscous time it must be contemporaneously regenerated because the characteristic diffusion time of a magnetic field is typically much shorter than a disk's viscous time. This is true for most thin accretion disks. Consequently, studying magentic fields in thin disks is usually synonymous with studying magnetic dynamos, a fact that is not commonly recognized in the literature. Progress in studying the structure of many accretion disks was achieved mainly because most disks can be regarded as two-dimensional flows in which vertical and radial structures are largely decoupled. By analogy, in a thin disk, one may expect that vertical and radial structures of the magnetic field are decoupled because the magnetic field diffuses more rapidly to the vertical boundary of the disk than along the radius. Thus, an asymptotic method, called an adiabatic approximation, can be applied to accretion disk dynamo. We can represent the solution to the dynamo equation in the form B = Q(r)b(r,z), where Q(r) describes the field distribution along the radius, while the field distribution across the disk is included in the vector function b, which parametrically depends on r and is normalized by the condition max (b(z)) = 1. The field distribution across the disk is established rapidly, while the radial

  19. Evidence for a correlation between mass accretion rates onto young stars and the mass of their protoplanetary disks

    CERN Document Server

    Manara, C F; Testi, L; Natta, A; Alcalá, J M; Williams, J P; Ansdell, M; Miotello, A; van der Marel, N; Tazzari, M; Carpenter, J; Guidi, G; Mathews, G S; Oliveira, I; Prusti, T; van Dishoeck, E F

    2016-01-01

    A relation between the mass accretion rate onto the central young star and the mass of the surrounding protoplanetary disk has long been theoretically predicted and observationally sought. For the first time, we have accurately and homogeneously determined the photospheric parameters, the mass accretion rate, and the disk mass for an essentially complete sample of young stars with disks in the Lupus clouds. Our work combines the results of surveys conducted with VLT/X-Shooter and ALMA. With this dataset we are able to test a basic prediction of viscous accretion theory, the existence of a linear relation between the mass accretion rate onto the central star and the total disk mass. We find a correlation between the mass accretion rate and the disk dust mass, with a ratio that is roughly consistent with the expected viscous timescale when assuming an ISM gas-to-dust ratio. This confirms that mass accretion rates are related to the properties of the outer disk. We find no correlation between mass accretion rate...

  20. CSI 2264: Characterizing Accretion-Burst Dominated Light Curves for Young Stars in NGC 2264

    CERN Document Server

    Stauffer, John; Baglin, Annie; Alencar, Silvia H P; Rebull, Luisa; Hillenbrand, Lynne A; Venuti, Laura; Turner, Neal J; Carpenter, John; Plavchan, Peter; Findeisen, Krzysztof; Carey, Sean; Terebey, Susan; Morales-Calderón, María; Bouvier, Jerome; Micela, Giusi; Flaccomio, Ettore; Song, Inseok; Gutermuth, Rob; Hartmann, Lee; Calvet, Nuria; Whitney, Barbara; Barrado, David; Vrba, Frederick J; Covey, Kevin; Herbst, William; Furesz, Gabor; Aigrain, Suzanne

    2014-01-01

    Based on more than four weeks of continuous high cadence photometric monitoring of several hundred members of the young cluster NGC 2264 with two space telescopes, NASA's Spitzer and the CNES CoRoT (Convection, Rotation, and planetary Transits), we provide high quality, multi-wavelength light curves for young stellar objects (YSOs) whose optical variability is dominated by short duration flux bursts, which we infer are due to enhanced mass accretion rates. These light curves show many brief -- several hour to one day -- brightenings at optical and near-infrared (IR) wavelengths with amplitudes generally in the range 5-50% of the quiescent value. Typically, a dozen or more of these bursts occur in a thirty day period. We demonstrate that stars exhibiting this type of variability have large ultraviolet (UV) excesses and dominate the portion of the u-g vs. g-r color-color diagram with the largest UV excesses. These stars also have large Halpha equivalent widths, and either centrally peaked, lumpy Halpha emission...

  1. CSI 2264: Accretion process in classical T Tauri stars in the young cluster NGC 2264

    CERN Document Server

    Sousa, Alana; Bouvier, Jérôme; Stauffer, John; Venuti, Laura; Hillenbrand, Lynne; Cody, Ann Marie; Teixeira, Paula; Guimarães, Marcelo; McGinnis, Pauline; Rebull, Luisa; Flaccomio, Ettore; Fürész, Gabor; Micela, Giuseppina; Gameiro, Jorge

    2016-01-01

    Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars, of the star-forming cluster NGC 2264, to the physical processes acting in the stellar and circumstellar environment, within a few stellar radii from the star. NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several accretion diagnostics and disk parameters. The morphology of the CoRoT light curve reflects the evolution of the accretion process and of the inner disk region. Accretion burst stars present high mass-accretion rates and optically thick inner disks. AA Tau-like systems, whose light curves are dominated by circumstellar dust obscuration, show intermediate mass-accretion rates and are located in the transition of thick to anemic disks. Classical T Tauri stars with spot-like light cur...

  2. Temperaments of young stars: Rapid mass-accretion rate changes in T Tauri and Herbig Ae stars

    CERN Document Server

    Costigan, Gráinne; Scholz, Aleks; Ray, Tom; Testi, Leonardo

    2014-01-01

    Variability in emission lines is a characteristic feature in young stars and can be used as a tool to study the physics of the accretion process. Here we present a study of H{\\alpha} variability in 15 T Tauri and Herbig Ae stars (K7-B2) over a wide range of time windows, from minutes, to hours, to days, and years. We assess the variability using linewidth measurements and the time series of line profiles. All objects show gradual, slow profile changes on time-scales of days. In addition, in three cases there is evidence for rapid variations in H{\\alpha} with typical time-scales of 10 min, which occurs in 10% of the total covered observing time. The mean accretion-rate changes, inferred from the line fluxes,are 0.01-0.07 dex for time-scales of < 1 hour, 0.04-0.4 dex for time-scales of days, and 0.13-0.52 dex for time-scales of years. In Costigan et al. 2012 we derived an upper limit finding that the intermediate (days) variability dominated over longer (years) variability. Here our new results, based on muc...

  3. On Magnetic Dynamos in Thin Accretion Disks around Compact and Young Stars

    Science.gov (United States)

    Stepinski, T. F.

    1993-01-01

    A variety of geometrically thin accretion disks commonly associated with such astronomical objects as X-ray binaries, cataclysmic variables, and protostars are likely to be seats of MHD dynamo actions. Thin disk geometry and the particular physical environment make accretion disk dynamos different from stellar, planetary, or even galactic dynamos. We discuss those particular features of disk dynamos with emphasis on the difference between protoplanetary disk dynamos and those associated with compact stars. We then describe normal mode solutions for thin disk dynamos and discuss implications for the dynamical behavior of dynamo-magnetized accretion disks.

  4. Formation of new stellar populations from gas accreted by massive young star clusters

    CERN Document Server

    Li, Chengyuan; Deng, Licai; Geller, Aaron M; Xin, Yu; Hu, Yi; Faucher-Giguere, Claude-Andre

    2016-01-01

    Stars in star clusters are thought to form in a single burst from a common progenitor cloud of molecular gas. However, massive, old globular clusters -- with ages greater than 10 billion years and masses of several hundred thousand solar masses -- often harbour multiple stellar populations, indicating that more than one star-forming event occurred during their lifetimes. Colliding stellar winds from late-stage, asymptotic-giant-branch stars are often invoked as second-generation star-formation trigger. The initial cluster masses should be at least 10 times more massive than they are today for this to work. However, large populations of clusters with masses greater than a few million solar masses are not found in the local Universe. Here we report on three 1-2 billion-year-old, massive star clusters in the Magellanic Clouds, which show clear evidence of burst-like star formation that occurred a few hundred million years after their initial formation era. We show that such clusters could accrete sufficient gas ...

  5. Magnetocentrifugally Driven Flows from Young Stars and Disks. VI. Accretion with a Multipole Stellar Field

    CERN Document Server

    Mohanty, Subhanjoy

    2008-01-01

    Previous analyses of magnetospheric accretion and outflow in classical T Tauri stars (CTTSs), within the context of both the X-wind model and other theoretical scenarios, have assumed a dipolar geometry for the stellar magnetic field if it were not perturbed by the presence of an accreting, electrically conducting disk. However, CTTS surveys reveal that accretion hot spots cover a small fraction of the stellar surface, and that the net field polarization on the stellar surface is small. Both facts imply that the magnetic field generated by the star has a complex non-dipolar structure. To address this discrepancy between theory and observations, we re-examine X-wind theory without the dipole constraint. Using simple physical arguments based on the concept of trapped flux, we show that a dipole configuration is in fact not essential. Independent of the precise geometry of the stellar magnetosphere, the requirement for a certain level of trapped flux predicts a definite relationship among various CTTS observable...

  6. Constraining the initial conditions and final outcomes of accretion processes around young stars and supermassive black holes

    Science.gov (United States)

    Stone, Jordan M.

    In this thesis I discuss probes of small spatial scales around young stars and protostars and around the supermassive black hole at the galactic center. I begin by describing adaptive optics-fed infrared spectroscopic studies of nascent and newborn binary systems. Binary star formation is a significant mode of star formation that could be responsible for the production of a majority of the galactic stellar population. Better characterization of the binary formation mechanism is important for better understanding many facets of astronomy, from proper estimates of the content of unresolved populations, to stellar evolution and feedback, to planet formation. My work revealed episodic accretion onto the more massive component of the pre-main sequence binary system UY Aur. I also showed changes in the accretion onto the less massive component, revealing contradictory indications of the change in accretion rate when considering disk-based and shock-based tracers. I suggested two scenarios to explain the inconsistency. First, increased accretion should alter the disk structure, puffing it up. This change could obscure the accretion shock onto the central star if the disk is highly inclined. Second, if accretion through the disk is impeded before it makes it all the way onto the central star, then increased disk tracers of accretion would not be accompanied by increased shock tracers. In this case mass must be piling up at some radius in the disk, possibly supplying the material for planet formation or a future burst of accretion. My next project focused on characterizing the atmospheres of very low-mass companions to nearby young stars. Whether these objects form in an extension of the binary-star formation mechanism to very low masses or they form via a different process is an open question. Different accretion histories should result in different atmospheric composition, which can be constrained with spectroscopy. I showed that 3--4mum spectra of a sample of these

  7. Massive Star Formation: Accreting from Companion

    Indian Academy of Sciences (India)

    X. Chen; J. S. Zhang

    2014-09-01

    We report the possible accretion from companion in the massive star forming region (G350.69–0.49). This region seems to be a binary system composed of a diffuse object (possible nebulae or UC HII region) and a Massive Young Stellar Object (MYSO) seen in Spitzer IRAC image. The diffuse object and MYSO are connected by the shock-excited 4.5 m emission, suggesting that the massive star may form through accreting material from the companion in this system.

  8. The Gaia-ESO Survey: chemical signatures of rocky accretion in a young solar-type star

    CERN Document Server

    Spina, L; Randich, S; Sacco, G G; Jeffries, R; Magrini, L; Franciosini, E; Meyer, M R; Tautvaišienė, G; Gilmore, G; Alfaro, E J; Prieto, C Allende; Bensby, T; Bragaglia, A; Flaccomio, E; Koposov, S E; Lanzafame, A C; Costado, M T; Hourihane, A; Lardo, C; Lewis, J; Monaco, L; Morbidelli, L; Sousa, S G; Worley, C C; Zaggia, S

    2015-01-01

    It is well known that newly formed planetary systems undergo processes of orbital reconfiguration and planetary migration. As a result, planets or protoplanetary objects may accrete onto the central star, being fused and mixed into its external layers. If the accreted mass is sufficiently high and the star has a sufficiently thin convective envelope, such events may result in a modification of the chemical composition of the stellar photosphere in an observable way, enhancing it with elements that were abundant in the accreted mass. The recent Gaia-ESO Survey observations of the 10-20 Myr old Gamma Velorum cluster have enabled identifying a star that is significantly enriched in iron with respect to other cluster members. In this Letter we further investigate the abundance pattern of this star, showing that its abundance anomaly is not limited to iron, but is also present in the refractory elements, whose overabundances are correlated with the condensation temperature. This finding strongly supports the hypot...

  9. The Magnetospheres of (Accreting Neutron Stars

    Directory of Open Access Journals (Sweden)

    Wilms J.

    2014-01-01

    Full Text Available I give an overview of the most important observational tools to study the magnetospheres of accreting neutron stars, with a focus on accreting neutron stars in high mass X-ray binary systems. Topics covered are the different types of accretion onto neutron stars and the structure of the accretion column, and how models for these can be tested with observations.

  10. Accretion disks in luminous young stellar objects

    CERN Document Server

    Beltran, M T

    2015-01-01

    An observational review is provided of the properties of accretion disks around young stars. It concerns the primordial disks of intermediate- and high-mass young stellar objects in embedded and optically revealed phases. The properties were derived from spatially resolved observations and therefore predominantly obtained with interferometric means, either in the radio/(sub)millimeter or in the optical/infrared wavelength regions. We make summaries and comparisons of the physical properties, kinematics, and dynamics of these circumstellar structures and delineate trends where possible. Amongst others, we report on a quadratic trend of mass accretion rates with mass from T Tauri stars to the highest mass young stellar objects and on the systematic difference in mass infall and accretion rates.

  11. Emission line diagnostics for accretion and outflows in young very low-mass stars and brown dwarfs

    Directory of Open Access Journals (Sweden)

    Stelzer B.

    2014-01-01

    Full Text Available We discuss accretion and outflow properties of three very low-mass young stellar objects based on broad-band mid-resolution X-Shooter/VLT spectra. Our targets (FU Tau A, 2M1207-39, and Par-Lup3-4 have spectral types between M5 and M8, ages between 1Myr and ~ 10Myr, and are known to be accreting from previous studies. The final objective of our project is the determination of mass outflow to accretion rate for objects near or within the substellar regime as a probe for the T Tauri phase of brown dwarfs and the investigation of variability in the accretion and outflow processes.

  12. Massive star formation by accretion. I. Disc accretion

    Science.gov (United States)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.

    2016-01-01

    Context. Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the Hertzsprung-Russell (HR) diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. Aims: We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the HR diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. Methods: We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. We compare them with previously published equivalent models. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the accretion history of most of the intermediate-mass stars. Results: In the numerical computation of the time derivative of the entropy, some treatment leads to an artificial loss of entropy and thus reduces the inflation that the accreting star undergoes along the birthline. In the case of cold disc accretion, the existence of a significant swelling during the accretion phase, which leads to radii ≳ 100 R⊙ and brings the star back to the red part of the HR diagram, depends sensitively on the initial conditions. For an accretion rate of 10-3M⊙ yr-1, only models starting from a core with a significant radiative region evolve back to the red part of the HR diagram. We also obtain that, in order to reproduce the observed upper envelope of pre-MS stars in the HR diagram with an accretion law deduced from the observed mass outflows in ultra-compact HII regions, the fraction of the

  13. Massive star formation by accretion I. Disc accretion

    CERN Document Server

    Haemmerlé, Lionel; Meynet, Georges; Maeder, André; Charbonnel, Corinne

    2016-01-01

    Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the HR diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the Hertzsprung-Russell (HR) diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the acc...

  14. Theory of Disk Accretion onto Magnetic Stars

    Directory of Open Access Journals (Sweden)

    Lai Dong

    2014-01-01

    Full Text Available Disk accretion onto magnetic stars occurs in a variety of systems, including accreting neutron stars (with both high and low magnetic fields, white dwarfs, and protostars. We review some of the key physical processes in magnetosphere-disk interaction, highlighting the theoretical uncertainties. We also discuss some applications to the observations of accreting neutron star and protostellar systems, as well as possible connections to protoplanetary disks and exoplanets.

  15. Theory of Disk Accretion onto Magnetic Stars

    CERN Document Server

    Lai, Dong

    2014-01-01

    Disk accretion onto magnetic stars occurs in a variety of systems, including accreting neutron stars (with both high and low magnetic fields), white dwarfs, and protostars. We review some of the key physical processes in magnetosphere-disk interaction, highlighting the theoretical uncertainties. We also discuss some applications to the observations of accreting neutron stars and protostellar systems, as well as possible connections to protoplanetary disks and exoplanets.

  16. Pycnonuclear burning and accreting neutron stars

    CERN Document Server

    Yakovlev, D G

    2002-01-01

    We outline the phenomenon of deep crustal heating in transiently accreting neutron stars. It is produced by nuclear transformations (mostly, by pycnonuclear reactions) in accreted matter while this matter sinks to densities rho > 10^{10} g/cc under the weight of freshly accreted material. We consider then thermal states of transiently accreting neutron stars (with mean mass accretion rates \\dot{M}=(10^{-14}-10^{-9}) M_\\odot/yr) determined by deep crustal heating. In a simplified fashion we study how the thermal flux emergent from such stars depends on the properties of superdense matter in stellar interiors. We analyze the most important regulators of the thermal flux: strong superfluidity in the cores of low-mass stars and fast neutrino emission (in nucleon, pion-condensed, kaon-condensed, or quark phases of dense matter) in the cores of high-mass stars. We compare the results with observations of soft X-ray transients in quiescent states.

  17. Probing thermonuclear burning on accreting neutron stars

    NARCIS (Netherlands)

    Keek, L.

    2008-01-01

    Neutron stars are the most compact stars that can be directly observed, which makes them ideal laboratories to study physics at extreme densities. Neutron stars in low-mass X-ray binaries accrete hydrogen and helium from a lower-mass companion star through Roche lobe overflow. This matter undergoes

  18. Plasma physics of accreting neutron stars

    Science.gov (United States)

    Ghosh, Pranab; Lamb, Frederick K.

    1991-01-01

    Plasma concepts and phenomena that are needed to understand X- and gamma-ray sources are discussed. The capture of material from the wind or from the atmosphere or envelope of a binary companion star is described and the resulting types of accretion flows discussed. The reasons for the formation of a magnetosphere around the neutron star are explained. The qualitative features of the magnetospheres of accreting neutron stars are then described and compared with the qualitative features of the geomagnetosphere. The conditions for stable flow and for angular and linear momentum conservation are explained in the context of accretion by magnetic neutron stars and applied to obtain rough estimates of the scale of the magnetosphere. Accretion from Keplerian disks is then considered in some detail. The radial structure of geometrically thin disk flows, the interaction of disk flows with the neutron star magnetosphere, and models of steady accretion from Keplerian disks are described. Accretion torques and the resulting changes in the spin frequencies of rotating neutron stars are considered. The predicted behavior is then compared with observations of accretion-powered pulsars. Magnetospheric processes that may accelerate particles to very high energies, producing GeV and, perhaps, TeV gamma-rays are discussed. Finally, the mechanisms that decelerate and eventually stop accreting plasma at the surfaces of strongly magnetic neutron stars are described.

  19. Probing thermonuclear burning on accreting neutron stars

    OpenAIRE

    Keek, L.

    2008-01-01

    Neutron stars are the most compact stars that can be directly observed, which makes them ideal laboratories to study physics at extreme densities. Neutron stars in low-mass X-ray binaries accrete hydrogen and helium from a lower-mass companion star through Roche lobe overflow. This matter undergoes thermonuclear burning in the neutron star envelope, creating carbon and heavier elements. The fusion process may proceed in an unstable manner, resulting in a thermonuclear runaway. Within one seco...

  20. Physical Environment of Accreting Neutron Stars

    Directory of Open Access Journals (Sweden)

    J. Wang

    2016-01-01

    Full Text Available Neutron stars (NSs powered by accretion, which are known as accretion-powered NSs, always are located in binary systems and manifest themselves as X-ray sources. Physical processes taking place during the accretion of material from their companions form a challenging and appealing topic, because of the strong magnetic field of NSs. In this paper, we review the physical process of accretion onto magnetized NS in X-ray binary systems. We, firstly, give an introduction to accretion-powered NSs and review the accretion mechanism in X-ray binaries. This review is mostly focused on accretion-induced evolution of NSs, which includes scenario of NSs both in high-mass binaries and in low-mass systems.

  1. Perspectives for the study of gas in protoplanetary disks and accretion/ejection phenomena in young stars with the near-IR spectrograph SPIROU at the CFHT

    Science.gov (United States)

    Carmona, A.; Bouvier, J.; Delfosse, X.

    2013-11-01

    Near-IR atomic and molecular transitions are powerful tools to trace the warm and hot gas in the circumstellar environment of young stars. Ro-vibrational transitions of H_2 and H_2O, and overtone transitions of CO at 2 μm centered at the stellar velocity trace hot (T˜ 1500 K) gas in the inner few AU of protoplanetary disks. H_2 near-IR lines displaying a blueshift of a few km/s probe molecular disk winds. H_2 lines presenting blueshifts of hundreds of km/s reveal hot shocked gas in jets. Atomic lines such as the HeI line at 10830 Å and the Hydrogen Paschen β and Brakett γ lines trace emission from accretion funnel flows and atomic disk winds. Bright forbidden atomic lines in the near-IR of species such as [Fe II], [N I], [S I], [S II], and [C I] trace atomic and ionized material in jets. The new near-IR high resolution spectrograph SPIROU planned for the Canada France Hawaii Telescope will offer the unique capability of combining high-spectral resolution (R˜75000) with a large wavelength coverage (0.98 to 2.35 μm) in one single exposure. This will provide us with the means of probing accretion funnel flows, winds, jets, and hot gas in the inner disk simultaneously. This opens the exiting possibility of investigating their combined behavior in time by the means of monitoring observations and systematic surveys. SPIROU will be a powerful tool to progress our understanding of the connexion between the accretion/ejection process, disk evolution, and planet formation.

  2. Millisecond phenomena in mass accreting neutron stars

    NARCIS (Netherlands)

    van der Klis, M.; Cohen, L.

    2007-01-01

    The past twelve years have seen the discovery, with NASA's Rossi X-ray Timing Explorer (RXTE), of several long-predicted phenomena associated with the accretion of matter onto a neutron star in a binary (double) star system. These phenomena are observed in the strong X-ray emission produced by these

  3. Magnetically Controlled Accretion Flows onto Young Stellar Objects

    CERN Document Server

    Adams, Fred C

    2011-01-01

    (abridged) Accretion from disks onto young stars is thought to follow magnetic field lines from the inner disk edge to the stellar surface. The accretion flow thus depends on the geometry of the magnetic field. This paper extends previous work by constructing a collection of orthogonal coordinate systems, including the corresponding differential operators, where one coordinate traces the magnetic field lines. This formalism allows for an (essentially) analytic description of the geometry and the conditions required for the flow to pass through sonic points. Using this approach, we revisit the problem of magnetically controlled accretion flow in a dipole geometry, and then generalize the treatment to consider magnetic fields with multiple components, including dipole, octupole, and split monopole contributions. This approach can be generalized further to consider more complex magnetic field configurations. Observations indicate that accreting young stars have substantial dipole and octupole components, and tha...

  4. Mixing and Accretion in lambda Bootis Stars

    CERN Document Server

    Turcotte, S

    2002-01-01

    Strong evidence for deep mixing has been uncovered for slowly rotating F, and A stars of the main sequence. As the accretion/diffusion model for the formation of lboo stars is heavily dependent on mixing in superficial regions, such deep mixing may have important repercussions on our understanding of these stars. It is shown that deep mixing at a level similar to that of FmAm stars increases the amount of matter that needs to be accreted by the stars with respect with the standard models by some three orders of magnitude. It is also shown that significantly larger accretion rates have to be maintained, as high as $10^{-11}$~M$_\\sun yr^{-1}$, to prevent meridional circulation from canceling the effect of accretion. The existence of old ($\\approx 1$~Gyr) is not a likely outcome of the present models for accretion/diffusion with or without deep mixing. It is argued that lboo stars are potentially very good diagnostics of mixing mechanisms in moderately fast rotators.

  5. Accretion Rates in Herbig Ae stars

    CERN Document Server

    López, R G; Testi, L; Habart, E

    2006-01-01

    Accretion rates from disks around pre-main sequence stars are of importance for our understanding of planetary formation and disk evolution. We provide in this paper estimates of the mass accretion rates in the disks around a large sample of Herbig Ae stars. We obtained medium resolution 2 micron spectra and used the results to compute values of Macc from the measured luminosity of the Br_gamma emission line, using a well established correlation between L(Br_gamma) and the accretion luminosity Lacc. We find that 80% of the stars, all of which have evidence of an associated circumstellar disk, are accreting matter, with rates 3x10^{-9} 10^{-7} Msun/yr. In most HAe stars the accretion rate is sufficiently low that the gas in the inner disk, inside the dust evaporation radius, is optically thin and does not prevent the formation of a puffed-up rim, where dust is directly exposed to the stellar radiation. When compared to the Macc values found for lower-mass stars in the star forming regions Taurus and Ophiuchus,...

  6. Probing neutron star physics using accreting neutron stars

    Directory of Open Access Journals (Sweden)

    Patruno A.

    2010-10-01

    Full Text Available We give an obervational overview of the accreting neutron stars systems as probes of neutron star physics. In particular we focus on the results obtained from the periodic timing of accreting millisecond X-ray pulsars in outburst and from the measurement of X-ray spectra of accreting neutron stars during quiescence. In the first part of this overview we show that the X-ray pulses are contaminated by a large amount of noise of uncertain origin, and that all these neutron stars do not show evidence of spin variations during the outburst. We present also some recent developments on the presence of intermittency in three accreting millisecond X-ray pulsars and investigate the reason why only a small number of accreting neutron stars show X-ray pulsations and why none of these pulsars shows sub-millisecond spin periods. In the second part of the overview we introduce the observational technique that allows the study of neutron star cooling in accreting systems as probes of neutron star internal composition and equation of state. We explain the phenomenon of the deep crustal heating and present some recent developments on several quasi persistent X-ray sources where a cooling neutron star has been observed.

  7. Cooling of Accretion-Heated Neutron Stars

    Indian Academy of Sciences (India)

    Rudy Wijnands; Nathalie Degenaar; Dany Page

    2017-09-01

    We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.

  8. The Final Fates of Accreting Supermassive Stars

    CERN Document Server

    Umeda, Hideyuki; Omukai, Kazuyuki; Yoshida, Naoki

    2016-01-01

    The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have less concentrated structure than fully-convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit $\\gtrsim 10^5~M_\\odot$ derived for the fully-convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with $\\dot M \\lesssim 0.1~M_\\odot~{\\rm yr}^{-1}$. With $\\dot{M} \\simeq 0.3 - 1~M_\\odot~{\\rm yr}^{-1}$, the star becomes GR-unstable during the helium-burning stage ...

  9. Planetesimal accretion in binary star systems

    CERN Document Server

    Marzari, Francesco; Scholl, Hans

    2007-01-01

    Numerical simulations of planetesimal accretion in circumprimary and circumbinary orbits are described. The secular perturbations by the com- panion star and gas drag are included in our models. We derive limits on the parameters of the binary system for which accretion and then planetary forma- tion are possible. In the circumbinary case we also outline the radial distance from the baricenter of the stars beyond which accumulation always occurs. Hy- drodynamical simulations are also presented to validate our N-body approach based on the axisymmetric approximation for the gas of the disk.

  10. Accretion, Outflows, and Winds of Magnetized Stars

    CERN Document Server

    Romanova, M M

    2016-01-01

    Many types of stars have strong magnetic fields that can dynamically influence the flow of circumstellar matter. In stars with accretion disks, the stellar magnetic field can truncate the inner disk and determine the paths that matter can take to flow onto the star. These paths are different in stars with different magnetospheres and periods of rotation. External field lines of the magnetosphere may inflate and produce favorable conditions for outflows from the disk-magnetosphere boundary. Outflows can be particularly strong in the propeller regime, wherein a star rotates more rapidly than the inner disk. Outflows may also form at the disk-magnetosphere boundary of slowly rotating stars, if the magnetosphere is compressed by the accreting matter. In isolated, strongly magnetized stars, the magnetic field can influence formation and/or propagation of stellar wind outflows. Winds from low-mass, solar-type stars may be either thermally or magnetically driven, while winds from massive, luminous O and B type stars...

  11. Perspectives for the study of gas in protoplanetary disks and accretion/ejection phenomena in young stars with the near-IR spectrograph SPIROU at the CFHT

    CERN Document Server

    Carmona, A; Delfosse, X

    2013-01-01

    Near-IR atomic and molecular transitions are powerful tools to trace the warm and hot gas in the circumstellar environment of young stars. Ro-vibrational transitions of H2 and H2O, and overtone transitions of CO at 2 micron centered at the stellar velocity trace hot (T~1500 K) gas in the inner few AU of protoplanetary disks. H2 near-IR lines displaying a blueshift of a few km/s probe molecular disk winds. H2 lines presenting blueshifts of hundreds of km/s reveal hot shocked gas in jets. Atomic lines such as the HeI line at 10830 A and the Hydrogen Paschen beta and Brakett gamma lines trace emission from accretion funnel flows and atomic disk winds. Bright forbidden atomic lines in the near-IR of species such as [Fe II], [N I], [S I], [S II], and [C I] trace atomic and ionized material in jets. The new near-IR high resolution spectrograph SPIROU planned for the Canada France Hawaii Telescope will offer the unique capability of combining high-spectral resolution (R~75000) with a large wavelength coverage (0.98 ...

  12. Viscosity in accretion discs. [for binary stars

    Science.gov (United States)

    Katz, J. I.

    1980-01-01

    Both HerX-1 and SS433 may contain accretion disks slaved to a precessing companion star. If so, it is possible to bound the effective viscosity in these disks. The results, in terms of the disk parameter alpha, are lower bounds of 0.01 for HerX-1 and of 0.1 for SS433.

  13. Massive star formation by accretion. II. Rotation: how to circumvent the angular momentum barrier?

    Science.gov (United States)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.; Klessen, R. S.

    2017-06-01

    Context. Rotation plays a key role in the star-formation process, from pre-stellar cores to pre-main-sequence (PMS) objects. Understanding the formation of massive stars requires taking into account the accretion of angular momentum during their PMS phase. Aims: We study the PMS evolution of objects destined to become massive stars by accretion, focusing on the links between the physical conditions of the environment and the rotational properties of young stars. In particular, we look at the physical conditions that allow the production of massive stars by accretion. Methods: We present PMS models computed with a new version of the Geneva Stellar Evolution code self-consistently including accretion and rotation according to various accretion scenarios for mass and angular momentum. We describe the internal distribution of angular momentum in PMS stars accreting at high rates and we show how the various physical conditions impact their internal structures, evolutionary tracks, and rotation velocities during the PMS and the early main sequence. Results: We find that the smooth angular momentum accretion considered in previous studies leads to an angular momentum barrier and does not allow the formation of massive stars by accretion. A braking mechanism is needed in order to circumvent this angular momentum barrier. This mechanism has to be efficient enough to remove more than two thirds of the angular momentum from the inner accretion disc. Due to the weak efficiency of angular momentum transport by shear instability and meridional circulation during the accretion phase, the internal rotation profiles of accreting stars reflect essentially the angular momentum accretion history. As a consequence, careful choice of the angular momentum accretion history allows circumvention of any limitation in mass and velocity, and production of stars of any mass and velocity compatible with structure equations.

  14. Spin equilibrium in strongly-magnetized accreting stars

    CERN Document Server

    D'Angelo, Caroline

    2016-01-01

    The spin rate of a strongly-magnetized accreting star is regulated by the interaction between the star's magnetic field and the accreting gas. These systems are often hypothesized to be in `spin equilibrium' with their surrounding accretion flows such that the net spin change of the star as a result of accretion is very small. This condition requires that the accretion rate changes more slowly than it takes the star to reach spin equilibrium. However, this is not true for most magnetically accreting stars, which have strongly variable accretion outbursts (by one to many orders of magnitude) on timescales much shorter than the time it would take to reach spin equilibrium. This paper examines how accretion outbursts affect the time a star takes to reach spin equilibrium and its final equilibrium spin period. I consider several different models for angular momentum loss -- where angular momentum is carried away in an outflow (the standard `propeller', centrifugally-launched outflow), where most angular momentum ...

  15. X-Shooter spectroscopy of young stellar objects: IV -- Accretion in low-mass stars and sub-stellar objects in Lupus

    CERN Document Server

    Alcalá, J M; Manara, C F; Spezzi, L; Stelzer, B; Frasca, A; Biazzo, K; Covino, E; Randich, S; Rigliaco, E; Testi, L; Comerón, F; Cupani, G; D'Elia, V

    2013-01-01

    We present X-Shooter/VLT observations of a sample of 36 accreting low-mass stellar and sub-stellar objects (YSOs) in the Lupus star forming region, spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun < Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (Lacc), and from it the accretion rate (Macc), is derived by modelling the excess emission, from the UV to the near-IR, as the continuum emission of a slab of hydrogen. The flux and luminosity (Ll) of a large number of emission lines of H, He, CaII, etc., observed simultaneously in the range from ~330nm to 2500nm, were computed. The luminosity of all the lines is well correlated with Lacc. We provide empirical relationships between Lacc and the luminosity of 39 emission lines, which have a lower dispersion as comp...

  16. Probing thermonuclear burning on accreting neutron stars

    Science.gov (United States)

    Keek, L.

    2008-12-01

    Neutron stars are the most compact stars that can be directly observed, which makes them ideal laboratories to study physics at extreme densities. Neutron stars in low-mass X-ray binaries accrete hydrogen and helium from a lower-mass companion star through Roche lobe overflow. This matter undergoes thermonuclear burning in the neutron star envelope, creating carbon and heavier elements. The fusion process may proceed in an unstable manner, resulting in a thermonuclear runaway. Within one second the entire surface is burned, which is observable as a sharp rise in the emitted X-ray flux: a type I X-ray burst. Afterwards the neutron star surface cools down on a timescale of ten to one hundred seconds. During these bursts the surface of an accreting neutron star can be observed directly, which makes them instrumental for studying this type of stars. We have studied rare kinds of X-ray bursts. One such rare burst is the superburst, which lasts a thousand times longer than an ordinary burst. Superbursts are thought to result from the explosive burning of a thick carbon layer, which lies deeper inside the neutron star, close to a layer known as the crust. A prerequisite for the occurrence of a superburst is a high enough temperature, which is set by the temperature of the crust and the heat conductivity of the envelope. The latter is lowered by the presence of heavy elements that are produced during normal X-ray bursts. Using a large set of observations from the Wide Field Camera's onboard the BeppoSAX satellite, we find that, at high accretion rate, sources which do not exhibit normal bursts likely have a longer superburst recurrence time, than the observed superburst recurrence time of one burster. We analyze in detail the first superburst from a transient source, which went into outburst only 55 days before the superburst. Recent models of the neutron star crust predict that this is too small a time to heat the crust sufficiently for superburst ignition, indicating

  17. Ages of young stars

    CERN Document Server

    Soderblom, David R; Jeffries, Rob D; Mamajek, Eric E; Naylor, Tim

    2013-01-01

    Determining the sequence of events in the formation of stars and planetary systems and their time-scales is essential for understanding those processes, yet establishing ages is fundamentally difficult because we lack direct indicators. In this review we discuss the age challenge for young stars, specifically those less than ~100 Myr old. Most age determination methods that we discuss are primarily applicable to groups of stars but can be used to estimate the age of individual objects. A reliable age scale is established above 20 Myr from measurement of the Lithium Depletion Boundary (LDB) in young clusters, and consistency is shown between these ages and those from the upper main sequence and the main sequence turn-off -- if modest core convection and rotation is included in the models of higher-mass stars. Other available methods for age estimation include the kinematics of young groups, placing stars in Hertzsprung-Russell diagrams, pulsations and seismology, surface gravity measurement, rotation and activ...

  18. Gravitational Waves from Fallback Accretion onto Neutron Stars

    CERN Document Server

    Piro, Anthony L

    2012-01-01

    Massive stars generally end their lives as neutron stars (NSs) or black holes (BHs), with NS formation typically occurring at the low mass end and collapse to a BH more likely at the high mass end. In an intermediate regime, with a mass range that depends on the uncertain details of rotation and mass loss during the star's life, a NS is initially formed which then experiences fallback accretion and collapse to a BH. The electromagnetic consequence of such an event is not clear. Depending on the progenitor's structure, possibilities range from a long gamma-ray burst to a Type II supernova (that may or may not be jet-powered) to a collapse with a weak electromagnetic signature. Gravitational waves (GWs) provide the exciting opportunity to peer through the envelope of a dying massive star and directly probe what is occurring inside. We explore whether fallback onto young NSs can be detected by ground-based interferometers. When the incoming material has sufficient angular momentum to form a disk, the accretion s...

  19. MN Lup: X-rays from a weakly accreting T Tauri star

    CERN Document Server

    Guenther, H M; Robrade, J; Wolk, S J

    2013-01-01

    Young T Tauri stars (TTS) are surrounded by an accretion disk, which over time disperses due to photoevaporation, accretion, and possibly planet formation. The accretion shock on the central star produces an UV/optical veiling continuum, line emission, and X-ray signatures. As the accretion rate decreases, the impact on the central star must change. In this article we study MN Lup, a young star where no indications of a disk are seen in IR observations. We present XMM-Newton and VLT/UVES observations, some of them taken simultaneously. The X-ray data show that MN Lup is an active star with L_X/L_bol close to the saturation limit. However, we find high densities (n_e > 3e10 /cm^3) in the X-ray grating spectrum. This can be well fitted using an accretion shock model with an accretion rate of 2e-11 M_sun/yr. Despite the simple Halpha line profile which has a broad component, but no absorption signatures as typically seen on accreting TTS, we find rotational modulation in Ca II K and in photospheric absorption li...

  20. X-shooting Herbig Ae/Be stars: Accretion probed by near-infrared He I emission

    NARCIS (Netherlands)

    Oudmaijer, R.D.; Van Den Ancker, M. E.; Baines, D.; Caselli, P.; Drew, J.E.; Hoare, M.G.; Lumsden, S.L.; Montesinos, B.; Sim, S.; Vink, J.S.; Wheelwright, H.E.; de Wit, W.J.M.

    2011-01-01

    The Herbig Ae/Be stars are intermediate mass pre-main sequence stars that bridge the gap between the low mass T Tauri stars and the Massive Young Stellar Objects. In this mass range, the acting star forming mechanism switches from magnetically controlled accretion to an as yet unknown mechanism, but

  1. Young Stars with SALT

    Science.gov (United States)

    Riedel, Adric R.; Alam, Munazza K.; Rice, Emily L.; Cruz, Kelle L.; Henry, Todd J.

    2017-05-01

    We present a spectroscopic and kinematic analysis of 79 nearby M dwarfs in 77 systems. All of these dwarfs are low-proper-motion southern hemisphere objects and were identified in a nearby star survey with a demonstrated sensitivity to young stars. Using low-resolution optical spectroscopy from the Red Side Spectrograph on the South African Large Telescope, we have determined radial velocities, H-alpha, lithium 6708 Å, and potassium 7699 Å equivalent widths linked to age and activity, and spectral types for all of our targets. Combined with astrometric information from literature sources, we identify 44 young stars. Eighteen are previously known members of moving groups within 100 pc of the Sun. Twelve are new members, including one member of the TW Hydra moving group, one member of the 32 Orionis moving group, 9 members of Tucana-Horologium, one member of Argus, and two new members of AB Doradus. We also find 14 young star systems that are not members of any known groups. The remaining 33 star systems do not appear to be young. This appears to be evidence of a new population of nearby young stars not related to the known nearby young moving groups. Based on observations made with the Southern African Large Telescope (SALT).

  2. Magnetic field evolution of accreting neutron stars

    CERN Document Server

    Istomin, Ya N

    2016-01-01

    The flow of a matter, accreting onto a magnetized neutron star, is accompanied by an electric current. The closing of the electric current occurs in the crust of a neutron stars in the polar region across the magnetic field. But the conductivity of the crust along the magnetic field greatly exceeds the conductivity across the field, so the current penetrates deep into the crust down up to the super conducting core. The magnetic field, generated by the accretion current, increases greatly with the depth of penetration due to the Hall conductivity of the crust is also much larger than the transverse conductivity. As a result, the current begins to flow mainly in the toroidal direction, creating a strong longitudinal magnetic field, far exceeding an initial dipole field. This field exists only in the narrow polar tube of $r$ width, narrowing with the depth, i.e. with increasing of the crust density $\\rho$, $r\\propto \\rho^{-1/4}$. Accordingly, the magnetic field $B$ in the tube increases with the depth, $B\\propto...

  3. Accretion shocks in the laboratory: Design of an experiment to study star formation

    Science.gov (United States)

    Young, R. P.; Kuranz, C. C.; Drake, R. P.; Hartigan, P.

    2017-06-01

    We present the design of a laboratory-astrophysics experiment to study magnetospheric accretion relevant to young, pre-main-sequence stars. Spectra of young stars show evidence of hotspots created when streams of accreting material impact the surface of the star and create shocks. The structures that form during this process are poorly understood, as the surfaces of young stars cannot be spatially resolved. Our experiment would create a scaled ;accretion shock; at a major (several kJ) laser facility. The experiment drives a plasma jet (the ;accretion stream;) into a solid block (the ;stellar surface;), in the presence of a parallel magnetic field analogous to the star's local field. We show that this experiment is well-scaled when the incoming jet has ρ ∼10-6 -10-5gcm-3 and u ∼ 100 - 200kms-1 in an imposed field of B ∼ 10 T. Such an experiment would represent an average accretion stream onto a pre-main sequence star with B ∼ 700 G.

  4. Forming an Early O-type Star Through Gas Accretion?

    CERN Document Server

    Zapata, Luis A; HO, Paul; Schilke, Peter; Garrod, Robin T; Rodriguez, Luis F; Menten, Karl

    2007-01-01

    We present high angular resolution ($\\sim$ 3$''$) and sensitive 1.3 mm continuum, cyanogen (CN) and vinyl cyanide (C$_2$H$_3$CN) line observations made with the Submillimeter Array (SMA) toward one of most highly obscured objects of the W51 IRS2 region, W51 North. We find that the CN line exhibits a pronounced inverse P-Cygni profile indicating that the molecular gas is infalling inwards this object with a mass accretion rate between 4 and 7 $\\times$ 10$^{-2}$ M$_\\odot$ yr$^{-1}$. The C$_2$H$_3$CN traces an east-west rotating molecular envelope that surrounds either a single obscured (proto)star with a kinematic mass of 40 M$_{\\odot}$ or a small central cluster of B-type stars and that is associated with a compact high velocity bipolar outflow traced by H$_2$O masers and SiO molecular emission. We thus confirm that the W51 North region is part of the growing list of young massive star forming regions that have been associated with infalling motions and with large mass accretion rates ($\\sim$ 10$^{-2}$ -- 10$^...

  5. Accretion Shocks in the Laboratory: Using the OMEGA Laser to Study Star Formation

    Science.gov (United States)

    Young, R. P.; Kuranz, C. C.; Li, C. K.; Hartigan, P.; Froula, D.; Fiksel, G.; Ross, J. S.; Chang, P. Y.; Klein, S.; Zylstra, A.; Sio, H. W.; Liao, A.; Barnak, D.

    2016-10-01

    We present an on-going series of experiments using the OMEGA laser (Laboratory for Laser Energetics) to study star formation. Spectra of young stars show evidence of hotspots created when streams of accreting material impact at the surface of the star to create accretion shocks. These accretion shocks are poorly understood, as the surfaces of young stars cannot be spatially resolved. Our experiment series creates a scaled ``accretion shock'' on the OMEGA laser by driving a plasma jet (the ``accretion stream'') into a solid block (the ``stellar surface''), in the presence of a parallel magnetic field analogous to the star's local field. Thus far, visible image data from this experimental series either shows very thin accretion shocks forming or does not show them forming at all. We intend to present this data, provide possible explanations for why shocks may not have formed, and discuss potential improvements to the experimental design. This work is funded by the U.S. Department of Energy, through the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, Grant Number DE-NA0002956, and the National Laser User Facility Program, Grant Number DE-NA0002719.

  6. A search for the lasts gasps of disk accretion in Orion T Tauri stars

    Science.gov (United States)

    Clark, Catherine; Briceno, Cesar; Calvet, Nuria; Hernandez, Jesus

    2017-01-01

    Using the echelle mode of the Michigan/Magellan Fiber System (M2FS) on the Magellan/Clay telescope at Las Campanas Observatory, we obtained high resolution spectra (R~35000) of a sample of ~4 - 10 Myr old T Tauri stars distributed in ten 0.5 deg diameter fields in the Orion OB1 association.We present here a search for accretion signatures among a sample of weak-line T Tauri stars (WTTS). These are young stars that on the basis of their classification in low-resolution spectra, are assumed to lack a primordial disk and therefore should not be actively accreting. We look for signatures of disk accretion at modest or low levels by measuring the width at 10% height of the H-alpha profile, and looking for a redshifted absorption feature. In parallel, we determine which WTTS among the M2FS sample have infrared excesses indicating a circumstellar disk, to see which disk-bearing WTTS also show indications of accretion. We propose that such WTTS accreting at low levels are T Tauri stars at or nearing the end of their accretion phase. Our goal is to build a large sample of these objects so that we can place statistical contraints on how long the accretion phase lasts in solar-like and low-mass stars.

  7. The accretion/ejection paradigm in young stellar objects: from HST and Herschel to JWST

    NARCIS (Netherlands)

    Podio, Linda

    2012-01-01

    Stellar jets and molecular outflows are observed in association with young accreting stars and are believed to play a key role in the star formation process. In this talk I will show how current and future space missions are of crucial importance to investigate the origin of stellar jets and their l

  8. Highly Variable Extinction and Accretion in the Jet-driving Class I Type Young Star PTF 10nvg (V2492 Cyg, IRAS 20496+4354)

    CERN Document Server

    Hillenbrand, Lynne A; Covey, Kevin R; Carpenter, John M; Cenko, S Bradley; Silverman, Jeffrey M; Muirhead, Philip; Fischer, William; Crepp, Justin R; Bloom, Joshua S; Filippenko, Alexei V

    2012-01-01

    [abbreviated] We report extensive photometry and spectroscopy of the highly variable young stellar object PTF 10nvg including optical, near-infrared, mid-infrared, and millimeter data. Following the 2010 maximum and subsequent fade, during 2011 and 2012 the source underwent additional episodes of brightening followed by several magnitude dimming events consistent with extinction variations. Further, a ~221 day period is derived; 2010 data taken when the source was near maximum brightness do not phase well to this period, however. Spectral evolution includes changes in slope and correlated changes in the prominence of TiO/VO/CO and atomic emission. These are anticorrelated with changes in forbidden emission which, along with H_2, dominate at faint epochs. Notably, night-to-night variations in several forbidden doublet strengths and ratios are observed. High-dispersion spectra in a variety of photometric states reveal line profiles for atomic species likely formed in an accretion flow and/or impact. The origin ...

  9. Echography of young stars reveals their evolution

    CERN Document Server

    Zwintz, K; Ryabchikova, T; Guenther, D; Aerts, C; Barnes, T G; Themessl, N; Lorenz, D; Cameron, C; Kuschnig, R; Pollack-Drs, S; Moravveji, E; Baglin, A; Matthews, J M; Moffat, A F J; Poretti, E; Rainer, M; Rucinski, S M; Sasselov, D; Weiss, W W

    2014-01-01

    We demonstrate that a seismic analysis of stars in their earliest evolutionary phases is a powerful method to identify young stars and distinguish their evolutionary states. The early star that is born from the gravitational collapse of a molecular cloud reaches at some point sufficient temperature, mass and luminosity to be detected. Accretion stops and the pre-main sequence star that emerges is nearly fully convective and chemically homogeneous. It will continue to contract gravitationally until the density and temperature in the core are high enough to start nuclear burning of hydrogen. We show that there is a relationship between detected pulsation properties for a sample of young stars and their evolutionary status illustrating the potential of asteroseismology for the early evolutionary phases.

  10. Young Stars with SALT

    CERN Document Server

    Riedel, Adric R; Rice, Emily L; Cruz, Kelle L; Henry, Todd J

    2016-01-01

    We present a spectroscopic and kinematic analysis of 79 nearby M dwarfs in 77 systems. All are low-proper-motion southern hemisphere objects and were identified in a nearby star survey with a demonstrated sensitivity to young stars. Using low-resolution optical spectroscopy from the Red Side Spectrograph (RSS) on the South African Large Telescope (SALT), we have determined radial velocities, H-alpha, Lithium 6708\\AA, and Potassium 7699\\AA~equivalent widths linked to age and activity, and spectral types for all our targets. Combined with astrometric information from literature sources, we identify 44 young stars. Eighteen are previously known members of moving groups within 100 parsecs of the Sun. Twelve are new members, including one member of the TW Hydra moving group, one member of the 32 Orionis moving group, nine members of Tucana-Horologium, one member of Argus, and two new members of AB Doradus. We also find fourteen young star systems that are not members of any known groups. The remaining 33 star syst...

  11. Old pre-main-sequence Stars: Disc reformation by Bondi-Hoyle accretion

    CERN Document Server

    Scicluna, P; Dale, J E; Testi, L

    2014-01-01

    Young stars show evidence of accretion discs which evolve quickly and disperse with an e-folding time of $\\sim$ 3Myr. This is in striking contrast with recent observations that suggest evidence for numerous $>30$ Myr old stars with an accretion disc in large star-forming complexes. We consider whether these observations of apparently old accretors could be explained by invoking Bondi-Hoyle accretion to rebuild a new disc around these stars during passage through a clumpy molecular cloud. We combine a simple Monte Carlo model to explore the capture of mass by such systems with a viscous evolution model to infer the levels of accretion that would be observed. We find that a significant fraction of stars may capture enough material via the Bondi-Hoyle mechanism to rebuild a disc of mass $\\gtrsim$ 1 minimum-mass solar nebula, and $\\lesssim 10\\%$ accrete at observable levels at any given time. A significant fraction of the observed old accretors may be explained with our proposed mechanism. Such accretion may prov...

  12. Accretion-disc model spectra for dwarf-nova stars

    OpenAIRE

    Idan, Irit; Lasota, Jean-Pierre; Hameury, Jean-Marie; Shaviv, Giora

    2008-01-01

    Radiation from accretion discs in cataclysmic variable stars (CVs) provides fundamental information about the properties of these close binary systems and about the physics of accretion in general. The detailed diagnostics of accretion disc structure can be achieved by including in its description all the relevant heating and cooling physical mechanism, in particular the convective energy transport that, although dominant at temperatures less than about 10 000 K, is usually not taken into acc...

  13. Accretion of Supersonic Winds on Boson Stars

    CERN Document Server

    Gracia-Linares, M

    2016-01-01

    We present the evolution of a supersonic wind interacting with a Boson Star (BS) and compare the resulting wind density profile with that of the shock cone formed when the wind is accreted by a non-rotating Black Hole (BH) of the same mass. The physical differences between these accretors are that a BS, unlike a BH has no horizon, it does not have a mechanical surface either and thus the wind is expected to trespass the BS. Despite these conditions, on the BS space-time the gas achieves a stationary flux with the gas accumulating in a high density elongated structure comparable to the shock cone formed behind a BH. The highest density resides in the center of the BS whereas in the case of the BH it is found on the downstream part of the BH near the event horizon. The maximum density of the gas is smaller in the BS than in the BH case. Our results indicate that the highest density of the wind is more similar on the BS to that on the BH when the BS has high self-interaction, when it is more compact and when the...

  14. Binary interactions with high accretion rates onto main sequence stars

    Science.gov (United States)

    Shiber, Sagiv; Schreier, Ron; Soker, Noam

    2016-07-01

    Energetic outflows from main sequence stars accreting mass at very high rates might account for the powering of some eruptive objects, such as merging main sequence stars, major eruptions of luminous blue variables, e.g., the Great Eruption of Eta Carinae, and other intermediate luminosity optical transients (ILOTs; red novae; red transients). These powerful outflows could potentially also supply the extra energy required in the common envelope process and in the grazing envelope evolution of binary systems. We propose that a massive outflow/jets mediated by magnetic fields might remove energy and angular momentum from the accretion disk to allow such high accretion rate flows. By examining the possible activity of the magnetic fields of accretion disks, we conclude that indeed main sequence stars might accrete mass at very high rates, up to ≈ 10-2 M ⊙ yr-1 for solar type stars, and up to ≈ 1 M ⊙ yr-1 for very massive stars. We speculate that magnetic fields amplified in such extreme conditions might lead to the formation of massive bipolar outflows that can remove most of the disk's energy and angular momentum. It is this energy and angular momentum removal that allows the very high mass accretion rate onto main sequence stars.

  15. Star Formation and Gas Accretion in Nearby Galaxies

    CERN Document Server

    Yim, Kijeong

    2016-01-01

    In order to quantify the relationship between gas accretion and star formation, we analyse a sample of 29 nearby galaxies from the WHISP survey which contains galaxies with and without evidence for recent gas accretion. We compare combined radial profiles of FUV (GALEX) and IR 24 {\\mu}m (Spitzer) characterizing distributions of recent star formation with radial profiles of CO (IRAM, BIMA, or CARMA) and HI (WSRT) tracing molecular and atomic gas contents to examine star formation efficiencies in symmetric (quiescent), asymmetric (accreting), and interacting (tidally disturbed) galaxies. In addition, we investigate the relationship between star formation rate and HI in the outer discs for the three groups of galaxies. We confirm the general relationship between gas surface density and star formation surface density, but do not find a significant difference between the three groups of galaxies.

  16. Theoretical Models of Superbursts on Accreting Neutron Stars

    CERN Document Server

    Cooper, R L; Cooper, Randall L.; Narayan, Ramesh

    2004-01-01

    We carry out a general-relativistic global linear stability analysis of the amassed carbon fuel on the surface of an accreting neutron star to determine the conditions under which superbursts occur. By comparing our results with observations, we are able to set constraints on neutron star parameters such as the stellar radius and neutrino cooling mechanism in the core, as well as the composition of the ashes where superbursts are triggered. Specifically, we find that accreting neutron stars with ordered crusts and highly efficient neutrino emission in their cores produce extremely energetic superbursts which are inconsistent with observations. Also, because of pycnonuclear burning of carbon, they do not have superbursts in the range of accretion rates at which superbursts are actually observed. Stars with less efficient neutrino emission produce bursts that agree better with observations. Stars with highly inefficient neutrino emission in their cores produce bursts that agree best with observations. Furthermo...

  17. Blocking Metal Accretion onto Population III Stars by Stellar Wind

    Science.gov (United States)

    Tanaka, Shuta J.; Chiaki, Gen; Tominaga, Nozomu; Susa, Hajime

    2017-08-01

    Low-mass population III (PopIII) stars of ≲ 0.8 {M}⊙ could survive up until the present. The nondetection of low-mass PopIII stars in our Galaxy has already put a stringent constraint on the initial mass function (IMF) of PopIII stars, suggesting that PopIII stars have a top-heavy IMF. On the other hand, some claim that the lack of such stars stems from metal enrichment of their surfaces by the accretion of heavy elements from the interstellar medium (ISM). We investigate the effects of the stellar wind on metal accretion onto low-mass PopIII stars because accretion of the local ISM onto the Sun is prevented by the solar wind, even for neutrals. The stellar wind and radiation of low-mass PopIII stars are modeled based on knowledge of nearby low-mass stellar systems, including our Sun. We find that low-mass PopIII stars traveling across the Galaxy form a stellar magnetosphere in most of their life. Once the magnetosphere is formed, most of the neutral interstellar particles are photoionized before reaching the stellar surface and are blown away by the wind. Especially, the accretion abundance of iron will be reduced by a factor of constraining the IMF of PopIII stars.

  18. On the Maximum Mass of Accreting Primordial Supermassive Stars

    Science.gov (United States)

    Woods, T. E.; Heger, Alexander; Whalen, Daniel J.; Haemmerlé, Lionel; Klessen, Ralf S.

    2017-06-01

    Supermassive primordial stars are suspected to be the progenitors of the most massive quasars at z ˜ 6. Previous studies of such stars were either unable to resolve hydrodynamical timescales or considered stars in isolation, not in the extreme accretion flows in which they actually form. Therefore, they could not self-consistently predict their final masses at collapse, or those of the resulting supermassive black hole seeds, but rather invoked comparison to simple polytropic models. Here, we systematically examine the birth, evolution, and collapse of accreting, non-rotating supermassive stars under accretion rates of 0.01-10 M ⊙ yr-1 using the stellar evolution code Kepler. Our approach includes post-Newtonian corrections to the stellar structure and an adaptive nuclear network and can transition to following the hydrodynamic evolution of supermassive stars after they encounter the general relativistic instability. We find that this instability triggers the collapse of the star at masses of 150,000-330,000 M ⊙ for accretion rates of 0.1-10 M ⊙ yr-1, and that the final mass of the star scales roughly logarithmically with the rate. The structure of the star, and thus its stability against collapse, is sensitive to the treatment of convection and the heat content of the outer accreted envelope. Comparison with other codes suggests differences here may lead to small deviations in the evolutionary state of the star as a function of time, that worsen with accretion rate. Since the general relativistic instability leads to the immediate death of these stars, our models place an upper limit on the masses of the first quasars at birth.

  19. Accretion to a Magnetized Neutron Star in the "Propeller" Regime

    CERN Document Server

    Toropina, O D; Lovelace, R V E

    2006-01-01

    We investigate spherical accretion to a rotating magnetized star in the "propeller" regime using axisymmetric resistive magnetohydrodynamic simulations. The regime is predicted to occur if the magnetospheric radius is larger than the corotation radius and smaller than the light cylinder radius. The simulations show that accreting matter is expelled from the equatorial region of the magnetosphere and that it moves away from the star in a supersonic, disk-shaped outflow. At larger radial distances the outflow slows down and becomes subsonic. The equatorial matter outflow is initially driven by the centrifugal force, but at larger distances the pressure gradient force becomes significant. We find the fraction of the Bondi accretion rate which accretes to the surface of the star.

  20. Observations on the Formation of Massive Stars by Accretion

    CERN Document Server

    Keto, E; Keto, Eric; Wood, Kenneth

    2006-01-01

    Observations of the H66a recombination line from the ionized gas in the cluster of newly formed massive stars, G10.6-0.4, show that most of the continuum emission derives from the dense gas in an ionized accretion flow that forms an ionized disk or torus around a group of stars in the center of the cluster. The inward motion observed in the accretion flow suggests that despite the equivalent luminosity and ionizing radiation of several O stars, neither radiation pressure nor thermal pressure has reversed the accretion flow. The observations indicate why the radiation pressure of the stars and the thermal pressure of the HII region are not effective in reversing the accretion flow. The observed rate of the accretion flow, 0.001 solar masses/yr, is sufficient to form massive stars within the time scale imposed by their short main sequence lifetimes. A simple model of disk accretion relates quenched HII regions, trapped hypercompact HII regions, and photo-evaporating disks in an evolutionary sequence.

  1. Close stars and accretion in Low Luminosity Active Galactic Nuclei

    CERN Document Server

    Nayakshin, S

    2004-01-01

    Quasar accretion disks are believed to form stars by self-gravity. Low Luminosity Active Galactic Nuclei (LLAGN) are much dimmer galactic centers, and are often believed to be quasars that ran out of gaseous fuel. LLAGN accretion disks should thus co-exist with thousands to millions of stars or proto-stars left from the previous stronger accretion activity. In principle, these stars may produce several important effects: (i) contribute to the optical/UV spectra of some LLAGN; (ii) reprocessing of the stellar radiation in the dusty disks could dominate the LLAGN infra-red spectra; (iii) deplete the (accretion) gas disk much faster than it can accrete onto the supper-massive black hole (SMBH); (iv) stars, individually or in groups, may slow down and modulate the accretion flow significantly due to their inertia. In this way they may produce the LLAGN cut-off disks; (v) alternatively, frequent enough stellar collisions and resulting stellar disruptions could keep the inner disk empty. Here we explore these ideas...

  2. Limiting Accretion onto Massive Stars by Fragmentation-Induced Starvation

    Energy Technology Data Exchange (ETDEWEB)

    Peters, Thomas; /ZAH, Heidelberg; Klessen, Ralf S.; /ZAH, Heidelberg /KIPAC, Menlo Park; Mac Low, Mordecai-Mark; /Amer. Museum Natural Hist.; Banerjee, Robi; /ZAH, Heidelberg

    2010-08-25

    Massive stars influence their surroundings through radiation, winds, and supernova explosions far out of proportion to their small numbers. However, the physical processes that initiate and govern the birth of massive stars remain poorly understood. Two widely discussed models are monolithic collapse of molecular cloud cores and competitive accretion. To learn more about massive star formation, we perform simulations of the collapse of rotating, massive, cloud cores including radiative heating by both non-ionizing and ionizing radiation using the FLASH adaptive mesh refinement code. These simulations show fragmentation from gravitational instability in the enormously dense accretion flows required to build up massive stars. Secondary stars form rapidly in these flows and accrete mass that would have otherwise been consumed by the massive star in the center, in a process that we term fragmentation-induced starvation. This explains why massive stars are usually found as members of high-order stellar systems that themselves belong to large clusters containing stars of all masses. The radiative heating does not prevent fragmentation, but does lead to a higher Jeans mass, resulting in fewer and more massive stars than would form without the heating. This mechanism reproduces the observed relation between the total stellar mass in the cluster and the mass of the largest star. It predicts strong clumping and filamentary structure in the center of collapsing cores, as has recently been observed. We speculate that a similar mechanism will act during primordial star formation.

  3. X-ray Diagnostics of Grain Depletion in Matter Accreting onto T Tauri Stars

    CERN Document Server

    Hartmann, J J D P T L

    2005-01-01

    Recent analysis of high resolution Chandra X-ray spectra has shown that the Ne/O abundance ratio is remarkably constant in stellar coronae. Based on this result, we point out the utility of the Ne/O ratio as a discriminant for accretion-related X-rays from T Tauri stars, and for probing the measure of grain-depletion of the accreting material in the inner disk. We apply the Ne/O diagnostic to the classical T Tauri stars BP Tau and TW Hya--the two stars found to date whose X-ray emission appears to originate, at least in part, from accretion activity. We show that TW Hya appears to be accreting material which is significantly depleted in O relative to Ne. In constrast, BP Tau has an Ne/O abundance ratio consistent with that observed for post-T Tauri stars. We interpret this result in terms of the different ages and evolutionary states of the circumstellar disks of these stars. In the young BP Tau disk (age 0.6 Myr) dust is still present near the disk corotation radius and can be ionized and accreted, re-releas...

  4. Effect of accretion on the pre-main-sequence evolution of low-mass stars and brown dwarfs

    Science.gov (United States)

    Vorobyov, Eduard I.; Elbakyan, Vardan; Hosokawa, Takashi; Sakurai, Yuya; Guedel, Manuel; Yorke, Harold

    2017-09-01

    Aims: The pre-main-sequence evolution of low-mass stars and brown dwarfs is studied numerically starting from the formation of a protostellar or proto-brown dwarf seed and taking into account the mass accretion onto the central object during the initial several Myr of evolution. Methods: The stellar evolution was computed using the STELLAR evolution code with recent modifications. The mass accretion rates were taken from numerical hydrodynamics models by computing the circumstellar disk evolution starting from the gravitational collapse of prestellar cloud cores of various mass and angular momentum. The resulting stellar evolution tracks were compared with the isochrones and isomasses calculated using non-accreting models. Results: We find that mass accretion in the initial several Myr of protostellar evolution can have a strong effect on the subsequent evolution of young stars and brown dwarfs. The disagreement between accreting and non-accreting models in terms of the total stellar luminosity L∗, stellar radius R∗, and effective temperature Teff depends on the thermal efficiency of accretion, that is, on the fraction of accretion energy that is absorbed by the central object. The largest mismatch is found for the cold accretion case, in which essentially all accretion energy is radiated away. The relative deviations in L∗ and R∗ in this case can reach 50% for objects 1.0 Myr old, and they remain notable even for objects 10 Myr old. In the hot and hybrid accretion cases, in which a constant fraction of accretion energy is absorbed, the disagreement between accreting and non-accreting models becomes less pronounced, but still remains notable for objects 1.0 Myr old. These disagreements may lead to an incorrect age estimate for objects of (sub-)solar mass when using the isochrones that are based on non-accreting models, as has also been noted previously. We find that objects with strong luminosity bursts exhibit notable excursions in the L∗-Teff diagram

  5. Magnetospheres and Disk Accretion in Herbig Ae/Be Stars

    CERN Document Server

    Muzerolle, J; Calvet, N; Hartmann, L

    2004-01-01

    We present evidence of magnetically-mediated disk accretion in Herbig Ae/Be stars. Magnetospheric accretion models of Balmer and sodium profiles calculated with appropriate stellar and rotational parameters are in qualitative agreement with the observed profiles of the Herbig Ae star UX Ori, and yield a mass accretion rate of ~ 10^{-8} Msun/yr. If more recent indications of an extremely large rotation rate for this object are correct, the magnetic field geometry must deviate from that of a standard dipole in order to produce line emission consistent with observed flux levels. Models of the associated accretion shock qualitatively explain the observed distribution of excess fluxes in the Balmer discontinuity for a large ensemble of Herbig Ae/Be stars, and imply typically small mass accretion rates, < 10^{-7} Msun/yr. In order for accretion to proceed onto the star, significant amounts of gas must exist inside the dust destruction radius, which is potentially problematic for recently advocated scenarios of "...

  6. SMBH Spherically Symmetric Accretion Regulated by Violent Star Formation Feedback

    CERN Document Server

    Silich, S; Tenorio-Tagle, G

    2008-01-01

    The mounting evidence for violent nuclear star formation in Seyfert galaxies has led us to consider the hydrodynamics of the matter reinserted by massive stars through strong stellar winds and supernovae, under the presence of a central massive BH. We show that in all cases there is a bimodal solution strongly weighted by the location of the stagnation radius (Rst), which splits the star cluster into two different zones. Matter reinserted within the stagnation volume is to be accreted by the BH while its outer counterpart would composed a star cluster wind. The mechanical power of the latter, ensures that there is no accretion of the ISM into the BH and thus the BH accretion and its luminosity is regulated by the star formation feedback. The location of the stagnation radius is a function of three parameters: the BH mass, the mechanical power (or mass) of the star formation event and the size of the star forming region. Here we present our self-consistent, stationary solution, discuss the accretion rates and ...

  7. Star-disc interaction in galactic nuclei: orbits and rates of accreted stars

    Science.gov (United States)

    Kennedy, Gareth F.; Meiron, Yohai; Shukirgaliyev, Bekdaulet; Panamarev, Taras; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-07-01

    We examine the effect of an accretion disc on the orbits of stars in the central star cluster surrounding a central massive black hole by performing a suite of 39 high-accuracy direct N-body simulations using state-of-the art software and accelerator hardware, with particle numbers up to 128k. The primary focus is on the accretion rate of stars by the black hole (equivalent to their tidal disruption rate for black holes in the small to medium mass range) and the eccentricity distribution of these stars. Our simulations vary not only the particle number, but disc model (two models examined), spatial resolution at the centre (characterized by the numerical accretion radius) and softening length. The large parameter range and physically realistic modelling allow us for the first time to confidently extrapolate these results to real galactic centres. While in a real galactic centre both particle number and accretion radius differ by a few orders of magnitude from our models, which are constrained by numerical capability, we find that the stellar accretion rate converges for models with N ≥ 32k. The eccentricity distribution of accreted stars, however, does not converge. We find that there are two competing effects at work when improving the resolution: larger particle number leads to a smaller fraction of stars accreted on nearly circular orbits, while higher spatial resolution increases this fraction. We scale our simulations to some nearby galaxies and find that the expected boost in stellar accretion (or tidal disruption, which could be observed as X-ray flares) in the presence of a gas disc is about a factor of 10. Even with this boost, the accretion of mass from stars is still a factor of ˜100 slower than the accretion of gas from the disc. Thus, it seems accretion of stars is not a major contributor to black hole mass growth.

  8. The Formation of Massive Star Systems by Accretion

    CERN Document Server

    Krumholz, Mark R; McKee, Christopher F; Offner, Stella S R; Cunningham, Andrew J

    2009-01-01

    Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to prevent them from growing by accretion. We present three-dimensional radiation-hydrodynamic simulations of the collapse of a massive prestellar core and find that radiation pressure does not halt accretion. Instead, gravitational and Rayleigh-Taylor instabilities channel gas onto the star system through non-axisymmetric disks and filaments that self-shield against radiation, while allowing radiation to escape through optically-thin bubbles. Gravitational instabilities cause the disk to fragment and form a massive companion to the primary star. Radiation pressure does not limit stellar masses, but the instabilities that allow accretion to continue lead to small multiple systems.

  9. Formation of primordial supermassive stars by burst accretion

    CERN Document Server

    Sakurai, Y; Yoshida, N; Yorke, H W

    2015-01-01

    A promising formation channel of SMBHs at redshift 6 is the so-called DC model, which posits that a massive seed BH forms through gravitational collapse of a $\\sim 10^5~M_\\odot$ SMS. We study the evolution of such a SMS growing by rapid mass accretion. In particular, we examine the impact of time-dependent mass accretion of repeating burst and quiescent phases that are expected to occur with a self-gravitating circumstellar disk. We show that the stellar evolution with such episodic accretion differs qualitatively from that expected with a constant accretion rate, even if the mean accretion rate is the same. Unlike the case of constant mass accretion, whereby the star expands roughly following $R_* \\simeq 2.6 \\times 10^3 R_\\odot (M_*/100~M_\\odot)^{1/2}$, the protostar can substantially contract during the quiescent phases between accretion bursts. The stellar effective temperature and ionizing photon emissivity increase accordingly as the star contracts, which can cause strong ionizing feedback and halt the m...

  10. Accretion disc flows around FU Orionis stars

    Science.gov (United States)

    Clarke, C. J.; Lin, D. N. C.; Papaloizou, J. C. B.

    1989-01-01

    The accretion disk model of FU Orionis systems in outburst is investigated by examining the time-dependent behavior of a disk around a low-mass protostar that accretes at 0.00001-0.0001 solar masses/yr. It is found that the disk may be stabilized against the thermal ionization instability by the effect of advective heat transport and that it may therefore exist in the quasi-steady-state observed in post-outburst FU Orionis systems. The disk models are used to discuss the cosmochemical consequences of possible FU Ori events during the evolution of the primordial solar nebula.

  11. The multipolar magnetic fields of accreting pre-main-sequence stars: B at the inner disk, B along the accretion flow, and B at the accretion shock

    CERN Document Server

    Gregory, Scott G; Hussain, Gaitee A J

    2016-01-01

    Zeeman-Doppler imaging studies have revealed the complexity of the large-scale magnetic fields of accreting pre-main-sequence stars. All have multipolar magnetic fields with the octupole component being the dominant field mode for many of the stars studied thusfar. Young accreting stars with fully convective interiors often feature simple axisymmetric magnetic fields with dipole components of order a kilo-Gauss (at least those of mass $\\gtrsim0.5\\,{\\rm M}_\\odot$), while those with substantially radiative interiors host more complex non-axisymmetric magnetic fields with dipole components of order a few 0.1 kilo-Gauss. Here, via several simple examples, we demonstrate that i). in most cases, the dipole component alone can be used to estimate the disk truncation radius (but little else); ii) due the presence of higher order magnetic field components, the field strength in the accretion spots is far in excess of that expected if a pure dipole magnetic field is assumed. (Fields of $\\sim$6$\\,{\\rm kG}$ have been mea...

  12. Thin accretion disks around cold Bose–Einstein condensate stars

    Energy Technology Data Exchange (ETDEWEB)

    Dănilă, Bogdan, E-mail: bogdan.danila22@gmail.com [Department of Physics, Babes-Bolyai University, Kogalniceanu Street, Cluj-Napoca (Romania); Harko, Tiberiu, E-mail: t.harko@ucl.ac.uk [Department of Mathematics, University College London, Gower Street, WC1E 6BT, London (United Kingdom); Kovács, Zoltán, E-mail: kovacsz2013@yahoo.com [Max-Fiedler-Str. 7, 45128, Essen (Germany)

    2015-05-09

    Due to their superfluid properties some compact astrophysical objects, like neutron or quark stars, may contain a significant part of their matter in the form of a Bose–Einstein condensate (BEC). Observationally distinguishing between neutron/quark stars and BEC stars is a major challenge for this latter theoretical model. An observational possibility of indirectly distinguishing BEC stars from neutron/quark stars is through the study of the thin accretion disks around compact general relativistic objects. In the present paper, we perform a detailed comparative study of the electromagnetic and thermodynamic properties of the thin accretion disks around rapidly rotating BEC stars, neutron stars and quark stars, respectively. Due to the differences in the exterior geometry, the thermodynamic and electromagnetic properties of the disks (energy flux, temperature distribution, equilibrium radiation spectrum, and efficiency of energy conversion) are different for these classes of compact objects. Hence in this preliminary study we have pointed out some astrophysical signatures that may allow one to observationally discriminate between BEC stars and neutron/quark stars.

  13. Thin accretion disks around cold Bose-Einstein condensate stars

    Energy Technology Data Exchange (ETDEWEB)

    Danila, Bogdan [Babes-Bolyai University, Department of Physics, Cluj-Napoca (Romania); Harko, Tiberiu [University College London, Department of Mathematics, London (United Kingdom); Kovacs, Zoltan

    2015-05-15

    Due to their superfluid properties some compact astrophysical objects, like neutron or quark stars, may contain a significant part of their matter in the form of a Bose-Einstein condensate (BEC). Observationally distinguishing between neutron/quark stars and BEC stars is a major challenge for this latter theoretical model. An observational possibility of indirectly distinguishing BEC stars from neutron/quark stars is through the study of the thin accretion disks around compact general relativistic objects. In the present paper, we perform a detailed comparative study of the electromagnetic and thermodynamic properties of the thin accretion disks around rapidly rotating BEC stars, neutron stars and quark stars, respectively. Due to the differences in the exterior geometry, the thermodynamic and electromagnetic properties of the disks (energy flux, temperature distribution, equilibrium radiation spectrum, and efficiency of energy conversion) are different for these classes of compact objects. Hence in this preliminary study we have pointed out some astrophysical signatures that may allow one to observationally discriminate between BEC stars and neutron/quark stars. (orig.)

  14. Advection of magnetic flux by accretion disks around neutron stars

    Science.gov (United States)

    Flores-Tulian, S.; Reisenegger, A.

    The aim of our research is to address why millisecond pulsars have relatively weak surface magnetic fields, of about 10^8 G, with a narrow spread. We propose that the accretion of plasma from the companion star fully screens the original neutron star field, but the accretion disk carries additional magnetic flux from the companion star, or itself can generate field by means of dynamo processes. For a strongly magnetized star, the field prevents the disk from approaching the star. The accretion is along the field lines and deposits the matter on the polar cap. Then, the accreted plasma flows, dragging with itself the magnetic field lines, from the pole to the equator (Payne & Melatos 2004). In a following stage, when the star becomes non-magnetic, because the field has been buried, the disk touches the star. We suggest that some effective mechanism of magnetic flux transport such as that proposed by Spruit & Uzdensky 2005 (or Bisnovatyi-Kogan & Lovelace 2007), operates and necessarily leads to a "strongly magnetized disk''. It becomes laminar because the magneto-rotational instability saturates (it is considered to be responsible for turbulence in the disk), and the magnetic difussivity is negligible. Then, the loss of angular momentum allowing the accretion is only caused by the magneto-centrifugal disk-wind (Blandford & Payne 1982). Meanwhile, the wind-driven transport of the magnetic flux by the disk re-magnetizes the star. This process continues until the Lorentz force due to the star's magnetic field forbids any further accretion of matter and magnetic flux, in the Ideal Magneto-Hydro-Dynamics approach. Additional of material can fall onto the star (but at lower rate) if some instability process sets in, allowing the diffusion of mass through the magnetic field lines (e.g the Interchange Instability, Spruit & Taam 1990). All these processes might lead to an asymptotic magnetic field of 10^8 G,as is inferred from observations. We are developing a self

  15. Modeling Rotational Evolution of Young T Tauri Stars

    Science.gov (United States)

    Aidle Esin, Ann; Baxter, E.; Corrales, L.

    2007-12-01

    Measurements of rotational periods of pre-main sequence stars in several young open clusters reveal a uniform trend. Stars with masses below 0.25 solar show a bimodal period distribution with fast and slow rotators clustered around 2 day and 8 day periods, respectively, while the period distribution of low-mass stars lacks the slow rotating component. In one popular interpretation of this observational result the slow rotators are identified with the "disk locked'' stars whose periods are fixed to the orbital periods at the inner edge of the accretion disk; the fast rotators are then assumed to have lost their connection to the disk. We argue that this scenario can account for observations only if the mass accretion rate in the disk declines with time. We construct a simple model for the period evolution in T Tauri stars that includes realistic prescriptions for the mass accretion rate, radius evolution and a better treatment of the transition between strong and weak accretion disk coupling. Using this model to simulate period distribution for a young cluster, we can qualitatively reproduce the observed results, but only if the accretion is allowed to continue after the disk and the star are no longer locked. This work was supported by the grant from the Research Corporation.

  16. Radio and infrared properties of young stars

    Science.gov (United States)

    Panagia, Nino

    1987-01-01

    Observing young stars, or more appropriately, pre-main-sequence (PMS) stars, in the infrared and at radio frequencies has the advantage over optical observation in that the heavy extinction associated with a star forming region is only a minor problem, so that the whole region can be studied thoroughly. Therefore, it means being able to: (1) search for stars and do statistical studies on the rate of star formation; (2) determine their luminosity, hence, to study luminosity functions and initial mass functions down to low masses; and (3) to study their spectra and, thus, to determine the prevailing conditions at and near the surface of a newly born star and its relations with the surrounding environment. The third point is of principal interest. The report limits itself to a consideration of the observations concerning the processes of outflows from, and accretion onto, PMS stars and the theory necessary to interpret them. Section 2 discusses the radiative processes relevant in stellar outflows. The main observational results are presented in Section 3. A discussion of the statistical properties of stellar winds from PMS stars are given in Section 4.

  17. Can neutron stars have auroras ? : electromagnetic coupling process between neutron star and magnetized accretion disk

    Science.gov (United States)

    Kimura, T.; Iwakiri, W. B.; Enoto, T.; Wada, T.; Tao, C.

    2015-12-01

    In the binary neutron star system, angular momentum transfer from accretion disk to a star is essential process for spin-up/down of stars. The angular momentum transfer has been well formulated for the accretion disk strongly magnetized by the neutron star [e.g., Ghosh and Lamb, 1978, 1979a, b]. However, the electromagnetic (EM) coupling between the neutron star and accretion disk has not been self-consistently solved in the previous studies although the magnetic field lines from the star are strongly tied with the accretion disk. In this study, we applied the planet-magnetosphere coupling process established for Jupiter [Hill, 1979] to the binary neutron star system. Angular momentum distribution is solved based on the torque balance between the neutron star's surface and accretion disk coupled by the magnetic field tensions. We found the EM coupling can transfer significantly larger fraction of the angular momentum from the magnetized accretion disk to the star than the unmagnetized case. The resultant spin-up rate is estimated to ~10^-14 [sec/sec] for the nominal binary system parameters, which is comparable with or larger than the other common spin-down/up processes: e.g., the magnetic dipole radiation spin-down. The Joule heating energy dissipated in the EM coupling is estimated to be up to ~10^36 [erg/sec] for the nominal binary system parameters. The release is comparable to that of gravitation energy directly caused by the matters accreting onto the neutron star. This suggests the EM coupling at the neutron star can accompany the observable radiation as auroras with a similar manner to those at the rotating planetary magnetospheres like Jupiter, Saturn, and other gas giants.

  18. Self-consistent evolution of accreting low-mass stars and brown dwarfs

    CERN Document Server

    Baraffe, I; Vorobyov, E I; Chabrier, G

    2016-01-01

    We present self-consistent calculations coupling numerical hydrodynamics simulations of collapsing pre-stellar cores and stellar evolution models of accreting objects. We analyse the main impact of consistent accretion history on the evolution and lithium depletion of young low-mass stars and brown dwarfs. These consistent models confirm the generation of a luminosity spread in Herzsprung-Russell diagrams at ages $\\sim$ 1-10 Myr. They also confirm that early accretion can produce objects with abnormal Li depletion, as found in a previous study that was based on arbitrary accretion rates. The results strengthen that objects with anomalously high level of Li depletion in young clusters should be extremely rare. We also find that early phases of burst accretion can produce coeval models of similar mass with a range of different Li surface abundances, and in particular with Li-excess compared to the predictions of non-accreting counterparts. This result is due to a subtle competition between the effect of burst a...

  19. Constraints on Bygone Nucleosynthesis of Accreting Neutron Stars

    Science.gov (United States)

    Meisel, Zach; Deibel, Alex

    2017-03-01

    Nuclear burning near the surface of an accreting neutron star produces ashes that, when compressed deeper by further accretion, alter the star’s thermal and compositional structure. Bygone nucleosynthesis can be constrained by the impact of compressed ashes on the thermal relaxation of quiescent neutron star transients. In particular, Urca cooling nuclei pairs in nuclear burning ashes that cool the neutron star crust via neutrino emission from {e}--capture/{β }--decay cycles and provide signatures of prior nuclear burning over the ˜century timescales it takes to accrete to the {e}--capture depth of the strongest cooling pairs. Using crust cooling models of the accreting neutron star transient MAXI J0556-332, we show that this source likely lacked Type I X-ray bursts and superbursts ≳120 years ago. Reduced nuclear physics uncertainties in rp-process reaction rates and {e}--capture weak transition strengths for low-lying transitions will improve nucleosynthesis constraints using this technique.

  20. Detecting gravitational wave emission from the known accreting neutron stars

    NARCIS (Netherlands)

    Watts, A.L.; Krishnan, B.; Bildsten, L.; Schutz, B.F.

    2008-01-01

    Detection of gravitational waves from accreting neutron stars (NSs) in our Galaxy, due to ellipticity or internal oscillation, would be a breakthrough in our understanding of compact objects and explain the absence of NSs rotating near the break-up limit. Direct detection, however, poses a formidabl

  1. Large-scale magnetic field in the accretion discs of young stars: the influence of magnetic diffusion, buoyancy and Hall effect

    Science.gov (United States)

    Khaibrakhmanov, S. A.; Dudorov, A. E.; Parfenov, S. Yu.; Sobolev, A. M.

    2017-01-01

    We investigate the fossil magnetic field in the accretion and protoplanetary discs using the Shakura and Sunyaev approach. The distinguishing feature of this study is the accurate solution of the ionization balance equations and the induction equation with Ohmic diffusion, magnetic ambipolar diffusion, buoyancy and the Hall effect. We consider the ionization by cosmic rays, X-rays and radionuclides, radiative recombinations, recombinations on dust grains and also thermal ionization. The buoyancy appears as the additional mechanism of magnetic flux escape in the steady-state solution of the induction equation. Calculations show that Ohmic diffusion and magnetic ambipolar diffusion constraint the generation of the magnetic field inside the `dead' zones. The magnetic field in these regions is quasi-vertical. The buoyancy constraints the toroidal magnetic field strength close to the disc inner edge. As a result, the toroidal and vertical magnetic fields become comparable. The Hall effect is important in the regions close to the borders of the `dead' zones because electrons are magnetized there. The magnetic field in these regions is quasi-radial. We calculate the magnetic field strength and geometry for the discs with accretion rates (10^{-8}-10^{-6}) {M}_{⊙} {yr}^{-1}. The fossil magnetic field geometry does not change significantly during the disc evolution while the accretion rate decreases. We construct the synthetic maps of dust emission polarized due to the dust grain alignment by the magnetic field. In the polarization maps, the `dead' zones appear as the regions with the reduced values of polarization degree in comparison to those in the adjacent regions.

  2. UV variability and accretion dynamics in the young open cluster NGC 2264

    CERN Document Server

    Venuti, Laura; Irwin, Jonathan; Stauffer, John; Hillenbrand, Lynne; Rebull, Luisa; Cody, Ann Marie; Alencar, Silvia; Micela, Giuseppina; Flaccomio, Ettore; Peres, Giovanni

    2015-01-01

    We explore UV and optical variability signatures for several hundred members of NGC 2264 (3 Myr). We performed simultaneous u- and r-band monitoring over two full weeks with CFHT/MegaCam. About 750 young stars are probed; 40% of them are accreting. Statistically distinct variability properties are observed for accreting and non-accreting cluster members. The accretors exhibit a significantly higher level of variability than the non-accretors, especially in the UV. The amount of u-band variability correlates statistically with UV excess in disk-bearing objects, which suggests that accretion and star-disk interaction are the main sources of variability. Cool magnetic spots, several hundred degrees colder than the photosphere and covering from 5 to 30% of the stellar surface, appear to be the leading factor of variability for the non-accreting stars. In contrast, accretion spots, a few thousand degrees hotter than the photosphere and covering a few percent of the stellar surface, best reproduce the variability o...

  3. Neutron drip transition in accreting and nonaccreting neutron star crusts

    CERN Document Server

    Chamel, N; Zdunik, J L; Haensel, P

    2015-01-01

    The neutron-drip transition in the dense matter constituting the interior of neutron stars generally refers to the appearance of unbound neutrons as the matter density reaches some threshold density $\\rho_\\textrm{drip}$. This transition has been mainly studied under the cold catalyzed matter hypothesis. However, this assumption is unrealistic for accreting neutron stars. After examining the physical processes that are thought to be allowed in both accreting and nonaccreting neutron stars, suitable conditions for the onset of neutron drip are derived and general analytical expressions for the neutron drip density and pressure are obtained. Moreover, we show that the neutron-drip transition occurs at lower density and pressure than those predicted within the mean-nucleus approximation. This transition is studied numerically for various initial composition of the ashes from X-ray bursts and superbursts using microscopic nuclear mass models.

  4. Accretion of dark matter by stars

    CERN Document Server

    Brito, Richard; Okawa, Hirotada

    2015-01-01

    Searches for dark matter imprints are one of the most active areas of current research. We focus here on light fields with mass $m_B$, such as axions and axion-like candidates. Using perturbative techniques and full-blown nonlinear Numerical Relativity methods, we show that (i) dark matter can pile up in the center of stars, leading to configurations and geometries oscillating with frequency which is a multiple of f=$2.5 10^{14}$ $m_B c^2$/eV Hz. These configurations are stable throughout most of the parameter space, and arise out of credible mechanisms for dark-matter capture. Stars with bosonic cores may also develop in other theories with effective mass couplings, such as (massless) scalar-tensor theories. We also show that (ii) collapse of the host star to a black hole is avoided by efficient gravitational cooling mechanisms.

  5. Accretion of dark matter by stars.

    Science.gov (United States)

    Brito, Richard; Cardoso, Vitor; Okawa, Hirotada

    2015-09-11

    Searches for dark matter imprints are one of the most active areas of current research. We focus here on light fields with mass m_{B}, such as axions and axionlike candidates. Using perturbative techniques and full-blown nonlinear numerical relativity methods, we show the following. (i) Dark matter can pile up in the center of stars, leading to configurations and geometries oscillating with a frequency that is a multiple of f=2.5×10^{14}(m_{B}c^{2}/eV)  Hz. These configurations are stable throughout most of the parameter space, and arise out of credible mechanisms for dark-matter capture. Stars with bosonic cores may also develop in other theories with effective mass couplings, such as (massless) scalar-tensor theories. We also show that (ii) collapse of the host star to a black hole is avoided by efficient gravitational cooling mechanisms.

  6. Large-scale magnetic field in the accretion discs of young stars: the influence of magnetic diffusion, buoyancy and Hall effect

    CERN Document Server

    Khaibrakhmanov, Sergey A; Parfenov, Sergey Yu; Sobolev, Andrey M

    2016-01-01

    We investigate the fossil magnetic field in the accretion and protoplanetary discs using the Shakura and Sunyaev approach. The distinguishing feature of this study is the accurate solution of the ionization balance equations and the induction equation with Ohmic diffusion, magnetic ambipolar diffusion, buoyancy and the Hall effect. We consider the ionization by cosmic rays, X-rays and radionuclides, radiative recombinations, recombinations onto dust grains, and also thermal ionization. The buoyancy appears as the additional mechanism of magnetic flux escape in the steady-state solution of the induction equation. Calculations show that Ohmic diffusion and magnetic ambipolar diffusion constraint the generation of the magnetic field inside the `dead' zones. The magnetic field in these regions is quasi-vertical. The buoyancy constraints the toroidal magnetic field strength close to the disc inner edge. As a result, the toroidal and vertical magnetic fields become comparable. The Hall effect is important in the re...

  7. Super-Eddington accretion on to a magnetized neutron star

    Science.gov (United States)

    Chashkina, Anna; Abolmasov, Pavel; Poutanen, Juri

    2017-09-01

    Most of ultraluminous X-ray sources are thought to be objects accreting above their Eddington limits. In the recently identified class of ultraluminous X-ray pulsars, accretor is a neutron star and thus has a fairly small mass with a small Eddington limit. The accretion disc structure around such an object affects important observables such as equilibrium period, period derivative and the size of the magnetosphere. We propose a model of a nearly standard accretion disc interacting with the magnetosphere only in a thin layer near the inner disc rim. Our calculations show that the size of the magnetosphere may be represented as the classical Alfvén radius times a dimensionless factor ξ which depends only on the disc thickness. In the case of radiation-pressure-dominated disc, the size of the magnetosphere does not depend on the mass accretion rate. In general, increasing the disc thickness leads to a larger magnetosphere size in units of the Alfvén radius. For large enough mass accretion rates and magnetic moments, it is important to take into account not only the pressure of the magnetic field and the radiation pressure inside the disc, but also the pressure of the radiation produced close to the surface of the neutron star in accretion column. The magnetospheric size may increase by up to factor of 2 as a result of the effects related to the disc thickness and the irradiation from the central source. Accounting for these effects reduces the estimate of the neutron star magnetic moment by a factor of several orders.

  8. Hot Radiative Accretion onto a Spinning Neutron Star

    CERN Document Server

    Medvedev, M V

    2004-01-01

    (Abridged) A new type of self-similar hot viscous radiative accretion flow onto a rapidly spinning neutron star has recently been discovered. This ``hot brake'' flow forms in the two-temperature zone (close to a central object), but at a sufficiently low accretion rate and a high spin it may extend in the radial direction beyond ~300 Schwarzchild radii into a one-temperature zone. When the spin of the star is small enough, the flow transforms smoothly to an advection-dominated accretion flow. All gas parameters (density, angular velocity, temperature, luminosity, angular momentum flux) except for the radial velocity are independent of the mass accretion rate. The radiative efficiency may be arbitrarily large as M-dot -> 0. The gas angular momentum is transported outward under most conditions, hence the central star is nearly always spun-down. The flow is convectively stable. We also find that themal conduction in the flow is strong enough to make the flow thermally stable. The very fact that the density, temp...

  9. Magnetically Controlled Spasmodic Accretion during Star Formation. II. Results

    Science.gov (United States)

    Tassis, Konstantinos; Mouschovias, Telemachos Ch.

    2005-01-01

    The problem of the late accretion phase of the evolution of an axisymmetric, isothermal magnetic disk surrounding a forming star has been formulated in a companion paper. The ``central sink approximation'' is used to circumvent the problem of describing the evolution inside the opaque central region for densities greater than 1011 cm-3 and radii smaller than a few AU. Only the electrons are assumed to be attached to the magnetic field lines, and the effects of both negatively and positively charged grains are accounted for. After a mass of 0.1 Msolar accumulates in the central cell (forming star), a series of magnetically driven outflows and associated outward-propagating shocks form in a quasi-periodic fashion. As a result, mass accretion onto the protostar occurs in magnetically controlled bursts. We refer to this process as spasmodic accretion. The shocks propagate outward with supermagnetosonic speeds. The period of dissipation and revival of the outflow decreases in time, as the mass accumulated in the central sink increases. We evaluate the contribution of ambipolar diffusion to the resolution of the magnetic flux problem of star formation during the accretion phase, and we find it to be very significant albeit not sufficient to resolve the entire problem yet. Ohmic dissipation is completely negligible in the disk during this phase of the evolution. The protostellar disk is found to be stable against interchange-like instabilities, despite the fact that the mass-to-flux ratio has temporary local maxima.

  10. MRI-driven Accretion onto Magnetized stars: Axisymmetric MHD Simulations

    CERN Document Server

    Romanova, Marina M; Koldoba, Alexander V; Lovelace, Richard V E

    2011-01-01

    We present the first results of a global axisymmetric simulation of accretion onto rotating magnetized stars from a turbulent, MRI-driven disk. The angular momentum is transported outward by the magnetic stress of the turbulent flow with a rate corresponding to a Shakura-Sunyaev viscosity parameter alpha\\approx 0.01-0.04. The result of the disk-magnetosphere interaction depends on the orientation of the poloidal field in the disk relative to that of the star at the disk-magnetosphere boundary. If fields have the same polarity, then the magnetic flux is accumulated at the boundary and blocks the accretion which leads to the accumulation of matter at the boundary. Subsequently, this matter accretes to the star in outburst before accumulating again. Hence, the cycling, `bursty' accretion is observed. If the disc and stellar fields have opposite polarity, then the field reconnection enhances the penetration of the disk matter towards the deeper field lines of the magnetosphere. However, the magnetic stress at the...

  11. An expanding disk around the young massive star AFGL 2591

    NARCIS (Netherlands)

    Wang, K. S.; van der Tak, F.; Hogerheijde, M.

    2011-01-01

    Recent detections of disks around young high-mass stars indicate their formation through accretion rather than coalescence, but the physical properties of these disks are poorly known. In this study, we used Plateau de Bure interferometric images to probe the environment of the nearby (˜1 kpc) and l

  12. A Substantial Dust Disk Surrounding an Actively Accreting First-Ascent Giant Star

    CERN Document Server

    Melis, C; Song, I; Rhee, J H; Metchev, S

    2009-01-01

    We report identification of the first unambiguous example of what appears to be a new class of first-ascent giant stars that are actively accreting gas and dust and that are surrounded by substantial dusty disks. These old stars, who are nearing the end of their lives, are experiencing a rebirth into characteristics typically associated with newborn stars. The F2-type first-ascent giant star TYC 4144 329 2 is in a wide separation binary system with an otherwise normal G8 IV star, TYC 4144 329 1. From Keck near-infrared imaging and high-resolution spectroscopy we are able to determine that these two stars are $\\sim$1 Gyr old and reside at a distance of $\\sim$550 pc. One possible explanation for the origin of the accreting material is common-envelope interaction with a low-mass stellar or sub-stellar companion. The gaseous and dusty material around TYC 4144 329 2, as it is similar to the primordial disks observed around young classical T Tauri stars, could potentially give rise to a new generation of planets an...

  13. Hubble Space Telescope Observations of Accretion-Induced Star Formation in the Tadpole Galaxy Kiso 5639

    CERN Document Server

    Elmegreen, Debra Meloy; Almeida, Jorge Sanchez; Munoz-Tunon, Casiana; Mendez-Abreu, Jairo; Gallagher, John S; Rafelski, Marc; Filho, Mercedes; Ceverino, Daniel

    2016-01-01

    The tadpole galaxy Kiso 5639 has a slowly rotating disk with a drop in metallicity at its star-forming head, suggesting that star formation was triggered by the accretion of metal-poor gas. We present multi-wavelength HST WFC3 images of UV through I band plus Halpha to search for peripheral emission and determine the properties of various regions. The head has a mass in young stars of ~10^6 Mo and an ionization rate of 6.4x10^51 s^{-1}, equivalent to ~2100 O9-type stars. There are four older star-forming regions in the tail, and an underlying disk with a photometric age of ~1 Gyr. The mass distribution function of 61 star clusters is a power law with a slope of -1.73+-0.51. Fourteen young clusters in the head are more massive than 10^4 Mo, suggesting a clustering fraction of 30%-45%. Wispy filaments of Halpha emission and young stars extend away from the galaxy. Shells and holes in the head HII region could be from winds and supernovae. Gravity from the disk should limit the expansion of the HII region, altho...

  14. Effect of Gas Accretion Disc Profile on Orbital Parameters of the Accreted Stars

    Science.gov (United States)

    Shukirgaliyev, Bekdaulet T.; Panamarev, Taras P.; Naurzbaeva, Aisha Zh.; Kalambay, Mukhagali T.; Makukov, Maxim A.; Vilkoviskij, Emmanuil Y.; Omarov, Chingis T.; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-10-01

    The results of studies of the effect of the gas disk and its profile on the dynamics of active galactic nuclei are presented. The study was conducted with a numerical model of galactic nucleus based on phiGRAPE+GPU comprising three subsystems - a central supermassive black hole, gaseous accretion disc, and compact stellar cluster. The evolution of the compact stellar cluster is modeled with direct integration (N-body simulation), while the black hole and gaseous disc are represented phenomenologically: the black hole is introduced as an external potential (fixed in space but variable in time due to black hole mass growth), and the gaseous disc is introduced as spatial time-independent density distribution. We examined and compared with each other orbital parameters of accreting stars for model of the galactic nucleus with gas disc of constant and variable thickness, as well as without gas. It was found that in the presence of a gaseous disk almost half of the accreted particles interact strongly with the gas and are captured by the disc before accretion, while more than 85% of particles are affected to some extent by the disc prior to accretion. This suggests that interaction of the stellar cluster with the gas disk in the galactic nucleus might lead to the formation of stellar disk in the central part of the nucleus.

  15. NEAR-ULTRAVIOLET EXCESS IN SLOWLY ACCRETING T TAURI STARS: LIMITS IMPOSED BY CHROMOSPHERIC EMISSION

    Energy Technology Data Exchange (ETDEWEB)

    Ingleby, Laura; Calvet, Nuria; Bergin, Edwin; McClure, Melissa [Department of Astronomy, University of Michigan, 830 Dennison Building, 500 Church Street, Ann Arbor, MI 48109 (United States); Herczeg, Gregory [Max-Planck-Institut fuer Extraterrestriche Physik, Postfach 1312, D-85741 Garching (Germany); Brown, Alexander; France, Kevin [Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309-0389 (United States); Alexander, Richard [Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH (United Kingdom); Edwards, Suzan [Department of Astronomy, Smith College, Northampton, MA 01063 (United States); Espaillat, Catherine; Brown, Joanna [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-78, Cambridge, MA 02138 (United States); Gregory, Scott G.; Hillenbrand, Lynne [Department of Astrophysics, California Institute of Technology, MC 249-17, Pasadena, CA 91125 (United States); Roueff, Evelyne; Abgrall, Herve [LUTH and UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon, Place J. Janssen, F-92195 Meudon (France); Valenti, Jeff [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Walter, Frederick [Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800 (United States); Johns-Krull, Christopher [Department of Physics and Astronomy, Rice University, Houston, TX 77005 (United States); Linsky, Jeffrey [JILA, University of Colorado and NIST, 440 UCB Boulder, CO 80309-0440 (United States); Ardila, David, E-mail: lingleby@umich.edu, E-mail: ncalvet@umich.edu, E-mail: gregoryh@mpe.mpg.de [NASA Herschel Science Center, California Institute of Technology, Mail Code 100-22, Pasadena, CA 91125 (United States); and others

    2011-12-20

    Young stars surrounded by disks with very low mass accretion rates are likely in the final stages of inner disk evolution and therefore particularly interesting to study. We present ultraviolet (UV) observations of the {approx}5-9 Myr old stars RECX-1 and RECX-11, obtained with the Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph on the Hubble Space Telescope, as well as optical and near-infrared spectroscopic observations. The two stars have similar levels of near-UV emission, although spectroscopic evidence indicates that RECX-11 is accreting and RECX-1 is not. The line profiles of H{alpha} and He I {lambda}10830 in RECX-11 show both broad and narrow redshifted absorption components that vary with time, revealing the complexity of the accretion flows. We show that accretion indicators commonly used to measure mass accretion rates, e.g., U-band excess luminosity or the Ca II triplet line luminosity, are unreliable for low accretors, at least in the middle K spectral range. Using RECX-1 as a template for the intrinsic level of photospheric and chromospheric emission, we determine an upper limit of 3 Multiplication-Sign 10{sup -10} M{sub Sun} yr{sup -1} for RECX-11. At this low accretion rate, recent photoevaporation models predict that an inner hole should have developed in the disk. However, the spectral energy distribution of RECX-11 shows fluxes comparable to the median of Taurus in the near-infrared, indicating that substantial dust remains. Fluorescent H{sub 2} emission lines formed in the innermost disk are observed in RECX-11, showing that gas is present in the inner disk, along with the dust.

  16. Accretion-induced variability links young stellar objects, white dwarfs, and black holes.

    Science.gov (United States)

    Scaringi, Simone; Maccarone, Thomas J; Körding, Elmar; Knigge, Christian; Vaughan, Simon; Marsh, Thomas R; Aranzana, Ester; Dhillon, Vikram S; Barros, Susana C C

    2015-10-01

    The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies.

  17. On radial oscillations in viscous accretion discs surrounding neutron stars

    Science.gov (United States)

    Chen, Xingming; Taam, Ronald E.

    1992-01-01

    Radial oscillations resulting from axisymmetric perturbations in viscous accretion disks surrounding neutron stars in X-ray binary systems have been investigated. Within the framework of the alpha-viscosity model a series of hydrodynamic calculations demonstrates that the oscillations are global for alpha of about 1. On the other hand, for alpha of 0.4 or less, the oscillations are local and confined to the disk boundaries. If viscous stresses acting in the radial direction are included, however, it is found that the disk can be stabilized. The application of such instabilities in accretion disks, without reference to the boundary layer region between the neutron star (or magnetosphere) and the inner edge of the disk, to the phenomenology of quasi-periodic oscillations is brought into question.

  18. The outer crust of non-accreting cold neutron stars

    CERN Document Server

    Ruster, S B; Schaffner-Bielich, J; Ruster, Stefan B.; Hempel, Matthias; Schaffner-Bielich, Jurgen

    2006-01-01

    The properties of the outer crust of non-accreting cold neutron stars are studied by using modern nuclear data and theoretical mass tables updating in particular the classic work of Baym, Pethick and Sutherland. Experimental data from the atomic mass table from Audi, Wapstra, and Thibault of 2003 is used and a thorough comparison of many modern theoretical nuclear models, relativistic and non-relativistic ones, is performed for the first time. In addition, the influences of pairing and deformation are investigated. State-of-the-art theoretical nuclear mass tables are compared in order to check their differences concerning the neutron dripline, magic neutron numbers, the equation of state, and the sequence of neutron-rich nuclei up to the dripline in the outer crust of non-accreting cold neutron stars.

  19. X-shooter study of accretion in Chamaeleon I. II. A steeper increase of accretion with stellar mass for very low-mass stars?

    Science.gov (United States)

    Manara, C. F.; Testi, L.; Herczeg, G. J.; Pascucci, I.; Alcalá, J. M.; Natta, A.; Antoniucci, S.; Fedele, D.; Mulders, G. D.; Henning, T.; Mohanty, S.; Prusti, T.; Rigliaco, E.

    2017-08-01

    The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star-forming region carried out using spectra taken with the ESO VLT/X-shooter spectrograph. The sample is nearly complete down to stellar masses (M⋆) 0.1 M⊙ for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broadband flux-calibrated medium resolution spectrum. The correlation between accretion luminosity to stellar luminosity, and of mass accretion rate to stellar mass in the logarithmic plane yields slopes of 1.9 ± 0.1 and 2.3 ± 0.3, respectively. These slopes and the accretion rates are consistent with previous results in various star-forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity lower than 0.45 L⊙ and for stellar masses lower than 0.3 M⊙ is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane that are empty of objects: one region at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second region is located just above the observational limits imposed by chromospheric emission, at M⋆ 0.3 - 0.4 M⊙. These are typical masses where photoevaporation is known to be effective. The mass accretion rates of this region are 10-10M⊙/yr, which is compatible with the value expected for photoevaporation to rapidly dissipate the inner disk. This work is

  20. Detecting gravitational wave emission from the known accreting neutron stars

    CERN Document Server

    Watts, Anna; Bildsten, Lars; Schutz, Bernard

    2008-01-01

    Detection of gravitational waves from accreting neutron stars (NSs) in our galaxy, due to ellipticity or internal oscillation, would be a breakthrough in our understanding of compact objects and explain the absence of NSs rotating near the break-up limit. Direct detection, however, poses a formidable challenge. Using the current data available on the properties of the accreting NSs in Low Mass X-Ray Binaries (LMXBs), we quantify the detectability for the known accreting NSs, considering various emission scenarios and taking into account the negative impact of parameter uncertainty on the data analysis process. Only a few of the persistently bright NSs accreting at rates near the Eddington limit are detectable by Advanced LIGO if they are emitting gravitational waves at a rate matching the torque from accretion. A larger fraction of the known population is detectable if the spin and orbital parameters are known in advance, especially with the narrow-band Advanced LIGO. We identify the most promising targets, a...

  1. Self-Similar Hot Accretion Flow onto a Neutron Star

    CERN Document Server

    Medvedev, M V; Medvedev, Mikhail V.; Narayan, Ramesh

    2000-01-01

    We consider hot, two-temperature, viscous accretion onto a rotating, unmagnetized neutron star. We assume Coulomb coupling betweenthe protons and electrons, and free-free cooling from the electrons. We show that the accretion flow has an extended settling region which can be described by means of two analytical self-similar solutions: a two-temperature solution which is valid in an inner zone, $r10^{2.5}$. In both zones the density varies as $\\rho\\propto r^{-2}$ and the angular velocity as $\\Omega\\propto r^{-3/2}$. We solve the flow equations numerically and confirm that the analytical solutions are accurate. The self-similar settling solution differs from the advection-dominated accretion flow discussed in the context of black hole accretion. The settling flow radiates the energy dissipated by viscosity; so it is not advection-dominated. Except for the radial velocity, all other gas properties - density, angular velocity, temperature, luminosity, angular momentum flux - are independent of the mass accretion ...

  2. Accretion phenomena in nearby star-forming dwarf galaxies

    Science.gov (United States)

    Annibali, F.; Tosi, M.; Aloisi, A.; Bellazzini, M.; Buzzoni, A.; Cignoni, M.; Ciotti, L.; Cusano, F.; Nipoti, C.; Sacchi, E.; Paris, D.; Romano, D.

    2017-03-01

    We present two pilot studies for the search and characterization of accretion events in star-forming dwarf galaxies. Our strategy consists of two complementary approaches: i) the direct search for stellar substructures around dwarf galaxies through deep wide-field imaging, and ii) the characterization of the chemical properties in these systems up to large galacto-centric distances. We show our results for two star-forming dwarf galaxies, the starburst irregular NGC 4449, and the extremely metal-poor dwarf DDO 68.

  3. Self-Similar Hot Accretion Flow onto a Neutron Star

    CERN Document Server

    Medvedev, M V

    2001-01-01

    We present analytical and numerical solutions which describe a hot, viscous, two-temperature accretion flow onto a neutron star or any other compact star with a surface. We assume Coulomb coupling between the protons and electrons, and free-free cooling from the electrons. Outside a thin boundary layer, where the accretion flow meets the star, we show that there is an extended settling region which is well-described by two self-similar solutions: (1) a two-temperature solution which is valid in an inner zone $r\\le10^{2.5}$ ($r$ is in Schwarzchild units), and (2) a one-temperature solution at larger radii. In both zones, $\\rho\\propto r^{-2}, \\Omega\\propto r^{-3/2}, v\\propto r^0,\\ T_p\\propto r^{-1}$; in the two-temperature zone, $T_e\\propto r^{-1/2}$. The luminosity of the settling zone arises from the rotational energy of the star as the star is braked by viscosity; hence the luminosity is independent of $\\dot M$. The settling solution is convectively and viscously stable and is unlikely to have strong winds o...

  4. Mapping the QCD Phase Transition with Accreting Compact Stars

    CERN Document Server

    Blaschke, David; Grigorian, Hovik

    2008-01-01

    We discuss an idea for how accreting millisecond pulsars could contribute to the understanding of the QCD phase transition in the high-density nuclear matter equation of state (EoS). It is based on two ingredients, the first one being a ``phase diagram'' of rapidly rotating compact star configurations in the plane of spin frequency and mass, determined with state-of-the-art hybrid equations of state, allowing for a transition to color superconducting quark matter. The second is the study of spin-up and accretion evolution in this phase diagram. We show that the quark matter phase transition leads to a characteristic line in the Omega-M plane, the phase border between neutron stars and hybrid stars with a quark matter core. Along this line a change in the pulsar's moment of inertia entails a waiting point phenomenon in the accreting millisecond X-ray pulsar (AMXP) evolution: most of these objects should therefore be found along the phase border in the Omega-M plane, which may be viewed as the AMXP analog of th...

  5. Spherically-symmetric Accretion onto a Black Hole at the Center of a Young Stellar Cluster

    CERN Document Server

    Silich, Sergiy; Hueyotl-Zahuantitla, Filiberto

    2008-01-01

    We present a self-consistent, bimodal stationary solution for spherically symmetric flows driven by young massive stellar clusters with a central supermassive black hole. We demonstrate that the hydrodynamic regime of the flow depends on the location of the cluster in the 3D (star cluster mechanical luminosity - BH mass - star cluster radius) parameter space. We show that a threshold mechanical luminosity (L_crit) separates clusters which evolve in the BH dominated regime frome those whose internal structure is strongly affected by the radiative cooling. In the first case(below the threshold energy) gravity of the BH separates the flow into two distinct zones: the inner accretion zone and the outer zone where the star cluster wind is formed. In the second case (above the critical luminosity), catastrophic cooling sets in inside the cluster. In this case the injected plasma becomes thermally unstable that inhibits a complete stationary solution. We compared the calculated accretion rates and the BH luminositie...

  6. The accretion dynamics of EX Lupi in quiescence:The star, the spot, and the accretion column

    CERN Document Server

    Sicilia-Aguilar, A; Roccatagliata, V; Cameron, A C; Kospal, A; Henning, Th; Abraham, P; Sipos, N

    2015-01-01

    EX Lupi is a young star, prototype of EXor variables. Its spectrum is very rich in emission lines, including many metallic lines. It has been also proposed to have a close companion. We use the metallic emission lines to study the accretion structures and to test the companion hypothesis. We analyse 54 spectra taken in 5 years of quiescence time. We study the line profile variability and the radial velocity of the metallic emission lines. We use the velocity signatures of different species with various excitation conditions and their time dependency to track the dynamics associated to accretion. We observe periodic velocity variations in the line components consistent with rotational modulation. The modulation is stronger for lines with higher excitation potentials. We propose that the narrow line components are produced in the post-shock region, while the broad components originate in the more extended, pre-shock material. All the emission lines suffer velocity modulation due to the rotation of the star. The...

  7. Luminosities of Disk--accreting Non--magnetic Neutron Stars

    CERN Document Server

    Thampan, A V

    2002-01-01

    Disk accretion onto a neutron star possessing a weak surface magnetic field ($B \\le 10^8$ G) provides interesting X-ray emission scenarios, and is relevant for understanding X-ray bursters and low-mass X-ray binaries (LMXBs). The standard (Newtonian) theory of disk-accretion predicts that the matter spiralling in from infinity loses one-half of its total gravitational energy in the extended disk, and the remainder in a narrow boundary layer girdling the neutron star. The ratio of the boundary layer luminosity to that from the disk ($E_{\\rm BL}/E_{\\rm D}$) is, therefore, unity. On incorporation of general relativity without rotation (Schwarzschild solution), $E_{\\rm BL}/E_{\\rm D}$ is seen to be as high as 6. We construct rotating sequences of neutron stars for three representative equations of state. We show here that for a neutron star rotating at a limit where centrifugal force balances the inward gravitational force, $E_{\\rm BL}/E_{\\rm D}\\sim 0$.

  8. Spectroscopic Evidence of Sporadic Gas Accretion onto the Herbig AE Stars with Non-Periodic Algol-Type Minima

    Science.gov (United States)

    Kozlova, O. V.; Grinin, V. P.; Rostopchina, A. N.

    We present the results of simultaneous spectroscopic and photometric observations for six isolated Herbig Ae stars with non-periodic Algol-like minima: UX Ori, BF Ori, CQ Tau, SV Cep, VX Gas, and WW Vul. In all cases the Ha line has the profile typical for axially symmetric accretion. In the spectra of five stars (CQ Tau is the exception) the strong He I 5876 Å line has been observed in absorption which is not typical of normal A stars. In three cases: UX Ori, VX Cas, and WW Vul, variability of this line is found. We did not observe any correlation between the strength of this line and the brightness of the stars. These observational facts are considered as evidence for gas accretion, which is probably an important property of young stars with non-periodic Algol-type minima.

  9. Magnetic propeller effect in the spectra of young stars

    Science.gov (United States)

    Grinin, V. P.; Potravnov, I. S.; Ilyin, I. V.; Shulman, S. G.

    2015-08-01

    The origin of the blueshifted narrow absorption components in the resonance sodium doublet lines observed in the spectra of some young stars is discussed. Such components are assumed to be formed by the interaction of the circumstellar gas with the stellar magnetosphere in the magnetic propeller regime. The results of observations for the post UX Ori star RZ Psc are discussed in detail. This star shows distinctive signatures of mass outflow in the absence of any clear accretion signatures. Such a picture is quite possible in the magnetic propeller regime. Estimates show that for this regime to be realized, the star must have a surface magnetic field of ~1 kG at an accretion rate that does not exceed 10-10 M . yr-1.

  10. The RMS Survey: Critical Tests of Accretion Models for the Formation of Massive Stars

    CERN Document Server

    Davies, Ben; Lumsden, Stuart L; Hosokawa, Takashi; Oudmaijer, Rene D; Urquhart, James S; Mottram, Joseph C; Stead, Joseph

    2011-01-01

    There is currently no accepted theoretical framework for the formation of the most massive stars, and the manner in which protostars continue to accrete and grow in mass beyond \\sim10Msun is still a controversial topic. In this study we use several prescriptions of stellar accretion and a description of the Galactic gas distribution to simulate the luminosities and spatial distribution of massive protostellar population of the Galaxy. We then compare the observables of each simulation to the results of the Red MSX Source (RMS) survey, a recently compiled database of massive young stellar objects. We find that the observations are best matched by accretion rates which increase as the protostar grows in mass, such as those predicted by the turbulent core and competitive accretion (i.e. Bondi-Hoyle) models. These 'accelerating accretion' models provide very good qualitative and quantitative fits to the data, though we are unable to distinguish between these two models on our simulations alone. We rule out models...

  11. Compression of matter in the center of accreting neutron stars

    CERN Document Server

    Bejger, M; Haensel, P; Fortin, M

    2011-01-01

    In order to estimate the feasibility of dense-matter phase transition, we study the evolution of central density and baryon chemical potential of accreting neutron stars. The thin-disk accretion with and without the magnetic field torque is compared with the spin-down scenario for a selection of recent equations of state. We compare the prevalent (in the recycled-pulsar context) Keplerian thin-disk model, in which the matter is accreted from the marginally-stable circular orbit, with the recent magnetic-torque model that takes into account the influence of stellar magnetic field on the effective inner boundary of the disk. Calculations are performed using a multi-domain spectral methods code in the framework of General Relativity. We consider three equations of state consistent with recently measured mass of PSR J1614-2230, 1.97+-0.04 Msun (one of them softened by the appearance of hyperons). In the case of no magnetic torque and efficient angular momentum transfer from the disk to the star, substantial centr...

  12. Magnetically Controlled Spasmodic Accretion During Star Formation. II. Results

    CERN Document Server

    Tassis, K; Tassis, Konstantinos; Mouschovias, Telemachos Ch.

    2004-01-01

    The problem of the late accretion phase of the evolution of an axisymmetric, isothermal magnetic disk surrounding a forming star has been formulated in a companion paper. The "central sink approximation" is used to circumvent the problem of describing the evolution inside the opaque central region for densities greater than 10^11 cm^-3 and radii smaller than a few AUs. Only the electrons are assumed to be attached to the magnetic field lines, and the effects of both negatively and positively charged grains are accounted for. After a mass of 0.1 solar mass accumulates in the central cell (forming star), a series of magnetically driven outflows and associated outward propagating shocks form in a quasi-periodic fashion. As a result, mass accretion onto the protostar occurs in magnetically controlled bursts. We refer to this process as spasmodic accretion. The shocks propagate outward with supermagnetosonic speeds. The period of dissipation and revival of the outflow decreases in time, as the mass accumulated in ...

  13. Comptonization and QPO Origins in Accreting Neutron Star Systems

    CERN Document Server

    Lee, H C; Lee, Hyong C.; Miller, Guy S.

    1997-01-01

    We develop a simple, time-dependent Comptonization model to probe the origins of spectral variability in accreting neutron star systems. In the model, soft ``seed photons'' are injected into a corona of hot electrons, where they are Compton upscattered before escaping as hard X-rays. The model describes how the hard X-ray spectrum varies when the properties of either the soft photon source or the Comptonizing medium undergo small oscillations. Observations of the resulting spectral modulations can determine whether the variability is due to (i) oscillations in the injection of seed photons, (ii) oscillations in the coronal electron density, or (iii) oscillations in the coronal energy dissipation rate. Identifying the origin of spectral variability should help clarify how the corona operates and its relation to the accretion disk. It will also help in finding the mechanisms underlying the various quasi-periodic oscillations (QPO) observed in the X-ray outputs of many accreting neutron star and black hole syste...

  14. Doppler Probe of Accretion onto a T Tauri star

    CERN Document Server

    Petrov, Peter P; Herczeg, Gregory J; Stempels, Henricus C; Walter, Frederick M

    2014-01-01

    The YY Ori stars are T Tauri stars with prominent time-variable redshifted absorption components that flank certain emission lines. One of the brightest in this class is S CrA, a visual double star. We have obtained a series of high-resolution spectra of the two components during four nights with the UVES spectrograph at the Very Large Telescope. We followed the spectral changes occurring in S CrA to derive the physical structure of the accreting gas. We found that both stars are very similar with regard to surface temperature, radius, and mass. Variable redshifted absorption components are particularly prominent in the SE component. During one night, this star developed a spectrum unique among the T Tauri stars: extremely strong and broad redshifted absorption components appeared in many lines of neutral and ionized metals, in addition to those of hydrogen and helium. The absorption depths of cooler, low ionization lines peak at low velocities - while more highly ionized lines have peak absorption depths at ...

  15. Photometric determination of the mass accretion rate of pre-main sequence stars. IV. Recent star formation in NGC 602

    CERN Document Server

    De Marchi, Guido; Panagia, Nino

    2013-01-01

    We have studied the young stellar populations in NGC 602, in the Small Magellanic Cloud, using a novel method that we have developed to combine Hubble Space Telescope photometry in the V, I, and Halpha bands. We have identified about 300 pre-main sequence (PMS) stars, all of which are still undergoing active mass accretion, and have determined their physical parameters (effective temperature, luminosity, age, mass and mass accretion rate). Our analysis shows that star formation has been present in this field over the last 60 Myr. In addition, we can recognise at least two clear, distinct, and prominent episodes in the recent past: one about 2 Myr ago, but still ongoing in regions of higher nebulosity, and one (or more) older than 30 Myr, encompassing both stars dispersed in the field and two smaller clusters located about 100 arcsec north of the centre of NGC 602. The relative locations of younger and older PMS stars do not imply a causal effect or triggering of one generation on the other. The strength of th...

  16. On the spreading layer emission in luminous accreting neutron stars

    CERN Document Server

    Revnivtsev, Mikhail G; Poutanen, Juri

    2013-01-01

    Emission of the neutron star surface potentially contains information about its size and thus of vital importance for high energy astrophysics. In spite of the wealth of data on the emission of luminous accreting neutron stars, the emission of their surfaces is hard to disentangle from their time averaged spectra. A recent X-ray transient source XTE J1701-462 has provided a unique dataset covering the largest ever observed luminosity range for a single source. In this paper, we extract the spectrum of the boundary layer between the inner part of the accretion disc and the neutron star surface with the help of maximally spectral model-independent method. We show compelling evidences that the energy spectrum of the boundary layer stays virtually the same over factor of 20 variations of the source luminosity. It is rather wide and cannot be described by a single temperature blackbody spectrum, probably because of the inhomogeneity of the boundary layer and a spread in the colour temperature. The observed maximum...

  17. Roche Accretion of stars close to massive black holes

    CERN Document Server

    Lixin,; Blandford, Roger D

    2011-01-01

    In this paper we consider Roche accretion in an Extreme Mass-Ratio Inspiral (EMRI) binary system formed by a star orbiting a massive black hole. The ultimate goal is to detect the mass and spin of the black hole and provide a test of general relativity in the strong-field regime from the resultant quasi-periodic signals. Before accretion starts, the stellar orbit is presumed to be circular and equatorial, and shrinks due to gravitational radiation. New fitting formulae are presented for the inspiral time and the radiation-reaction torque in the relativistic regime. If the inspiralling star fills its Roche lobe outside the Innermost Stable Circular Orbit (ISCO) of the hole, gas will flow through the inner Lagrange point (L1) to the hole. We give new relativistic interpolation formulae for the volume enclosed by the Roche lobe. If this mass-transfer happens on a time scale faster than the thermal time scale but slower than the dynamical time scale, the star will evolve adiabatically, and, in most cases, will re...

  18. Studying Young Stars with Large Spectroscopic Surveys

    CERN Document Server

    Martell, Sarah L

    2015-01-01

    Galactic archaeology is the study of the history of star formation and chemical evolution in the Milky Way, based on present-day stellar populations. Studies of young stars are a key anchor point for Galactic archaeology, since quantities like the initial mass function and the star formation rate can be studied directly in young clusters and star forming regions. Conversely, massive spectroscopic Galactic archaeology surveys can be used as a data source for young star studies.

  19. Kinematics of the inner thousand AU region around the young massive star AFGL 2591-VLA3: a massive disk candidate?

    NARCIS (Netherlands)

    Wang, K.-S.; van der Tak, F. F. S.; Hogerheijde, M. R.

    2012-01-01

    Context. Recent detections of disks around young high-mass stars support the idea of massive star formation through accretion rather than coalescence, but the detailed kinematics in the equatorial region of the disk candidates is not well known, which limits our understanding of the accretion proces

  20. The Burst Mode of Accretion in Primordial Star Formation

    CERN Document Server

    DeSouza, A L; Basu, S

    2012-01-01

    We present simulation results for the formation and long-term evolution of a primordial protostellar disk harbored by a first star. Using a 2+1D nonaxisymmetric thin disk numerical simulation, together with a barotropic relation for the gas, we are able to probe ~20 kyr of the disk's evolution. During this time period we observe fragmentation leading to loosely bound gaseous clumps within the disk. These are then torqued inward and accreted onto the growing protostar, giving rise to a burst phenomenon. The luminous feedback produced by this mechanism may have important consequences for the subsequent growth of the protostar.

  1. Accretion-induced luminosity spreads in young clusters: evidence from stellar rotation

    CERN Document Server

    Littlefair, S P; Mayne, N J; Saunders, Eric; Jeffries, R D

    2011-01-01

    We present an analysis of the rotation of young stars in the associations Cepheus OB3b, NGC 2264, NGC 2362 and the Orion Nebula Cluster (ONC). We discover a correlation between rotation rate and position in a colour-magnitude diagram (CMD) such that stars which lie above an empirically determined median pre-main sequence rotate more rapidly than stars which lie below this sequence. The same correlation is seen, with a high degree of statistical significance, in each association studied here. If position within the CMD is interpreted as being due to genuine age spreads within a cluster, then the stars above the median pre-main sequence would be the youngest stars. This would in turn imply that the most rapidly rotating stars in an association are the youngest, and hence those with the largest moments of inertia and highest likelihood of ongoing accretion. Such a result does not fit naturally into the existing picture of angular momentum evolution in young stars, where the stars are braked effectively by their ...

  2. Modeling of Disk-Star Interaction: Different Regimes of Accretion and Variability

    CERN Document Server

    Romanova, Marina M; Long, Min; Lovelace, Richard V E

    2008-01-01

    The appearance and time variability of accreting millisecond X-ray pulsars (hereafter AMXPs, e.g. Wijnands & van der Klis 1998) depends strongly on the accretion rate, the effective viscosity and the effective magnetic diffusivity of the disk-magnetosphere boundary. The accretion rate is the main parameter which determines the location of the magnetospheric radius of the star for a given stellar magnetic field. We introduce a classification of accreting neutron stars as a function of the accretion rate and show the corresponding stages obtained from our global 3D magnetohydrodynamic (MHD) simulations and from our axisymmetric MHD simulations. We discuss the expected variability features in each stage of accretion, both periodic and quasi-periodic (QPOs). We conclude that the periodicity may be suppressed at both very high and very low accretion rates. In addition the periodicity may disappear when ordered funnel flow accretion is replaced by disordered accretion through the interchange instability.

  3. Young Stars in the Cygnus OB2 Association

    Science.gov (United States)

    Hartigan, Patrick; Cleeves, Ilse

    2009-08-01

    Cygnus OB2 is by far the dominant region of massive star formation within 2-3 kpc of the Sun, but its stellar content is almost completely unknown owing to large amounts of visual extinction. Using NEWFIRM last fall we surveyed a large area of over two square degrees centered on the main cluster of O and B stars, and found the central part of the cluster to be teeming with at least several hundred, and probably over a thousand stars with near-IR excesses typical of circumstellar disks. With this proposal we aim to begin the process of acquiring spectral types for the entire sample of IR-excess and X-ray bright stars. This census will lead to masses, ages, and accretion rates for each object. With over 100 O stars, Cyg OB2 dwarfs the young clusters in Orion, and promises to provide a truly unique large sample of young stars that can address many of the major outstanding questions of star formation, such as IMF variations, binarity, triggering, mass segregation, accretion history, and disk evolution in a statistically significant way.

  4. Star-disc interaction in galactic nuclei: orbits and rates of accreted stars

    CERN Document Server

    Kennedy, Gareth F; Shukirgaliyev, Bekdaulet; Panamarev, Taras; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-01-01

    We examine the effect of an accretion disc on the orbits of stars in the central star cluster surrounding a central massive black hole by performing a suite of 39 high-accuracy direct N-body simulations using state-of-the art software and accelerator hardware, with particle numbers up to 128k. The primary focus is on the accretion rate of stars by the black hole (equivalent to their tidal disruption rate for black holes in the small to medium mass range) and the eccentricity distribution of these stars. Our simulations vary not only the particle number, but disc model (two models examined), spatial resolution at the centre (characterised by the numerical accretion radius) and softening length. The large parameter range and physically realistic modelling allow us for the first time to confidently extrapolate these results to real galactic centres. While in a real galactic centre both particle number and accretion radius differ by a few orders of magnitude from our models, which are constrained by numerical cap...

  5. Outflow and Accretion in Massive Star Forming Regions

    CERN Document Server

    Klaassen, P D

    2007-01-01

    In order to distinguish between the various components of massive star forming regions (i.e. infalling, outflowing and rotating gas structures) within our own Galaxy, we require high angular resolution observations which are sensitive to structures on all size scales. To this end, we present observations of the molecular and ionized gas towards massive star forming regions at 230 GHz from the SMA (with zero spacing from the JCMT) and at 22 and 23 GHz from the VLA at arcsecond or better resolution. These observations (of sources such as NGC7538, W51e2 and K3-50A) form an integral part of a multi-resolution study of the molecular and ionized gas dynamics of massive star forming regions (i.e. Klaassen & Wilson 2007). Through comparison of these observations with 3D radiative transfer models, we hope to be able to distinguish between various modes of massive star formation, such as ionized or halted accretion (i.e Keto 2003 or Klaassen et al. 2006 respectively).

  6. The effect of rotation on the stability of nuclear burning in accreting neutron stars

    NARCIS (Netherlands)

    Keek, L.; Langer, N.; in 't Zand, J.J.M.

    2009-01-01

    Hydrogen and/or helium accreted by a neutron star from a binary companion may undergo thermonuclear fusion. Different burning regimes are discerned at different mass accretion rates. Theoretical models predict helium fusion to proceed as a thermonuclear runaway for accretion rates below the

  7. A Survey of Chemical Separation in Accreting Neutron Stars

    Science.gov (United States)

    Mckinven, Ryan; Cumming, Andrew; Medin, Zach; Schatz, Hendrik

    2016-06-01

    The heavy element ashes of rp-process hydrogen and helium burning in accreting neutron stars are compressed to high density where they freeze, forming the outer crust of the star. We calculate the chemical separation on freezing for a number of different nuclear mixtures resulting from a range of burning conditions for the rp-process. We confirm the generic result that light nuclei are preferentially retained in the liquid and heavy nuclei in the solid. This is in agreement with the previous study of a 17-component mixture of rp-process ashes by Horowitz et al., but extends that result to a much larger range of compositions. We also find an alternative phase separation regime for the lightest ash mixtures which does not demonstrate this generic behavior. With a few exceptions, we find that chemical separation reduces the expected {Q}{{imp}} in the outer crust compared to the initial rp-process ash, where {Q}{{imp}} measures the mean-square dispersion in atomic number Z of the nuclei in the mixture. We find that the fractional spread of Z plays a role in setting the amount of chemical separation and is strongly correlated to the divergence between the two/three-component approximations and the full component model. The contrast in Y e between the initial rp-process ashes and the equilibrium liquid composition is similar to that assumed in earlier two-component models of compositionally driven convection, except for very light compositions which produce nearly negligible convective driving. We discuss the implications of these results for observations of accreting neutron stars.

  8. MHD Simulations of Magnetized Stars in the Propeller Regime of Accretion

    Directory of Open Access Journals (Sweden)

    Lii Patrick

    2014-01-01

    Full Text Available Accreting magnetized stars may be in the propeller regime of disc accretion in which the angular velocity of the stellar magnetosphere exceeds that of the inner disc. In these systems, the stellar magnetosphere acts as a centrifugal barrier and inhibits matter accretion onto the rapidly rotating star. Instead, the matter accreting through the disc accumulates at the disc-magnetosphere interface where it picks up angular momentum and is ejected from the system as a wide-angled outflow which gradually collimates at larger distances from the star. If the ejection rate is lower than the accretion rate, the matter will accumulate at the boundary faster than it can be ejected; in this case, accretion onto the star proceeds through an episodic accretion instability in which the episodes of matter accumulation are followed by a brief episode of simultaneous ejection and accretion of matter onto the star. In addition to the matter dominated wind component, the propeller outflow also exhibits a well-collimated, magnetically-dominated Poynting jet which transports energy and angular momentum away from the star. The propeller mechanism may explain some of the weakly-collimated jets and winds observed around some T Tauri stars as well as the episodic variability present in their light curves. It may also explain some of the quasi-periodic variability observed in cataclysmic variables, millisecond pulsars and other magnetized stars.

  9. Low-level accretion in neutron star X-ray binaries

    NARCIS (Netherlands)

    Wijnands, R.; Degenaar, N.; Armas Padilla, M.; Altamirano, D.; Cavecchi, Y.; Linares, M.; Bahramian, A.; Heinke, C.O.

    2015-01-01

    We search the literature for reports on the spectral properties of neutron star low-mass X-ray binaries when they have accretion luminosities between 1034 and 1036 erg s−1, corresponding to roughly 0.01-1 per cent of the Eddington accretion rate for a neutron star. We found that in this luminosity r

  10. A search for nearby young stars among the flare stars

    CERN Document Server

    König, B; Hambaryan, V; Neuh\\"auser, Ralph; Hambaryan, Valeri

    2001-01-01

    Flare stars were discovered in the late 1940s in the solar vicinity and were named UV Cet-type variables (classical FSs). Among the FSs within 100 pc we search for young stars. For the search we take spectra with sufficient resolution to resolve Lithium at 6707 \\AA and Calcium at 6718 \\AA of all the stars. The real young stars are prime targets for the search of extra-solar planets by direct imaging.

  11. Mechanical Properties of non-accreting Neutron Star Crusts

    CERN Document Server

    Hoffman, Kelsey

    2012-01-01

    The mechanical properties of a neutron star crust, such as breaking strain and shear modulus, have implications for the detection of gravitational waves from a neutron star as well as bursts from Soft Gamma-ray Repeaters (SGRs). These properties are calculated here for three different crustal compositions for a non-accreting neutron star that results from three different cooling histories, as well as for a pure iron crust. A simple shear is simulated using molecular dynamics to the crustal compositions by deforming the simulation box. The breaking strain and shear modulus are found to be similar in the four cases, with a breaking strain of ~0.1 and a shear modulus of ~10^{30} dyne cm^{-2} at a density of \\rho = 10^{14} g cm^{-3} for simulations with an initially perfect BCC lattice. With these crustal properties and the observed properties of {PSR J2124-3358} the predicted strain amplitude of gravitational waves for a maximally deformed crust is found to be greater than the observational upper limits from LIG...

  12. He-Accreting WDs: AM CVn stars with WD Donors

    CERN Document Server

    Piersanti, Luciano; Tornambe', Amedeo

    2015-01-01

    We study the physical and evolutionary properties of the "WD family" of AM CVn stars by computing realistic models of IDD systems. We evaluate self-consistently both the mass transfer rate from the donor, as determined by GW emission and interaction with the binary companion, and the thermal response of the accretor to mass deposition. We find that, after the onset of mass transfer, all the considered systems undergo a strong non-dynamical He-flash. However, due to the compactness of these systems, the expanding accretors fill their Roche lobe very soon, thus preventing the efficient heating of the external layers of the accreted CO WDs. Moreover, due to the loss of matter from the systems, the orbital separations enlarge and mass transfer comes to a halt. The further evolution depends on the value of \\mdot\\, after the donors fill again their lobe. On one hand, if the accretion rate, as determined by the actual value of (M_don,M_acc), is high enough, the accretors experience several He-flashes of decreasing s...

  13. Interactions between exoplanets and the winds of young stars

    Directory of Open Access Journals (Sweden)

    Vidotto A. A.

    2014-01-01

    Full Text Available The topology of the magnetic field of young stars is important not only for the investigation of magnetospheric accretion, but also responsible in shaping the large-scale structure of stellar winds, which are crucial for regulating the rotation evolution of stars. Because winds of young stars are believed to have enhanced mass-loss rates compared to those of cool, main-sequence stars, the interaction of winds with newborn exoplanets might affect the early evolution of planetary systems. This interaction can also give rise to observational signatures which could be used as a way to detect young planets, while simultaneously probing for the presence of their still elusive magnetic fields. Here, we investigate the interaction between winds of young stars and hypothetical planets. For that, we model the stellar winds by means of 3D numerical magnetohydrodynamic simulations. Although these models adopt simplified topologies of the stellar magnetic field (dipolar fields that are misaligned with the rotation axis of the star, we show that asymmetric field topologies can lead to an enhancement of the stellar wind power, resulting not only in an enhancement of angular momentum losses, but also intensifying and rotationally modulating the wind interactions with exoplanets.

  14. General relativistic spectra of accretion disks around rotating neutron stars

    CERN Document Server

    Bhattacharya, S; Thampan, A V

    2000-01-01

    General relativistic spectra from accretion disks around rotating neutron stars in the appropriate space-time geometry for several different equation of state, spin rates and mass of the compact object have been computed. The analysis involves the computation of the relativistically corrected radial temperature profiles and the effect of Doppler and gravitational red-shifts on the spectra. Light bending effects have been omitted for simplicity. The relativistic spectrum is compared with the Newtonian one and it is shown that the difference between the two is primarily due to the different radial temperature profile for the relativistic and Newtonian disk solutions. To facilitate direct comparison with observations, a simple empirical function has been presented which describes the numerically computed relativistic spectra well. This empirical function (which has three parameters including normalization) also describes the Newtonian spectrum adequately. Thus the function can in principle be used to distinguish...

  15. Mapping accretion and its variability in the young open cluster NGC 2264: a study based on u-band photometry

    CERN Document Server

    Venuti, Laura; Flaccomio, Ettore; Alencar, Silvia H P; Irwin, Jonathan; Stauffer, John R; Cody, Ann Marie; Teixeira, Paula S; Sousa, Alana P; Micela, Giuseppina; Cuillandre, Jean-Charles; Peres, Giovanni

    2014-01-01

    We aim at characterizing the accretion properties of several hundred members of the star-forming cluster NGC 2264 (3 Myr). We performed a deep u,g,r,i mapping and a simultaneous u+r monitoring of the region with CFHT/MegaCam in order to directly probe the accretion process from UV excess measurements. Photometric properties and stellar parameters are determined homogeneously for about 750 monitored young objects, spanning the mass range 0.1-2 Mo. About 40% are classical (accreting) T Tauri stars, based on various diagnostics (H_alpha, UV and IR excesses). The remaining non-accreting members define the (photospheric+chromospheric) reference UV emission level over which flux excess is detected and measured. We revise the membership status of cluster members based on UV accretion signatures and report a new population of 50 CTTS candidates. A large range of UV excess is measured for the CTTS population, varying from a few 0.1 to 3 mag. We convert these values to accretion luminosities and obtain mass accretion r...

  16. Jets from Young Stars I: Models and Constraints

    Science.gov (United States)

    Ferreira, Jonathan; Dougados, Catherine; Whelan, Emma

    2007-09-01

    This volume contains the edited lecture notes of the First JETSET School on Jets from Young Stars: Models and Constraints, held by the Marie Curie Research and Training Network on JET Simulations, Experiments and Theory. At this school the lecturers gave an introduction to observational properties and basic models describing the launching and collimation mechanisms of jets. The first half of the book is devoted to general observational constraints, covering the outflow phenomenon in young stars, the identification of magneto-centrifugal processes as the main jet driving mechanism, and the magnetic interaction between the star and its accretion disc. The second half of the book is devoted to theoretical knowledge of magneto-hydrodynamic processes pertinent to the jet launching mechanism in young stars. This comprises a general introduction to magneto-hydrodynamics, a description of the role of MHD processes in Standard Accretion Discs, and the physics of steady state MHD o! utflows, from the basic concepts and equations to modern self-similar solutions. Further lectures detail the various classes of steady magnetic-wind models currently discussed in the context of protostellar jets.

  17. Accretion Rates for T Tauri Stars Using Nearly Simultaneous Ultraviolet and Optical Spectra

    CERN Document Server

    Ingleby, Laura; Herczeg, Gregory; Blaty, Alex; Walter, Frederick; Ardila, David; Alexander, Richard; Edwards, Suzan; Espaillat, Catherine; Gregory, Scott G; Hillenbrand, Lynne; Brown, Alexander

    2013-01-01

    We analyze the accretion properties of 21 low mass T Tauri stars using a dataset of contemporaneous near ultraviolet (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph (STIS) and the ground based Small and Medium Aperture Research Telescope System (SMARTS), a unique dataset because of the nearly simultaneous broad wavelength coverage. Our dataset includes accreting T Tauri stars (CTTS) in Taurus, Chamaeleon I, $\\eta$ Chamaeleon and the TW Hydra Association. For each source we calculate the accretion rate by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high density, low filling factor accretion spots co-exist with low density, high filling factor spots. By fitting the UV and optical spectra wi...

  18. Stellar parameters and accretion rate of the transition disk star HD 142527 from X-shooter

    Energy Technology Data Exchange (ETDEWEB)

    Mendigutía, I.; Fairlamb, J.; Oudmaijer, R. D. [School of Physics and Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT (United Kingdom); Montesinos, B. [Centro de Astrobiología, Departamento de Astrofísica (CSIC-INTA), ESAC Campus, P.O. Box 78, E-28691 Villanueva de la Cañada, Madrid (Spain); Najita, J. R. [National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719 (United States); Brittain, S. D. [Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978 (United States); Van den Ancker, M. E., E-mail: I.Mendigutia@leeds.ac.uk [European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching b. München (Germany)

    2014-07-20

    HD 142527 is a young pre-main-sequence star with properties indicative of the presence of a giant planet and/or a low-mass stellar companion. We have analyzed an X-Shooter/Very Large Telescope spectrum to provide accurate stellar parameters and accretion rate. The analysis of the spectrum, together with constraints provided by the spectral energy distribution fitting, the distance to the star (140 ± 20 pc), and the use of evolutionary tracks and isochrones, led to the following set of parameters: T{sub eff} = 6550 ± 100 K, log g = 3.75 ± 0.10, L{sub *}/L{sub ☉} = 16.3 ± 4.5, M{sub *}/M{sub ☉} = 2.0 ± 0.3, and an age of 5.0 ± 1.5 Myr. This stellar age provides further constraints to the mass of the possible companion estimated by Biller et al., being between 0.20 and 0.35 M{sub ☉}. Stellar accretion rates obtained from UV Balmer excess modeling and optical photospheric line veiling, and from the correlations with several emission lines spanning from the UV to the near-IR, are consistent with each other. The mean value from all previous tracers is 2 (±1) × 10{sup –7} M{sub ☉} yr{sup –1}, which is within the upper limit gas flow rate from the outer to the inner disk recently provided by Cassasus et al.. This suggests that almost all gas transferred between both components of the disk is not trapped by the possible planet(s) in between but fall onto the central star, although it is discussed how the gap flow rate could be larger than previously suggested. In addition, we provide evidence showing that the stellar accretion rate of HD 142527 has increased by a factor ∼7 on a timescale of 2 to 5 yr.

  19. Stellar Parameters and Accretion Rate of the Transition Disk Star HD 142527 from X-Shooter

    Science.gov (United States)

    Mendigutía, I.; Fairlamb, J.; Montesinos, B.; Oudmaijer, R. D.; Najita, J. R.; Brittain, S. D.; van den Ancker, M. E.

    2014-07-01

    HD 142527 is a young pre-main-sequence star with properties indicative of the presence of a giant planet and/or a low-mass stellar companion. We have analyzed an X-Shooter/Very Large Telescope spectrum to provide accurate stellar parameters and accretion rate. The analysis of the spectrum, together with constraints provided by the spectral energy distribution fitting, the distance to the star (140 ± 20 pc), and the use of evolutionary tracks and isochrones, led to the following set of parameters: T eff = 6550 ± 100 K, log g = 3.75 ± 0.10, L */L ⊙ = 16.3 ± 4.5, M */M ⊙ = 2.0 ± 0.3, and an age of 5.0 ± 1.5 Myr. This stellar age provides further constraints to the mass of the possible companion estimated by Biller et al., being between 0.20 and 0.35 M ⊙. Stellar accretion rates obtained from UV Balmer excess modeling and optical photospheric line veiling, and from the correlations with several emission lines spanning from the UV to the near-IR, are consistent with each other. The mean value from all previous tracers is 2 (±1) × 10-7 M ⊙ yr-1, which is within the upper limit gas flow rate from the outer to the inner disk recently provided by Cassasus et al.. This suggests that almost all gas transferred between both components of the disk is not trapped by the possible planet(s) in between but fall onto the central star, although it is discussed how the gap flow rate could be larger than previously suggested. In addition, we provide evidence showing that the stellar accretion rate of HD 142527 has increased by a factor ~7 on a timescale of 2 to 5 yr.

  20. Accretion discs around young stellar objects and the proto-sun

    Science.gov (United States)

    Lin, D. N. C.

    1989-01-01

    Observed IR and UV excesses have widely been interpreted as signatures for accretion disks around young stellar objects. Analyses of the observed properties of these disks are important for the investigation of star formation as well as the dynamics of the protoplanetary disk out of which the solar system was formed. Accretion-disk theories suggest that evolution of protoplanetary disks is determined by the efficiency of angular momentum transport. During the formation stages, the disk dynamics are regulated by mixing of infalling material and disk gas. In the outermost regions of the disk, self-gravity may promote the growth of nonaxisymmetric perturbations which can transfer angular momentum outwards. After infall has ceased, convectively driven turbulence can redistribute angular momentum with an evolutionary timescale of 0.1 - 1 Myr. Convection in protoplanetary disks may eventually be stabilized by surface heating as the disk material is depleted.

  1. Physical Conditions of Accreting Gas in T Tauri Star Systems

    CERN Document Server

    Bary, Jeffrey S; Skrutskie, Michael F; Wilson, John C; Peterson, Dawn E; Nelson, Matthew J

    2008-01-01

    We present results from a low resolution (R~300) near-infrared spectroscopic variability survey of actively accreting T Tauri stars (TTS) in the Taurus-Auriga star forming region. Paschen and Brackett series H I recombination lines were detected in 73 spectra of 15 classical T Tauri systems. The values of the Pan/PaB, Brn/BrG, and BrG/Pan H I line ratios for all observations exhibit a scatter of < 20% about the weighted mean, not only from source to source, but also for epoch-to-epoch variations in the same source. A representative or `global' value was determined for each ratio in both the Paschen and Brackett series as well as the BrG/Pan line ratios. A comparison of observed line ratio values was made to those predicted by the temperature and electron density dependent models of Case B hydrogen recombination line theory. The measured line ratios are statistically well-fit by a tightly constrained range of temperatures (T < 2000 K) and electron densities 1e9 < n_e < 1e10 cm^-3. A comparison of t...

  2. Black hole accretion versus star formation rate: theory confronts observations

    CERN Document Server

    Volonteri, Marta; Netzer, Hagai; Bellovary, Jillian; Dotti, Massimo; Governato, Fabio

    2015-01-01

    We use a suite of hydrodynamical simulations of galaxy mergers to compare star formation rate (SFR) and black hole accretion rate (BHAR) for galaxies before the interaction ('stochastic' phase), during the 'merger' proper, lasting ~0.2-0.3 Gyr, and in the 'remnant' phase. We calculate the bi-variate distribution of SFR and BHAR and define the regions in the SFR-BHAR plane that the three phases occupy. No strong correlation between BHAR and galaxy-wide SFR is found. A possible exception are galaxies with the highest SFR and the highest BHAR. We also bin the data in the same way used in several observational studies, by either measuring the mean SFR for AGN in different luminosity bins, or the mean BHAR for galaxies in bins of SFR. We find that the apparent contradiction or SFR versus BHAR for observed samples of AGN and star forming galaxies is actually caused by binning effects. The two types of samples use different projections of the full bi-variate distribution, and the full information would lead to unamb...

  3. A Survey of Chemical Separation in Accreting Neutron Stars

    CERN Document Server

    Mckinven, Ryan; Medin, Zach; Schatz, Hendrik

    2016-01-01

    The heavy element ashes of rp-process hydrogen and helium burning in accreting neutron stars are compressed to high density where they freeze, forming the outer crust of the star. We calculate the chemical separation on freezing for a number of different nuclear mixtures resulting from a range of burning conditions for the rp-process. We confirm the generic result that light nuclei are preferentially retained in the liquid and heavy nuclei in the solid. This is in agreement with the previous study of a 17-component mixture of rp-process ashes by Horowitz et al. (2007), but extends that result to a much larger range of compositions. We also find an alternate phase separation regime for the lightest ash mixtures which does not demonstrate this generic behaviour. With a few exceptions, we find that chemical separation reduces the expected $Q_{\\rm imp}$ in the outer crust compared to the initial rp-process ash, where $Q_{\\rm imp}$ measures the mean-square dispersion in atomic number $Z$ of the nuclei in the mixtu...

  4. Tuning up for Gravitational Wave Detection in Accreting Neutron Stars

    Science.gov (United States)

    Galloway, Duncan; Steeghs, Danny; Ransom, Scott

    Rapidly-rotating neutron stars are the only candidates for persistent gravitational wave emis-sion, for which a targeted search can be performed based on the spin period measured from electromagnetic (e.g. radio and X-ray) observations. Apart from the expected weakness of the emission, the principal difficulty for such searches is the lack of precision in measurements of the spin as well as the other physical parameters of the system. I present a pilot program of optical and infra-red observations of the stellar counterparts to X-ray bright accreting neutron stars, in order to measure (or improve the precision of) the binary parameters. These measurements will allow optimisation of future gravitational wave searches, and will also facilitate searches of the extensive X-ray timing data from NASA's Rossi X-ray Timing Explorer, to measure the spin frequency (for those systems where it is not precisely known). Observations such as these will provide the best possible chance for detecting the gravitational wave emission from these systems.

  5. Stellar parameters and accretion rate of the transition disk star HD 142527 from X-Shooter

    CERN Document Server

    Mendigutía, I; Montesinos, B; Oudmaijer, R D; Najita, J R; Brittain, S D; Ancker, M E van den

    2014-01-01

    HD 142527 is a young pre-main sequence star with properties indicative of the presence of a giant planet or/and a low-mass stellar companion. We have analyzed an X-Shooter/Very Large Telescope spectrum to provide accurate stellar parameters and accretion rate. The analysis of the spectrum, together with constraints provided by the SED fitting, the distance to the star (140 +- 20 pc) and the use of evolutionary tracks and isochrones, lead to the following set of parameters T_eff = 6550 +- 100 K, log g = 3.75 +- 0.10, L_*/L_sun = 16.3 +- 4.5, M_*/M_sun = 2.0 +- 0.3 and an age of 5.0 +- 1.5 Myr. This stellar age provides further constrains to the mass of the possible companion estimated by Biller et al. (2012), being in-between 0.20 and 0.35 M_sun. Stellar accretion rates obtained from UV Balmer excess modelling, optical photospheric line veiling, and from the correlations with several emission lines spanning from the UV to the near-IR, are consistent to each other. The mean value from all previous tracers is 2 ...

  6. Growth of a Protostar and a Young Circumstellar Disk with High Mass Accretion Rate onto the Disk

    CERN Document Server

    Ohtani, Takuya

    2013-01-01

    The growing process of both a young protostar and a circumstellar disk is investigated. Viscous evolution of a disk around a single star is considered with a model where a disk increases its mass by dynamically accreting envelope and simultaneously loses its mass via viscous accretion onto the central star. We focus on the circumstellar disk with high mass accretion rate onto the disk $\\dot{M}=8.512c_{\\rm s}^3/G$ as a result of dynamical collapse of rotating molecular cloud core. We study the origin of the surface density distribution and the origin of the disk-to-star mass ratio by means of numerical calculations of unsteady viscous accretion disk in one-dimensional axisymmetric model. It is shown that the radial profiles of the surface density $\\Sigma$, azimuthal velocity $v_{\\phi}$, and mass accretion rate $\\dot{M}$ in the inner region approach to the quasi-steady state. Profile of the surface density distribution in the quasi-steady state is determined as a result of angular momentum transport rather than...

  7. Accretion by a Neutron Star Moving at a High Kick Velocity in the Supernova Ejecta

    Institute of Scientific and Technical Information of China (English)

    Xu Zhang; Ye Lu; Yong-Heng Zhao

    2007-01-01

    We suggest a two-dimensional time dependent analytic model to describe the accretion of matter onto a neutron star moving at a high speed across the ejecta left in the aftermath of a supernova explosion. The formation of a strange star resulting from the accretion is also addressed. The newborn neutron star is assumed to move outward at a kick velocity of vns ~ 103 km s-1, and the accretion flow is treated as a dust flow. When the neutron star travels across the ejecta with high speed, it sweeps up material, and when the accreted mass has reached a critical value, the neutron star will undergo a phase transition,for instance, to become a strange star. Our results show that the accretion rate decreases in a complicated way in time, not just a power law dependence: it drops much faster than the power law derived by Colpi et al. We also found that the total accreted mass and the phase transition of the neutron star depend sensitively on the velocity of supernova ejecta.

  8. Photon Bubbles in Young Massive Stars

    CERN Document Server

    Turner, N J; Socrates, A; Blaes, Omer M

    2004-01-01

    Spectroscopic studies indicate that gas in the photospheres of young O stars moves at speeds up to the sound speed. We show, using two-dimensional radiation MHD calculations and results from a local linear analysis, that the motions may be due to photon bubble instability if young O stars have magnetic fields.

  9. Photon Bubbles in Young Massive Stars

    Science.gov (United States)

    Turner, N. J.; Yorke, H. W.; Socrates, A.; Blaes, O. M.

    2004-12-01

    Spectroscopic studies indicate that gas in the photospheres of young O stars moves at speeds up to the sound speed. We show, using two-dimensional radiation MHD calculations and results from a local linear analysis, that the motions may be due to photon bubble instability if young O stars have magnetic fields.

  10. An ultraluminous X-ray source powered by an accreting neutron star

    DEFF Research Database (Denmark)

    Bachetti, M.; Harrison, F. A.; Walton, D. J.;

    2014-01-01

    the Eddington limit for a 1.4-solar-mass object, or more than ten times brighter than any known accreting pulsar. This implies that neutron stars may not be rare in the ultraluminous X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects....

  11. Unstable Disk Accretion to Magnetized Stars: First Global 3D MHD Simulations

    CERN Document Server

    Romanova, Marina M; Lovelace, Richard V E

    2007-01-01

    We report the first global three-dimensional (3D) MHD simulations of disk accretion onto a rotating magnetized star through the Rayleigh-Taylor instability. In this regime, the accreting matter typically forms 2 to 7 vertically elongated "tongues" which penetrate deep into the magnetosphere, until they are stopped by the strong field. Subsequently, the matter is channeled along the field lines to the surface of the star, forming hot spots. The number, position and shape of the hot spots vary with time, so that the light-curves associated with the hot spots are stochastic. A magnetized star may be in the stable (with funnel streams) or unstable (with random tongues) regime of accretion, and consequently have significantly different observational properties. A star may switch between these two regimes depending on the accretion rate.

  12. Gravitational radiation and gamma-ray bursts from accreting neutron stars

    Energy Technology Data Exchange (ETDEWEB)

    Mosquera Cuesta, H.J.; Araujo, J.C.N. de; Aguiar, O.D. [Instituto Nacional de Pesquisas Espaciais (INPE), Sao Jose dos Campos, SP (Brazil). Div. de Astrofisica]. E-mail: herman@das.inpe.br; jcarlos@das.inpe.br; odylio@das.inpe.br; Horvath, J.E. [Sao Paulo Univ., SP (Brazil). Inst. Astronomico e Geofisico]. E-mail: foton@orion.iagusp.usp.br

    2000-07-01

    It is well known that hydrodynamic instabilities can be induced in rapidly rotating low magnetic field neutron stars, which accrete mass from a companion in both high and low mass X-ray binaries. (author)

  13. Excitation of turbulence in accretion disks of binary stars by non-linear perturbations

    Science.gov (United States)

    Kurbatov, E. P.; Bisikalo, D. V.

    2017-06-01

    Accretion disks in binary systems can experience hydrodynamical influences at both their inner and outer edges. The former is typical for protoplanetary disks around young T Tauri stars, while the latter is typical for circumstellar disks in close binaries. This influence excites perturbations with various scales and amplitudes in the disk. The nonlinear evolution of perturbations with a finite, but small amplitude against the background of a sub-Keplerian flow is investigated. Nonlinear effects at the fronts of perturbation waves lead to the formation of discontinuities in the density and radial velocity; i.e., to formation of shocks. The tangential flow in the neighborhood of the shock becomes equivalent to a flow in a boundary layer. Due to an instability of the tangential flow, the disk becomes turbulent. The characteristics of the turbulence depend on the parameters of the perturbations, but the Shakura-Syunyaev α parameter does not exceed 0.1.

  14. Mass loss from very young massive stars

    Science.gov (United States)

    Henning, Th.

    The physics of mass loss from very young massive stars is reviewed, and mass-loss rates are determined for several objects on the basis of published observational data. The observational evidence for mass loss of 0.0001-0.001 solar mass/yr with velocity 10-60 km/s, dynamical timescale 1000-100,000 yr, and kinetic energy (1-100) x 10 to the 38th W from these objects is chracterized; techniques for estimating mass-loss rates from H recombination lines, CO line profiles maser data, and IR-continuum observations are described; rates for molecular outflows and ionized winds are presented in tabels; and theoretical models developed to explain the mechanism driving bipolar mass loss are examined critically. It is found that neither radiation pressure on dust grins nor the ionized winds can drive the molecular outflow. The models considered most probable are those involving production of holes by original spherical stellar winds (Canto, 1980, rotationally driven magnetic pressure (Draine, 1983), and infall from an accretion disk (Torbett, 1984).

  15. The Eruption of the Candidate Young Star ASASSN-15qi

    CERN Document Server

    Herczeg, Gregory J; Shappee, Benjamin J; Chen, Ping; Hillenbrand, Lynne A; Jose, Jessy; Kochanek, Christopher S; Prieto, Jose L; Stanek, K Z; Kaplan, Kyle; Holoein, Thomas W -S; Mairs, Steve; Johnstone, Doug; Gully-Santiago, Michael; Zhu, Zhaohuan; Smith, Martin C; Bersier, David; Mulders, Gijs D; Filippenko, Alexei V; Ayani, Kazuya; Brimacombe, Joseph; Brown, Jonathan S; Connelley, Michael; Harmanen, Jussi; Ito, Ryosuke; Kawabata, Koji S; Maehara, Hiroyuki; Takata, Koji; Yuk, Heechan; Zheng, WeiKang

    2016-01-01

    Outbursts on young stars are usually interpreted as accretion bursts caused by instabilities in the disk or the star-disk connection. However, some protostellar outbursts may not fit into this framework. In this paper, we analyze optical and near-infrared spectra and photometry to characterize the 2015 outburst of the probable young star ASASSN-15qi. The $\\sim 3.5$ mag brightening in the $V$ band was sudden, with an unresolved rise time of less than one day. The outburst decayed exponentially by 1 mag for 6 days and then gradually back to the pre-outburst level after 200 days. The outburst is dominated by emission from $\\sim10,000$ K gas. An explosive release of energy accelerated matter from the star in all directions, seen in a spectacular cool, spherical wind with a maximum velocity of 1000 km/s. The wind and hot gas both disappeared as the outburst faded and the source the source returned to its quiescent F-star spectrum. Nebulosity near the star brightened with a delay of 10-20 days. Fluorescent excitati...

  16. Brown dwarf accretion: Nonconventional star formation over very long timescales

    Directory of Open Access Journals (Sweden)

    Ćirković Milan M.

    2005-01-01

    Full Text Available We investigate the process of accretion of interstellar gas by the Galactic population of brown dwarfs over very long timescales typical for physical eschatology. In particular, we use the classical Hoyle-Lyttleton-Bondi accretion model to investigate the rate at which brown dwarfs collect enough additional mass to become red dwarfs, accretion-induced changes in the mass function of the low- mass objects, and the corresponding accretion heating of brown dwarfs. In addition, we show how we can make the definition of the final mass function for stellar objects more precise.

  17. Magnetically elevated accretion disks in active galactic nuclei: broad emission line regions and associated star formation

    CERN Document Server

    Begelman, Mitchell C

    2016-01-01

    We propose that the accretion disks fueling active galactic nuclei are supported vertically against gravity by a strong toroidal ($\\phi-$direction) magnetic field that develops naturally as the result of an accretion disk dynamo. The magnetic pressure elevates most of the gas carrying the accretion flow at $R$ to large heights $z > 0.1 R$ and low densities, while leaving a thin dense layer containing most of the mass --- but contributing very little accretion --- around the equator. We show that such a disk model leads naturally to the formation of a broad emission line region through thermal instability. Extrapolating to larger radii, we demonstrate that local gravitational instability and associated star formation are strongly suppressed compared to standard disk models for AGN, although star formation in the equatorial zone is predicted for sufficiently high mass supply rates. This new class of accretion disk models thus appears capable of resolving two longstanding puzzles in the theory of AGN fueling: th...

  18. The Structure of the Accretion Flow on pre-main-sequence stars

    Science.gov (United States)

    Calvet, Nuria

    1999-07-01

    We propose to test an essential prediction of the magnetospheric accretion model for T Tauri stars. STIS echelle spectra will be used to search for the relatively narrow high-temperature emission lines that must result from the magnetospheric accretion shock, but are not expected in the previous, alternative boundary layer model. By combining the results from high temperature {10^5 K} lines, accessible only with HST, with optical lines and optical-UV continuum emission, we will develop physically self-consistent models of accretion shock structure. The geometrically distribution of the emitting gas as derived from our results will test theories of mass-loading of magnetic field lines at the magnetosphere-disk interface. Analysis of the UV emission lines will also provide improved calibrations between ultraviolet continuum emission and accretion luminosities, and thus improve estimates of mass accretion rates for T Tauri stars.

  19. Rayleigh-Taylor-Unstable Accretion and Variability of Magnetized Stars: Global Three-Dimensional Simulations

    CERN Document Server

    Kulkarni, Akshay K

    2008-01-01

    We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It produces tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, $\\Theta\\lesssim30^\\circ$, between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hot spots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accre...

  20. Accretion to Magnetized Stars through the Rayleigh-Taylor Instability: Global Three-Dimensional Simulations

    CERN Document Server

    Kulkarni, Akshay K

    2008-01-01

    We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It manifests itself in the form of tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, $\\Theta\\lesssim30^\\circ$, between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hot spots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermitten...

  1. Formation of Massive Primordial Stars: Intermittent UV Feedback with Episodic Mass Accretion

    Science.gov (United States)

    Hosokawa, Takashi; Hirano, Shingo; Kuiper, Rolf; Yorke, Harold W.; Omukai, Kazuyuki; Yoshida, Naoki

    2016-06-01

    We present coupled stellar evolution (SE) and 3D radiation-hydrodynamic (RHD) simulations of the evolution of primordial protostars, their immediate environment, and the dynamic accretion history under the influence of stellar ionizing and dissociating UV feedback. Our coupled SE RHD calculations result in a wide diversity of final stellar masses covering 10 {M}⊙ ≲ M * ≲ 103 {M}⊙ . The formation of very massive (≳250 {M}⊙ ) stars is possible under weak UV feedback, whereas ordinary massive (a few ×10 {M}⊙ ) stars form when UV feedback can efficiently halt the accretion. This may explain the peculiar abundance pattern of a Galactic metal-poor star recently reported by Aoki et al., possibly the observational signature of very massive precursor primordial stars. Weak UV feedback occurs in cases of variable accretion, in particular when repeated short accretion bursts temporarily exceed 0.01 {M}⊙ {{{yr}}}-1, causing the protostar to inflate. In the bloated state, the protostar has low surface temperature and UV feedback is suppressed until the star eventually contracts, on a thermal adjustment timescale, to create an H ii region. If the delay time between successive accretion bursts is sufficiently short, the protostar remains bloated for extended periods, initiating at most only short periods of UV feedback. Disk fragmentation does not necessarily reduce the final stellar mass. Quite the contrary, we find that disk fragmentation enhances episodic accretion as many fragments migrate inward and are accreted onto the star, thus allowing continued stellar mass growth under conditions of intermittent UV feedback. This trend becomes more prominent as we improve the resolution of our simulations. We argue that simulations with significantly higher resolution than reported previously are needed to derive accurate gas mass accretion rates onto primordial protostars.

  2. Investigating Binarity and Active Accretion in a New Class of AGB Stars with FUV Excesses

    Science.gov (United States)

    Sahai, Raghvendra

    2013-09-01

    We propose a pilot survey for X-ray emission from AGB stars that are candidates for having binary companions with active accretion. These objects were identified via our innovative technique to search for FUV/NUV excesses in AGB stars using GALEX. The detection (or non-detection) of X-rays from this sample will enable us to begin testing models for the origin of the UV-excesses, leading to vital breakthroughs in our understanding of accretion-related phenomena and binarity in AGB stars. A larger survey, optimised using results fron this study, will be proposed in future cycles.

  3. THE FIRST X-SHOOTER OBSERVATIONS OF JETS FROM YOUNG STARS

    NARCIS (Netherlands)

    Bacciotti, F.; Whelan, E. T.; Alcala, J. M.; Nisini, B.; Podio, L.; Randich, S.; Stelzer, B.; Cupani, G.

    2011-01-01

    We present the first pilot study of jets from young stars conducted with X-shooter, on the ESO/Very Large Telescope. As it offers simultaneous, high-quality spectra in the range 300-2500 nm, X-shooter is uniquely important for spectral diagnostics in jet studies. We chose to probe the accretion/ejec

  4. A strong shallow heat source in the accreting neutron star MAXI J0556-332

    CERN Document Server

    Deibel, Alex; Brown, Edward F; Page, Dany

    2015-01-01

    An accretion outburst in an X-ray transient deposits material onto the neutron star primary; this accumulation of matter induces reactions in the neutron star's crust. During the accretion outburst these reactions heat the crust out of thermal equilibrium with the core. When accretion halts, the crust cools to its long-term equilibrium temperature on observable timescales. Here we examine the accreting neutron star transient MAXI J0556-332, which is the hottest transient, at the start of quiescence, observed to date. Models of the quiescent light curve require a large deposition of heat in the shallow outer crust from an unknown source. The additional heat injected is $\\approx 4\\textrm{-}10\\,\\mathrm{MeV}$ per accreted nucleon; when the observed decline in accretion rate at the end of the outburst is accounted for, the required heating increases to $\\approx 6\\textrm{-}16\\,\\mathrm{MeV}$. This shallow heating is still required to fit the lightcurve even after taking into account a second accretion episode, uncer...

  5. Magnetically elevated accretion disks in active galactic nuclei: broad emission line regions and associated star formation

    Science.gov (United States)

    Begelman, Mitchell C.; Silk, Joseph

    2016-10-01

    We propose that the accretion disks fueling active galactic nuclei are supported vertically against gravity by a strong toroidal (φ -direction) magnetic field that develops naturally as the result of an accretion disk dynamo. The magnetic pressure elevates most of the gas carrying the accretion flow at R to large heights z ˜ 0.1 R and low densities, while leaving a thin dense layer containing most of the mass - but contributing very little accretion - around the equator. We show that such a disk model leads naturally to the formation of a broad emission line region through thermal instability. Extrapolating to larger radii, we demonstrate that local gravitational instability and associated star formation are strongly suppressed compared to standard disk models for AGN, although star formation in the equatorial zone is predicted for sufficiently high mass supply rates. This new class of accretion disk models thus appears capable of resolving two longstanding puzzles in the theory of AGN fueling: the formation of broad emission line regions and the suppression of fragmentation thought to inhibit accretion at the required rates. We show that the disk of stars that formed in the Galactic Center a few million years ago could have resulted from an episode of magnetically elevated accretion at ˜0.1 of the Eddington limit.

  6. On the existence of accretion-driven bursts in massive star formation

    CERN Document Server

    Meyer, D M -A; Kuiper, R; Kley, W

    2016-01-01

    Accretion-driven luminosity outbursts are a vivid manifestation of variable mass accretion onto protostars. They are known as the so-called FU Orionis phenomenon in the context of low-mass protostars. More recently, this process has been found in models of primordial star formation. Using numerical radiation hydrodynamics simulations, we stress that present-day forming massive stars also experience variable accretion and show that this process is accompanied by luminous outbursts induced by the episodic accretion of gaseous clumps falling from the circumstellar disk onto the protostar. Consequently, the process of accretion-induced luminous flares is also conceivable in the high-mass regime of star formation and we propose to regard this phenomenon as a general mechanism that can affect protostars regardless of their mass and/or the chemical properties of the parent environment in which they form. In addition to the commonness of accretion-driven outbursts in the star formation machinery, we conjecture that l...

  7. Hypercritical Accretion onto a Newborn Neutron Star and Magnetic Field Submergence

    CERN Document Server

    Bernal, Cristian G; Lee, William H

    2012-01-01

    We present magnetohydrodynamic numerical simulations of the late post-supernova hypercritical accretion to understand its effect on the magnetic field of the new-born neutron star. We consider as an example the case of a magnetic field loop protruding from the star's surface. The accreting matter is assumed to be non magnetized and, due to the high accretion rate, matter pressure dominates over magnetic pressure. We find that an accretion envelope develops very rapidly and once it becomes convectively stable the magnetic field is easily buried and pushed into the newly forming neutron star crust. However, for low enough accretion rates the accretion envelope remains convective for an extended period of time and only partial submergence of the magnetic field occurs due to a residual field that is maintained at the interface between the forming crust and the convective envelope. In this latter case, the outcome should be a weakly magnetized neutron star with a likely complicated field geometry. In our simulatio...

  8. Accretion Rates for T Tauri Stars Using Nearly Simultaneous Ultraviolet and Optical Spectra

    Science.gov (United States)

    Ingleby, Laura; Calvet, Nuria; Herczeg, Gregory; Blaty, Alex; Walter, Frederick; Ardila, David; Alexander, Richard; Edwards, Suzan; Espaillat, Catherine; Gregory, Scott G.; Hillenbrand, Lynne; Brown, Alexander

    2013-04-01

    We analyze the accretion properties of 21 low-mass T Tauri stars using a data set of contemporaneous near-UV (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph and the ground-based Small and Medium Aperture Research Telescope System, a unique data set because of the nearly simultaneous broad wavelength coverage. Our data set includes accreting T Tauri stars in Taurus, Chamaeleon I, η Chamaeleon, and the TW Hydra Association. For each source we calculate the accretion rate (\\dot{M}) by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high-density, low filling factor accretion spots coexist with low-density, high filling factor spots. By fitting the UV and optical spectra with multiple accretion components, we can explain excesses which have been observed in the near-IR. Comparing our estimates of \\dot{M} to previous estimates, we find some discrepancies; however, they may be accounted for when considering assumptions for the amount of extinction and variability in optical spectra. Therefore, we confirm many previous estimates of the accretion rate. Finally, we measure emission line luminosities from the same spectra used for the \\dot{M} estimates, to produce correlations between accretion indicators (Hβ, Ca II K, C II], and Mg II) and accretion properties obtained simultaneously.

  9. The Beta Pictoris Phenomenon in A-Shell Stars: Detection of Accreting Gas

    Science.gov (United States)

    Grady, C. A.; Perez, Mario R.; Talavera, A.; McCollum, B.; Rawley, L. A.; England, M. N.; Schlegel, M.

    1996-01-01

    We present the results of an expanded survey of A-shell stars using IUE high-dispersion spectra and find accreting, circumstellar gas in the line of sight to nine stars, in addition to the previously identified beta Pic, HR 10, and 131 Tau, which can be followed to between +70 and 100 km/s relative to the star. Two of the program stars, HD 88195 and HD 148283, show variable high-velocity gas. Given the small number of IUE spectra for our program stars, detection of high-velocity, accreting gas in 2/3 of the A-shell stars sampled indicates that accretion is an intrinsic part of the A-shell phenomenon and that beta Pic is not unique among main-sequence A stars in exhibiting such activity. Our program stars, as a group, have smaller column densities of high-velocity gas and smaller near-IR excesses compared with beta Pic. These features are consistent with greater central clearing of a remnant debris disk, compared with beta Pic, and suggest that the majority of field A-shell stars are older than beta Pic.

  10. Mass accretion rates in self-regulated disks of T Tauri stars

    CERN Document Server

    Vorobyov, E I

    2008-01-01

    We have studied numerically the evolution of protostellar disks around intermediate and upper mass T Tauri stars (0.25 M_sun < M_st < 3.0 M_sun) that have formed self-consistently from the collapse of molecular cloud cores. In the T Tauri phase, disks settle into a self-regulated state, with low-amplitude nonaxisymmetric density perturbations persisting for at least several million years. Our main finding is that the global effect of gravitational torques due to these perturbations is to produce disk accretion rates that are of the correct magnitude to explain observed accretion onto T Tauri stars. Our models yield a correlation between accretion rate M_dot and stellar mass M_st that has a best fit M_dot \\propto M_st^{1.7}, in good agreement with recent observations. We also predict a near-linear correlation between the disk accretion rate and the disk mass.

  11. X-ray emission from classical T Tauri stars: Accretion shocks and coronae?

    CERN Document Server

    Guenther, H M; Robrade, J; Liefke, C

    2007-01-01

    Classical T Tauri stars (CTTS) are surrounded by actively accreting disks. According to current models material falls along the magnetic field lines from the disk with more or less free-fall velocity onto the star, where the plasma heats up and generates X-rays. We want to quantitatively explain the observed high energy emission and measure the infall parameters from the data. Absolute flux measurements allow to calculate the filling factor and the mass accretion rate.We use a numerical model of the hot accretion spot and solve the conservation equations. A comparison to data from XMM-Newton and Chandra shows that our model reproduces the main features very well. It yields for TW Hya a filling factor of 0.3% and a mass accretion rate 2e-10 M_sun/yr.

  12. Brackett γ radiation from the inner gaseous accretion disk, magnetosphere, and disk wind region of Herbig AeBe stars

    Science.gov (United States)

    Tambovtseva, L. V.; Grinin, V. P.; Weigelt, G.

    2016-05-01

    Various disk and outflow components such as the magnetosphere, the disk wind, the gaseous accretion disk, and other regions may contribute to the hydrogen line emission of young Herbig AeBe stars. Non-LTE modeling was performed to show the influence of the model parameters of each emitting region on the intensity and shape of the Brγ line profile, to present the spatial brightness distribution of each component, and to compare the contribution of each component to the total line emission. The modeling shows that the disk wind is the dominant contributor to the Brγ line rather than the magnetosphere and inner gaseous accretion disk. The contribution of the disk wind region to the Hα line is also considered.

  13. Enhanced accretion rates of stars on Super-massive Black Holes by star-disk interactions in galactic nuclei

    CERN Document Server

    Just, Andreas; Makukov, Maxim; Berczik, Peter; Omarov, Chingis; Spurzem, Rainer; Vilkoviskij, Emanuel Y

    2012-01-01

    We investigate the dynamical interaction of a central star cluster surrounding a super-massive black hole and a central accretion disk. The dissipative force acting on stars in the disk leads to an enhanced mass flow towards the super-massive black hole and to an asymmetry in the phase space distribution due to the rotating accretion disk. The accretion disk is considered as a stationary Keplerian rotating disk, which is vertically extended in order to employ a fully self-consistent treatment of stellar dynamics including the dissipative force originating from star-gas ram pressure effects. The stellar system is treated with a direct high-accuracy N-body integration code. A star-by-star representation, desirable in N-body simulations, cannot be extended to real particle numbers yet. Hence, we carefully discuss the scaling behavior of our model with regard to particle number and tidal accretion radius. The main idea is to find a family of models for which the ratio of two-body relaxation time and dissipation t...

  14. X-ray deficiency on strong accreting T Tauri stars - Comparing Orion with Taurus

    CERN Document Server

    Bustamante, Ignacio; Bouy, Hervé; Manara, Carlo; Ribas, Álvaro; Riviere-Marichalar, Pablo

    2015-01-01

    Depending on whether a T Tauri star accretes material from its circumstellar disk or not, different X-ray emission properties can be found. The accretion shocks produce cool heating of the plasma, contributing to the soft X-ray emission from the star. Using X-ray data from the Chandra Orion Ultra-deep Project and accretion rates that were obtained with the Hubble Space Telescope/WFPC2 photometric measurements in the Orion Nebula Cluster, we studied the relation between the accretion processes and the X-ray emissions of a coherent sample of T Tauri sources in the region. We performed regression and correlation analyses of our sample of T Tauri stars between the X-ray parameters, stellar properties, and the accretion measurements. We find that a clear anti-correlation is present between the residual X-ray luminosity and the accretion rates in our samples in Orion that is consistent with that found on the XMM-Newton Extended Survey of the Taurus molecular cloud (XEST) study. We provide a catalog with X-ray lumin...

  15. Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency

    Science.gov (United States)

    Sakurai, Y.; Vorobyov, E. I.; Hosokawa, T.; Yoshida, N.; Omukai, K.; Yorke, H. W.

    2016-06-01

    The formation of supermassive stars (SMSs) is a potential pathway to seed supermassive black holes in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper, we study the evolution of an accreting SMS and its UV emissivity with realistic variable accretion from a circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the protostellar accretion until the stellar mass exceeds 104 M⊙. The disc fragments by gravitational instability, creating many clumps that migrate inward to fall on to the star. The resulting accretion history is highly time-dependent: short episodic accretion bursts are followed by longer quiescent phases. We show that the disc for the direct collapse model is more unstable and generates greater variability than normal Pop III cases. Next, we conduct a stellar evolution calculation using the obtained accretion history. Our results show that, regardless of the variable accretion, the stellar radius monotonically increases with almost constant effective temperature at Teff ≃ 5000 K as the stellar mass increases. The resulting UV feedback is too weak to hinder accretion due to the low flux of stellar UV photons. The insensitivity of stellar evolution to variable accretion is attributed to the fact that time-scales of variability, ≲103 yr, are too short to affect the stellar structure. We argue that this evolution will continue until the SMS collapses to produce a black hole by the general relativistic instability after the mass reaches ≳105 M⊙.

  16. Variable X-Ray and UV emission from AGB stars: Accretion activity associated with binarity

    Science.gov (United States)

    Sahai, Raghvendra; Sanz-Forcada, Jorge; Sánchez Contreras, Carmen

    2016-07-01

    Almost all of our current understanding of the late evolutionary stages of (1 — 8) Mʘ stars is based on single-star models. However, binarity can drastically affect late stellar evolution, producing dramatic changes in the history and geometry of mass loss that occurs in stars as they evolve off the AGB to become planetary nebulae (PNe). A variety of binary models have been proposed, which can lead to the generation of accretion disks and magnetic fields, which in turn produce the highly collimated jets that have been proposed as the primary agents for the formation of bipolar and multipolar PNe. However, observational evidence of binarity in AGB stars is sorely lacking simply these stars are very luminous and variable, invalidating standard techniques for binary detection. Using an innovative technique of searching for UV emission from AGB stars with GALEX, we have identified a class of AGB stars with far- ultraviolet excesses (fuvAGB stars), that are likely candidates for active accretion associated with a binary companion. We have carried out a pilot survey for X-ray emission from fuvAGB stars. The X-ray fluxes are found to vary in a stochastic or quasi-periodic manner on roughly hour-long times-scales, and simultaneous UV observations show similar variations in the UV fluxes. We discuss several models for the X-ray emission and its variability and find that the most likely scenario for the origin of the X-ray (and FUV) emission involves accretion activity around a main-sequence companion star, with confinement by strong magnetic fields associated with the companion and/or an accretion disk around it.

  17. X-Shooter study of accretion in $\\rho$-Ophiucus: very low-mass stars and brown dwarfs

    CERN Document Server

    Manara, C F; Natta, A; Alcalá, J M

    2015-01-01

    We present new VLT/X-Shooter optical and NIR spectra of a sample of 17 candidate young low-mass stars and BDs in the rho-Ophiucus cluster. We derived SpT and Av for all the targets, and then we determined their physical parameters. All the objects but one have M*<0.6 Msun, and 8 have mass below or close to the hydrogen-burning limit. Using the intensity of various emission lines present in their spectra, we determined the Lacc and Macc for all the objects. When compared with previous works targeting the same sample, we find that, in general, these objects are not as strongly accreting as previously reported, and we suggest that the reason is our more accurate estimate of the photospheric parameters. We also compare our findings with recent works in other slightly older star-forming regions to investigate possible differences in the accretion properties, but we find that the accretion properties for our targets have the same dependence on the stellar and substellar parameters as in the other regions. This l...

  18. Turbulence-driven Polar Winds from T Tauri Stars Energized by Magnetospheric Accretion

    CERN Document Server

    Cranmer, Steven R

    2008-01-01

    Pre-main-sequence stars are observed to be surrounded by both accretion flows and some kind of wind or jet-like outflow. Recent work by Matt and Pudritz has suggested that if classical T Tauri stars exhibit stellar winds with mass loss rates about 0.1 times their accretion rates, the wind can carry away enough angular momentum to keep the stars from being spun up unrealistically by accretion. This paper presents a preliminary set of theoretical models of accretion-driven winds from the polar regions of T Tauri stars. These models are based on recently published self-consistent simulations of the Sun's coronal heating and wind acceleration. In addition to the convection-driven MHD turbulence (which dominates in the solar case), we add another source of wave energy at the photosphere that is driven by the impact of plasma in neighboring flux tubes undergoing magnetospheric accretion. This added energy, determined quantitatively from the far-field theory of MHD wave generation, is sufficient to produce T Tauri-l...

  19. Comparison between accretion-related properties of Herbig Ae/Be and T Tauri stars

    CERN Document Server

    Mendigutía, I

    2013-01-01

    This paper summarizes several results concerning the comparison between accretion-related properties of cool (T Tauri; T < 7000 K, M < 1 Msun and hot (Herbig Ae/Be; 7000 < T(K) < 13000; 1 < M(Msun) < 6) pre-main sequence (PMS) stars. This comparison gives insight into the analogies/differences in the physics of the star-disk interaction and in the physical mechanisms driving disk dissipation. Several optical and near-IR line luminosities used for low-mass objects are also valid to estimate typical accretion rates for intermediate-mass stars under similar empirical expressions. In contrast, the Halpha width at 10% of peak intensity is used as an accretion tracer for T Tauris, but is not reliable to estimate accretion rates for Herbig Ae/Bes. This can be explained as a consequence of the different stellar rotation rates that characterize both types of stars. In addition, there are similar trends when the accretion rate is related to the near-IR colours and disk masses, suggesting that viscous ...

  20. 3D MHD Simulations of accreting neutron stars: evidence of QPO emission from the surface

    CERN Document Server

    Bachetti, Matteo; Kulkarni, Akshay; Burderi, Luciano; di Salvo, Tiziana; .,

    2009-01-01

    3D Magnetohydrodynamic simulations show that when matter accretes onto neutron stars, in particular if the misalignment angle is small, it does not constantly fall at a fixed spot. Instead, the location at which matter reaches the star moves. These moving hot spots can be produced both during stable accretion, where matter falls near the magnetic poles of the star, and unstable accretion, characterized by the presence of several tongues of matter which fall on the star near the equator, due to Rayleigh-Taylor instabilities. Precise modeling with Monte Carlo simulations shows that those movements could be observed as high frequency Quasi Periodic Oscillations. We performed a number of new simulation runs with a much wider set of parameters, focusing on neutron stars with a small misalignment angle. In most cases we observe oscillations whose frequency is correlated with the mass accretion rate $\\dot{M}$. Moreover, in some cases double QPOs appear, each of them showing the same correlation with $\\dot{M}$.

  1. The Early History of Stellar Spin: the Theory of Accretion onto Young Stellar Objects

    Directory of Open Access Journals (Sweden)

    Pudritz Ralph E.

    2014-01-01

    Full Text Available The interaction of the magnetospheres of forming stars with their surrounding protostellar disks results in magnetospheric accretion flow onto the star. How is the associated angular momentum of accreting material channelled? The resolution of this issue is crucial for understanding the origin of the spins of pre main sequence stars. A significant fraction of these rotate very slowly, which indicates that an efficient angular momentum transport mechanism is at work to counteract the strong accretion spin up torques. We review the observational, theoretical, and computational advances in the field and argue that an accretion powered stellar winds together with highly time variable mass ejections from the disk/magnetosphere interface is a likely solution.

  2. An IUE Atlas of Pre-Main-Sequence Stars. II. Far-Ultraviolet Accretion Diagnostics in T Tauri Stars

    Science.gov (United States)

    Johns-Krull, Christopher M.; Valenti, Jeff A.; Linsky, Jeffrey L.

    2000-08-01

    We use our ultraviolet (UV) atlas of pre-main-sequence stars constructed from all useful, short-wavelength, low-resolution spectra in the International Ultraviolet Explorer (IUE) satellite Final Archive to analyze the short-wavelength UV properties of 49 T Tauri stars (TTSs). We compare the line and continuum fluxes in these TTSs with each other and with previously published parameters of these systems, including rotation rate, infrared excess, and mass accretion rate. The short-wavelength continuum in the classical TTSs (CTTSs) appears to originate in a ~10,000 K optically thick plasma, while in the naked TTSs (NTTSs-stars without dusty disks) the continuum appears to originate in the stellar atmosphere. We show that all of the TTSs in our sample lie in the regime of ``saturated'' magnetic activity due to their small Rossby numbers. However, while some of the TTSs show emission line surface fluxes consistent with this saturation level, many CTTSs show significantly stronger emission than predicted by saturation. In these stars, the emission line luminosity in the high ionization lines present in the spectrum between 1200 and 2000 Å correlates well with the mass accretion rate. Therefore, we conclude that the bulk of the short-wavelength emission seen in CTTSs results from accretion related processes and not from dynamo-driven magnetic activity. Using CTTSs with known mass accretion rates, we calibrate the relationship between M and LC IV to derive the mass accretion rate for some CTTSs which for various reasons have never had their mass accretion rates measured. Finally, several of the CTTSs show strong emission from molecular hydrogen. While emission from H2 cannot form in gas at a temperature of ~105 K, the strength of the molecular hydrogen emission is nevertheless well correlated with all the other emissions displayed in the IUE short-wavelength bandpass. This suggests that the H2 emission is in fact fluorescent emission pumped by the emission (likely Ly

  3. Clumpy wind accretion in supergiant neutron star high mass X-ray binaries

    Science.gov (United States)

    Bozzo, E.; Oskinova, L.; Feldmeier, A.; Falanga, M.

    2016-05-01

    The accretion of the stellar wind material by a compact object represents the main mechanism powering the X-ray emission in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. In this work we present the first attempt to simulate the accretion process of a fast and dense massive star wind onto a neutron star, taking into account the effects of the centrifugal and magnetic inhibition of accretion ("gating") due to the spin and magnetic field of the compact object. We made use of a radiative hydrodynamical code to model the nonstationary radiatively driven wind of an O-B supergiant star and then place a neutron star characterized by a fixed magnetic field and spin period at a certain distance from the massive companion. Our calculations follow, as a function of time (on a total timescale of several hours), the transitions of the system through all different accretion regimes that are triggered by the intrinsic variations in the density and velocity of the nonstationary wind. The X-ray luminosity released by the system is computed at each time step by taking into account the relevant physical processes occurring in the different accretion regimes. Synthetic lightcurves are derived and qualitatively compared with those observed from classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. Although a number of simplifications are assumed in these calculations, we show that taking into account the effects of the centrifugal and magnetic inhibition of accretion significantly reduces the average X-ray luminosity expected for any neutron star wind-fed binary. The present model calculations suggest that long spin periods and stronger magnetic fields are favored in order to reproduce the peculiar behavior of supergiant fast X-ray transients in the X-ray domain.

  4. A Global Jet/Circulation Model for Young Stars

    CERN Document Server

    Lery, T; Fiege, J D; Ray, T P; Frank, A; Bacciotti, F

    2002-01-01

    Powerful, highly collimated jets, surrounded by bipolar molecular outflows, are commonly observed near Young Stellar Objects (YSOs). In the usual theoretical picture of star formation, a jet is ejected from a magnetized accretion disk, with a molecular outflow being driven either by the jet or by a wider wind coming from the disk. Here, we propose an alternative global model for the flows surrounding YSOs. In addition to a central accretion-ejection engine driving the jet, the molecular outflow is powered by the infalling matter and follows a circulation pattern around the central object without necessarily being entrained by a jet. It is shown that the model produces a heated pressure-driven outflow with magneto-centrifugal acceleration and collimation. We report solutions for the three different parts of this self-similar model, i.e. the jet, the infalling envelope and the circulating matter that eventually forms the molecular outflow. This new picture of the accretion/outflow phase provides a possible expl...

  5. Observable Signatures of Classical T Tauri Stars Accreting in an Unstable Regime

    Directory of Open Access Journals (Sweden)

    Kurosawa Ryuichi

    2014-01-01

    Full Text Available We discuss key observational signatures of Classical T Tauri stars (CTTSs accreting through Rayleigh-Taylor instability, which occurs at the interface between an accretion disk and a stellar magnetosphere. In this study, the results of global 3-D MHD simulations of accretion flows, in both stable and unstable regimes, are used to predict the variability of hydrogen emission lines and light curves associated with those two distinctive accretion flow patterns. In the stable regime, a redshifted absorption component (RAC periodically appears in some hydrogen lines, but only during a fraction of a stellar rotation period. In the unstable regime, the RAC is present rather persistently during a whole stellar rotation period, and its strength varies non-periodically. The latter is caused by multiple accreting streams, formed randomly due to the instability, passing across the line of sight to an observer during one stellar rotation. This results in the quasi-stationarity appearance of the RAC because at least one of the accretion stream is almost always in the line of sight to an observer. In the stable regime, two stable hot spots produce a smooth and periodic light curve that shows only one or two peaks per stellar rotation. In the unstable regime, multiple hot spots formed on the surface of the star, produce the stochastic light curve with several peaks per rotation period.

  6. The S2 star as a probe of the accretion disk of Sgr A*

    CERN Document Server

    Giannios, Dimitrios

    2013-01-01

    How accretion proceeds around the massive black hole in the Galactic center and other highly sub-Eddington accretors remains poorly understood. The orbit of the S2 star in the Galactic center passes through the accretion disk of the massive black hole and any observational signature from such interaction may be used as an accretion probe. Because of its early stellar type, S2 is expected to possess a fairly powerful wind. We show here that the ram pressure of the accretion disk shocks the stellar wind fairly close to the star. The shocked fluid reaches a temperature of ~ 1 keV and cools efficiently through optically thin, thermal bremsstrahlung emission. The radiation from the shocked wind peaks around the epoch of the pericenter passage of the star at a luminosity potentially comparable to the quiescent emission detected from Sgr A*. Detection of shocked wind radiation can constrain the density of the accretion disk at a distance of several thousands of gravitational radii from the black hole.

  7. TRANSITIONAL DISKS AROUND YOUNG LOW MASS STARS

    Directory of Open Access Journals (Sweden)

    P. D'Alessio

    2009-01-01

    have been interpreted as produced by disks with inner holes, which have been classi ed as \\Transitional Disks". These disks are considered the evolutionary link between the full disks typically found around the young T Tauri and Herbig Ae stars, and the debris disks, found around some main sequence stars. In this contribution we summarize the observed/inferred characteristics of these transitional disks and also some of the models proposed to explain their peculiar geometry.

  8. DISK-RELATED BURSTS AND FADES IN YOUNG STARS

    Energy Technology Data Exchange (ETDEWEB)

    Findeisen, Krzysztof; Hillenbrand, Lynne; Levitan, David; Sesar, Branimir [Cahill Center for Astronomy and Astrophysics, California Institute of Technology, MC 249-17, Pasadena, CA 91125 (United States); Ofek, Eran [Benoziyo Center for Astrophysics, Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100 (Israel); Laher, Russ; Surace, Jason, E-mail: krzys@astro.caltech.edu, E-mail: lah@astro.caltech.edu [Spitzer Science Center, California Institute of Technology, MC 314-6, Pasadena, CA 91125 (United States)

    2013-05-01

    We present first results from a new, multiyear, time domain survey of young stars in the North America Nebula complex using the Palomar Transient Factory. Our survey is providing an unprecedented view of aperiodic variability in young stars on timescales of days to years. The analyzed sample covers R{sub PTF} Almost-Equal-To 13.5-18 and spans a range of mid-infrared color, with larger-amplitude optical variables (exceeding 0.4 mag root mean squared) more likely to have mid-infrared evidence for circumstellar material. This paper characterizes infrared excess stars with distinct bursts above or fades below a baseline of lower-level variability, identifying 41 examples. The light curves exhibit a remarkable diversity of amplitudes, timescales, and morphologies, with a continuum of behaviors that cannot be classified into distinct groups. Among the bursters, we identify three particularly promising sources that may represent theoretically predicted short-timescale accretion instabilities. Finally, we find that fading behavior is approximately twice as common as bursting behavior on timescales of days to years, although the bursting and fading duty cycle for individual objects often varies from year to year.

  9. Disk-Related Bursts and Fades in Young Stars

    CERN Document Server

    Findeisen, Krzysztof; Ofek, Eran; Levitan, David; Sesar, Branimir; Laher, Russ; Surace, Jason

    2013-01-01

    We present first results from a new, multiyear, time domain survey of young stars in the North America Nebula complex using the Palomar Transient Factory. Our survey is providing an unprecedented view of aperiodic variability in young stars on timescales of days to years. The analyzed sample covers R_PTF = 13.5-18 and spans a range of mid-infrared color, with larger-amplitude optical variables (exceeding 0.4 mag root-mean-squared) more likely to have mid-infrared evidence for circumstellar material. This paper characterizes infrared excess stars with distinct bursts above or fades below a baseline of lower-level variability, identifying 41 examples. The light curves exhibit a remarkable diversity of amplitudes, timescales, and morphologies, with a continuum of behaviors that can not be classified into distinct groups. Among the bursters, we identify three particularly promising sources that may represent theoretically predicted short-timescale accretion instabilities. Finally, we find that fading behavior is ...

  10. A Multi-Fiber Spectroscopic Search for Low-mass Young Stars in Orion OB1

    Science.gov (United States)

    Loerincs, Jacqueline; Briceno, Cesar; Calvet, Nuria; Mateo, Mario L.; Hernandez, Jesus

    2017-01-01

    We present here results of a low resolution spectroscopic followup of candidate low-mass pre-main sequence stars in the Orion OB1 association. Our targets were selected from the CIDA Variability Survey of Orion (CVSO), and we used the Michigan/Magellan Fiber Spectrograph (M2FS) on the Magellan Clay 6.5m telescope to obtain spectra of 500 candidate T Tauri stars distributed in seven 0.5 deg diameter fields, adding to a total area of ~5.5 deg2. We identify young stars by looking at the distinctive Hα 6563 Å emission and Lithium Li I 6707 Å absorption features characteristic of young low mass pre-main sequence stars. Furthermore, by measuring the strength of their Hα emission lines, confirmed T Tauri stars can be classified as either Classical T Tauris (CTTS) or Weak-line T Tauris (WTTS), which give indication of whether the star is actively accreting material from a gas and dust disk surrounding the star, which may be the precursor of a planetary system. We confirm a total of 90 T Tauri stars, of which 50% are newly identified young members of Orion; out of the 49 new detections,15 are accreting CTTS, and of these all but one are found in the OB1b sub-region. This result is in line with our previous findings that this region is much younger than the more extended Orion OB1a sub-association. The M2FS results add to our growing census of young stars in Orion, that is allowing us to characterize in a systematic and consistent way the distribution of stellar ages across the entire complex, in order to building a complete picture of star formation in this, one of nearest most active sites of star birth.

  11. Critical condition for the propeller effect in systems with magnetized neutron stars accreting from geometrically thin accretion disks

    Science.gov (United States)

    Ertan, Unal

    2016-07-01

    The inner disk radius around a magnetized neutron star in the spin-down phase is usually assumed to be close to the radius at which the viscous and magnetic stresses are balanced. With different assumptions, this radius is estimated to be very close the Alfven radius. Furthermore, it is commonly assumed that the propeller mechanism can expel the matter from the system when this radius is found to be greater than the co-rotation radius. In the present work, we have shown with simple analytical calculations from the first principles that a steady-state propeller mechanism cannot be established at the radius where the viscous and the magnetic torques are balanced. We have found that a steady-state propeller phase can be built up with an inner disk radius that is at least ~10 - 30 times smaller than the Alfven radius depending on the current mass-flow rate of the disk, the field strength and the rotational period of the source. This result also indicates that the critical accretion rate for the accretion-propeller transition is orders of magnitude smaller than the rate found by equating the Alfven and the co-rotation radii. Our results are consistent with the properties of recently discovered transitional millisecond pulsars which show transitions between the rotational powered radio pulsar and the accretion powered X-ray pulsar states.

  12. Magnetospheric accretion and spin-down of the prototypical classical T Tauri star AA Tau

    Science.gov (United States)

    Donati, J.-F.; Skelly, M. B.; Bouvier, J.; Gregory, S. G.; Grankin, K. N.; Jardine, M. M.; Hussain, G. A. J.; Ménard, F.; Dougados, C.; Unruh, Y.; Mohanty, S.; Aurière, M.; Morin, J.; Farès, R.; MAPP Collaboration

    2010-12-01

    From observations collected with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope (CFHT) and with the NARVAL spectropolarimeter at the Télescope Bernard Lyot (TBL), we report the detection of Zeeman signatures on the prototypical classical T Tauri star AA Tau, both in photospheric lines and accretion-powered emission lines. Using time series of unpolarized and circularly polarized spectra, we reconstruct at two epochs maps of the magnetic field, surface brightness and accretion-powered emission of AA Tau. We find that AA Tau hosts a 2-3 kG magnetic dipole tilted at ≃20° to the rotation axis, and of presumably dynamo origin. We also show that the magnetic poles of AA Tau host large cool spots at photospheric level and accretion regions at chromospheric level. The accretion rate at the surface of AA Tau at the time of our observations (estimated from the emission in the He I D3 line mainly) is strongly variable, ranging from -9.6 to -8.5 and equal to -9.2 in average (in logarithmic scale and in M⊙ yr-1); this is an order of magnitude smaller than the disc accretion rate at which the magnetic truncation radius (below which the disc is disrupted by the stellar magnetic field) matches the corotation radius (where the Keplerian period equals the stellar rotation period) - a necessary condition for accretion to occur. It suggests that AA Tau is largely in the propeller regime, with most of the accreting material in the inner disc regions being expelled outwards and only a small fraction accreted towards the surface of the star. The strong variability in the observed surface mass accretion rate and the systematic time-lag of optical occultations (by the warped accretion disc) with respect to magnetic and accretion-powered emission maxima also support this conclusion. Our results imply that AA Tau is being actively spun-down by the star/disc magnetic coupling and appears as an ideal laboratory for studying angular momentum losses of forming suns

  13. Cyclotron line signatures of thermal and magnetic mountains from accreting neutron stars

    CERN Document Server

    Priymak, Maxim; Lasky, Paul

    2014-01-01

    Cyclotron resonance scattering features (CRSFs) in the X-ray spectrum of an accreting neutron star are modified differently by accretion mounds sustained by magnetic and thermocompositional gradients. It is shown that one can discriminate, in principle, between mounds of different physical origins by studying how the line energy, width, and depth of a CRSF depend on the orientation of the neutron star, accreted mass, surface temperature distribution, and equation of state. CRSF signatures including gravitational light bending are computed for both phase-resolved and phase-averaged spectra on the basis of self-consistent Grad-Shafranov mound equilibria satisfying a global flux-freezing constraint. The prospects of multimessenger X-ray and gravitational-wave observations with future instruments are canvassed briefly.

  14. The Ionization of Accretion Flows in High Mass Star Formation: W51e2

    CERN Document Server

    Keto, Eric

    2008-01-01

    Previous observations show that the hypercompact HII region W51e2 is surrounded by a massive molecular accretion flow centered on the HII region. New observations of the H53alpha radio recombination line made with the VLA at 0.45 arc second angular resolution show a velocity gradient in the ionized gas within the HII region of > 500 kms-1 pc-1 comparable to the velocity gradient seen in the molecular accretion flow. New CO line observations made with the SMA at arc second angular resolution detect a molecular bipolar outflow immediately around the W51e2 HII region and extending along the axis of rotation of the molecular flow. These observations are consistent with an evolutionary phase for high mass star formation in which a newly formed massive star first begins to ionize its surroundings including its own accretion flow.

  15. Star formation in accretion discs : from the Galactic Center to Active Galactic Nuclei

    CERN Document Server

    Collin, Suzy

    2007-01-01

    Keplerian accretion discs around massive black holes (MBHs) are gravitationally unstable beyond a few hundredths of parsec and should collapse to form stars. Indeed an accretion/star formation episode took place a few millions years ago in the Galactic Center (GC). This raises the question of how the disc can survive in AGN and quasars and continue to transport matter towards the black hole. We study the accretion/star formation process, with one aim in mind, to show that a spectrum similar to the observed AGN one can be produced by the disc. We compute models of stationary accretion discs, both continuous and clumpy. Continuous discs must be maintained in a state of marginal stability for the rate of star formation to remain modest, so they require additional heating and transport of angular momentum. Non-viscous heating can be provided by stellar illumination, but momentum transport by supernovae is insufficient to sustain a marginal state, except at the very periphery of the disc. In clumpy discs it is pos...

  16. Relativistic lines and reflection from the inner accretion disks around neutron stars

    NARCIS (Netherlands)

    Cackett, E.M.; Miller, J.M.; Ballantyne, D.R.; Barret, D.; Bhattacharyya, S.; Boutelier, M.; Miller, M.C.; Strohmayer, T.E.; Wijnands, R.

    2010-01-01

    A number of neutron star low-mass X-ray binaries (LMXBs) have recently been discovered to show broad, asymmetric Fe K emission lines in their X-ray spectra. These lines are generally thought to be the most prominent part of a reflection spectrum, originating in the inner part of the accretion disk w

  17. Clumpy wind accretion in supergiant neutron star high mass X-ray binaries

    CERN Document Server

    Bozzo, E; Feldmeier, A; Falanga, M

    2016-01-01

    The accretion of the stellar wind material by a compact object represents the main mechanism powering the X-ray emission in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. In this work we present the first attempt to simulate the accretion process of a fast and dense massive star wind onto a neutron star, taking into account the effects of the centrifugal and magnetic inhibition of accretion ("gating") due to the spin and magnetic field of the compact object. We made use of a radiative hydrodynamical code to model the non-stationary radiatively driven wind of an O-B supergiant star and then place a neutron star characterized by a fixed magnetic field and spin period at a certain distance from the massive companion. Our calculations follow, as a function of time (on a total time scale of several hours), the transition of the system through all different accretion regimes that are triggered by the intrinsic variations in the density and velocity of the non-stationary wind. Th...

  18. Star formation in accretion discs: from the Galactic center to active galactic nuclei

    Science.gov (United States)

    Collin, S.; Zahn, J.-P.

    2008-01-01

    Context: Keplerian accretion discs around massive black holes (MBHs) are gravitationally unstable beyond a few hundredths of a parsec, and they should collapse to form stars. It has indeed been shown recently that an accretion/star formation episode took place a few million years ago in the Galactic center (GC). This raises the question of how the disc can survive in AGN and quasars and continue to transport matter towards the black hole. Aims: We study the accretion/star formation process in quasars and AGN with one aim in mind: to show that a spectrum similar to the observed one can be produced by the disc. Methods: We compute models of stationary accretion discs that are either continuous or clumpy. Continuous discs must be maintained in a state of marginal stability so that the rate of star formation remains modest and the disc is not immediately destroyed. The disc then requires additional heating and additional transport of angular momentum. In clumpy discs, the momentum transport is provided by cloud interactions. Results: Non-viscous heating can be provided by stellar illumination, but in the case of continuous discs, even momentum transport by supernovae is insufficient for sustaining a marginal state, except at the very periphery of the disc. In clumpy discs it is possible to account for the required accretion rate through interactions between clouds, but this model is unsatisfactory because its parameters are tightly constrained without any physical justification. Conclusions: Finally one must appeal to non-stationary discs with intermittent accretion episodes like those that occurred in the GC, but such a model is probably not applicable either to luminous high redshift quasars or to radio-loud quasars.

  19. Probing the effects of a thermonuclear X-ray burst on the neutron star accretion flow with NuSTAR

    CERN Document Server

    Degenaar, N; Chakrabarty, D; Kara, E; Altamirano, D; Miller, J M; Fabian, A C

    2016-01-01

    Observational evidence has been accumulating that thermonuclear X-ray bursts ignited on the surface of neutron stars influence the surrounding accretion flow. Here, we exploit the excellent sensitivity of NuSTAR up to 79 keV to analyze the impact of an X-ray burst on the accretion emission of the neutron star LMXB 4U 1608-52. The ~200 s long X-ray burst occurred during a hard X-ray spectral state, and had a peak intensity of ~30-50 per cent of the Eddington limit with no signs of photospheric radius expansion. Spectral analysis suggests that the accretion emission was enhanced up to a factor of ~5 during the X-ray burst. We also applied a linear unsupervised decomposition method, namely non-negative matrix factorization (NMF), to study this X-ray burst. We find that the NMF performs well in characterizing the evolution of the burst emission and is a promising technique to study changes in the underlying accretion emission in more detail than is possible through conventional spectral fitting. For the burst of ...

  20. The accretion/ejection paradigm in young stellar objects: from HST and Herschel to JWST

    Science.gov (United States)

    Podio, Linda

    2012-07-01

    Stellar jets and molecular outflows are observed in association with young accreting stars and are believed to play a key role in the star formation process. In this talk I will show how current and future space missions are of crucial importance to investigate the origin of stellar jets and their link to the accretion process. Thanks to its high angular (˜0.1") resolution, HST has been the first telescope allowing us to investigate the jet physics at optical/UV wavelengths down to the heart of the launching mechanism. We recently analysed a datacube of the jet emitted by the T Tauri star DG Tau obtaining spatio-kinematical maps of the hot atomic gas in the jet and of its physical conditions (Maurri et al., submitted). These data confirm the predictions of theoretical models including the fact that jets may extract the excess angular momentum from the system. In the last two years Herschel has further improved our comprehension of the ejection process observing the far infrared counterpart of fast and collimated atomic jets. PACS and HIFI observations, acquired within the GASPS (GAS in Protoplanetary Systems) Open Time Key Project (PI: B. Dent), show that T Tauri stars driving optical jets are also associated with a warm gas component emitting not only atomic ([OI], [CII]) but also molecular (high-J CO, H_2O, OH) lines. The comparison with Class 0 outflows highlights a clear evolutionary trend: the emission associated with evolved Class I/II sources is fainter and more compact and the estimated mass loss rates and lines cooling are one to two orders of magnitudes lower (Podio et al., to be submitted). The arrival of JWST will fill-in the gap between HST and Herschel opening a new window in the near and mid-infrared range at unprecedented angular resolution (down to 0.03"). This will allow resolving the emission in both atomic (e.g., [FeII]) and molecular (e.g., H_2) lines and understanding if the molecular gas is entrained by the atomic jet or launched with it

  1. Binarity and Accretion: An Extended Survey of AGB stars with FUV Excesses

    Science.gov (United States)

    Sahai, Raghvendra

    2013-10-01

    Based on the exciting results of our AO-12 pilot survey, we propose an extended survey for X-ray emission from AGB stars that are candidates for having binary companions with active accretion. These objects were identified via our innovative technique to search for FUV/NUV excesses in AGB stars using GALEX. The detection (or non-detection) of X-rays from this sample will enable us to begin testing models for the origin of the UV-excesses, leading to vital breakthroughs in our understanding of accretion-related phenomena and binarity in AGB stars. A larger project that includes time-monitoring of specific objects, optimised using results from this study, will be proposed in future cycles.

  2. The inner disc radius in the propeller phase and accretion-propeller transition of neutron stars

    Science.gov (United States)

    Ertan, Ünal

    2017-04-01

    We have investigated the critical conditions required for a steady propeller effect for magnetized neutron stars with optically thick, geometrically thin accretion discs. We have shown through simple analytical calculations that a steady-state propeller mechanism cannot be sustained at an inner disc radius where the viscous and magnetic stresses are balanced. The radius calculated by equating these stresses is usually found to be close to the conventional Alfvén radius for spherical accretion, rA. Our results show that: (1) a steady propeller phase can be established with a maximum inner disc radius that is at least ∼15 times smaller than rA depending on the mass-flow rate of the disc, rotational period and strength of the magnetic dipole field of the star, (2) the critical accretion rate corresponding to the accretion-propeller transition is orders of magnitude lower than the rate estimated by equating rA to the co-rotation radius. Our results are consistent with the properties of the transitional millisecond pulsars that show transitions between the accretion powered X-ray pulsar and the rotational powered radio-pulsar states.

  3. Photon Bubbles in the Circumstellar Envelopes of Young Massive Stars

    CERN Document Server

    Turner, N J; Yorke, H W

    2007-01-01

    We show that the optically-thick dusty envelopes surrounding young high-mass stars are subject to the photon bubble instability. The infrared radiation passing through the envelope amplifies magnetosonic disturbances, with growth rates in our local numerical radiation MHD calculations that are consistent with a linear analysis. Modes with wavelengths comparable to the gas pressure scale height grow by more than two orders of magnitude in a thousand years, reaching non-linear amplitudes within the envelope lifetime. If the magnetic pressure in the envelope exceeds the gas pressure, the instability develops into trains of propagating shocks. Radiation escapes readily through the low-density material between the shocks, enabling accretion to continue despite the Eddington limit imposed by the dust opacity. The supersonic motions arising from the photon bubble instability can help explain the large velocity dispersions of hot molecular cores, while conditions in the shocked gas are suitable for maser emission. We...

  4. Formation of Massive Primordial Stars: Intermittent UV Feedback with Episodic Mass Accretion

    CERN Document Server

    Hosokawa, Takashi; Kuiper, Rolf; Yorke, Harold W; Omukai, Kazuyuki; Yoshida, Naoki

    2015-01-01

    We present coupled stellar evolution (SE) and 3D radiation-hydrodynamic (RHD) simulations of the evolution of primordial protostars, their immediate environment, and the dynamic accretion history under the influence of stellar ionizing and dissociating UV feedback. Our coupled SE-RHD calculations result in a wide diversity of final stellar masses covering $10~M_\\odot \\lesssim M_* \\lesssim 10^3~M_\\odot$. The formation of very massive ($\\gtrsim 250~M_\\odot$) stars is possible under weak UV feedback, whereas ordinary massive (a few $\\times 10~M_\\odot$) stars form when UV feedback can efficiently halt the accretion. Weak UV feedback occurs in cases of variable accretion, in particular when repeated short accretion bursts temporarily exceed $0.01~M_\\odot~{\\rm yr}^{-1}$, causing the protostar to inflate. In the bloated state, the protostar has low surface temperature and UV feedback is suppressed until the star eventually contracts, on a thermal adjustment timescale, to create an HII region. If the delay time betwe...

  5. Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency

    CERN Document Server

    Sakurai, Yuya; Hosokawa, Takashi; Yoshida, Naoki; Omukai, Kazuyuki; Yorke, Harold W

    2015-01-01

    The formation of SMSs is a potential pathway to seed SMBHs in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper we study the evolution of an accreting SMS and its UV emissivity under conditions of realistic variable accretion from a self-gravitating circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the long-term protostellar accretion until the stellar mass exceeds $10^4~M_\\odot$. The disc fragments due to gravitational instability, creating a number of small clumps that rapidly migrate inward to fall onto the star. The resulting accretion history is thus highly time-dependent: short episodic accretion bursts are followed by longer, relative quiescent phases. We show that the circumstellar disc for the so-called direct collapse model is more unstable and generates greater variability over shorter timescales than normal Pop III cases. We conduct a post-process stellar evolution calculati...

  6. Conditions for water ice lines and Mars-mass exomoons around accreting super-Jovian planets at 1 - 20 AU from Sun-like stars

    CERN Document Server

    Heller, René

    2015-01-01

    Exomoon detections might be feasible with NASA's Kepler or ESA's upcoming PLATO mission or the ground-based E-ELT. To use observational resources most efficiently we need to know where the largest, most easily detected moons can form. We explore the possibility of large exomoons by following the movement of water (H2O) ice lines in the accretion disks around young super-Jovian planets. We want to know how different heating sources in those disks affect the H2O ice lines. We simulate 2D rotationally symmetric accretion disks in hydrostatic equilibrium around super-Jovian exoplanets. The energy terms in our semi-analytical model -- (1) viscous heating, (2) planetary illumination, (3) accretional heating, and (4) stellar illumination -- are fed by precomputed planet evolution tracks. We consider planets accreting 1 to 12 Jupiter masses at distances between 1 and 20 AU to a Sun-like star. Accretion disks around Jupiter-mass planets closer than ~4.5 AU to Sun-like stars do not feature H2O ice lines, but the most m...

  7. Visible AO Observations at Halpha for Accreting Young Planets

    Science.gov (United States)

    Close, L. M.; Follette, K.; Males, J. R.; Morzinski, K.; Rodigas, T. J.; Hinz, P.; Wu, Y.-L.; Apai, D.; Najita, J.; Puglisi, A.; Esposito, S.; Riccardi, A.; Bailey, V.; Xompero, M.; Briguglio, R.; Weinberger, A.

    2014-01-01

    We utilized the new high-order (250-378 mode) Magellan Adaptive Optics system (MagAO) to obtain very high-resolution science in the visible with MagAO's VisAO CCD camera. In the good-median seeing conditions of Magellan (0.5-0.7'') we find MagAO delivers individual short exposure images as good as 19 mas optical resolution. Due to telescope vibrations, long exposure (60s) r' (0.63μm) images are slightly coarser at FWHM = 23-29 mas (Strehl ~ 28%) with bright (R Orion Trapezium θ1 Ori A, B, and C cluster members were obtained with VisAO. In particular, the 32 mas binary θ1 Ori C 1 C 2 was easily resolved in non-interferometric images for the first time. Relative positions of the bright trapezium binary stars were measured with ~ 0.6-5 mas accuracy. In the second commissioning run we were able to correct 378 modes and achieved good contrasts (Strehl>20% on young transition disks at Hα). We discuss the contrasts achieved at Hα and the possibility of detecting low mass (~ 1-5 Mjup) planets (past 5AU) with our new SAPPHIRES survey with MagAO at Hα.

  8. A nova re-accretion model for J-type carbon stars

    CERN Document Server

    Sengupta, S; Lau, H H B

    2013-01-01

    The J-type carbon (J)-stars constitute 10-15% of the observed carbon stars in both our Galaxy and the Large Magellanic Cloud (LMC). They are characterized by strong 13C absorption bands with low 12C/13C ratios along with other chemical signatures peculiar for typical carbon stars, e.g. a lack of s-process enhancement. Most of the J-stars are dimmer than the N-type carbon stars some of which, by hot-bottom burning, make 13C only in a narrow range of masses. We investigate a binary-star formation channel for J-stars involving re-accretion of carbon-rich nova ejecta on main-sequence companions to low-mass carbon-oxygen white-dwarfs. The subsequent evolution of the companion stars in such systems is studied with a rapid binary evolutionary code to predict chemical signatures of nova pollution in systems which merge into giant single stars. A detailed population synthesis study is performed to estimate the number of these mergers and compare their properties with observed J-stars. Our results predict that such nov...

  9. The Role for the Inner Disk in Mass Accretion to the Star in the Early Phase of Star Formation

    CERN Document Server

    Ohtani, Takuya; Tsuribe, Toru; Vorobyov, Eduard I

    2014-01-01

    A physical mechanism that drives FU Orionis-type outbursts is reconsidered. We study the effect of inner part of a circumstellar disk covering a region from near the central star to the radius of approximately $5$ AU (hereafter, the inner disk). Using the fluctuated mass accretion rate onto the inner disk $\\dot{M}_{\\rm out}$, we consider the viscous evolution of the inner disk and the time variability of the mass accretion rate onto the central star $\\dot{M}_{\\rm in}$ by means of numerical calculation of an unsteady viscous accretion disk in a one-dimensional axisymmetric model. First, we calculate the evolution of the inner disk assuming an oscillating $\\dot{M}_{\\rm out}$. It is shown that the time variability of $\\dot{M}_{\\rm in}$ does not coincide with $\\dot{M}_{\\rm out}$ due to viscous diffusion. Second, we investigate the properties of spontaneous outbursts with temporally constant $\\dot{M}_{\\rm out}$. Outburst occur only in a limited range of mass accretion rates onto the inner disk $10^{-10}<\\dot{M}...

  10. Star formation and accretion in the circumnuclear disks of active galaxies

    CERN Document Server

    Wutschik, Stephanie; Palmer, Thomas S

    2013-01-01

    We explore the evolution of supermassive black holes (SMBH) centered in a circumnuclear disk (CND) as a function of the mass supply from the host galaxy and considering different star formation laws, which may give rise to a self-regulation via the injection of supernova-driven turbulence. A system of equations describing star formation, black hole accretion and angular momentum transport was solved for an axisymmetric disk in which the gravitational potential includes contributions from the black hole, the disk and the hosting galaxy. Our model extends the framework provided by Kawakatu et al. (2008) by separately considering the inner and outer part of the disk, and by introducing a potentially non-linear dependence of the star formation rate on the gas surface density and the turbulent velocity. The star formation recipes are calibrated using observational data for NGC 1097, while the accretion model is based on turbulent viscosity as a source of angular momentum transport in a thin viscous accretion disk....

  11. On the accretion properties of young stellar objects in the L1615/L1616 cometary cloud

    CERN Document Server

    Biazzo, K; Frasca, A; Zusi, M; Getman, F; Covino, E; Gandolfi, D

    2014-01-01

    We present the results of FLAMES/UVES and FLAMES/GIRAFFE spectroscopic observations of 23 low-mass stars in the L1615/L1616 cometary cloud, complemented with FORS2 and VIMOS spectroscopy of 31 additional stars in the same cloud. L1615/L1616 is a cometary cloud where the star formation was triggered by the impact of the massive stars in the Orion OB association. From the measurements of the lithium abundance and radial velocity, we confirm the membership of our sample to the cloud. We use the equivalent widths of the H$\\alpha$, H$\\beta$, and the HeI $\\lambda$5876, $\\lambda$6678, $\\lambda$7065 \\AA$ $emission lines to calculate the accretion luminosities, $L_{\\rm acc}$, and the mass accretion rates, $\\dot M_{\\rm acc}$. We find in L1615/L1616 a fraction of accreting objects ($\\sim 30\\%$), which is consistent with the typical fraction of accretors in T associations of similar age ($\\sim 3$ Myr). The mass accretion rate for these stars shows a trend with the mass of the central object similar to that found for othe...

  12. Angular momentum transport in accretion disk boundary layers around weakly magnetized stars

    DEFF Research Database (Denmark)

    Pessah, M.E.; Chan, C.-K.

    2013-01-01

    The standard model for turbulent shear viscosity in accretion disks is based on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. This implies that the turbulent stress must be negative and thus transport angular momentum inwards......, in the boundary layer where the accretion disk meets the surface of a weakly magnetized star. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability (MRI......) is inefficient in disk regions where, as expected in boundary layers, the angular frequency increases with radius. Motivated by the need of a deeper understanding of the behavior of an MHD fluid in a differentially rotating background that deviates from a Keplerian profile, we study the dynamics of MHD waves...

  13. High energy signatures of quasi-spherical accretion onto rotating, magnetized neutron star in the ejector-accretor intermediate state

    CERN Document Server

    Bednarek, W

    2015-01-01

    We consider a simple scenario for the accretion of matter onto a neutron star in order to understand processes in the inner pulsar magnetosphere during the transition stage between different accretion modes. A simple quasi-spherical accretion process onto rotating, magnetized compact object is analyzed in order to search for the radiative signatures which could appear during transition between ejecting and accreting modes. It is argued that different accretion modes can be present in a single neutron star along different magnetic field lines for specific range of parameters characterising the pulsar (rotational period, surface magnetic field strength) and the density of surrounding medium. The radiation processes characteristic for the ejecting pulsar, i.e. curvature and synchrotron radiation produced by primary electrons in the pulsar outer gap, are expected to be modified by the presence of additional thermal radiation from the neutron star surface. We predict that during the transition from the pure ejecto...

  14. Interplay between diffusion, accretion and nuclear reactions in the atmospheres of Sirius and Przybylski's star

    CERN Document Server

    Yushchenko, A; Goriely, S; Shavrina, A; Kang, Y W; Rostopchin, S; Valyavin, G; Mkrtichian, D; Hatzes, A; Lee, B C; Kim, C; Yushchenko, Alexander; Gopka, Vera; Goriely, Stephane; Shavrina, Angelina; Kang, Young Woon; Rostopchin, Sergey; Valyavin, Gennady; Mkrtichian, David; Hatzes, Artie; Lee, Byeong-Cheol; Kim, Chulhee

    2006-01-01

    The abundance anomalies in chemically peculiar B-F stars are usually explained by diffusion of chemical elements in the stable atmospheres of these stars. But it is well known that Cp stars with similar temperatures and gravities show very different chemical compositions. We show that the abundance patterns of several stars can be influenced by accretion and (or) nuclear reactions in stellar atmospheres. We report the result of determination of abundances of elements in the atmosphere of hot Am star: Sirius A and show that Sirius A was contaminated by s-process enriched matter from Sirius B (now a white dwarf). The second case is Przybylski's star. The abundance pattern of this star is the second most studied one after the Sun with the abundances determined for about 60 chemical elements. Spectral lines of radioactive elements with short decay times were found in the spectrum of this star. We report the results of investigation on the stratification of chemical elements in the atmosphere of Przybylski's star ...

  15. GR-AMRVAC code applications: accretion onto compact objects, boson stars versus black holes

    Science.gov (United States)

    Meliani, Z.; Grandclément, P.; Casse, F.; Vincent, F. H.; Straub, O.; Dauvergne, F.

    2016-08-01

    In the close vicinity of a compact object strong gravity imprints its signature onto matter. Systems that contain at least one compact object are observed to exhibit extreme physical properties and typically emit highly energetic radiation. The nature of the compact objects that produce the strongest gravitational fields is to date not settled. General relativistic numerical simulations of fluid dynamics around black holes, neutron stars, and other compact objects such as boson stars (BSs) may give invaluable insights into this fundamental question. In order to study the behavior of fluid in the strong gravity regime of an arbitrary compact object we develop a new general relativistic hydrodynamics code. To this end we extend the existing versatile adaptive mesh refinement code MPI-AMRVAC into a general relativistic hydrodynamics framework and adapt it for the use of numerically given spacetime metrics. In the present article we study accretion flows in the vicinity of various types of BSs whose numerical metrics are calculated by the KADATH spectral solver library. We design specific tests to check the reliability of any code intending to study BSs and compare the solutions with those obtained in the context of Schwarzschild black holes. We perform the first ever general relativistic hydrodynamical simulations of gas accretion by a BS. The behavior of matter at small distances from the center of a BS differs notably from the black hole case. In particular we demonstrate that in the context of Bondi spherical accretion the mass accretion rate onto non-rotating BSs remains constant whereas it increases for Schwarzschild black holes. We also address the scenario of non-spherical accretion onto BSs and show that this may trigger mass ejection from the interior of the BS. This striking feature opens the door to forthcoming investigations regarding accretion-ejection flows around such types of compact objects.

  16. Role of local absorption on the X-ray emission from MHD accretion shocks in classical T Tauri stars

    OpenAIRE

    Bonito; Orlando,; Argiroffi; Miceli; Reale,, S; Peres,, Marco A; Matsakos; Stehle; Ibgui

    2014-01-01

    Accretion processes onto classical T Tauri stars (CTTSs) are believed to generate shocks at the stellar surface due to the impact of supersonic downflowing plasma. Although current models of accretion streams provide a plausible global picture of this process, several aspects are still unclear. For example, the observed X-ray luminosity in accretion shocks is, in general, well below the predicted value. A possible explanation discussed in the literature is in terms of significant absorption o...

  17. The mode of gas accretion onto star-forming galaxies

    CERN Document Server

    Marinacci, F; Fraternali, F; Nipoti, C; Ciotti, L; Londrillo, P

    2010-01-01

    It is argued that galaxies like ours sustain their star formation by transferring gas from an extensive corona to the star-forming disc. The transfer is effected by the galactic fountain -- cool clouds that are shot up from the plane to kiloparsec heights above the plane. The Kelvin-Helmholtz instability strips gas from these clouds. If the pressure and the the metallicity of the corona are high enough, the stripped gas causes a similar mass of coronal gas to condense in the cloud's wake. Hydrodynamical simulations of cloud-corona interaction are presented. These confirm the existence of a critical ablation rate above which the corona is condensed, and imply that for the likely parameters of the Galactic corona this rate lies near the actual ablation rate of clouds. In external galaxies trails of HI behind individual clouds will not be detectable, although the integrated emission from all such trails should be significant. Parts of the trails of the clouds that make up the Galaxy's fountain should be observab...

  18. Magnetospheric accretion on the fully-convective classical T Tauri star DN Tau

    CERN Document Server

    Donati, JF; Alencar, SHP; Hussain, G; Bouvier, J; Jardine, MM; Menard, F; Dougados, C; Romanova, MM

    2013-01-01

    We report here results of spectropolarimetric observations of the classical T Tauri star DN Tau carried out (at 2 epochs) with ESPaDOnS at the Canada-France-Hawaii Telescope within the `Magnetic Protostars and Planets' programme. We infer that DN Tau, with a photospheric temperature of 3,950+-50 K, a luminosity of 0.8+-0.2 Lsun and a rotation period of 6.32 d, is a ~2Myr-old fully-convective 0.65+-0.05 Msun star with a radius of 1.9+-0.2 Dsun, viewed at an inclination of 35+-10degr. Clear circularly-polarized Zeeman signatures are detected in both photospheric and accretion-powered emission lines, probing longitudinal fields of up to 1.8 kG (in the He1 D3 accretion proxy). Rotational modulation of Zeeman signatures, detected both in photospheric and accretion lines, is different between our 2 runs, providing further evidence that fields of cTTSs are generated by non-stationary dynamos. Using tomographic imaging, we reconstruct maps of the large-scale field, of the photospheric brightness and of the accretion-...

  19. Time-dependent, compositionally driven convection in the oceans of accreting neutron stars

    CERN Document Server

    Medin, Zach

    2014-01-01

    We discuss the effect of chemical separation as matter freezes at the base of the ocean of an accreting neutron star, and the subsequent enrichment of the ocean in light elements and inward transport of heat through convective mixing. We extend the steady-state results of Medin & Cumming 2011 to transiently accreting neutron stars, by considering the time-dependent cases of heating during accretion outbursts and cooling during quiescence. Convective mixing is extremely efficient, flattening the composition profile in about one convective turnover time (weeks to months at the base of the ocean). During accretion outbursts, inward heat transport has only a small effect on the temperature profile in the outer layers until the ocean is strongly enriched in light elements, a process that takes hundreds of years to complete. During quiescence, however, inward heat transport rapidly cools the outer layers of the ocean while keeping the inner layers hot. We find that this leads to a sharp drop in surface emission...

  20. Accretion in the Rho-Oph pre-main sequence stars

    CERN Document Server

    Natta, A; Testi, L

    2006-01-01

    The aim of this paper is to provide a measurement of the mass accretion rate in a large, complete sample of objects in the core of the star forming region Rho-Oph. The sample includes most of the objects (104 out of 111) with evidence of a circumstellar disk from mid-infrared photometry; it covers a stellar mass range from about 0.03 to 3 Msun and it is complete to a limiting mass of ~0.05 Msun. We used J and K-band spectra to derive the mass accretion rate of each object from the intensity of the hydrogen recombination lines, Pab or Brg. For comparison, we also obtained similar spectra of 35 diskless objects. The results show that emission in these lines is only seen in stars with disks, and can be used as an indicator of accretion. However, the converse does not hold, as about 50% of our disk objects do not have detectable line emission. The measured accretion rates show a strong correlation with the mass of the central object (Macc ~ Mstar^1.8+-0.2) and a large spread, of two orders of magnitude at least, ...

  1. The relation between accretion rates and the initial mass function in hydrodynamical simulations of star formation

    CERN Document Server

    Maschberger, Th; Clarke, C J; Moraux, E

    2013-01-01

    We analyse a hydrodynamical simulation of star formation. Sink particles in the simulations which represent stars show episodic growth, which is presumably accretion from a core that can be regularly replenished in response to the fluctuating conditions in the local environment. The accretion rates follow $\\dot{m} \\propto m^{2/3}$, as expected from accretion in a gas-dominated potential, but with substantial variations over-laid on this. The growth times follow an exponential distribution which is tapered at long times due to the finite length of the simulation. The initial collapse masses have an approximately lognormal distribution with already an onset of a power-law at large masses. The sink particle mass function can be reproduced with a non-linear stochastic process, with fluctuating accretion rates $\\propto m^{2/3}$, a distribution of seed masses and a distribution of growth times. All three factors contribute equally to the form of the final sink mass function. We find that the upper power law tail of...

  2. Do stellar winds prevent the formation of supermassive stars by accretion?

    Science.gov (United States)

    Nakauchi, Daisuke; Hosokawa, Takashi; Omukai, Kazuyuki; Saio, Hideyuki; Nomoto, Ken'ichi

    2017-03-01

    Supermassive stars (SMSs; ∼105 M⊙) formed from metal-free gas in the early Universe attract attention as progenitors of supermassive black holes observed at high redshifts. To form SMSs by accretion, central protostars must accrete at as high rates as ∼0.1-1 M⊙ yr-1. Such protostars have very extended structures with bloated envelopes, like supergiant stars, and are called supergiant protostars (SGPSs). Under the assumption of hydrostatic equilibrium, SGPSs have density-inverted layers, where the luminosity becomes locally super-Eddington, near the surface. If the envelope matter is allowed to flow out, however, a stellar wind could be launched and hinder the accretion growth of SGPSs before reaching the supermassive regime. We examine whether radiation-driven winds are launched from SGPSs by constructing steady and spherically symmetric wind solutions. We find that the wind velocity does not reach the escape velocity in any case considered. This is because once the temperature falls below ∼104 K, the opacity plummet drastically owing to the recombination of hydrogen and the acceleration ceases suddenly. This indicates that, in realistic non-steady cases, even if outflows are launched from the surface of SGPSs, they would fall back again. Such a 'wind' does not result in net mass-loss and does not prevent the growth of SGPSs. In conclusion, SGPSs will grow to SMSs and eventually collapse to massive black holes of ∼105 M⊙, as long as the rapid accretion is maintained.

  3. Supersonic turbulence, filamentary accretion,and the rapid assembly of massive stars and disks

    CERN Document Server

    Banerjee, R; Anderson, D W; Banerjee, Robi; Pudritz, Ralph E.; Anderson, Dave W.

    2006-01-01

    We present a detailed computational study of the assembly of protostellar disks and massive stars in molecular clouds with supersonic turbulence. We follow the evolution of large scale filamentary structures in a cluster-forming clump down to protostellar length scales by means of very highly resolved, 3D adaptive mesh refined (AMR) simulations, and show how accretion disks and massive stars form in such environments. We find that an initially elongated cloud core which has a slight spin from oblique shocks collapses first to a filament and later develops a turbulent disk close to the center of the filament. The continued large scale flow that shocks with the filament maintains the high density and pressure within it. Material within the cooling filament undergoes gravitational collapse and an outside-in assembly of a massive protostar. Our simulations show that very high mass accretion rates of up to 10^-2 Msol/yr and high, supersonic, infall velocities result from such filamentary accretion. Accretion at th...

  4. Extremes of the jet-accretion power relation of blazars, as explored by NuSTAR

    CERN Document Server

    Sbarrato, T; Tagliaferri, G; Perri, M; Madejski, G M; Stern, D; Boggs, S E; Christensen, F E; Craig, W W; Hailey, C J; Harrison, F A; Zhang, W W

    2015-01-01

    Hard X-ray observations are crucial to study the non-thermal jet emission from high-redshift, powerful blazars. We observed two bright z>2 flat spectrum radio quasars (FSRQs) in hard X-rays to explore the details of their relativistic jets and their possible variability. S5 0014+81 (at z=3.366) and B0222+185 (at z=2.690) have been observed twice by the Nuclear Spectroscopic Telescope Array (NuSTAR) simultaneously with Swift/XRT, showing different variability behaviours. We found that NuSTAR is instrumental to explore the variability of powerful high-redshift blazars, even when no gamma-ray emission is detected. The two sources have proven to have respectively the most luminous accretion disk and the most powerful jet among known blazars. They are located at the extreme end of the jet-accretion disk relation previously found for gamma-ray detected blazars.

  5. Magnetic Field Effect on β+ Decay in the Crusts of Accreting Neutron Stars

    Institute of Scientific and Technical Information of China (English)

    ZHANG Jie; LIU Men-Quan; LUO Zhi-Quan

    2007-01-01

    Based on shell model of nuclei,the influence of a high magnetic field on ββ+ decay in the crusts of accreting neutron stars is analyzed.The magnetic field effect on 54Mn is discussed.The results show that a weak magnetic field makes little effect on β+ decay but a strong magnetic field (B > 1011 G) improves β+ decay rates obviously.The conclusion derived will benefit to develop further research on nuclear astrophysics in the future.

  6. Abbott Wave-Triggered Runaway in Line-Driven Winds from Stars and Accretion Disks

    OpenAIRE

    2001-01-01

    Line-driven winds from stars and accretion disks are accelerated by scattering in numerous line transitions. The wind is believed to adopt a unique critical solution, out of the infinite variety of shallow and steep solutions. We study the inherent dynamics of the transition towards the critical wind. A new runaway wind mechanism is analyzed in terms of radiative-acoustic (Abbott) waves which are responsible for shaping the wind velocity law and fixing the mass loss. Three different flow type...

  7. Spectral variability of classical T Tauri stars accreting in an unstable regime

    CERN Document Server

    Kurosawa, Ryuichi

    2013-01-01

    Classical T Tauri stars (CTTSs) are variable in different time-scales. One type of variability is possibly connected with the accretion of matter through the Rayleigh-Taylor instability that occurs at the interface between an accretion disc and a stellar magnetosphere. In this regime, matter accretes in a several temporarily formed accretion streams or `tongues' which appear in random locations, and produce stochastic photometric and line variability. We use the results of global three-dimensional magnetohydrodynamic simulations of matter flows in both stable and unstable accretion regimes to calculate time-dependent hydrogen line profiles and study their variability behaviours. In the stable regime, some hydrogen lines (e.g. H-beta, H-gamma, H-delta, Pa-beta and Br-gamma) show a redshifted absorption component only during a fraction of a stellar rotation period, and its occurrence is periodic. However, in the unstable regime, the redshifted absorption component is present rather persistently during a whole s...

  8. Magnetospheric accretion and spin-down of the prototypical classical T Tauri star AATau

    CERN Document Server

    Donati, JF; Bouvier, J; Gregory, SG; Grankin, KN; Jardine, MM; Hussain, GAJ; Menard, F; Dougados, C; Unruh, Y; Mohanty, S; Auriere, M; Morin, J; Fares, R

    2010-01-01

    From observations collected with the ESPaDOnS & NARVAL spectropolarimeters at CFHT and TBL, we report the detection of Zeeman signatures on the prototypical classical TTauri star AATau, both in photospheric lines and accretion-powered emission lines. Using time series of unpolarized and circularly polarized spectra, we reconstruct at two epochs maps of the magnetic field, surface brightness and accretion-powered emission of AATau. We find that AATau hosts a 2-3kG magnetic dipole tilted at ~20deg to the rotation axis, and of presumably dynamo origin. We also show that the magnetic poles of AATau host large cool spots at photospheric level and accretion regions at chromospheric level. The logarithmic accretion rate at the surface of AATau at the time of our observations is strongly variable, ranging from -9.6 to -8.5 and equal to -9.2 in average (in Msun/yr); this is an order of magnitude smaller than the disc accretion rate at which the magnetic truncation radius (below which the disc is disrupted by the s...

  9. Accretion disk assembly and survival during the disruption of a neutron star by a black hole

    CERN Document Server

    Ramirez-Ruiz, E; Ramirez-Ruiz, Enrico; Lee, William H.

    2003-01-01

    We study the formation of accretion disks resulting from dynamical three dimensional binary coalescence calculations, where a neutron star is tidally disrupted before being swallowed by its black hole companion. By subsequently assuming azimuthal symmetry we are able to follow the time dependence of the disk structure for a few tenths of a second. Although the disruption of a neutron star leads to a situation where violent instabilities redistribute mass and angular momentum within a few dynamical timescales, enough gas mass remains in the orbiting debris to catalyse the extraction of energy from the hole at a rate adequate to power a short-lived gamma ray burst.

  10. Evolution of emission line activity in intermediate mass young stars

    CERN Document Server

    Manoj, P; Maheswar, G; Muneer, S

    2006-01-01

    We present optical spectra of 45 intermediate mass Herbig Ae/Be stars. Together with the multi-epoch spectroscopic and photometric data compiled for a large sample of these stars and ages estimated for individual stars by using pre-main sequence evolutionary tracks, we have studied the evolution of emission line activity in them. We find that, on average, the H_alpha emission line strength decreases with increasing stellar age in HAeBe stars, indicating that the accretion activity gradually declines during the PMS phase. This would hint at a relatively long-lived (a few Myr) process being responsible for the cessation of accretion in Herbig Ae/Be stars. We also find that the accretion activity in these stars drops substantially by ~ 3 Myr. This is comparable to the timescale in which most intermediate mass stars are thought to lose their inner disks, suggesting that inner disks in intermediate mass stars are dissipated rapidly after the accretion activity has fallen below a certain level. We, further find a r...

  11. Laboratory analogue of a supersonic accretion column in a binary star system

    Science.gov (United States)

    Cross, J. E.; Gregori, G.; Foster, J. M.; Graham, P.; Bonnet-Bidaud, J. -M.; Busschaert, C.; Charpentier, N.; Danson, C. N.; Doyle, H. W.; Drake, R. P.; Fyrth, J.; Gumbrell, E. T.; Koenig, M.; Krauland, C.; Kuranz, C. C.; Loupias, B.; Michaut, C.; Mouchet, M.; Patankar, S.; Skidmore, J.; Spindloe, C.; Tubman, E. R.; Woolsey, N.; Yurchak, R.

    2016-01-01

    Astrophysical flows exhibit rich behaviour resulting from the interplay of different forms of energy—gravitational, thermal, magnetic and radiative. For magnetic cataclysmic variable stars, material from a late, main sequence star is pulled onto a highly magnetized (B>10 MG) white dwarf. The magnetic field is sufficiently large to direct the flow as an accretion column onto the poles of the white dwarf, a star subclass known as AM Herculis. A stationary radiative shock is expected to form 100–1,000 km above the surface of the white dwarf, far too small to be resolved with current telescopes. Here we report the results of a laboratory experiment showing the evolution of a reverse shock when both ionization and radiative losses are important. We find that the stand-off position of the shock agrees with radiation hydrodynamic simulations and is consistent, when scaled to AM Herculis star systems, with theoretical predictions. PMID:27291065

  12. Laboratory analogue of a supersonic accretion column in a binary star system.

    Science.gov (United States)

    Cross, J E; Gregori, G; Foster, J M; Graham, P; Bonnet-Bidaud, J-M; Busschaert, C; Charpentier, N; Danson, C N; Doyle, H W; Drake, R P; Fyrth, J; Gumbrell, E T; Koenig, M; Krauland, C; Kuranz, C C; Loupias, B; Michaut, C; Mouchet, M; Patankar, S; Skidmore, J; Spindloe, C; Tubman, E R; Woolsey, N; Yurchak, R; Falize, É

    2016-06-13

    Astrophysical flows exhibit rich behaviour resulting from the interplay of different forms of energy-gravitational, thermal, magnetic and radiative. For magnetic cataclysmic variable stars, material from a late, main sequence star is pulled onto a highly magnetized (B>10 MG) white dwarf. The magnetic field is sufficiently large to direct the flow as an accretion column onto the poles of the white dwarf, a star subclass known as AM Herculis. A stationary radiative shock is expected to form 100-1,000 km above the surface of the white dwarf, far too small to be resolved with current telescopes. Here we report the results of a laboratory experiment showing the evolution of a reverse shock when both ionization and radiative losses are important. We find that the stand-off position of the shock agrees with radiation hydrodynamic simulations and is consistent, when scaled to AM Herculis star systems, with theoretical predictions.

  13. Accretion and Dynamical Interactions in Small-N Star-Forming Clusters I. N=5 case

    CERN Document Server

    Delgado-Donate, E J; Bate, M R

    2003-01-01

    We present results from high-resolution hydrodynamical simulations which explore the effects of small scale clustering in star-forming regions. A large ensemble of small-N clusters with 5 stellar seeds have been modelled and the resulting properties of stars and brown dwarfs statistically derived and compared with observational data. Close dynamical interactions between the protostars and competitive accretion driven by the cloud collapse are shown to produce a distribution of final masses which is bimodal, with most of the mass residing in the binary components. When convolved with a suitable core mass function, the final distribution of masses resembles the observed IMF, both in the stellar and sub-stellar regime. Binaries and single stars are found to constitute two kinematically distinct populations, with about half of the singles attaining velocities > 2 km/s, which might deprive low mass star-forming regions of their lightest members in a few crossing times. The eccentricity distribution of binaries and...

  14. Galactic Halo Stars in Phase Space A Hint of Satellite Accretion?

    CERN Document Server

    Brook, C B; Gibson, B K; Flynn, C; Brook, Chris B.; Kawata, Daisuke; Gibson, Brad K.; Flynn, Chris

    2003-01-01

    The present day chemical and dynamical properties of the Milky Way bear the imprint of the Galaxy's formation and evolutionary history. One of the most enduring and critical debates surrounding Galactic evolution is that regarding the competition between ``satellite accretion'' and ``monolithic collapse''; the apparent strong correlation between orbital eccentricity and metallicity of halo stars was originally used as supporting evidence for the latter. While modern-day unbiased samples no longer support the claims for a significant correlation, recent evidence has been presented by Chiba & Beers (2000,AJ,119,2843) for the existence of a minor population of high-eccentricity metal-deficient halo stars. It has been suggested that these stars represent the signature of a rapid (if minor) collapse phase in the Galaxy's history. Employing velocity- and integrals of motion-phase space projections of these stars, coupled with a series of N-body/Smoothed Particle Hydrodynamic (SPH) chemodynamical simulations, we...

  15. FUV Emission from AGB Stars: Modeling Accretion Activity Associated with a Binary Companion

    Science.gov (United States)

    Stevens, Alyx Catherine; Sahai, Raghvendra

    2012-01-01

    It is widely believed that the late stages of evolution for Asymptotic Giant Branch (AGB) stars are influenced by the presence of binary companions. Unfortunately, there is a lack of direct observational evidence of binarity. However, more recently, strong indirect evidence comes from the discovery of UV emission in a subsample of these objects (fuvAGB stars). AGB stars are comparatively cool objects (< or =3000 K), thus their fluxes falls off drastically for wavelengths 3000 Angstroms and shorter. Therefore, ultraviolet observations offer an important, new technique for detecting the binary companions and/or associated accretion activity. We develop new models of UV emission from fuvAGB stars constrained by GALEX photometry and spectroscopy of these objects. We compare the GALEX UV grism spectra of the AGB M7 star EY Hya to predictions using the spectral synthesis code Cloudy, specifically investigating the ultraviolet wavelength range (1344-2831 Angstroms). We investigate models composed of contributions from a photoionized "hot spot" due to accretion activity around the companion, and "chromospheric" emission from collisionally ionized plasma, to fit the UV observations.

  16. Self-Similar Hot Accretion Flow onto a Rotating Neutron Star Structure and Stability

    CERN Document Server

    Medvedev, M V; Medvedev, Mikhail; Narayan, Ramesh

    2001-01-01

    We present analytical and numerical solutions which describe a hot, viscous, two-temperature accretion flow onto a rotating neutron star or any other rotating compact star with a surface. We assume Coulomb coupling between the protons and electrons, and free-free cooling from the electrons. Outside a thin boundary layer, where the accretion flow meets the star, we show that there is an extended settling region which is well-described by two self-similar solutions: (i) a two-temperature solution which is valid in an inner zone $r\\le10^{2.5}$ ($r$ is in Schwarzchild units), and (ii) a one-temperature solution at larger radii. In both zones, $\\rho\\propto r^{-2}, \\Omega\\propto r^{-3/2}, v\\propto r^0, T_p\\propto r^{-1}$; in the two-temperature zone, $T_e\\propto r^{-1/2}$. The luminosity of the settling zone arises from the rotational energy of the star as the star is braked by viscosity. Hence the luminosity and the flow parameters (density, temperature, angular velocity) are independent of $\\dot M$. The settling ...

  17. Generation of magnetic field on the accretion disk around a proto-first-star

    Energy Technology Data Exchange (ETDEWEB)

    Shiromoto, Yuki; Susa, Hajime [Department of Physics, Konan University, Kobe 658-8501 (Japan); Hosokawa, Takashi, E-mail: susa@konan-u.ac.jp [Department of Physics and Research Center for the Early Universe, The University of Tokyo, Tokyo 113-0033 (Japan)

    2014-02-20

    The generation process of a magnetic field around a proto-first-star is studied. Utilizing the recent numerical results of proto-first-star formation based on radiation hydrodynamics simulations, we assess the magnetic field strength generated by the radiative force and the Biermann battery effect. We find that a magnetic field of ∼10{sup –9} G is generated on the surface of the accretion disk around the proto-first-star. The field strength on the accretion disk is smaller by two orders of magnitude than the critical value, above which the gravitational fragmentation of the disk is suppressed. Thus, the generated seed magnetic field hardly affect the dynamics of on-site first star formation directly, unless an efficient amplification process is taken into consideration. We also find that the generated magnetic field is continuously blown out from the disk on the outflows to the poles, that are driven by the thermal pressure of photoheated gas. The strength of the diffused magnetic field in low-density regions is ∼10{sup –14}-10{sup –13} G at n {sub H} = 10{sup 3} cm{sup –3}, which could play an important role in the next generation star formation, as well as the seeds of the magnetic field in the present-day universe.

  18. Mass accretion flows in the high-mass star forming complex NGC 6334

    Science.gov (United States)

    Sánchez-Monge, Á.; Schilke, P.; Zernickel, A.; Schmiedeke, A.; Möller, Th.; Qin, S.-L.

    2016-05-01

    The formation of high-mass stars is one of the major topics of astrophysical research, in particular the process of accretion from large-scale clouds down to small-scale cores. We have selected the nearby, filamentary, high-mass star forming complex NGC 6334 to study the gas velocity at different scales and probe the infall rates onto the protostellar cores embedded in the NGC 6334-I and I(N) clusters. This study makes use of single-dish and interferometric submillimeter observations, complemented with 3D numerical non-LTE radiative transfer modeling. We measure a mass accretion rate of 10-5 M⊙ yr-1 throughout the filament increasing up to 10-3 M⊙ yr-1 towards the densest regions where high-mass stars are forming. At smaller scales, our 3D model is consistent with accretion rates of 10-3 M⊙ yr-1 towards the clusters, and 10-4 M⊙ yr-1 onto the protostars.

  19. Is Main Sequence Galaxy Star Formation Controlled by Halo Mass Accretion?

    CERN Document Server

    Rodriguez-Puebla, Aldo; Behroozi, Peter; Faber, S M

    2015-01-01

    It is known that the galaxy stellar-to-halo mass ratio (SHMR) is nearly independent of redshift from z=0-4. This motivates us to construct a toy model in which we assume that the SMHR for central galaxies measured at redshift z~0 is independent of redshift, which implies that the star formation rate (SFR) is determined by the halo mass accretion rate, a phenomenon we call Stellar-Halo Accretion Rate Coevolution (SHARC). Moreover, we show here that the ~0.3 dex dispersion of the halo mass accretion rate (MAR) is similar to the observed dispersion of the SFR on the main sequence. In the context of bathtub-type models of galaxy formation, SHARC leads to mass-dependent constraints on the relation between SFR and MAR. The SHARC assumption is no doubt over-simplified, but we expect it to be possibly valid for central galaxies with stellar masses of 10^9 - 10^10.5 M_sol that are on the star formation main sequence. Such galaxies represent most of the life history of M_* galaxies, and therefore most of the star forma...

  20. Highly efficient star formation in NGC 5253 possibly from stream-fed accretion

    CERN Document Server

    Turner, J L; Benford, D J; Consiglio, S M; Ho, P T P; Kovács, A; Meier, D S; Zhao, J -H

    2015-01-01

    A local dwarf galaxy, NGC 5253, has a young super star cluster that may provide an example of highly efficient star formation. Here we report the detection and imaging, with the Submillimeter Array, of the J= 3-2 rotational transition of CO at the location of the massive cluster associated with the supernebula. The gas cloud is hot, dense, quiescent, and extremely dusty. Its gas-to-dust ratio is lower than the Galactic value, which we attribute to dust enrichment by Wolf-Rayet stars within the embedded star cluster. Its star formation efficiency exceeds 50%, ten times higher than clouds in the Milky Way: this cloud is a factory of stars and soot. We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy.

  1. The structure of young star clusters

    CERN Document Server

    Gladwin, P P; Boffin, H M J; Whitworth, A P

    1998-01-01

    We analyse and compare the clustering of young stars in Chamaeleon I and Taurus. We compute the mean surface-density of companion stars \\bar{N} as a function of angular displacement \\theta from each star. We then fit exponents of the power laws are 1.97 and 0.28, with the elbow at ~0.011 degrees. For Taurus, we obtain 2.02 and 0.87, with the elbow at ~0.013 deg. For both star clusters the observational data make quite large systematic excursions from the best fitting curve in the binary regime. These excursions may be attributable to evolutionary effects of the types discussed recently by Nakajima et al. and Bate et al. In the clustering regime the data conform to the best fitting curve very well. We also calculate the box-dimensions for the two star clusters. However, the limited dynamic range makes these estimates simply descriptors of the large-scale clustering, and not admissible evidence for fractality. We also propose two algorithms for objectively generating maps of constant stellar surface-density in ...

  2. ANISOTROPY OF X-RAY BURSTS FROM NEUTRON STARS WITH CONCAVE ACCRETION DISKS

    Energy Technology Data Exchange (ETDEWEB)

    He, C.-C. [College of Physics, Jilin University, Changchun 130012 (China); Keek, L., E-mail: jordanhe1994@gmail.com [CRESST and X-ray Astrophysics Laboratory NASA/GSFC, Greenbelt, MD 20771 (United States)

    2016-03-01

    Emission from neutron stars and accretion disks in low-mass X-ray binaries is anisotropic. The non-spherical shape of the disk as well as blocking of the neutron star by the disk make the observed flux dependent on the inclination angle of the disk with respect to the line of sight. This is of importance for the interpretation of thermonuclear X-ray bursts from neutron stars. Because part of the X-ray burst is reflected off the disk, the observed burst flux depends on the anisotropies for both direct emission from the neutron star and reflection off the disk. This influences measurements of source distance, mass accretion rate, and constraints on the neutron star’s equation of state. Previous predictions of the anisotropy factors assumed a geometrically flat disk. Detailed observations of two so-called superbursts allowed for the direct and the reflected burst fluxes to each be measured separately. The reflection fraction was much higher than what the anisotropies of a flat disk can account for. We create numerical models to calculate the anisotropy factors for different disk shapes, including concave disks. We present the anisotropy factors of the direct and reflected burst fluxes separately, as well as the anisotropy of the persistent flux. Reflection fractions substantially larger than unity are produced in the case where the inner accretion disk increases steeply in height, such that part of the star is blocked from view. Such a geometry could possibly be induced by the X-ray burst if X-ray heating causes the inner disk to puff up.

  3. Disc-jet coupling in low-luminosity accreting neutron stars

    Science.gov (United States)

    Tudor, V.; Miller-Jones, J. C. A.; Patruno, A.; D'Angelo, C. R.; Jonker, P. G.; Russell, D. M.; Russell, T. D.; Bernardini, F.; Lewis, F.; Deller, A. T.; Hessels, J. W. T.; Migliari, S.; Plotkin, R. M.; Soria, R.; Wijnands, R.

    2017-09-01

    In outburst, neutron star X-ray binaries produce less powerful jets than black holes at a given X-ray luminosity. This has made them more difficult to study as they fade towards quiescence. To explore whether neutron stars power jets at low accretion rates (LX ≲ 1036 erg s-1), we investigate the radio and X-ray properties of three accreting millisecond X-ray pulsars (IGR J17511-3057, SAX J1808.4-3658 and IGR J00291+5934) during their outbursts in 2015, and of the non-pulsing neutron star Cen X-4 in quiescence (2015) and in outburst (1979). We did not detect the radio counterpart of IGR J17511-3057 in outburst or of Cen X-4 in quiescence, but did detect IGR J00291+5934 and SAX J1808.4-3658, showing that at least some neutron stars launch jets at low accretion rates. While the radio and X-ray emission in IGR J00291+5934 seem to be tightly correlated, the relationship in SAX J1808.4-3658 is more complicated. We find that SAX J1808.4-3658 produces jets during the reflaring tail, and we explore a toy model to ascertain whether the radio emission could be attributed to the onset of a strong propeller. The lack of a universal radio/X-ray correlation, with different behaviours in different neutron star systems (with various radio/X-ray correlations; some being radio faint and others not), points at distinct disc-jet interactions in individual sources, while always being fainter in the radio band than black holes at the same X-ray luminosity.

  4. Advances in understanding young high-mass stars using optical interferometry

    CERN Document Server

    de Wit, W J

    2012-01-01

    The closest examples of high-mass star birth occurs in deeply embedded environments at kiloparsec distances. Although much progress has been made, an observationally validated picture of the dominant processes which allows the central hydrostatic object to grow in mass has yet to be established. The observational technique of optical interferometry has demonstrated its potential in the field of high-mass star formation by delivering a milli-arcsecond infrared view on the complex accretion environment. We provide an overview of the scientific results obtained with multi-aperture telescope arrays and briefly discuss future instruments and their anticipated impact on our understanding of massive young stellar objects.

  5. EVIDENCE OF HOT HIGH VELOCITY PHOTOIONIZED PLASMA FALLING ON ACTIVELY ACCRETING T TAURI STARS

    Energy Technology Data Exchange (ETDEWEB)

    Gómez de Castro, Ana Ines [Grupo de Investigación Complutense AEGORA and S.D. Astronomía y Geodesia, Fac. de CC Matemáticas, Universidad Complutense, E-28040 Madrid (Spain)

    2013-10-01

    The He II (1640 Å) line and the resonance doublet of N V (UV1) provide a good diagnostic tool to constrain the excitation mechanism of hot (T{sub e} > 40,000 K) atmospheric/magnetospheric plasmas in T Tauri stars (TTSs). Making use of the data available in the Hubble Space Telescope archive, this work shows that there are at least two distinct physical components contributing to the radiation in these tracers: the accretion flow sliding on the magnetosphere and the atmosphere. The N V profiles in most sources are symmetric and at rest with respect to the star. The velocity dispersion of the profile increases from non-accreting (σ = 40 km s{sup –1}) to accreting (σ = 120 km s{sup –1}) TTSs, suggesting that the macroturbulence field in the line formation region decreases as the stars approach the main sequence. Evidence of the N V line being formed in a hot solar-like wind has been found in RW Aur, HN Tau, and AA Tau. The He II profile has a strong narrow component that dominates the line flux; the dispersion of this component ranges from 20 to 60 km s{sup –1}. Current data suggest that both accretion shocks and atmospheric emission might contribute to the line flux. In some sources, the He II line shows a broad and redward-shifted emission component often accompanied by semiforbidden O III] emission that has a critical electron density of ∼3.4 × 10{sup 10} cm{sup 3}. In spite of their different origins (inferred from the kinematics of the line formation region), N V and He II fluxes are strongly correlated, with only the possible exception of some of the heaviest accretors.

  6. A spectroscopic survey of Herbig Ae/Be stars with X-Shooter - II. Accretion diagnostic lines

    Science.gov (United States)

    Fairlamb, J. R.; Oudmaijer, R. D.; Mendigutia, I.; Ilee, J. D.; van den Ancker, M. E.

    2017-02-01

    The Herbig Ae/Be stars (HAeBes) allow an exploration of the properties of pre-main-sequence(PMS) stars above the low-mass range (8 M_{⊙}). This paper is the second in a series exploring accretion in 91 HAeBes with Very Large Telescope/X-shooter spectra. Equivalent width measurements are carried out on 32 different lines, spanning the UV (ultraviolet) to NIR (near infrared), in order to obtain their line luminosities. The line luminosities were compared to accretion luminosities that were determined directly from measurements of a UV excess. When detected, emission lines always demonstrate a correlation with the accretion luminosity, regardless of detection frequency. The average relationship between accretion luminosity and line luminosity is found to be L_acc ∝ L_line1.16 ± 0.15. This is in agreement with the findings in Classical T Tauri stars, although the HAeBe relationship is generally steeper, particularly towards the Herbig Be mass range. Since all observed lines display a correlation with the accretion luminosity, all of them can be used as accretion tracers. This has increased the number of accretion diagnostic lines in HAeBes 10-fold. However, questions still remain on the physical origin of each line, which may not be due to accretion.

  7. Star formation and black hole accretion activity in rich local clusters of galaxies

    CERN Document Server

    Bianconi, Matteo; Fadda, Dario

    2016-01-01

    We present a study of the star formation and central black hole accretion activity of the galaxies hosted in the two nearby (z$\\sim$0.2) rich galaxy clusters Abell 983 and 1731. Aims: We are able to quantify both the obscured and unobscured star formation rates, as well as the presence of active galactic nuclei (AGN) as a function of the environment in which the galaxy is located. Methods: We targeted the clusters with unprecedented deep infrared Spitzer observations (0.2 mJy @ 24 micron), near-IR Palomar imaging and optical WIYN spectroscopy. The extent of our observations ($\\sim$ 3 virial radii) covers the vast range of possible environments, from the very dense cluster centre to the very rarefied cluster outskirts and accretion regions. Results: The star forming members of the two clusters present star formation rates comparable with those measured in coeval field galaxies. The analysis of the spatial arrangement of the spectroscopically confirmed members reveals an elongated distribution for A1731 with re...

  8. Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW Hydrae

    CERN Document Server

    Johns-Krull, C M; Valenti, J A; Jeffers, S V; Piskunov, N E; Kuchukhov, O; Makaganiuk, V; Stempels, H C; Snik, F; Keller, C; Rodenhuis, M

    2013-01-01

    We present high spectral resolution ($R\\approx108,000$) Stokes $V$ polarimetry of the Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the \\ion{He}{1} emission lines at 5876 \\AA\\ and 6678 \\AA. The \\ion{He}{1} lines in these CTTSs contain both narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission components which may come from either a wind or the large scale magnetospheric accretion flow. We detect strong polarization in the narrow component of the two \\ion{He}{1} emission lines in both stars. We observe a maximum implied field strength of $6.05 \\pm 0.24$ kG in the 5876 \\AA\\ line of GQ Lup, making it the star with the highest field strength measured in this line for a CTTS. We find field strengths in the two \\ion{He}{1} lines that are c...

  9. Generation of Magnetic Field on the Accretion Disk around a Proto-First-Star

    CERN Document Server

    Shiromoto, Yuki; Hosokawa, Takashi

    2014-01-01

    The generation process of magnetic field around a proto-first-star is studied. Utilizing the recent numerical result of proto-first-star formation based upon the radiation hydrodynamics simulations, we assess the magnetic field strength generated by the radiative force and the Biermann battery effect. We find that magnetic field of \\sim 10^{-9} G is generated on the surface of the accretion disk around the proto-first-star. The field strength on the accretion disk is smaller by two orders of magnitude than the critical value, above which the gravitational fragmentation of the disk is suppressed. Thus, the generated seed magnetic field hardly affect the dynamics of on-site first star formation directly, unless efficient amplification process is taken into consideration. We also find that the generated magnetic field is continuously blown out from the disk on the outflows to the poles, that are driven by the thermal pressure of photoheated gas. The strength of the diffused magnetic field in low density regions ...

  10. Highly variable young massive stars in ATLASGAL clumps

    CERN Document Server

    Kumar, M S N; Lucas, P W; Thompson, M A

    2016-01-01

    High-amplitude variability in Young Stellar Objects (YSOs) is usually associated with episodic accretion events. It has not been observed so far in massive YSOs. Here, the high-amplitude variable star sample of ContrerasPe\\~{n}a et al.(2016) has been used to search for highly-variable($\\Delta$K$\\ge$1\\,mag) sources coinciding with dense clumps mapped using the 850\\mum continuum emission by the ATLASGAL survey. 18 variable sources are centred on the sub-mm clump peaks, and coincide ($$2 mag, significantly higher compared to the mean variability of the entire VVV sample. The light curves of these objects sampled between 2010-2015 display rising, declining, or quasi-periodic behaviour but no clear periodicity. Light-curve analysis using Plavchan method show that the most prominent phased signals have periods of a few hundred days. The nature and time-scale of variations found in 6.7 Ghz methanol maser emission (MME) in massive stars are similar to that of the VYSO light curves. We argue that the origin of the obs...

  11. On the origin of the correlations between the accretion luminosity and emission line luminosities in pre-main-sequence stars

    Science.gov (United States)

    Mendigutía, I.; Oudmaijer, R. D.; Rigliaco, E.; Fairlamb, J. R.; Calvet, N.; Muzerolle, J.; Cunningham, N.; Lumsden, S. L.

    2015-09-01

    Correlations between the accretion luminosity and emission line luminosities (Lacc and Lline) of pre-main-sequence (PMS) stars have been published for many different spectral lines, which are used to estimate accretion rates. Despite the origin of those correlations is unknown, this could be attributed to direct or indirect physical relations between the emission line formation and the accretion mechanism. This work shows that all (near-UV/optical/near-IR) Lacc-Lline correlations are the result of the fact that the accretion luminosity and the stellar luminosity (L*) are correlated, and are not necessarily related with the physical origin of the line. Synthetic and observational data are used to illustrate how the Lacc-Lline correlations depend on the Lacc-L* relationship. We conclude that because PMS stars show the Lacc-L* correlation immediately implies that Lacc also correlates with the luminosity of all emission lines, for which the Lacc-Lline correlations alone do not prove any physical connection with accretion but can only be used with practical purposes to roughly estimate accretion rates. When looking for correlations with possible physical meaning, we suggest that Lacc/L* and Lline/L* should be used instead of Lacc and Lline. Finally, the finding that Lacc has a steeper dependence on L* for T Tauri stars than for intermediate-mass Herbig Ae/Be stars is also discussed. That is explained from the magnetospheric accretion scenario and the different photospheric properties in the near-UV.

  12. CSI 2264: Investigating rotation and its connection with disk accretion in the young open cluster NGC 2264

    Science.gov (United States)

    Venuti, L.; Bouvier, J.; Cody, A. M.; Stauffer, J. R.; Micela, G.; Rebull, L. M.; Alencar, S. H. P.; Sousa, A. P.; Hillenbrand, L. A.; Flaccomio, E.

    2017-02-01

    Context. The low spin rates measured for solar-type stars at an age of a few Myr ( 10% of the break-up velocity) indicate that some mechanism of angular momentum regulation must be at play in the early pre-main sequence. This may be associated with magnetospheric accretion and star-disk interaction, as suggested by observations that disk-bearing objects (CTTS) are slower rotators than diskless sources (WTTS) in young star clusters. Aims: We characterize the rotation properties for members of the star-forming region NGC 2264 ( 3 Myr) as a function of mass, and investigate the accretion-rotation connection at an age where about 50% of the stars have already lost their disks. Methods: We examined a sample of 500 cluster members (40% with disks, 60% without disks), distributed in mass between 0.15 and 2 M⊙, whose photometric variations were monitored in the optical for 38 consecutive days with the CoRoT space observatory. Light curves were analyzed for periodicity using three different techniques: the Lomb-Scargle periodogram, the autocorrelation function and the string-length method. Periods were searched in the range between 0.17 days (i.e., 4 h, twice the data sampling adopted) and 19 days (half the total time span). Period detections were confirmed using a variety of statistical tools (false alarm probability, Q-statistics), as well as visual inspection of the direct and phase-folded light curves. Results: About 62% of sources in our sample were found to be periodic; the period detection rate is 70% among WTTS and 58% among CTTS. The vast majority of periodic sources exhibit rotational periods shorter than 13 d. The period distribution obtained for the cluster consists of a smooth distribution centered around P = 5.2 d with two peaks, located respectively at P = 1-2 d and at P = 3-4 d. A separate analysis of the rotation properties for CTTS and WTTS indicates that the P = 1-2 d peak is associated with the latter, while both groups contribute to the P = 3-4 d peak

  13. A Theoretical Model of Non-conservative Mass Transfer with Non-uniform Mass Accretion Rate in Close Binary Stars

    CERN Document Server

    Gharami, Prabir; Rahaman, Farook

    2014-01-01

    Mass transfer in close binaries is often non-conservative and the modeling of this kind of mass transfer is mathematically challenging as in this case due to the loss of mass as well as angular momentum the governing system gets complicated and uncertain. In the present work a new mathematical model has been prescribed for the non-conservative mass transfer in a close binary system taking in to account the gradually decreasing profile of the mass accretion rate by the accreting star with respect to time as well as with respect to the increase in mass of the accreting star. The process of mass transfer is understood to occur up to a critical mass limit of the accreting star beyond which this process may cease to work.

  14. Metal-rich accretion and thermohaline instabilities in exoplanets-host stars: consequences on the light elements abundances

    CERN Document Server

    Théado, Sylvie

    2011-01-01

    The early evolution of planetary systems is expected to depend on various periods of disk matter accretion onto the central star, which may include the accretion of metal-rich matter after the star settles on the main sequence. When this happens, the accreted material is rapidly mixed within the surface convective zone and induces an inverse mean-molecular-weight gradient, unstable for thermohaline convection. The induced mixing, which dilutes the metal excess, may also have important consequences on the light elements abundances. We model and analyze this process, and present the results according to various possible accretion scenarios. We give a detailed discussion of the different ways of treating thermohaline mixing, as proposed by previous authors, and we converge on a consistent view, including the most recent numerical simulations. We show how the observations of light elements in stars can be used as tracers of such events.

  15. New Young Star Candidates in CG4 and Sa101

    CERN Document Server

    Rebull, L M; Hoette, V; Kim, J S; Laine, S; Foster, M; Laher, R; Legassie, M; Mallory, C R; McCarron, K; Sherry, W H

    2011-01-01

    The CG4 and Sa101 regions together cover a region of ~0.5 square degree in the vicinity of a "cometary globule" that is part of the Gum Nebula. There are seven previously identified young stars in this region; we have searched for new young stars using mid- and far-infrared data (3.6 to 70 microns) from the Spitzer Space Telescope, combined with ground-based optical data and near-infrared data from the Two-Micron All-Sky Survey (2MASS). We find infrared excesses in all 6 of the previously identified young stars in our maps, and we identify 16 more candidate young stars based on apparent infrared excesses. Most (73%) of the new young stars are Class II objects. There is a tighter grouping of young stars and young star candidates in the Sa101 region, in contrast to the CG4 region, where there are fewer young stars and young star candidates, and they are more dispersed. Few likely young objects are found in the "fingers" of the dust being disturbed by the ionization front from the heart of the Gum Nebula.

  16. Discovery and Observations of ASASSN-13db, an EXor Accretion Event on a Low-Mass T Tauri Star

    CERN Document Server

    Holoien, Thomas W -S; Stanek, Krzysztof Z; Kochanek, Christopher S; Shappee, B J; Zhu, Z; Sicilia-Aguilar, A; Grupe, D; Croxall, K; Adams, J; Simon, J D; McGraw, N Morell S M; Wagner, R M; Basu, U; Beacom, J F; Bersier, D; Brimacombe, J; Jencson, J; Pojmanski, G; Starrfield, S G; Szczygieł, D M; Woodward, C E

    2014-01-01

    We discuss ASASSN-13db, an EXor accretion event on the young stellar object (YSO) SDSS J051011.01$-$032826.2 (hereafter SDSSJ0510) discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN). Using archival photometric data of SDSSJ0510 we construct a pre-outburst spectral energy distribution (SED) and find that it is consistent with a low-mass class II YSO near the Orion star forming region ($d \\sim 420$ pc). We present follow-up photometric and spectroscopic observations of the source after the $\\Delta V \\sim-$3.7 mag outburst that began in September 2013. These data indicate an increase in temperature and luminosity consistent with an accretion rate of $\\sim10^{-7}$ $\\rm{M}_\\odot$ yr$^{-1}$, three-to-five orders of magnitude greater than in quiescence. Spectroscopic observations show a forest of narrow emission lines dominated by neutral metallic lines from Fe I and some low-ionization lines. The properties of ASASSN-13db are similar to those of the EXor prototype EX~Lupi in late 2008 during its st...

  17. ASTRO-H White Paper - Stars -- Accretion, Shocks, Charge Exchanges and Magnetic Phenomena

    CERN Document Server

    Tsuboi, Y; Audard, M; Hamaguchi, K; Leutenegger, M A; Maeda, Y; Mori, K; H,; Murakami,; Sugawara, Y; Tsujimoto, M

    2014-01-01

    X-ray emission from stars has origins as diverse as the stars themselves: accretion shocks, shocks generated in wind-wind collisions, or release of magnetic energy. Although the scenarios responsible for X-ray emission are thought to be known, the physical mechanisms operating are in many cases not yet fully understood. Full testing of many of these mechanisms requires high energy resolution, large effective area, and coverage of broad energy bands. The loss of the X-ray calorimeter spectrometer on board ASTRO-E2 was a huge blow to the field; it would have provided a large sample of high resolution spectra of stars with high signal-to-noise ratio. Now, with the advent of the ASTRO-H Soft X-ray Spectrometer and Hard X-ray Imager, we will be able to examine some of the hot topics in stellar astrophysics and solve outstanding mysteries.

  18. Helium Ignition on Accreting Neutron Stars with a New Triple-α Reaction Rate

    Science.gov (United States)

    Peng, Fang; Ott, Christian D.

    2010-12-01

    We investigate the effect of a new triple-α reaction rate from Ogata et al. on helium ignition conditions on accreting neutron stars and on the properties of the subsequent type I X-ray burst. We find that the new rate leads to significantly lower ignition column density for accreting neutron stars at low accretion rates. We compare the results of our ignition models for a pure helium accretor to observations of bursts in ultracompact X-ray binaries (UCXBs), which are believed to have nearly pure helium donors. For \\dot{m}> 0.001 \\dot{m}_{{Edd}}, the new triple-α reaction rate from Ogata et al. predicts a maximum helium ignition column of ~3 × 109 g cm-2, corresponding to a burst energy of ~4 × 1040 erg. For \\dot{m}˜ 0.01 \\dot{m}_{{Edd}} at which intermediate long bursts occur, the predicted burst energies are at least a factor of 10 too low to explain the observed energies of such bursts in UCXBs. This finding adds to the doubts cast on the triple-α reaction rate of Ogata et al. by the low-mass stellar evolution results of Dotter & Paxton.

  19. Helium Ignition on Accreting Neutron Stars with a New Triple-alpha Reaction Rate

    CERN Document Server

    Peng, Fang

    2010-01-01

    We investigate the effect of a new triple-alpha reaction rate from Ogata et al. (2009) on helium ignition conditions on accreting neutron stars and on the properties of the subsequent type I X-ray burst. We find that the new rate leads to significantly lower ignition column density for accreting neutron stars at low accretion rates. We compare the results of our ignition models for a pure helium accretor to observations of bursts in ultra-compact X-ray binary (UCXBs), which are believed to have nearly pure helium donors. For mdot > 0.001 mdot_Edd, the new triple-alpha reaction rate from Ogata et al. (2009) predicts a maximum helium ignition column of ~ 3 x 10^9 g cm^{-2}, corresponding to a burst energy of ~ 4 x 10^{40} ergs. For mdot ~ 0.01 mdot_Edd at which intermediate long bursts occur, the predicted burst energies are at least a factor of 10 too low to explain the observed energies of such bursts in UCXBs. This finding adds to the doubts cast on the triple-alpha reaction rate of Ogata et al. (2009) by th...

  20. Do Stellar Winds Prevent the Formation of Supermassive Stars by Accretion?

    CERN Document Server

    Nakauchi, Daisuke; Omukai, Kazuyuki; Saio, Hideyuki; Nomoto, Ken'ichi

    2016-01-01

    Supermassive stars (SMS; ~ 10^5 M_sun) formed from metal-free gas in the early Universe attract attention as progenitors of supermassive black holes observed at high redshifts. To form SMSs by accretion, central protostars must accrete at as high rates as ~ 0.1-1 M_sun/yr. Such protostars have very extended structures with bloated envelopes, like super-giant stars, and are called super-giant protostars (SGPSs). Under the assumption of hydrostatic equilibrium, SGPSs have density inverted layers, where the luminosity becomes locally super-Eddington, near the surface. If the envelope matter is allowed to flow out, however, a stellar wind could be launched and hinder the accretion growth of SGPSs before reaching the supermassive regime. We examine whether radiation-driven winds are launched from SGPSs by constructing steady and spherically symmetric wind solutions. We find that the wind velocity does not reach the escape velocity in any case considered. This is because once the temperature falls below ~ 10^4 K, t...

  1. Carbon production on accreting neutron stars in a new regime of stable nuclear burning

    CERN Document Server

    Keek, L

    2015-01-01

    Accreting neutron stars exhibit Type I X-ray bursts from both frequent hydrogen/helium flashes as well as rare carbon flashes. The latter (superbursts) ignite in the ashes of the former. Hydrogen/helium bursts, however, are thought to produce insufficient carbon to power superbursts. Stable burning could create the required carbon, but this was predicted to only occur at much larger accretion rates than where superbursts are observed. We present models of a new steady-state regime of stable hydrogen and helium burning that produces pure carbon ashes. Hot CNO burning of hydrogen heats the neutron star envelope and causes helium to burn before the conditions of a helium flash are reached. This takes place when the mass accretion rate is around 10% of the Eddington limit: close to the rate where most superbursts occur. We find that increased heating at the base of the envelope sustains steady-state burning by steepening the temperature profile, which increases the amount of helium that burns before a runaway can...

  2. Carbon Synthesis in Steady-State Hydrogen and Helium Burning On Accreting Neutron Stars

    CERN Document Server

    Stevens, Jeremy; Cumming, Andrew; Cyburt, Richard; Schatz, Hendrik

    2014-01-01

    Superbursts from accreting neutron stars probe nuclear reactions at extreme densities ($\\rho \\approx 10^{9}~g\\,cm^{-3}$) and temperatures ($T>10^9~K$). These bursts ($\\sim$1000 times more energetic than type I X-ray bursts) are most likely triggered by unstable ignition of carbon in a sea of heavy nuclei made during the rp-process of regular type I X-ray bursts (where the accumulated hydrogen and helium are burned). An open question is the origin of sufficient amounts of carbon, which is largely destroyed during the rp-process in X-ray bursts. We explore carbon production in steady-state burning via the rp-process, which might occur together with unstable burning in systems showing superbursts. We find that for a wide range of accretion rates and accreted helium mass fractions large amounts of carbon are produced, even for systems that accrete solar composition. This makes stable hydrogen and helium burning a viable source of carbon to trigger superbursts. We also investigate the sensitivity of the results to...

  3. Young star clusters in circumnuclear starburst rings

    CERN Document Server

    de Grijs, Richard; Jia, Siyao; Ho, Luis C; Anders, Peter

    2016-01-01

    We analyse the cluster luminosity functions (CLFs) of the youngest star clusters in three galaxies exhibiting prominent circumnuclear starburst rings. We focus specifically on NGC 1512 and NGC 6951, for which we have access to H$\\alpha$ data that allow us to unambiguously identify the youngest sample clusters. To place our results on a firm statistical footing, we first explore in detail a number of important technical issues affecting the process from converting the observational data into the spectral-energy distributions of the objects in our final catalogues. The CLFs of the young clusters in both galaxies exhibit approximate power-law behaviour down to the 90 per cent observational completeness limits, thus showing that star cluster formation in the violent environments of starburst rings appears to proceed similarly as that elsewhere in the local Universe. We discuss this result in the context of the density of the interstellar medium in our starburst-ring galaxies.

  4. Young star clusters in circumnuclear starburst rings

    Science.gov (United States)

    de Grijs, Richard; Ma, Chao; Jia, Siyao; Ho, Luis C.; Anders, Peter

    2017-03-01

    We analyse the cluster luminosity functions (CLFs) of the youngest star clusters in two galaxies exhibiting prominent circumnuclear starburst rings. We focus specifically on NGC 1512 and NGC 6951, for which we have access to Hα data that allow us to unambiguously identify the youngest sample clusters. To place our results on a firm statistical footing, we first explore in detail a number of important technical issues affecting the process from converting the observational data into the spectral energy distributions of the objects in our final catalogues. The CLFs of the young clusters in both galaxies exhibit approximate power-law behaviour down to the 90 per cent observational completeness limits, thus showing that star cluster formation in the violent environments of starburst rings appears to proceed similarly as that elsewhere in the local Universe. We discuss this result in the context of the density of the interstellar medium in our starburst-ring galaxies.

  5. Jets from Young Stars: The Need for MHD Collimation and Acceleration Processes

    Science.gov (United States)

    Cabrit, Sylvie

    This lecture revisits in the light of recent data the main lines of evidence indicating that MHD processes play a crucial role in jets from young stars. Measurements of jet collimation and jet ejection-accretion efficiencies are reviewed and compared at various evolutionary stages (from protostars to optically revealed objects). It is then shown that they cannot satisfactorily be accounted for by purely hydrodynamical processes. MHD magneto-centrifugal ejection (combining magnetic self-collimation and magnetic acceleration) appears as the most effective mechanism able to reproduce the observed jet properties at all evolutionary phases. The jets would then be intimately linked to angular momentum extraction from the accreting disk and/or star.

  6. Photospheric Spot Temperature Models of Young Stars in the Orion Nebula Cluster

    Science.gov (United States)

    Miller, M. J.; Stassun, K. G.; Jensen, E. L. N.

    2003-12-01

    We apply a simple photospheric spot temperature model to photometric variability measurements of T Tauri stars in the Trapezium region of the Orion Nebula Cluster. Our aim is to search for the relationship, if any, between spot temperatures and stellar rotation periods to better understand the relationship between accretion and angular momentum regulation in T Tauri stars. Current magnetic disk-locking models of young stars ascribe spot temperatures hotter than the photosphere to signatures of active accretion from a circumstellar disk. If accretion acts to brake stellar rotation, spot temperatures hotter than the photosphere should be more prevalent among slow rotators. From the variability amplitudes at four wavelengths (B, V, R, I), we determine spot temperatures and the areal coverage of the spot on the stellar surface. The results of our model show that we can unambiguously distinguish spots hotter than the photosphere from spots cooler than the photosphere for most stars. We present the results of our search for correlations between spot temperatures and previously determined rotation periods.

  7. Spectroscopic signatures of magnetospheric accretion in Herbig Ae/Be stars. I. The case of HD101412

    CERN Document Server

    Schöller, M; Cahuasqui, J A; Drake, N A; Hubrig, S; Petr-Gotzens, M G; Savanov, I S; Wolff, B; Gonzalez, J F; Mysore, S; Ilyin, I; Jarvinen, S P; Stelzer, B

    2016-01-01

    Models of magnetically-driven accretion and outflows reproduce many observational properties of T Tauri stars. This concept is not well established for the more massive Herbig Ae/Be stars. We intend to examine the magnetospheric accretion in Herbig Ae/Be stars and search for rotational modulation using spectroscopic signatures, in this first paper concentrating on the well-studied Herbig Ae star HD101412. We used near-infrared spectroscopic observations of the magnetic Herbig Ae star HD101412 to test the magnetospheric character of its accretion disk/star interaction. We reduced and analyzed 30 spectra of HD101412, acquired with the CRIRES and X-shooter spectrographs installed at the VLT (ESO, Chile). The spectroscopic analysis was based on the He I lambda 10,830 and Pa gamma lines, formed in the accretion region. We found that the temporal behavior of these diagnostic lines in the near-infrared spectra of HD101412 can be explained by rotational modulation of line profiles generated by accreting gas with a pe...

  8. The Sizes of the Nearest Young Stars

    Science.gov (United States)

    McCarthy, Kyle; White, Russel J.

    2012-06-01

    We present moderate resolution (R ~ 3575) optical spectra of 19 known or suspected members of the AB Doradus and β Pictoris Moving Groups, obtained with the DeVeny Spectrograph on the 72 inch Perkins telescope at Lowell Observatory. For four of five recently proposed members, signatures of youth such as Li I 6708 Å absorption and Hα emission further strengthen the case for youth and membership. The lack of detected lithium in the proposed β Pic member TYC 2211-1309-1 implies that it is older than all other K-type members and weakens the case for membership. Effective temperatures are determined via line ratio analyses for the 11 F, G, and early-K stars observed, and via spectral comparisons for the eight late-K and M stars observed. We assemble updated candidate membership lists for these moving groups that account for known binarity. Currently, the AB Dor Moving Group contains 127 proposed members and the β Pic Moving Group holds 77 proposed members. We then use temperature, luminosity, and distance estimates to predict angular diameters for these stars; the motivation is to identify stars that can be spatially resolved with long-baseline optical/infrared interferometers in order to improve age estimates for these groups and to constrain evolutionary models at young ages. Considering the portion of the sky accessible to northern hemisphere facilities (decl. > - 30), six stars have diameters large enough to be spatially resolved (θ > 0.4 mas) with the CHARA Array, which currently has the world's longest baseline of 331 m; this subsample includes the low-mass M2.5 member of AB Dor, GJ 393, which is likely to still be pre-main sequence. For southern hemisphere facilities (decl. < + 30), 18 stars have diameters larger than this limiting size, including the low-mass debris disk star AU Mic (0.72 mas). However, the longest baselines of southern hemisphere interferometers (160 m) are only able to resolve the largest of these, the B6 star α Gru (1.17 mas) proposed

  9. Accreting Neutron Stars in Low-Mass X-Ray Binary Systems

    CERN Document Server

    Lamb, Frederick K

    2007-01-01

    Using the Rossi X-ray Timing Explorer (RossiXTE), astronomers have discovered that disk-accreting neutron stars with weak magnetic fields produce three distinct types of high-frequency X-ray oscillations. These oscillations are powered by release of the binding energy of matter falling into the strong gravitational field of the star or by the sudden nuclear burning of matter that has accumulated in the outermost layers of the star. The frequencies of the oscillations reflect the orbital frequencies of gas deep in the gravitational field of the star and/or the spin frequency of the star. These oscillations can therefore be used to explore fundamental physics, such as strong-field gravity and the properties of matter under extreme conditions, and important astrophysical questions, such as the formation and evolution of millisecond pulsars. Observations using RossiXTE have shown that some two dozen neutron stars in low-mass X-ray binary systems have the spin rates and magnetic fields required to become milliseco...

  10. Appearance of innermost stable circular orbits of accretion discs around rotating neutron stars

    CERN Document Server

    Torok, G; Adamek, K; Urbancova, G

    2014-01-01

    The innermost stable cicular orbit (ISCO) of an accretion disc orbiting a neutron star (NS) is often assumed a unique prediction of general relativity. However, it has been argued that ISCO also appears around highly elliptic bodies described by Newtonian theory. In this sense, the behaviour of an ISCO around a rotating oblate neutron star is formed by the interplay between relativistic and Newtonian effects. Here we briefly explore the consequences of this interplay using a straightforward analytic approach as well as numerical models that involve modern NS equations of state. We examine the ratio K between the ISCO radius and the radius of the neutron star. We find that, with growing NS spin, the ratio K first decreases, but then starts to increase. This non-monotonic behaviour of K can give rise to a neutron star spin interval in which ISCO appears for two very different ranges of NS mass. This may strongly affect the distribution of neutron stars that have an ISCO (ISCO-NS). When (all) neutron stars are d...

  11. Highly efficient star formation in NGC 5253 possibly from stream-fed accretion.

    Science.gov (United States)

    Turner, J L; Beck, S C; Benford, D J; Consiglio, S M; Ho, P T P; Kovács, A; Meier, D S; Zhao, J-H

    2015-03-19

    Gas clouds in present-day galaxies are inefficient at forming stars. Low star-formation efficiency is a critical parameter in galaxy evolution: it is why stars are still forming nearly 14 billion years after the Big Bang and why star clusters generally do not survive their births, instead dispersing to form galactic disks or bulges. Yet the existence of ancient massive bound star clusters (globular clusters) in the Milky Way suggests that efficiencies were higher when they formed ten billion years ago. A local dwarf galaxy, NGC 5253, has a young star cluster that provides an example of highly efficient star formation. Here we report the detection of the J = 3→2 rotational transition of CO at the location of the massive cluster. The gas cloud is hot, dense, quiescent and extremely dusty. Its gas-to-dust ratio is lower than the Galactic value, which we attribute to dust enrichment by the embedded star cluster. Its star-formation efficiency exceeds 50 per cent, tenfold that of clouds in the Milky Way. We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy.

  12. Investigating Active Accretion, Flare Activity and a 50 Million Degree Corona in the cool AGB Star, Y Gem

    Science.gov (United States)

    Sahai, Raghvendra

    2014-09-01

    We propose to make multi-epoch observations of the X-ray (and radio contiuum) emission from a cool AGB star, Y Gem. This star shows relatively strong X-ray emission implying the presence of 50 million degree coronal gas, and highly-variable FUV and NUV fluxes - likely evidence of variable accretion onto a magnetized accretion disk in a binary system. Y Gem is the most prominent member of a new class of AGB stars with FUV excesses identified using the GALEX archive. Our study will allow us to study the never-before explored phase of an active accretion disk in an AGB star with a binary companion. Such disks are believed to ultimately produce collimated jets that transform AGB circumsetllar envelopes into bipolar planetary nebulae.

  13. Fusion of neutron rich oxygen isotopes in the crust of accreting neutron stars

    CERN Document Server

    Horowitz, C J; Berry, D K

    2007-01-01

    Fusion reactions in the crust of an accreting neutron star are an important source of heat, and the depth at which these reactions occur is important for determining the temperature profile of the star. Fusion reactions depend strongly on the nuclear charge $Z$. Nuclei with $Z\\le 6$ can fuse at low densities in a liquid ocean. However, nuclei with $Z=8$ or 10 may not burn until higher densities where the crust is solid and electron capture has made the nuclei neutron rich. We calculate the $S$ factor for fusion reactions of neutron rich nuclei including $^{24}$O + $^{24}$O and $^{28}$Ne + $^{28}$Ne. We use a simple barrier penetration model. The $S$ factor could be further enhanced by dynamical effects involving the neutron rich skin. This possible enhancement in $S$ should be studied in the laboratory with neutron rich radioactive beams. We model the structure of the crust with molecular dynamics simulations. We find that the crust of accreting neutron stars may contain micro-crystals or regions of phase sep...

  14. The Impact of Feedback During Massive Star Formation by Core Accretion

    CERN Document Server

    Tanaka, Kei E I; Zhang, Yichen

    2016-01-01

    We study the impact of feedback during the formation of massive stars that are accreting from massive gas cores using analytic method. MHD-driven disk winds are known to be the primary feedback mechanism for low-mass star formation. Radiative feedback processes are also expected to become significant for more massive protostars. We model feedback from disk winds, radiation pressure, photoevaporation and stellar winds, while following protostellar evolution in a series of models of collapsing massive cores. We find disk winds are the dominant feedback mechanism for massive star formation from cores in clump environments with surface densities Sigma>0.3g/cm2. >90% of total momentum is input by the disk wind, however radiation pressure also assists in widening the outflow cavity. Photoevaporation is of relatively minor importance due to dust attenuation of ionizing photons. Mass-loss and momentum feedback from stellar winds have very minor effects during the accretion stage. We find the SFE from the cores to be ...

  15. Carbon Shell or Core Ignitions in White Dwarfs Accreting from Helium Stars

    CERN Document Server

    Brooks, Jared; Schwab, Josiah; Paxton, Bill

    2016-01-01

    White dwarfs accreting from helium stars can stably burn at the accreted rate and avoid the challenge of mass loss associated with unstable Helium burning that is a concern for many Type Ia supernovae scenarios. We study binaries with helium stars of mass $1.25 M_\\odot\\le M_{\\rm{He}} \\le 1.8 M_\\odot$, which have lost their hydrogen rich envelopes in an earlier common envelope event and now orbit with periods ($P_{\\rm orb}$) of several hours with non-rotating $0.84$ and $1.0 M_\\odot$ C/O WDs. The helium stars fill their Roche lobes (RLs) after exhaustion of central helium and donate helium on their thermal timescales (${\\sim}10^5$yr). As shown by others, these mass transfer rates coincide with the steady helium burning range for WDs, and grow the WD core up to near the Chandrasekhar mass ($M_{\\rm Ch}$) and a core carbon ignition. We show here, however, that many of these scenarios lead to an ignition of hot carbon ashes near the outer edge of the WD and an inward going carbon flame that does not cause an explo...

  16. Magnetically elevated accretion discs in active galactic nuclei: broad emission-line regions and associated star formation

    Science.gov (United States)

    Begelman, Mitchell C.; Silk, Joseph

    2017-01-01

    We propose that the accretion discs fueling active galactic nuclei (AGN) are supported vertically against gravity by a strong toroidal (φ-direction) magnetic field that develops naturally as the result of an accretion disc dynamo. The magnetic pressure elevates most of the gas carrying the accretion flow at R to large heights z ≳ 0.1R and low densities, while leaving a thin dense layer containing most of the mass - but contributing very little accretion - around the equator. We show that such a disc model leads naturally to the formation of a broad emission-line region through thermal instability. Extrapolating to larger radii, we demonstrate that local gravitational instability and associated star formation are strongly suppressed compared to standard disc models for AGN, although star formation in the equatorial zone is predicted for sufficiently high mass supply rates. This new class of accretion disc models thus appears capable of resolving two longstanding puzzles in the theory of AGN fueling: the formation of broad emission-line regions and the suppression of fragmentation thought to inhibit accretion at the required rates. We show that the disc of stars that formed in the Galactic Center a few million years ago could have resulted from an episode of magnetically elevated accretion at ≳ 0.1 of the Eddington limit.

  17. UV spectral variability in the Herbig Ae star HR 5999. 11: The accretion interpretation

    Science.gov (United States)

    Perez, M. R.; Grady, C. A.; The, P. S.

    1993-01-01

    We report recent IUE high- and low-dispersion observations with the IUE long wavelength camera (LWP) and short wavelength camera (SWP) of the Herbig Ae star HR 5999. We have found a dramatic change in the structure of the Mg II h and k lines (2795.5, 2802.7 A) along with some continuum flux excesses especially at the short end of the SWP camera. LWP high dispersion observations of HR 5999 obtained between 1979 and 1990, at times of comparatively low UV continuum fluxes, exhibit P Cygni type m profiles in the Mg II resonance doublet. In contrast, observations made from September 1990 through March 16-18, 1992, with high W continuum fluxes, present Mg II lines with reverse P Cygni profiles indicative of some active episodic accretion. Accreting gas can also be detected in the additional red wings of the various Fe II and Mn II absorption lines, with velocities up to +300-350 km/s (September 1990). By September 10, 1992 the Mg II profile had returned to the type III P Cygni profile similar to those from earlier spectra. The correlation between the presence of large column densities of accreting gas and the continuum light variations supports suggestions by several authors that HR 5999 is surrounded by an optically thick, viscously heated accretion disk. Detection of accreting gas in the line of sight to HR 5999 permits us to place constraints on our viewing geometry for this system. A discussion is included comparing the spectral and physical similarities between HR 5999 and the more evolved proto-planetary candidate system, beta Pictoris.

  18. Spectroscopic signatures of magnetospheric accretion in Herbig Ae/Be stars. I. The case of HD 101412

    Science.gov (United States)

    Schöller, M.; Pogodin, M. A.; Cahuasquí, J. A.; Drake, N. A.; Hubrig, S.; Petr-Gotzens, M. G.; Savanov, I. S.; Wolff, B.; González, J. F.; Mysore, S.; Ilyin, I.; Järvinen, S. P.; Stelzer, B.

    2016-07-01

    Context. Models of magnetically-driven accretion and outflows reproduce many observational properties of T Tauri stars. This concept is not well established for the more massive Herbig Ae/Be stars. Aims: We intend to examine the magnetospheric accretion in Herbig Ae/Be stars and search for rotational modulation using spectroscopic signatures, in this first paper concentrating on the well-studied Herbig Ae star HD 101412. Methods: We used near-infrared spectroscopic observations of the magnetic Herbig Ae star HD 101412 to test the magnetospheric character of its accretion disk/star interaction. We reduced and analyzed 30 spectra of HD 101412, acquired with the CRIRES and X-shooter spectrographs installed at the VLT (ESO, Chile). The spectroscopic analysis was based on the He iλ10 830 and Paγ lines, formed in the accretion region. Results: We found that the temporal behavior of these diagnostic lines in the near-infrared spectra of HD 101412 can be explained by rotational modulation of line profiles generated by accreting gas with a period P = 20.53d±1.68d. The discovery of this period, about half of the magnetic rotation period Pm = 42.076d previously determined from measurements of the mean longitudinal magnetic field, indicates that the accreted matter falls onto the star in regions close to the magnetic poles intersecting the line-of-sight two times during the rotation cycle. We intend to apply this method to a larger sample of Herbig Ae/Be stars. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 087.C-0124(A), 088.C-0218(A,B,C,E), 090.C-0331(A), and 092.C-0126(A).

  19. Measurement of Hard Lags and Coherences in the X-Ray Flux of Accreting Neutron Stars and Comparison with Accreting Black Holes

    OpenAIRE

    Ford, Eric C.; Van Der Klis, Michiel; Mendez, Mariano; van Paradijs, Jan; Kaaret, Philip

    1998-01-01

    Using the Rossi X-ray Timing Explorer we have measured lags of the 9 to 33 keV photons relative to the 2 to 9 keV photons in the timing noise between 0.01 and 100 Hz in the accreting neutron stars 4U 0614+091 and 4U 1705-44. We performed similar measurements on the accreting black hole candidates Cyg X-1 and GX 339-4 as a comparison. During the observations these sources were all in low (hard) states. We find phase lags of between 0.03 and 0.2 radians in all these sources, with a variation in...

  20. Long-term Accretion Variations of the Magnetic Cataclysmic Variable Star QQ Vulpecula

    Science.gov (United States)

    Cooper Rose, Sanaea; Kafka, Stella; Jorgenson, Regina; Carr, Derrick; Childs, Francesca; Christenson, Holly; Karim, Md. Tanveer; Konchady, Tarini; Walker, Gary E.; Honeycutt, R. K.

    2017-01-01

    Magnetic cataclysmic variable stars have brightness variations that repeat with each revolution of the two stars about the center of mass of the system. However, in the case of QQ Vulpecula (QQ Vul), this brightness variation pattern changes in the long term. This study makes use of two decades worth of data from the Roboscope Telescope as well as data from the American Association of Variable Star Observers (AAVSO) database to examine the long-term evolution of QQ Vul’s phase curves. Nightly observations using the Maria Mitchell Association's Vestal and Loines Observatories supplemented this analysis by clarifying short-term brightness variation. The long-term data was divided into four commonly observed behavioral types ranging from a double peaked curve of ~15.5 magnitude to a ~15.0 magnitude curve that had a primary minimum and a slow, linear rise in brightness in place of the secondary minimum. The nightly data kept within the confines of these categories, though the secondary minimum in the nightly data never vanished. No periodicity was found in the long-term variations. The model often invoked to explain the double peaked curve consists of single pole accretion in which a partial self-eclipse causes the secondary minimum and cyclotron beaming causes the primary minimum. However, the long-term variation may indicate a changing accretion rate, which may manifest itself in changes to the shape, size, or location of the accretion spot on the white dwarf such that it lessens or removes the secondary minimum. This project was supported by the NSF REU grant AST-1358980, the Massachusetts Space Grant, and the Nantucket Maria Mitchell Association.

  1. The evolution of accretion in young stellar objects: Strong accretors at 3-10 Myr

    Energy Technology Data Exchange (ETDEWEB)

    Ingleby, Laura; Calvet, Nuria; Hartmann, Lee; Miller, Jon; McClure, Melissa [Department of Astronomy, University of Michigan, 830 Dennison Building, 500 Church Street, Ann Arbor, MI 48109 (United States); Hernández, Jesus; Briceno, Cesar [Centro de Investigaciones de Astronomía (CIDA), Mérida, 5101-A (Venezuela, Bolivarian Republic of); Espaillat, Catherine, E-mail: lingleby@umich.edu, E-mail: ncalvet@umich.edu, E-mail: cce@bu.edu [Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States)

    2014-07-20

    While the rate of accretion onto T Tauri stars is predicted to decline with age, objects with strong accretion have been detected at ages of up to 10 Myr. We analyze a sample of these old accretors, identified by having a significant U band excess and infrared emission from a circumstellar disk. Objects were selected from the ∼3 Myr σ Ori, 4-6 Myr Orion OB1b, and 7-10 Myr Orion OB1a star forming associations. We use high-resolution spectra from the Magellan Inamori Kyocera Echelle to estimate the veiling of absorption lines and calculate extinction for our T Tauri sample. We also use observations obtained with the Magellan Echellette and, in a few cases, the SWIFT Ultraviolet and Optical Telescope to estimate the excess produced in the accretion shock, which is then fit with accretion shock models to estimate the accretion rate. We find that even objects as old as 10 Myr may have high accretion rates, up to ∼10{sup –8} M{sub ☉} yr{sup –1}. These objects cannot be explained by viscous evolution models, which would deplete the disk in shorter timescales unless the initial disk mass is very high, a situation that is unstable. We show that the infrared spectral energy distribution of one object, CVSO 206, does not reveal evidence of significant dust evolution, which would be expected during the 10 Myr lifetime. We compare this object to predictions from photoevaporation and planet formation models and suggest that neither of these processes have had a strong impact on the disk of CVSO 206.

  2. Accretion disks around neutron and strange stars in $\\mathcal{R}^2$ gravity

    CERN Document Server

    Staykov, Kalin V; Yazadjiev, Stoytcho S

    2016-01-01

    We study the electromagnetic spectrum of accretion disks around neutron and strange stars in $\\mathcal{R}^2$ gravity. Both static and rapidly rotating models are investigated. The results are compared with the General Relativistic results. We found difference between the results in both theories of about 50\\% for the electromagnetic flux and about 20\\% in the luminosity for models with equal mass and angular velocity in both theories. The observed differences are much lower for models rotating with Kelperian velocity and with equal masses.

  3. The First X-shooter Observations of Jets from Young Stars

    CERN Document Server

    Bacciotti, Francesca; Alcala', Juan M; Nisini, Brunella; Podio, Linda; Randich, Sofia; Stelzer, Beate; Cupani, Guido

    2011-01-01

    We present the first pilot study of jets from young stars conducted with X-shooter, on ESO/VLT. As it offers simultaneous, high quality spectra in the range 300-2500 nm X-shooter is uniquely important for spectral diagnostics in jet studies. We chose to probe the accretion/ejection mechanisms at low stellar masses examining two targets with well resolved continuous jets lying on the plane of the sky, ESO-HA 574 in Chamaleon I, and Par-Lup3-4 in Lupus III. The mass of the latter is close to the sub-stellar boundary (Mstar=0.13 Msun). A large number of emission lines probing regions of different excitation are identified, position-velocity diagrams are presented and mass outflow/accretion rates are estimated. Comparison between the two objects is striking. ESO-HA 574 is a weakly accreting star for which we estimate a mass accretion rate of log(\\dot{Macc}) = -10.8 \\pm 0.5 (in Msun/yr), yet it drives a powerful jet with \\dot{Mout} ~ 1.5-2.7 10^-9 Msun/yr. These values can be reconciled with a magneto-centrifugal ...

  4. Magnetized accretion

    Science.gov (United States)

    Heyvaerts, J.

    This lecture reviews in simple terms the general subject of large scale magnetic field coupling to plasma flows in the vicinity of accreting compact stars. The relevant astrophysical phenomenology is summarized. Disk interaction with the magnetosphere of accreting stars is first discussed, in particular the structure of the magnetopause, its stability and plasma ejection in so-called propeller systems. The physics of accretion/ejection is then considered. Acceleration and focusing mechanisms of jets from accretion disks around compact stars or black holes and the question of the self-consistency of accretion and ejection are described. By contrast, small scale MHD turbulence in disks is not discussed, neither are accretion columns near the polar caps of neutron stars or white dwarfs. The reader is only assumed to have some basic knowledge of astrophysics and of fluid mechanics and electromagnetism.

  5. Star Cluster Formation in Cosmological Simulations. I. Properties of Young Clusters

    Science.gov (United States)

    Li, Hui; Gnedin, Oleg Y.; Gnedin, Nickolay Y.; Meng, Xi; Semenov, Vadim A.; Kravtsov, Andrey V.

    2017-01-01

    We present a new implementation of star formation in cosmological simulations by considering star clusters as a unit of star formation. Cluster particles grow in mass over several million years at the rate determined by local gas properties, with high time resolution. The particle growth is terminated by its own energy and momentum feedback on the interstellar medium. We test this implementation for Milky Way-sized galaxies at high redshift by comparing the properties of model clusters with observations of young star clusters. We find that the cluster initial mass function is best described by a Schechter function rather than a single power law. In agreement with observations, at low masses the logarithmic slope is α ≈ 1.8{--}2, while the cutoff at high mass scales with the star formation rate (SFR). A related trend is a positive correlation between the surface density of the SFR and fraction of stars contained in massive clusters. Both trends indicate that the formation of massive star clusters is preferred during bursts of star formation. These bursts are often associated with major-merger events. We also find that the median timescale for cluster formation ranges from 0.5 to 4 Myr and decreases systematically with increasing star formation efficiency. Local variations in the gas density and cluster accretion rate naturally lead to the scatter of the overall formation efficiency by an order of magnitude, even when the instantaneous efficiency is kept constant. Comparison of the formation timescale with the observed age spread of young star clusters provides an additional important constraint on the modeling of star formation and feedback schemes.

  6. Non-stationary dynamo & magnetospheric accretion processes of the classical T Tauri star V2129 Oph

    CERN Document Server

    Donati, JF; Walter, FM; Gregory, SG; Skelly, MB; Hussain, GAJ; Flaccomio, E; Argiroffi, C; Grankin, KN; Jardine, MM; Menard, F; Dougados, C; Romanova, MM

    2010-01-01

    We report here the first results of a multi-wavelength campaign focussing on magnetospheric accretion processes of the classical TTauri star (cTTS) V2129Oph. In this paper, we present spectropolarimetric observations collected in 2009 July with ESPaDOnS at the Canada-France-Hawaii Telescope (CFHT). Circularly polarised Zeeman signatures are clearly detected, both in photospheric absorption and accretion-powered emission lines, from time-series of which we reconstruct new maps of the magnetic field, photospheric brightness and accretion-powered emission at the surface of V2129Oph using our newest tomographic imaging tool -- to be compared with those derived from our old 2005 June data set, reanalyzed in the exact same way. We find that in 2009 July, V2129Oph hosts octupolar & dipolar field components of about 2.1 & 0.9kG respectively, both tilted by about 20deg with respect to the rotation axis; we conclude that the large-scale magnetic topology changed significantly since 2005 June (when the octupole ...

  7. Outflows from accretion disks formed in neutron star mergers: effect of black hole spin

    CERN Document Server

    Fernández, Rodrigo; Metzger, Brian D; Quataert, Eliot

    2014-01-01

    The accretion disk that forms after a neutron star merger is a source of neutron-rich ejecta. The ejected material contributes to a radioactively-powered electromagnetic transient, with properties that depend sensitively on the composition of the outflow. Here we investigate how the spin of the black hole remnant influences mass ejection on the thermal and viscous timescales. To this end, we carry out two-dimensional, time-dependent hydrodynamic simulations of merger remnant accretion disks including viscous angular momentum transport and approximate neutrino self-irradiation. The gravity of the spinning black hole is included via a pseudo-Newtonian potential. We find that a disk around a spinning black hole ejects more mass, up to a factor of several, relative to the non-spinning case. The enhanced mass loss is due to energy release by accretion occurring deeper in the gravitational potential, raising the disk temperature and hence the rate of viscous heating in regions where neutrino cooling is ineffective....

  8. Accretion-driven core collapse and the collisional formation of massive stars

    CERN Document Server

    Bonnell, C Clarke I

    2008-01-01

    We consider the conditions required for a cluster core to shrink, by adiabatic accretion of gas from the surrounding cluster, to densities such that stellar collisions are a likely outcome. We show that the maximum densities attained, and hence the viability of collisions, depends on a competition between core shrinkage (driven by accretion) and core puffing up (driven by relaxation effects). The expected number of collisions scales as $N_{core}^{5/3} \\tilde v^2$ where $N_{core}$ is the number of stars in the cluster core and $\\tilde v$ is the free fall velocity of the parent cluster (gas reservoir). Thus whereas collisions are very unlikely in a relatively low mass, low internal velocity system such as the Orion Nebula Cluster, they become considerably more important at the mass and velocity scale characteristic of globular clusters. Thus stellar collisions in response to accretion induced core shrinkage remains a viable prospect in more massive clusters, and may contribute to the production of intermediate ...

  9. 3D Gray Radiative Properties of Accretion Shocks in Young Stellar Objects

    Science.gov (United States)

    Ibgui, L.; Orlando, S.; Stehlé, C.; Chièze, J.-P.; Hubeny, I.; Lanz, T.; de Sá, L.; Matsakos, T.; González, M.; Bonito, R.

    2014-01-01

    We address the problem of the contribution of radiation to the structure and dynamics of accretion shocks on Young Stellar Objects. Solving the 3D RTE (radiative transfer equation) under our "gray LTE approach", i.e., using appropriate mean opacities computed in local thermodynamic equilibrium, we post-process the 3D MHD (magnetohydrodynamic) structure of an accretion stream impacting the stellar chromosphere. We find a radiation flux of ten orders of magnitude larger than the accreting energy rate, which is due to a large overestimation of the radiative cooling. A gray LTE radiative transfer approximation is therefore not consistent with the given MHD structure of the shock. Further investigations are required to clarify the role of radiation, by relaxing both the gray and LTE approximations in RHD (radiation hydrodynamics) simulations. Post-processing the obtained structures through the resolution of the non-LTE monochromatic RTE will provide reference radiation quantities against which RHD approximate solutions will be compared.

  10. Bondi-Hoyle-Lyttleton Accretion onto a Protoplanetary Disk

    CERN Document Server

    Moeckel, Nickolas

    2009-01-01

    Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star's birth and the dispersal of the cluster's gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary disk; the potential importance of this `tail-end' accretion for planet formation was recently highlighted by Throop & Bally (2008). While accretion onto a point mass is successfully modeled by the classical Bondi-Hoyle-Lyttleton solutions, the more complicated case of accretion onto a star-disk system defies analytic solution. In this paper we investigate via direct hydrodynamic simulations the accretion of dense interstellar material onto a star with an associated gaseous protoplanetary disk. We discuss the changes to the structure of the accretion flow caused by the disk, and vice versa. We find that immersion in a dense accretion flow can redistribute disk material such ...

  11. Effects of a New Triple-alpha Reaction on X-ray Bursts of a Helium Accreting Neutron Star

    CERN Document Server

    Matsuo, Y; Noda, T; Saruwatari, M; Ono, M; Hashimoto, M; Fujimoto, M

    2011-01-01

    The effects of a new triple-$\\alpha$ reaction rate (OKK rate) on the helium flash of a helium accreting neutron star in a binary system have been investigated. Since the ignition points determine the properties of a thermonuclear flash of type I X-ray bursts, we examine the cases of different accretion rates, $dM/dt (\\dot{M})$, of helium from $3\\times10^{-10} M_{\\odot} \\rm yr^{-1}$ to $3\\times10^{-8} M_{\\odot} \\rm yr^{-1}$, which could cover the observed accretion rates. We find that for the cases of low accretion rates, nuclear burnings are ignited at the helium layers of rather low densities. As a consequence, helium deflagration would be triggered for all cases of lower accretion rate than $\\dot{M}\\simeq 3\\times10^{-8} M_{\\odot} \\rm yr^{-1}$. We find that OKK rate could be barely consistent with the available observations of the X-ray bursts on the helium accreting neutron star. However this coincidence is found to depend on the properties of crustal heating and the neutron star model.We suggest that OKK r...

  12. Dichotomy Between Black Hole and Neutron Star Accretion: Effect of Hard Surface

    Science.gov (United States)

    Dhang, Prasun; Mukhopadhyay, Banibrata; Sharma, Prateek

    2016-07-01

    Estimates of accretion rate on to compact objects have been explored based on the well-known, spherically symmetric, inviscid, steady-state solution given by Bondi. This solution assumes that there is a sink of mass at the center -- which in case of a black hole (BH) corresponds to the advection of matter across the event horizon. Other stars, such as a neutron star (NS), have surfaces and hence the infalling matter has to come to rest at the surface. We study the initial value problem in which the matter distribution is uniform and at rest at time t=0 with different inner radial boundary conditions for BHs and NSs: inflow boundary condition valid for BHs; and reflective or settling boundary condition for NSs. We obtain a similarity solution for the flow with inner inflow and reflective boundary conditions (assuming a cold ambient medium) and compare with numerical simulations of the Euler equations. One-dimensional simulations show the formation of an outward propagating and a standing shock in NS system for reflective and settling boundary conditions respectively. Two-dimensional simulations show that both these flows are unstable (locally to convection and globally to a standing shock instability). Numerical simulations show that in steady state, spherical accretion rate on to a NS for reflective boundary condition is suppressed by orders of magnitude compared to that on to a BH.

  13. The quenching of star formation in accretion-driven clumpy turbulent tori of active galactic nuclei

    CERN Document Server

    Vollmer, B

    2013-01-01

    Galactic gas-gas collisions involving a turbulent multiphase ISM share common ISM properties: dense extraplanar gas visible in CO, large linewidths (>= 50 km/s), strong mid-infrared H_2 line emission, low star formation activity, and strong radio continuum emission. Gas-gas collisions can occur in the form of ICM ram pressure stripping, galaxy head-on collisions, compression of the intragroup gas and/or galaxy ISM by an intruder galaxy which flies through the galaxy group at a high velocity, or external gas accretion on an existing gas torus in a galactic center. We suggest that the common theme of all these gas-gas interactions is adiabatic compression of the ISM leading to an increase of the turbulent velocity dispersion of the gas. The turbulent gas clouds are then overpressured and star formation is quenched. Within this scenario we developed a model for turbulent clumpy gas disks where the energy to drive turbulence is supplied by external infall or the gain of potential energy by radial gas accretion wi...

  14. Accretion-related properties of Herbig Ae/Be stars. Comparison with T Tauris

    CERN Document Server

    Mendigutía, I; Montesinos, B; Eiroa, C; Meeus, G; Merín, B; Oudmaijer, R D

    2012-01-01

    We look for trends relating the mass accretion rate (Macc) and the stellar ages (t), spectral energy distributions (SEDs), and disk masses (Mdisk) for a sample of 38 HAeBe stars, comparing them to analogous correlations found for classical T Tauri stars. Our goal is to shed light on the timescale and physical processes that drive evolution of intermediate-mass pre-main sequence objects. Macc shows a dissipation timescale \\tau = 1.3^{+1.0}_{-0.5} Myr from an exponential law fit, while a power law yields Macc(t) \\propto t^{-\\eta}, with \\eta = 1.8^{+1.4}_{-0.7}. This result is based on our whole HAeBe sample (1-6 Msun), but the accretion rate decline most probably depends on smaller stellar mass bins. The near-IR excess is higher and starts at shorter wavelengths (J and H bands) for the strongest accretors. Active and passive disks are roughly divided by 2 x 10^{-7} Msun/yr. The mid-IR excess and the SED shape from the Meeus et al. classification are not correlated with Macc. We find Macc \\propto Mdisk^{1.1 +- 0...

  15. Coronal geometry at low mass-accretion rates from XMM and NuSTAR spectra

    Science.gov (United States)

    Fuerst, F.; NuSTAR Binaries Team; NuSTAR AGN Team

    2016-06-01

    At very low Eddington luminosities the structure and physics of the accretion flow around a black hole are still debated, in particular in the inner most regions. By making sensitive measurements of the relativistic blurring of the X-ray reflection spectrum we investigate these physics, a task for which XMM-Newton, in combination with hard X-ray coverage provided by NuSTAR or Hitomi, is ideally suited and will continue to be unique for years to come. I will present results from XMM and NuSTAR observations of the radio-galaxy Cen A and of the X-ray binary GRS 1739-278 during the decline of its outburst. While Cen A shows a prominent iron line, the broad-band spectrum shows no evidence of reflection. This lack of reflection can best be explained by a jet origin of the hard X-rays or a significantly truncated accretion disk. The iron line can be self-consistently explained when assuming an optically thick torus surrounding the super-massive black-hole. The broad-band X-ray spectrum of GRS 1739-278 can be well described by a simple power-law or Comptonization continuum. A weak relativistic reflection model results in a small but significant improvement of the statistical quality of the fit. This relativistic model indicates a strongly truncated disk.

  16. Compositionally-driven convection in the oceans of accreting neutron stars

    CERN Document Server

    Medin, Zach

    2010-01-01

    We discuss the effect of chemical separation as matter freezes at the base of the ocean of an accreting neutron star, and argue that the retention of light elements in the liquid acts as a source of buoyancy that drives a slow but continual mixing of the ocean, enriching it substantially in light elements, and leading to a relatively uniform composition with depth. We first consider the timescales associated with different processes that can redistribute elements in the ocean, including convection, sedimentation, crystallization, and diffusion. We then calculate the steady state structure of the ocean of a neutron star for an illustrative model in which the accreted hydrogen and helium burns to produce a mixture of O and Se. Even though the H/He burning produces only 2% oxygen by mass, the steady state ocean has an oxygen abundance more than ten times larger, almost 40% by mass. Furthermore, we show that the convective motions transport heat inwards, with a flux of ~ 0.2 MeV per nucleon for an O-Se ocean, hea...

  17. Photometry and Polarization of the UXor Type Young Star GM Cep

    Science.gov (United States)

    Huang, Po-Chieh; Chen, Chang-Yao; Hu, Chia-Ling; Chen, Wen-Ping

    2015-08-01

    UX Orionis stars, or UXORs, are a sub-type of Herbig Ae/be or T Tauri stars exhibiting sporadic extinction of stellar light due to circumstellar dust obscuration. GM Cep is such a UXOR in the young (~4 Myr) open cluster Trumper 37 at ~900 pc, showing prominent infrared access, H-alpha emission, and abrupt brightness variation. Here we present intense multi-color photometric monitoring from 2009 to 2015, together with the century-long photometric behavior reported in the literature, to add to the study by Chen et al. (2012) that GM Cep showed (i) sporadic brightening on a time scale of days due to young stellar accretion, (ii) occultation events, each lasting for a couple months, with a probable recurrence time of about two years, (iii) normal dust reddening as the star became redder when dimmer, (iv) the unusual “blueing” phenomena near the brightness minima when the star appeared bluer when dimmer. The occultation events may be caused by a dust clump, signifying the density inhomogeneity in a young stellar disk from grain coagulation to planetesimal formation. We present evidence of possible radial drift of the clump toward the star, stretching longer along the orbit and thinner in the line of sight. GM Cep is moderately polarized, from 4% to 9% in g, r, and i bands, with the level of polarization anticorrelated with the brightness in the bright state, during which the dust clump is back-scattering stellar light.

  18. Impact of initial models and variable accretion rates on the pre-main-sequence evolution of massive and intermediate-mass stars and the early evolution of H II regions

    Science.gov (United States)

    Haemmerlé, Lionel; Peters, Thomas

    2016-05-01

    Massive star formation requires the accretion of gas at high rate while the star is already bright. Its actual luminosity depends sensitively on the stellar structure. We compute pre-main-sequence tracks for massive and intermediate-mass stars with variable accretion rates and study the evolution of stellar radius, effective temperature and ionizing luminosity, starting at 2 M⊙ with convective or radiative structures. The radiative case shows a much stronger swelling of the protostar for high accretion rates than the convective case. For radiative structures, the star is very sensitive to the accretion rate and reacts quickly to accretion bursts, leading to considerable changes in photospheric properties on time-scales as short as 100-1000 yr. The evolution for convective structures is much less influenced by the instantaneous accretion rate, and produces a monotonically increasing ionizing flux that can be many orders of magnitude smaller than in the radiative case. For massive stars, it results in a delay of the H II region expansion by up to 10 000 yr. In the radiative case, the H II region can potentially be engulfed by the star during the swelling, which never happens in the convective case. We conclude that the early stellar structure has a large impact on the radiative feedback during the pre-main-sequence evolution of massive protostars and introduces an important uncertainty that should be taken into account. Because of their lower effective temperatures, our convective models may hint at a solution to an observed discrepancy between the luminosity distribution functions of massive young stellar objects and compact H II regions.

  19. Differential Radial Velocities and Stellar Parameters of Nearby Young Stars

    CERN Document Server

    Yelda, D P S

    2006-01-01

    Radial velocity searches for substellar mass companions have focused primarily on stars older than 1 Gyr. Increased levels of stellar activity in young stars hinders the detection of solar system analogs and therefore there has been a prejudice against inclusion of young stars in radial velocity surveys until recently. Adaptive optics surveys of young stars have given us insight into the multiplicity of young stars but only for massive, distant companions. Understanding the limit of the radial velocity technique, restricted to high-mass, close-orbiting planets and brown dwarfs, we began a survey of young stars of various ages. While the number of stars needed to carry out full analysis of the problems of planetary and brown dwarf population and evolution is large, the beginning of such a sample is included here. We report on 61 young stars ranging in age from beta Pic association (~12 Myr) to the Ursa Majoris association (~300 Myr). This initial search resulted in no stars showing evidence for companions grea...

  20. YoungStar in Wisconsin: Analysis of Data as of July 2014. YoungStar Progress Report #5

    Science.gov (United States)

    Wisconsin Council on Children and Families, 2014

    2014-01-01

    YoungStar is a program of the Department of Children and Families (DCF) designed to improve the quality of child care for Wisconsin children. YoungStar is designed to: (1) evaluate and rate the quality of care given by child care providers; (2) help parents choose the best child care for their kids; (3) support providers with tools and training to…

  1. Topics in theoretical astrophysics: Precession of warped disks, oscillations of presupernova stars, and thermal evolution and nucleosynthesis of young neutron stars

    Science.gov (United States)

    Shirakawa, Akiko

    This thesis consists of three parts. In the first part, we study the magnetically driven precession of warped disks. An accretion disk around a rotating magnetized star is subjected to the magnetic torques that induce warping and precession of the disk. We study the global hydrodynamical warping/ precession modes of the disk under the combined influences of the magnetic torques, relativistic frame dragging, and the classical precession due to oblateness of the neutron star. We apply our analysis to two types of accreting systems: low-mass X-ray binaries (LMXBs) and accreting X-ray pulsars. We argue that some features of low-frequency quasi-periodic oscillations (QPOs) in LMXBs and milli-Hertz QPOs in accreting X-ray pulsars can be explained by the magnetically driven precession of warped disks. The second part is related to the hydrodynamically-driven mechanism for asymmetric supernova explosions/neutron star kicks. We explore the possibility that the gravity modes in the core of a presupernova star may be amplified in the silicon burning shell to produce the global asymmetric perturbations that lead to an asymmetric supernova explosion. By performing a linear analysis of the oscillations in the cores of presupernova stars, we estimate the growth/ damping rates of the modes. We find that most of the modes are damping modes with a few exceptions. We also find that, even for a growing mode, the timescale of mode growth is much longer than the remaining time before the core collapse. We conclude that the gravity modes in a presupernova core cannot provide the global asymmetric perturbations that lead to an asymmetric supernova explosion. In the last part, we attempt to predict the innate chemical composition of a neutron star atmosphere. There has been great progress in X-ray observations and now thermal radiation from neutron stars is being studied in detail. There has also been significant progress in modeling thermal spectra from neutron stars. However, the unknown

  2. YOUNG “DIPPER” STARS IN UPPER SCO AND OPH OBSERVED BY K2

    Energy Technology Data Exchange (ETDEWEB)

    Ansdell, M.; Williams, J. P. [Institute for Astronomy, University of Hawai‘i at Mānoa, Honolulu, HI (United States); Gaidos, E. [Department of Geology and Geophysics, University of Hawai‘i at Mānoa, Honolulu, HI (United States); Rappaport, S. A. [Physics Department and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA (United States); Mann, A. W. [Department of Astronomy, The University of Texas at Austin, Austin, TX (United States); Wyatt, M. C.; Kennedy, G. [Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge (United Kingdom); Boyajian, T. S. [Department of Astronomy, Yale University, New Haven, CT (United States); Jacobs, T. L.; LaCourse, D. M.; Jek, K. J.

    2016-01-10

    We present ten young (≲10 Myr) late-K and M dwarf stars observed in K2 Campaign 2 that host protoplanetary disks and exhibit quasi-periodic or aperiodic dimming events. Their optical light curves show ∼10–20 dips in flux over the 80-day observing campaign with durations of ∼0.5–2 days and depths of up to ∼40%. These stars are all members of the ρ Ophiuchus (∼1 Myr) or Upper Scorpius (∼10 Myr) star-forming regions. To investigate the nature of these “dippers” we obtained: optical and near-infrared spectra to determine stellar properties and identify accretion signatures; adaptive optics imaging to search for close companions that could cause optical variations and/or influence disk evolution; and millimeter-wavelength observations to constrain disk dust and gas masses. The spectra reveal Li i absorption and Hα emission consistent with stellar youth (<50 Myr), but also accretion rates spanning those of classical and weak-line T Tauri stars. Infrared excesses are consistent with protoplanetary disks extending to within ∼10 stellar radii in most cases; however, the sub-millimeter observations imply disk masses that are an order of magnitude below those of typical protoplanetary disks. We find a positive correlation between dip depth and WISE-2 (Wide-field Infrared Survey Explorer-2) excess, which we interpret as evidence that the dipper phenomenon is related to occulting structures in the inner disk, although this is difficult to reconcile with the weakly accreting aperiodic dippers. We consider three mechanisms to explain the dipper phenomenon: inner disk warps near the co-rotation radius related to accretion; vortices at the inner disk edge produced by the Rossby Wave Instability; and clumps of circumstellar material related to planetesimal formation.

  3. Photoionization Models for the Inner Gaseous Disks of Herbig Be Stars: Evidence against Magnetospheric Accretion?

    Science.gov (United States)

    Patel, P.; Sigut, T. A. A.; Landstreet, J. D.

    2017-02-01

    We investigate the physical properties of the inner gaseous disks of three hot Herbig B2e stars, HD 76534, HD 114981, and HD 216629, by modeling CFHT-ESPaDOns spectra using non-LTE radiative transfer codes. We assume that the emission lines are produced in a circumstellar disk heated solely by photospheric radiation from the central star in order to test whether the optical and near-infrared emission lines can be reproduced without invoking magnetospheric accretion. The inner gaseous disk density was assumed to follow a simple power-law in the equatorial plane, and we searched for models that could reproduce observed lines of H i (Hα and Hβ), He i, Ca ii, and Fe ii. For the three stars, good matches were found for all emission line profiles individually; however, no density model based on a single power-law was able to reproduce all of the observed emission lines. Among the single power-law models, the one with the gas density varying as ˜10-10(R */R)3 g cm-3 in the equatorial plane of a 25 R * (0.78 au) disk did the best overall job of representing the optical emission lines of the three stars. This model implies a mass for the Hα-emitting portion of the inner gaseous disk of ˜10-9 M *. We conclude that the optical emission line spectra of these HBe stars can be qualitatively reproduced by a ≈1 au, geometrically thin, circumstellar disk of negligible mass compared to the central star in Keplerian rotation and radiative equilibrium. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l”Univers of the Centre National de la Recherche Scientique of France, and the University of Hawaii.

  4. GRB060602B = Swift J1749.4-2807: an unusual transiently accreting neutron-star X-ray binary

    OpenAIRE

    Wijnands, R.; Rol, E.; Cackett, E.; Starling, R. L. C.; Remillard, R.A.

    2007-01-01

    We present an analysis of the Swift BAT and XRT data of GRB060602B, which is most likely an accreting neutron star in a binary system and not a gamma-ray burst. Our analysis shows that the BAT burst spectrum is consistent with a thermonuclear flash (type-I X-ray burst) from the surface of an accreting neutron star in a binary system. The X-ray binary nature is further confirmed by the report of a detection of a faint point source at the position of the XRT counterpart of the burst in archival...

  5. Self-gravitating accretion discs

    OpenAIRE

    Lodato, G.

    2008-01-01

    I review recent progresses in the dynamics and the evolution of self-gravitating accretion discs. Accretion discs are a fundamental component of several astrophysical systems on very diverse scales, and can be found around supermassive black holes in Active Galactic Nuclei (AGN), and also in our Galaxy around stellar mass compact objects and around young stars. Notwithstanding the specific differences arising from such diversity in physical extent, all these systems share a common feature whe...

  6. Numerical Solution of the Radiative Transfer Equation: X-Ray Spectral Formation from Cylindrical Accretion onto a Magnetized Neutron Star

    Science.gov (United States)

    Fairnelli, R.; Ceccobello, C.; Romano, P.; Titarchuk, L.

    2011-01-01

    Predicting the emerging X-ray spectra in several astrophysical objects is of great importance, in particular when the observational data are compared with theoretical models. This requires developing numerical routines for the solution of the radiative transfer equation according to the expected physical conditions of the systems under study. Aims. We have developed an algorithm solving the radiative transfer equation in the Fokker-Planck approximation when both thermal and bulk Comptonization take place. The algorithm is essentially a relaxation method, where stable solutions are obtained when the system has reached its steady-state equilibrium. Methods. We obtained the solution of the radiative transfer equation in the two-dimensional domain defined by the photon energy E and optical depth of the system pi using finite-differences for the partial derivatives, and imposing specific boundary conditions for the solutions. We treated the case of cylindrical accretion onto a magnetized neutron star. Results. We considered a blackbody seed spectrum of photons with exponential distribution across the accretion column and for an accretion where the velocity reaches its maximum at the stellar surface and at the top of the accretion column, respectively. In both cases higher values of the electron temperature and of the optical depth pi produce flatter and harder spectra. Other parameters contributing to the spectral formation are the steepness of the vertical velocity profile, the albedo at the star surface, and the radius of the accretion column. The latter parameter modifies the emerging spectra in a specular way for the two assumed accretion profiles. Conclusions. The algorithm has been implemented in the XPEC package for X-ray fitting and is specifically dedicated to the physical framework of accretion at the polar cap of a neutron star with a high magnetic field (approx > 10(exp 12) G). This latter case is expected to be of typical accreting systems such as X

  7. A Spectroscopic Examination of Accretion Diagnostics for Near Solar Mass Stars in IC 348

    Science.gov (United States)

    Dahm, S. E.

    2008-08-01

    High-resolution optical and moderate-resolution near-infrared spectra were obtained for 40 near solar mass (~2.0-0.5 M sun) members of the 2-3 Myr old cluster IC 348 in order to examine established accretion diagnostics and the coupling between inner disk gas and hot, micron-sized dust grains inferred from thermal and mid-infrared excesses. The stellar sample was drawn from the cluster census of Luhman in 2003 with membership being confirmed by radial velocity analysis and the presence of strong Li I λ6708 absorption. Of the stars included in this survey, 12 were classified by Lada in 2006 as hosting primordial, optically thick circumstellar disks, 5 as weak or transition disk systems, and 23 as non-excess stars using the measured slope of the stellar spectral energy distribution (SED) through the four Infrared Array Camera channels (3.6-8.0 μm) of Spitzer Space Telescope. Using the velocity width of Hα as an accretion indicator, we find that 11 primordial disk candidates are suspected accretors, suggesting a strong correlation between gaseous inner disks and optically thick dust emission. Of the five weak or transition disk systems observed, two (L21 and L67) exhibit spectroscopic features indicative of accretion. The presence of gas within the inner disk of these systems, which are free of infrared excess emission shortward of ~4.5 μm, may place constraints upon the physical mechanism responsible for inner disk clearing. Mass accretion rates (\\dot{M}) were determined for all suspected accretors using continuum excess measurements near λ6500 and established relationships between He I λ5876, Hα, Ca II λ8542, Paβ, and Brγ line fluxes and accretion luminosity. \\dot{M} values were found to range from log \\dot{M} = -8.7 to -7.2 M sun yr-1, with a median value of -8.1 M sun yr-1. Magnetospheric accretion models of Hα, Paβ, and Brγ emission by Muzerolle et al. and Kurosawa et al. are found to be in relative agreement with observed fluxes and derived \\dot

  8. LONG-ORBITAL-PERIOD PREPOLARS CONTAINING EARLY K-TYPE DONOR STARS. BOTTLENECK ACCRETION MECHANISM IN ACTION

    Energy Technology Data Exchange (ETDEWEB)

    Tovmassian, G.; González–Buitrago, D.; Zharikov, S. [Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 877, Ensenada, Baja California, 22800 México (Mexico); Reichart, D. E.; Haislip, J. B.; Ivarsen, K. M.; LaCluyze, A. P.; Moore, J. P. [Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Campus Box 3255, Chapel Hill, NC 27599 (United States); Miroshnichenko, A. S., E-mail: gag@astro.unam.mx, E-mail: dgonzalez@astro.unam.mx, E-mail: zhar@astro.unam.mx [Department of Physics and Astronomy, University of North Carolina at Greensboro, Greensboro, NC 27402-6170 (United States)

    2016-03-01

    We studied two objects identified as cataclysmic variables (CVs) with periods exceeding the natural boundary for Roche-lobe-filling zero-age main sequence (ZAMS) secondary stars. We present observational results for V1082 Sgr with a 20.82 hr orbital period, an object that shows a low luminosity state when its flux is totally dominated by a chromospherically active K star with no signs of ongoing accretion. Frequent accretion shutoffs, together with characteristics of emission lines in a high state, indicate that this binary system is probably detached, and the accretion of matter on the magnetic white dwarf takes place through stellar wind from the active donor star via coupled magnetic fields. Its observational characteristics are surprisingly similar to V479 And, a 14.5 hr binary system. They both have early K-type stars as donor stars. We argue that, similar to the shorter-period prepolars containing M dwarfs, these are detached binaries with strong magnetic components. Their magnetic fields are coupled, allowing enhanced stellar wind from the K star to be captured and channeled through the bottleneck connecting the two stars onto the white dwarf’s magnetic pole, mimicking a magnetic CV. Hence, they become interactive binaries before they reach contact. This will help to explain an unexpected lack of systems possessing white dwarfs with strong magnetic fields among detached white+red dwarf systems.

  9. On the origin of the correlations between the accretion luminosity and emission line luminosities in pre-main sequence stars

    CERN Document Server

    Mendigutía, I; Rigliaco, E; Fairlamb, J R; Calvet, N; Muzerolle, J; Cunningham, N; Lumsden, S L

    2015-01-01

    Correlations between the accretion luminosity and emission line luminosities (L_acc and L_line) of pre-main sequence (PMS) stars have been published for many different spectral lines, which are used to estimate accretion rates. Despite the origin of those correlations is unknown, this could be attributed to direct or indirect physical relations between the emission line formation and the accretion mechanism. This work shows that all (near-UV/optical/near-IR) L_acc-L_line correlations are the result of the fact that the accretion luminosity and the stellar luminosity (L_star) are correlated, and are not necessarily related with the physical origin of the line. Synthetic and observational data are used to illustrate how the L_acc-L_line correlations depend on the L_acc-L_star relationship. We conclude that because PMS stars show the L_acc-L_star correlation immediately implies that L_acc also correlates with the luminosity of all emission lines, for which the L_acc-L_line correlations alone do not prove any phy...

  10. A Direct Measurement of the Heat Release in the Outer Crust of the Transiently Accreting Neutron Star XTE J1709-267

    NARCIS (Netherlands)

    N. Degenaar; R. Wijnands; J.M. Miller

    2013-01-01

    The heating and cooling of transiently accreting neutron stars provides a powerful probe of the structure and composition of their crust. Observations of superbursts and cooling of accretion-heated neutron stars require more heat release than is accounted for in current models. Obtaining firm constr

  11. ZEIT: Searching for Young Stars in K2

    Science.gov (United States)

    Morris, Nathan; Mann, Andrew W.

    2017-01-01

    Nearby young, open clusters such as the Hyades, Pleiades, and Praesepe provide an important reference point for the properties of stellar systems in general. In each cluster, all stars are of the same known age. As such, observations of planetary systems around these stars can be used to gain insight into the early stages of planetary system formation. K2, the revived Kepler mission, has provided a vast number of light curves for young stars in the clusters and elsewhere in the K2 field. We aim to compute rotational periods from sunspot patterns for all K2 target stars and use gyrochronometric relationships derived from cluster stars to determine their ages. From there, we will search for planets around young stars outside the clusters with the ultimate goal of shedding light on how planets and planetary systems evolve with time.

  12. A sparse population of young stars in Cepheus

    Science.gov (United States)

    Klutsch, A.

    2010-12-01

    Once mixed in the ambient galactic plane stellar population, young stars are virtually indiscernible because neither their global photometric properties nor the presence of nearby gas can help to disentangle them from older ones. Nevertheless, the study of the RasTyc sample revealed 4 lithium-rich field stars displaying the same space motion, which are located within a few degrees from each other on the celestial sphere near the Cepheus-Cassiopeia complex and at a similar distance from the Sun. Both physical and kinematical indicators show that all these stars are young, with ages in the range 10-30 Ma. Multivariate analysis methods were used to select optical counterparts of XMM-Newton / ROSAT All-Sky Survey X-ray sources cross-identified with late-type stars around these 4 young stars. Recent intermediate- and high-resolution spectroscopic observations of this sample allowed to discover additional lithium-rich sources. The preliminary results show that some of them share the same space motion as the 4 original stars. They have properties rather similar to the members of the TW~Hydrae association, although they are slightly older and located in the northern hemisphere. Nearby young stars in the field are of great importance to understand the recent local history of star formation, as well as to give new insight into the process of star formation outside standard star-forming regions and to study the evolution of circumstellar discs.

  13. X-Shooter spectroscopy of young stellar objects: V - Slow winds in T Tauri stars

    CERN Document Server

    Natta, A; Alcalá, J M; Rigliaco, E; Covino, E; Stelzer, B; D'Elia, V

    2014-01-01

    Disks around T Tauri stars are known to lose mass, as best shown by the profiles of forbidden emission lines of low ionization species. At least two separate kinematic components have been identified, one characterised by velocity shifts of tens to hundreds km/s (HVC) and one with much lower velocity of few km/s (LVC). The HVC are convincingly associated to the emission of jets, but the origin of the LVC is still unknown. In this paper we analyze the forbidden line spectrum of a sample of 44 mostly low mass young stars in Lupus and $\\sigma$-Ori observed with the X-Shooter ESO spectrometer. We detect forbidden line emission of [OI], [OII], [SII], [NI], and [NII], and characterize the line profiles as LVC, blue-shifted HVC and red-shifted HVC. We focus our study on the LVC. We show that there is a good correlation between line luminosity and both L$_{star}$ and the accretion luminosity (or the mass-accretion rate) over a large interval of values (L$_{star}$ $\\sim 10^{-2} - 1$ L$_\\odot$; L$_{acc}$ $\\sim 10^{-5} ...

  14. Young "Dipper" Stars in Upper Sco and $\\rho$ Oph Observed by K2

    CERN Document Server

    Ansdell, M; Rappaport, S A; Jacobs, T L; LaCourse, D M; Jek, K J; Mann, A W; Wyatt, M C; Kennedy, G; Williams, J P; Boyajian, T S

    2015-01-01

    We present ten young ($\\lesssim$10 Myr) late-K and M dwarf stars observed in K2 Campaign 2 that host protoplanetary disks and exhibit quasi-periodic or aperiodic dimming events. Their optical light curves show $\\sim$10-20 dips in flux over the 80-day observing campaign with durations of $\\sim$0.5-2 days and depths of up to $\\sim$40%. These stars are all members of the $\\rho$ Ophiuchus ($\\sim$1 Myr) or Upper Scorpius ($\\sim$10 Myr) star-forming regions. To investigate the nature of these "dippers" we obtained: optical and near-infrared spectra to determine stellar properties and identify accretion signatures; adaptive optics imaging to search for close companions that could cause optical variations and/or influence disk evolution; and millimeter-wavelength observations to constrain disk dust and gas masses. The spectra reveal Li I absorption and H$\\alpha$ emission consistent with stellar youth (<50 Myr), but also accretion rates spanning those of classical and weak-line T Tauri stars. Infrared excesses are ...

  15. The Eating Habits of Milky Way Mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    CERN Document Server

    Deason, Alis J; Wechsler, Risa H

    2016-01-01

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW) mass M_vir ~ 10^12.1 M_sun) halos using a suite of 45 zoom-in, dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z=0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M_star ~ 10^8-10^10 M_sun. Halos with more quiescent accretion histories tend to have lower mass progenitors (10^8-10^9 M_sun), and lower overall accreted stellar masses. Ultra-faint mass (M_star 10^8 M_sun can contribute a considerable fraction (~20-60 %) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surv...

  16. Magnetic fields during the early stages of massive star formation I: Accretion and disk evolution

    CERN Document Server

    Seifried, D; Klessen, R S; Duffin, D; Pudritz, R E

    2011-01-01

    We present simulations of collapsing 100 M_\\sun mass cores in the context of massive star formation. The effect of variable initial rotational and magnetic energies on the formation of massive stars is studied in detail. We focus on accretion rates and on the question under which conditions massive Keplerian disks can form in the very early evolutionary stage of massive protostars. For this purpose, we perform 12 simulations with different initial conditions extending over a wide range in parameter space. The equations of magnetohydrodynamics (MHD) are solved under the assumption of ideal MHD. We find that the formation of Keplerian disks in the very early stages is suppressed for a mass-to-flux ratio normalised to the critical value \\mu below 10, in agreement with a series of low-mass star formation simulations. This is caused by very efficient magnetic braking resulting in a nearly instantaneous removal of angular momentum from the disk. For weak magnetic fields, corresponding to \\mu > 10, large-scale, cent...

  17. Magnetically Controlled Spasmodic Accretion During Star Formation. I. Formulation of the Problem and Method of Solution

    CERN Document Server

    Tassis, K; Tassis, Konstantinos; Mouschovias, Telemachos Ch.

    2004-01-01

    We formulate the problem of the late accretion phase of the evolution of an isothermal magnetic disk surrounding a forming star. The evolution is described by the six-fluid MHD equations, accounting for the presence of neutrals, atomic and molecular ions, electrons, and neutral, positively, and negatively charged grains. Only the electron fluid is assumed to be attached to the magnetic field, in order to investigate the effect of the detachment of the ions from the magnetic field lines that begins at densities as low as 10^8 cm^-3. The "central sink approximation" is used to circumvent the problem of describing the evolution inside the opaque central region for densities greater than 10^11 cm^-3. In this way, the structure and evolution of the isothermal disk surrounding the forming star can be studied at late times without having to implement the numerically costly radiative transfer required by the physics of the opaque core. The mass and magnetic flux accumulating in the forming star arecalculated , as are...

  18. Low-level accretion in neutron-star X-ray binaries

    CERN Document Server

    Wijnands, R; Padilla, M Armas; Altamirano, D; Cavecchi, Y; Linares, M; Bahramian, A; Heinke, C O

    2014-01-01

    We search the literature for reports on the spectral properties of neutron-star low-mass X-ray binaries when they have accretion luminosities between 1E34 and 1E36 ergs/s. We found that in this luminosity range the photon index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the spectrum softens). Such behaviour has been reported before for individual sources, but here we demonstrate that very likely most (if not all) neutron-star systems behave in a similar manner and possibly even follow a universal relation. When comparing the neutron-star systems with black-hole systems, it is clear that most black-hole binaries have significantly harder spectra at luminosities of 1E34 - 1E35 erg/s. Despite a limited number of data points, there are indications that these spectral differences also extend to the 1E35 - 1E36 erg/s range. We note, however, that the system in our sample which has the hardest spectra is in fact a neutron-s...

  19. Revisiting the pre-main-sequence evolution of stars. I. Importance of accretion efficiency and deuterium abundance

    Science.gov (United States)

    Kunitomo, Masanobu; Guillot, Tristan; Takeuchi, Taku; Ida, Shigeru

    2017-03-01

    Context. Protostars grow from the first formation of a small seed and subsequent accretion of material. Recent theoretical work has shown that the pre-main-sequence (PMS) evolution of stars is much more complex than previously envisioned. Instead of the traditional steady, one-dimensional solution, accretion may be episodic and not necessarily symmetrical, thereby affecting the energy deposited inside the star and its interior structure. Aims: Given this new framework, we want to understand what controls the evolution of accreting stars. Methods: We use the MESA stellar evolution code with various sets of conditions. In particular, we account for the (unknown) efficiency of accretion in burying gravitational energy into the protostar through a parameter, ξ, and we vary the amount of deuterium present. Results: We confirm the findings of previous works that, in terms of evolutionary tracks on an Hertzprung-Russell (H-R) diagram, the evolution changes significantly with the amount of energy that is lost during accretion. We find that deuterium burning also regulates the PMS evolution. In the low-entropy accretion scenario, the evolutionary tracks in the H-R diagram are significantly different from the classical tracks and are sensitive to the deuterium content. A comparison of theoretical evolutionary tracks and observations allows us to exclude some cold accretion models (ξ 0) with low deuterium abundances. Conclusions: We confirm that the luminosity spread seen in clusters can be explained by models with a somewhat inefficient injection of accretion heat. The resulting evolutionary tracks then become sensitive to the accretion heat efficiency, initial core entropy, and deuterium content. In this context, we predict that clusters with a higher D/H ratio should have less scatter in luminosity than clusters with a smaller D/H. Future work on this issue should include radiation-hydrodynamic simulations to determine the efficiency of accretion heating and further

  20. Stellar Masses in the Mysterious Young Triple Star System AS 205

    Science.gov (United States)

    Encalada, Frankie; Rosero, Viviana A.; Prato, Lisa A.; Bruhns, Sara

    2015-01-01

    The lack of accurate absolute mass measurements for young, low-mass pre-main sequence stars is problematic for the calibration of stellar evolutionary track models. An on-going program to increase the sample of young star masses begins with mass ratio measurements in spectroscopic binaries. By the end of its 5-year duration, the GAIA all-sky mission will provide new astrometric measurements for young spectroscopic binaries down to separations of tens of microarcseconds, yielding absolute masses for double-lined systems. We obtain mass ratios by taking high-resolution spectra of young double-lined spectroscopic binaries over a few epochs to construct a radial velocity versus phase diagram. For the young spectroscopic binary AS 205B, using eight of our own spectra supplied by the CSHELL instrument on the IRTF at Mauna Kea, plus one from the literature, we estimate a period of approximately 140 days, an eccentricity of 0.7, and a mass-ratio of 0.5. This spectroscopic system comprises the secondary in a 1.4'' visual binary in which both the A and B components are surrounded by optically thick, actively accreting disks, making AS 205B a member of that rare class of young spectroscopic binaries with a primordial circumbinary disk.

  1. Absolute parameters of young stars: QZ Carinae

    Science.gov (United States)

    Walker, W. S. G.; Blackford, M.; Butland, R.; Budding, E.

    2017-09-01

    New high-resolution spectroscopy and BVR photometry together with literature data on the complex massive quaternary star QZ Car are collected and analysed. Absolute parameters are found as follows. System A: M1 = 43 (±3), M2 = 19 (+3 -7), R1 = 28 (±2), R2 = 6 (±2), (⊙); T1 ∼ 28 000, T2 ∼ 33 000 K; System B: M1 = 30 (±3), M2 = 20 (±3), R1 = 10 (±0.5), R2 = 20 (±1), (⊙); T1 ∼ 36 000, T2 ∼ 30 000 K (model dependent temperatures). The wide system AB: Period = 49.5 (±1) yr, Epochs, conjunction = 1984.8 (±1), periastron = 2005.3 (±3) yr, mean separation = 65 (±3), (au); orbital inclination = 85 (+5 -15) deg, photometric distance ∼2700 (±300) pc, age = 4 (±1) Myr. Other new contributions concern: (a) analysis of the timing of minima differences (O - C)s for the eclipsing binary (System B); (b) the width of the eclipses, pointing to relatively large effects of radiation pressure; (c) inferences from the rotational widths of lines for both Systems A and B; and (d) implications for theoretical models of early-type stars. While feeling greater confidence on the quaternary's general parametrization, observational complications arising from strong wind interactions or other, unclear, causes still inhibit precision and call for continued multiwavelength observations. Our high-inclination value for the AB system helps to explain failures to resolve the wide binary in the previous years. The derived young age independently confirms membership of QZ Car to the open cluster Collinder 228.

  2. Long-term evolution of accretion disks around the neutron star in Be/X-ray binaries

    CERN Document Server

    Hayasaki, K; Hayasaki, Kimitake; Okazaki, Atsuo T.

    2005-01-01

    we study the long-term evolution of the accretion disk around the neutron star in Be/X-ray binaries. We confirm the earlier result by Hayasaki & Okazaki (2004) that the disk evolves via a two-stage process, which consists of the initial developing stage and the later developed stage. The peak mass-accretion rate is distributed around apastron after the disk is fully developed. This indicates that the modulation of the mass accretion rate is essentially caused by an inward propagation of the one-armed spiral wave. The X-ray luminosity peak around the apastron could provide circumstatial evidence for an persistent disk around the neutron star in Be/X-ray binaries.

  3. A Compton reflection dominated spectrum in a peculiar accreting neutron star

    CERN Document Server

    Rea, N; Israel, G L; Matt, G; Zane, S; Segreto, A; Oosterbroek, T; Orlandini, M; Rea, Nanda; Stella, Luigi; Israel, Gian Luca; Matt, Giorgio; Zane, Silvia; Segreto, Alberto; Oosterbroek, Tim

    2005-01-01

    We report on a puzzling event occurred during a long BeppoSAX observation of the slow-rotating binary pulsar GX 1+4. During this event, lasting about 1 day, the source X-ray flux was over a factor 10 lower than normal. The low-energy pulsations disappeared while at higher energies they were shifted in phase. The spectrum taken outside this low-intensity event was well fitted by an absorbed cut-off power law, and exhibited a broad iron line at ~6.5 keV probably due to the blending of the neutral (6.4 keV) and ionised (6.7 keV) K_alpha iron lines. The spectrum during the event was Compton reflection dominated and it showed two narrow iron lines at ~6.4 keV and ~7.0 keV, the latter never revealed before in this source. We also present a possible model for this event in which a variation of the accretion rate thickens a torus-like accretion disc which hides for a while the direct neutron star emission from our line of sight. In this scenario the Compton reflected emission observed during the event is well explain...

  4. Physical processes in the strong magnetic fields of accreting neutron stars

    Science.gov (United States)

    Meszaros, P.

    1984-01-01

    Analytical formulae are fitted to observational data on physical processes occurring in strong magnetic fields surrounding accreting neutron stars. The propagation of normal modes in the presence of a quantizing magnetic field is discussed in terms of a wave equation in Fourier space, quantum electrodynamic effects, polarization and mode ellipticity. The results are applied to calculating the Thomson scattering, bremsstrahlung and Compton scattering cross-sections, which are a function of the frequency, angle and polarization of the magnetic field. Numerical procedures are explored for solving the radiative transfer equations. When applied to modeling X ray pulsars, a problem arises in the necessity to couple the magnetic angle and frequency dependence of the cross-sections with the hydrodynamic equations. The use of time-dependent averaging and approximation techniques is indicated.

  5. SKA studies of nearby galaxies: star-formation, accretion processes and molecular gas across all environments

    CERN Document Server

    Beswick, R J; Perez-Torres, M A; Richards, A M S; Aalto, S; Alberdi, A; Argo, M K; van Bemmel, I; Conway, J E; Dickinson, C; Fenech, D M; Gray, M D; Klockner, H-R; Murphy, E J; Muxlow, T W B; Peel, M; Rushton, A P; Schinnerer, E

    2014-01-01

    The SKA will be a transformational instrument in the study of our local Universe. In particular, by virtue of its high sensitivity (both to point sources and diffuse low surface brightness emission), angular resolution and the frequency ranges covered, the SKA will undertake a very wide range of astrophysical research in the field of nearby galaxies. By surveying vast numbers of nearby galaxies of all types with $\\mu$Jy sensitivity and sub-arcsecond angular resolutions at radio wavelengths, the SKA will provide the cornerstone of our understanding of star-formation and accretion activity in the local Universe. In this chapter we outline the key continuum and molecular line science areas where the SKA, both during phase-1 and when it becomes the full SKA, will have a significant scientific impact.

  6. X-shooter spectroscopy of young stellar objects in Lupus. Accretion properties of class II and transitional objects

    Science.gov (United States)

    Alcalá, J. M.; Manara, C. F.; Natta, A.; Frasca, A.; Testi, L.; Nisini, B.; Stelzer, B.; Williams, J. P.; Antoniucci, S.; Biazzo, K.; Covino, E.; Esposito, M.; Getman, F.; Rigliaco, E.

    2017-03-01

    The mass accretion rate, Ṁacc, is a key quantity for the understanding of the physical processes governing the evolution of accretion discs around young low-mass (M⋆ ≲ 2.0 M⊙) stars and substellar objects (YSOs). We present here the results of a study of the stellar and accretion properties of the (almost) complete sample of class II and transitional YSOs in the Lupus I, II, III and IV clouds, based on spectroscopic data acquired with the VLT/X-shooter spectrograph. Our study combines the dataset from our previous work with new observations of 55 additional objects. We have investigated 92 YSO candidates in total, 11 of which have been definitely identified with giant stars unrelated to Lupus. The stellar and accretion properties of the 81 bona fide YSOs, which represent more than 90% of the whole class II and transition disc YSO population in the aforementioned Lupus clouds, have been homogeneously and self-consistently derived, allowing for an unbiased study of accretion and its relationship with stellar parameters. The accretion luminosity, Lacc, increases with the stellar luminosity, L⋆, with an overall slope of 1.6, similar but with a smaller scatter than in previous studies. There is a significant lack of strong accretors below L⋆ ≈ 0.1 L⊙, where Lacc is always lower than 0.01 L⋆. We argue that the Lacc - L⋆ slope is not due to observational biases, but is a true property of the Lupus YSOs. The log Ṁacc - log M⋆ correlation shows a statistically significant evidence of a break, with a steeper relation for M⋆ ≲ 0.2 M⊙ and a flatter slope for higher masses. The bimodality of the Ṁacc - M⋆ relation is confirmed with four different evolutionary models used to derive the stellar mass. The bimodal behaviour of the observed relationship supports the importance of modelling self-gravity in the early evolution of the more massive discs, but other processes, such as photo-evaporation and planet formation during the YSO's lifetime, may

  7. Hypervelocity stars from young stellar clusters in the Galactic Centre

    CERN Document Server

    Fragione, Giacomo; Kroupa, Pavel

    2016-01-01

    The enormous velocities of the so called hypervelocity stars (HVSs) derive, likely, from close interactions with massive black holes, binary stars encounters or supernova explosions. In this paper, we investigate the origin of hypervelocity stars as consequence of the close interaction between the Milky Way central massive black hole and a passing-by young stellar cluster. We found that both single and binary HVSs may be generated in a burst-like event, as the cluster passes near the orbital pericentre. High velocity stars will move close to the initial cluster orbital plane and in the direction of the cluster orbital motion at the pericentre. The binary fraction of these HVS jets depends on the primordial binary fraction in the young cluster. The level of initial mass segregation determines the value of the average mass of the ejected stars. Some binary stars will merge, continuing their travel across and out of the Galaxy as blue stragglers.

  8. Long orbital period pre-polars containing an early K-type donor stars. Bottleneck accretion mechanism in action

    CERN Document Server

    Tovmassian, G; Zharikov, S; Reichart, D E; Haislip, J B; Ivarsen, K M; LaCluyze, A P; Moore, J P; Miroshnichenko, A S

    2016-01-01

    We studied two objects identified as a Cataclysmic Variables (CVs) with periods exceeding the natural boundary for Roche lobe filling ZAMS secondary stars. We present observational results for V1082 Sgr with 20.82 h orbital period, an object that shows low luminosity state, when its flux is totally dominated by a chromospherically active K- star with no signs of ongoing accretion. Frequent accretion shut-offs, together with characteristics of emission lines in a high state, indicate that this binary system is probably detached and the accretion of matter on the magnetic white dwarf takes place through stellar wind from the active donor star via coupled magnetic fields. Its observational characteristics are surprisingly similar to V479 And, a 14.5 h binary system. They both have early K-type stars as a donor star. We argue, that similar to the shorter period pre-polars containing M-dwarfs, these are detached binaries with strong magnetic components. Their magnetic fields are coupled, allowing enhanced stellar ...

  9. THE EATING HABITS OF MILKY WAY-MASS HALOS: DESTROYED DWARF SATELLITES AND THE METALLICITY DISTRIBUTION OF ACCRETED STARS

    Energy Technology Data Exchange (ETDEWEB)

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H., E-mail: adeason@stanford.edu [Kavli Institute for Particle Astrophysics and Cosmology and Physics Department, Stanford University, Stanford, CA 94305 (United States)

    2016-04-10

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M{sub vir} ∼ 10{sup 12.1} M{sub ⊙}) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M{sub star} ∼ 10{sup 8}–10{sup 10}M{sub ⊙}. Halos with more quiescent accretion histories tend to have lower mass progenitors (10{sup 8}–10{sup 9} M{sub ⊙}), and lower overall accreted stellar masses. Ultra-faint mass (M{sub star} < 10{sup 5} M{sub ⊙}) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (∼2%–5%) of the very metal-poor stars with [Fe/H] < −2. Dwarfs with masses 10{sup 5} < M{sub star}/M{sub ⊙} < 10{sup 8} provide a substantial amount of the very metal-poor stellar material (∼40%–80%), and even relatively metal-rich dwarfs with M{sub star} > 10{sup 8} M{sub ⊙} can contribute a considerable fraction (∼20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil”; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

  10. Rotational periods of solar-mass young stars in Orion

    NARCIS (Netherlands)

    Marilli, E.; Frasca, A.; Covino, E.; Alcalá, J.M.; Catalano, S.; Fernández, M.; Arellano Ferro, A.; Rubio Herrera, E.A.; Spezzi, L.

    2007-01-01

    Context: The evolution of the angular momentum in young low-mass stars is still a debated issue. The stars presented here were discovered as X-ray sources in the ROSAT All-Sky Survey (RASS) of the Orion complex and subsequently optically identified thanks to both low and high resolution spectroscopy

  11. Full 3-D MHD calculations of accretion flow Structure in magnetic cataclysmic variable stars with strong and complex magnetic fields

    CERN Document Server

    Zhilkin, A G; Mason, P A; 10.1134/S1063772912040087

    2012-01-01

    We performed 3D MHD calculations of stream accretion in cataclysmic variable stars for which the white dwarf primary star possesses a strong and complex magnetic field. These calculations are motivated by observations of polars; cataclysmic variables containing white dwarfs with magnetic fields sufficiently strong to prevent the formation of an accretion disk. So an accretion stream flows from the L1 point and impacts directly onto one or more spots on the surface of the white dwarf. Observations indicate that the white dwarf, in some binaries, possesses a complex (non-dipolar) magnetic field. We perform simulations of 10 polars or equivalently one asynchronous polar at 10 different beat phases. Our models have an aligned dipole plus quadrupole magnetic field centered on the white dwarf primary. We find that for a sufficiently strong quadrupole component an accretion spot occurs near the magnetic equator for slightly less than half of our simulations while a polar accretion zone is active for most of the rest...

  12. Nuclear star formation activity and black hole accretion in nearby Seyfert galaxies

    CERN Document Server

    Esquej, P; González-Martín, O; Hönig, S F; Caballero, A Hernán; Roche, P F; Almeida, C Ramos; Mason, R E; Díaz-Santos, T; Levenson, N A; Aretxaga, I; Espinosa, J M Rodríguez; Packham, C

    2013-01-01

    Recent theoretical and observational works indicate the presence of a correlation between the star formation rate (SFR) and the active galactic nuclei (AGN) luminosity (and, therefore, the black hole accretion rate) of Seyfert galaxies. This suggests a physical connection between the gas forming stars on kpc scales and the gas on sub-pc scales that is feeding the black hole. We compiled the largest sample of Seyfert galaxies to date with high angular resolution (0.4-0.8 arcsec) mid-infrared (8-13 micron) spectroscopy. The sample includes 29 Seyfert galaxies drawn from the AGN Revised Shapley-Ames catalogue. At a median distance of 33 Mpc, our data allow us to probe nuclear regions on scales of 65 pc (median value). We found no general evidence of suppression of the 11.3 micron polycyclic aromatic hydrocarbon (PAH) emission in the vicinity of these AGN, and used this feature as a proxy for the SFR. We detected the 11.3 micron PAH feature in the nuclear spectra of 45% of our sample. The derived nuclear SFRs are...

  13. The Formation of Population III Stars in Gas Accretion Stage: Effects of Magnetic Fields

    CERN Document Server

    Machida, Masahiro N

    2013-01-01

    The formation of Population III stars is investigated using resistive magnetohydrodynamic simulations. Starting from a magnetized primordial prestellar cloud, we calculate the cloud evolution several hundreds of years after first protostar formation, resolving the protostellar radius. When the natal minihalo field strength is weaker than B \\lesssim 10^-13 (n/1 cm^-3)^-2/3 G (n is the hydrogen number density), magnetic effects can be ignored. In this case, fragmentation occurs frequently and a stellar cluster forms, in which stellar mergers and mass exchange between protostars contribute to the mass growth of these protostars. During the early gas accretion phase, the most massive protostar remains near the cloud centre, whereas some of the less massive protostars are ejected. The magnetic field significantly affects Population III star formation when B_amb \\gtrsim 10^-12 (n/1 cm^-3)^-2/3 G. In this case, because the angular momentum around the protostar is effectively transferred by both magnetic braking and ...

  14. The Star Formation and Nuclear Accretion Histories of Normal Galaxies in the AGES Survey

    CERN Document Server

    Watson, Casey R; Forman, William R; Hickox, Ryan C; Jones, Christine J; Brown, Michael J I; Brand, Kate; Dey, Arjun; Jannuzi, Buell T; Kenter, Almus T; Murray, Steve S; Vikhlinin, Alexey; Eisenstein, Daniel J; Fazio, Giovani G; Green, Paul J; McNamara, Brian R; Rieke, Marcia; Shields, Joseph C

    2009-01-01

    We combine IR, optical and X-ray data from the overlapping, 9.3 square degree NOAO Deep Wide-Field Survey (NDWFS), AGN and Galaxy Evolution Survey (AGES), and XBootes Survey to measure the X-ray evolution of 6146 normal galaxies as a function of absolute optical luminosity, redshift, and spectral type over the largely unexplored redshift range 0.1 < z < 0.5. Because only the closest or brightest of the galaxies are individually detected in X-rays, we use a stacking analysis to determine the mean properties of the sample. Our results suggest that X-ray emission from spectroscopically late-type galaxies is dominated by star formation, while that from early-type galaxies is dominated by a combination of hot gas and AGN emission. We find that the mean star formation and supermassive black hole accretion rate densities evolve like (1+z)^3, in agreement with the trends found for samples of bright, individually detectable starburst galaxies and AGN. Our work also corroborates the results of many previous stack...

  15. Reflection spectra from an accretion disc illuminated by a neutron star X-ray burst

    CERN Document Server

    Ballantyne, D R

    2004-01-01

    Recent time-resolved X-ray spectra of a neutron star undergoing a superburst revealed an Fe K line and edge consistent with reprocessing from the surrounding accretion disc. Here, we present models of X-ray reflection from a constant density slab illuminated by a blackbody, the spectrum emitted by a neutron star burst. The calculations predict a prominent Fe K line and a rich soft X-ray line spectrum which is superimposed on a strong free-free continuum. The lines slowly vanish as the ionization parameter of the slab is increased, but the free-free continuum remains dominant at energiesless than 1 keV. The reflection spectrum has a quasi-blackbody shape only at energies greater than 3 keV. If the incident blackbody is added to the reflection spectrum, the Fe K equivalent width varies between 100 and 300 eV depending on the ionization parameter and the temperature, kT, of the blackbody. The equivalent width is correlated with kT, and therefore we predict a strong Fe K line when an X-ray burst is at its brighte...

  16. N-body simulations of planetary accretion around M dwarf stars

    CERN Document Server

    Ogihara, Masahiro

    2009-01-01

    We have investigated planetary accretion from planetesimals in terrestrial planet regions inside the ice line around M dwarf stars through N-body simulations including tidal interactions with disk gas. Because of low luminosity of M dwarfs, habitable zones (HZs) are located in inner regions. In the close-in HZ, type-I migration and the orbital decay induced by eccentricity damping are efficient according to the high disk gas density in the small orbital radii. In the case of full efficiency of type-I migration predicted by the linear theory, we found that protoplanets that migrate to the vicinity of the host star undergo close scatterings and collisions, and 4 to 6 planets eventually remain in mutual mean motion resonances and their orbits have small eccentricities and they are stable both before and after disk gas decays. In the case of slow migration, the resonant capture is so efficient that densely-packed ~ 40 small protoplanets remain in mutual mean motion resonances. In this case, they start orbit cross...

  17. The g-mode Excitation in the Proto Neutron Star by the Standing Accretion Shock Instability

    CERN Document Server

    Yoshida, S; Yamada, S; Yoshida, Shijun; Ohnishi, Naofumi; Yamada, Shoichi

    2007-01-01

    The so-called "acoustic revival mechanism" of core-collapse supernova proposed recently by the Arizona group is an interesting new possibility. Aiming to understand the elementary processes involved in the mechanism, we have calculated the eigen frequencies and eigen functions for the g-mode oscillations of a non-rotating proto neutron star. The possible excitation of these modes by the standing accretion shock instability, or SASI, is discussed based on these eigen functions. We have formulated the forced oscillations of $g$-modes by the external pressure perturbations exerted on the proto neutron star surface. The driving pressure fluctuations have been adopted from our previous computations of the axisymmetric SASI in the non-linear regime. We have paid particular attention to low l modes, since these are the modes that are dominant in SASI and that the Arizona group claimed played an important role in their acoustic revival scenario. Here l is the index of the spherical harmonic functions, $Y_l^m$. Althou...

  18. Spread of Matter over a Neutron-Star Surface During Disk Accretion: Deceleration of Rapid Rotation

    CERN Document Server

    Sunyaev, R A

    2011-01-01

    The problem of disk accretion onto the surface of a neutron star with a weak magnetic field at a luminosity exceeding several percent of Eddington is reduced to the problem of the braking of a hypersonic flow with a velocity that is 0.4-0.5 of the speed of light above the base of the spreading layer -- a dense atmosphere made up of previously fallen matter. We show that turbulent braking in the Prandtl-Karman model with universally accepted coefficients for terrestrial conditions and laboratory experiments and a ladder of interacting gravity waves in a stratified quasi-exponential atmosphere at standard Richardson numbers lead to a spin-up of the massive zone that extends to the ocean made up of a plasma with degenerate electrons. Turbulent braking in the ocean at the boundary with the outer solid crust reduces the rotation velocity to the solid-body rotation velocity of the star. This situation should lead to strong heating of deep atmospheric layers and to the switch-off of the explosive helium burning mech...

  19. Progressive star formation in the young galactic super star cluster NGC 3603

    CERN Document Server

    Beccari, Giacomo; De Marchi, Guido; Paresce, Francesco; Young, Erick; Andersen, Morten; Panagia, Nino; Balick, Bruce; Bond, Howard; Calzetti, Daniela; Carollo, C Marcella; Disney, Michael J; Dopita, Michael A; Frogel, Jay A; Hall, Donald N B; Holtzman, Jon A; Kimble, Randy A; McCarthy, Patrick J; O'Connell, Robert W; Saha, Abhijit; Silk, Joseph I; Trauger, John T; Walker, Alistair R; Whitmore, Bradley C; Windhorst, Rogier A

    2010-01-01

    Early release science observations of the cluster NGC3603 with the WFC3 on the refurbished HST allow us to study its recent star formation history. Our analysis focuses on stars with Halpha excess emission, a robust indicator of their pre-main sequence (PMS) accreting status. The comparison with theoretical PMS isochrones shows that 2/3 of the objects with Halpha excess emission have ages from 1 to 10 Myr, with a median value of 3 Myr, while a surprising 1/3 of them are older than 10 Myr. The study of the spatial distribution of these PMS stars allows us to confirm their cluster membership and to statistically separate them from field stars. This result establishes unambiguously for the first time that star formation in and around the cluster has been ongoing for at least 10-20 Myr, at an apparently increasing rate.

  20. A spectroscopic survey of Herbig Ae/Be stars with X-Shooter II: Accretion diagnostic lines

    CERN Document Server

    Fairlamb, John R; Mendigutia, Ignacio; Ilee, John D; Ancker, Mario E van den

    2016-01-01

    The Herbig Ae/Be stars (HAeBes) allow an exploration of the properties of Pre-Main Sequence(PMS) stars above the low-mass range ($8{\\rm\\thinspace M_{\\odot}}$). This paper is the second in a series exploring accretion in 91 HAeBes with Very Large Telescope/X-shooter spectra. Equivalent width measurements are carried out on 32 different lines, spanning the UV to NIR, in order to obtain their line luminosities. The line luminosities were compared to accretion luminosities, which were determined directly from measurements of an UV-excess. When detected, emission lines always demonstrate a correlation with the accretion luminosity, regardless of detection frequency. The average relationship between accretion luminosity and line luminosity is found to be ${\\thinspace L_{\\rm acc}}\\propto{\\thinspace L_{\\rm line}}^{1.16 \\pm 0.15}$. This is in agreement with the findings in Classical T Tauri stars, although the HAeBe relationship is generally steeper, particularly towards the Herbig Be mass range. Since all observed li...

  1. Formation of an O-Star Cluster by Hierarchical Accretion in G20.08-0.14 N

    CERN Document Server

    Galván-Madrid, Roberto; Zhang, Qizhou; Kurtz, Stan; Rodríguez, Luis F; Ho, Paul T P

    2009-01-01

    Spectral line and continuum observations of the ionized and molecular gas in G20.08-0.14 N explore the dynamics of accretion over a range of spatial scales in this massive star forming region. Very Large Array observations of NH_3 at 4'' angular resolution show a large scale (0.5 pc) molecular accretion flow around and into a star cluster with three small, bright HII regions. Higher resolution (0.4'') observations with the Submillimeter Array in hot core molecules (CH_3CN, OCS, and SO_2) and the VLA in NH_3, show that the two brightest and smallest HII regions are themselves surrounded by smaller scale (0.05 pc) accretion flows. The axes of rotation of the large and small scale flows are aligned, and the time scale for the contraction of the cloud is short enough, 0.1 Myr, for the large scale accretion flow to deliver significant mass to the smaller scales within the star formation time scale. The flow structure appears to be continuous and hierarchical from larger to smaller scales. Millimeter radio recombin...

  2. Role of local absorption on the X-ray emission from MHD accretion shocks in classical T Tauri stars

    Directory of Open Access Journals (Sweden)

    Bonito

    2014-01-01

    Full Text Available Accretion processes onto classical T Tauri stars (CTTSs are believed to generate shocks at the stellar surface due to the impact of supersonic downflowing plasma. Although current models of accretion streams provide a plausible global picture of this process, several aspects are still unclear. For example, the observed X-ray luminosity in accretion shocks is, in general, well below the predicted value. A possible explanation discussed in the literature is in terms of significant absorption of the emission due to the thick surrounding medium. Here we consider a 2D MHD model describing an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere. The model includes all the relevant physics, namely the gravity, the thermal conduction, and the radiative cooling, and a realistic description of the unperturbed stellar atmosphere (from the chromosphere to the corona. From the model results, we synthesize the X-ray emission emerging from the hot slab produced by the accretion shock, exploring different configurations and strengths of the stellar magnetic field. The synthesis includes the local absorption by the thick surrounding medium and the Doppler shift of lines due to the component of plasma velocity along the line-of-sight. We explore the effects of absorption on the emerging X-ray spectrum, considering different inclinations of the accretion stream with respect to the observer. Finally we compare our results with the observations.

  3. The Eating Habits of Milky Way-mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    Science.gov (United States)

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-04-01

    We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (Mvir ˜ 1012.1 M⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with Mstar ˜ 108-1010M⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (108-109 M⊙), and lower overall accreted stellar masses. Ultra-faint mass (Mstar 108 M⊙ can contribute a considerable fraction (˜20%-60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil” a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

  4. Magnetically Controlled Spasmodic Accretion during Star Formation. I. Formulation of the Problem and Method of Solution

    Science.gov (United States)

    Tassis, Konstantinos; Mouschovias, Telemachos Ch.

    2005-01-01

    We formulate the problem of the late accretion phase of the evolution of an isothermal magnetic disk surrounding a forming star. The evolution is described by the six-fluid MHD equations, accounting for the presence of neutrals, atomic and molecular ions, electrons, and neutral, positively, and negatively charged grains. Only the electron fluid is assumed to be attached to the magnetic field, in order to investigate the effect of the detachment of the ions from the magnetic field lines that begins at densities as low as 108 cm-3. The ``central sink approximation'' is used to circumvent the problem of describing the evolution inside the opaque central region for densities greater than 1011 cm-3. In this way, the structure and evolution of the isothermal disk surrounding the forming star can be studied at late times without having to implement the numerically costly radiative transfer required by the physics of the opaque core. The mass and magnetic flux accumulating in the forming star are calculated, as are their effects on the structure & evolution of the surrounding disk. The numerical method of solution first uses an adaptive grid and later, after a central region a few AU in radius becomes opaque, switches to a stationary but nonuniform grid with a central sink cell. It also involves an implicit time integrator, an advective difference scheme that possesses the transportive property, a second-order difference approximation of forces inside a cell, an integral approximation of the gravitational and magnetic fields, and tensor artificial viscosity that permits an accurate investigation of the formation and evolution of shocks in the neutral fluid.

  5. STAR FORMATION IN DENSE CLUSTERS

    Energy Technology Data Exchange (ETDEWEB)

    Myers, Philip C., E-mail: pmyers@cfa.harvard.edu [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

    2011-12-10

    A model of core-clump accretion with equally likely stopping describes star formation in the dense parts of clusters, where models of isolated collapsing cores may not apply. Each core accretes at a constant rate onto its protostar, while the surrounding clump gas accretes as a power of protostar mass. Short accretion flows resemble Shu accretion and make low-mass stars. Long flows resemble reduced Bondi accretion and make massive stars. Accretion stops due to environmental processes of dynamical ejection, gravitational competition, and gas dispersal by stellar feedback, independent of initial core structure. The model matches the field star initial mass function (IMF) from 0.01 to more than 10 solar masses. The core accretion rate and the mean accretion duration set the peak of the IMF, independent of the local Jeans mass. Massive protostars require the longest accretion durations, up to 0.5 Myr. The maximum protostar luminosity in a cluster indicates the mass and age of its oldest protostar. The distribution of protostar luminosities matches those in active star-forming regions if protostars have a constant birthrate but not if their births are coeval. For constant birthrate, the ratio of young stellar objects to protostars indicates the star-forming age of a cluster, typically {approx}1 Myr. The protostar accretion luminosity is typically less than its steady spherical value by a factor of {approx}2, consistent with models of episodic disk accretion.

  6. Variability in young very low mass stars: two surprises from spectrophotometric monitoring

    Science.gov (United States)

    Bozhinova, I.; Scholz, A.; Eislöffel, J.

    2016-05-01

    We present simultaneous photometric and spectroscopic observations of seven young and highly variable M dwarfs in star-forming regions in Orion, conducted in four observing nights with FOcal Reducer and low dispersion Spectrograph2 at European Southern Observatory/VLT. All seven targets show significant photometric variability in the I band, with amplitudes between 0.1-0.8 mag, The spectra, however, remain remarkably constant, with spectral type changes less than 0.5 subtypes. Thus, the brightness changes are not caused by veiling that `fills in' absorption features. Three objects in the σ Ori cluster (age ˜3 Myr) exhibit strong Hα emission and Hα variability, in addition to the continuum variations. Their behaviour is mostly consistent with the presence of spots with temperature of ˜300 K above the photosphere and filling factors between 0.2-0.4, in contrast to typical hotspots observed in more massive stars. The remaining targets near ɛ Ori, likely to be older, show eclipse-like light curves, no significant Hα activity and are better represented by variable extinction due to circumstellar material. Interestingly, two of them show no evidence of infrared excess emission. Our study shows that high-amplitude variability in young very low mass stars can be caused by different phenomena than in more massive T Tauri stars and can persist when the disc has disappeared and accretion has ceased.

  7. Photo-reverberation Mapping of a Protoplanetary Accretion Disk around a T Tauri Star

    CERN Document Server

    Meng, Huan Y A; Rieke, George H; Cody, Ann Marie; Güth, Tina; Stauffer, John; Covey, Kevin; Carey, Sean; Ciardi, David; Duran-Rojas, Maria C; Gutermuth, Robert A; Morales-Calderón, María; Rebull, Luisa M; Watson, Alan M

    2016-01-01

    Theoretical models and spectroscopic observations of newborn stars suggest that protoplantary disks have an inner "wall" at a distance set by the disk interaction with the star. Around T Tauri stars, the size of this disk hole is expected to be on a 0.1-AU scale that is unresolved by current adaptive optics imaging, though some model-dependent constraints have been obtained by near-infrared interferometry. Here we report the first measurement of the inner disk wall around a solar-mass young stellar object, YLW 16B in the {\\rho} Ophiuchi star-forming region, by detecting the light travel time of the variable radiation from the stellar surface to the disk. Consistent time lags were detected on two nights, when the time series in H (1.6 {\\mu}m) and K (2.2 {\\mu}m) bands were synchronized while the 4.5 {\\mu}m emission lagged by 74.5 +/- 3.2 seconds. Considering the nearly edge-on geometry of the disk, the inner rim should be 0.084 AU from the protostar on average, with an error of order 0.01 AU. This size is likel...

  8. On the new braking index of PSR B0540-69: further support for magnetic field growth of neutron stars following submergence by fallback accretion

    CERN Document Server

    Ekşi, K Yavuz

    2016-01-01

    The magnetic fields of the nascent neutron stars could be submerged to the crust by rapid fallback accretion and could diffuse to the surface later in life. According to this field burial scenario young pulsars may have growing magnetic fields which is known to result in less-than-three braking indices; larger braking indices implying longer field-growth time-scales. A nascent neutron star with a larger kick velocity would accrete less amount of matter and would have a rapidly growing magnetic field, leading to a larger discrepancy with the braking index expected from a constant field. Such an inverse relation between the field growth time-scale inferred from the braking indices and space velocity of pulsars was claimed in the past as a prediction of the field-burial scenario. With a braking index of $n\\sim 2$ and large space velocity PSR B0540-69 was then an outlier in the claimed relation. The recently measured small braking index of the object as $n \\sim 0.03$ implies a much shorter time-scale for the fiel...

  9. Disk wind and magnetospheric accretion in emission from the Herbig Ae star MWC 480

    Science.gov (United States)

    Tambovtseva, L. V.; Grinin, V. P.; Potravnov, I. S.; Mkrtichian, D. E.

    2016-09-01

    The young Herbig Ae star MWC 480 (HD 31648) is one of the comprehensively spectroscopically studied stars in the ultraviolet, optical, and infrared spectral ranges. Using non-LTE modeling of its hydrogen spectrum, we have calculated the contribution to the hydrogen emission from such important regions of the circumstellar environment as the disk wind and the magnetosphere. We have used our own observations of the stellar spectrum performed with the 2.4-m telescope at the Thai National Observatory to quantitatively check our theoretical calculations. In addition, all of the visible and infrared spectra available in the literature have been used for a qualitative comparison. The modeling results have revealed a significant role of the magneto-centrifugal disk wind in the formation of atomic hydrogen emission. The cause of the emission line variability in the spectrum ofMWC 480 is discussed.

  10. Angular Momentum in Disk Wind Revealed in the Young Star MWC 349A

    Science.gov (United States)

    Zhang, Qizhou; Claus, Brian; Watson, Linda; Moran, James

    2017-03-01

    Disk winds are thought to play a critical role in star birth. As winds extract excess angular momentum from accretion disks, matter in the disk can be transported inward to the star to fuel mass growth. However, observational evidence of wind carrying angular momentum has been very limited. We present Submillimeter Array (SMA) observations of the young star MWC 349A in the H26α and H30α recombination lines. The high signal-to-noise ratios made possible by the maser emission process allow us to constrain the relative astrometry of the maser spots to milli-arcsecond precision. Previous observations of the H30α line with the SMA and the Plateau de Bure interferometer (PdBI) showed that masers are distributed in the disk and wind. Our new high-resolution observations of the H26α line reveal differences in spatial distribution from that of the H30α line. H26α line masers in the disk are excited in a thin annulus with a radius of about 25 au, while the H30α line masers are formed in a slightly larger annulus with a radius of 30 au. This is consistent with expectations for maser excitation in the presence of an electron density variation of approximately R ‑4. In addition, the H30α and H26α line masers arise from different parts in the wind. This difference is also expected from maser theory. The wind component of both masers exhibits line-of-sight velocities that closely follow a Keplerian law. This result provides strong evidence that the disk wind extracts significant angular momentum, thereby facilitating mass accretion in the young star.

  11. The origin of S-stars and a young stellar disk: distribution of debris stars of a sinking star cluster

    CERN Document Server

    Fujii, Michiko; Funato, Yoko; Makino, Junichiro

    2010-01-01

    Within the distance of 1 pc from the Galactic center (GC), more than 100 young massive stars have been found. The massive stars at 0.1--1 pc from the GC are located in one or two disks, while those within 0.1 pc from the GC, S-stars, have an isotropic distribution. How these stars are formed is not well understood, especially for S-stars. Here we propose that a young star cluster with an intermediate-mass black hole (IMBH) can form both the disks and S-stars. We performed a fully self-consistent $N$-body simulation of a star cluster near the GC. Stars escaped from the tidally disrupted star cluster were carried to the GC due to an 1:1 mean motion resonance with the IMBH formed in the cluster. In the final phase of the evolution, the eccentricity of the IMBH becomes very high. In this phase, stars carried by the 1:1 resonance with the IMBH were dropped from the resonance and their orbits are randomized by a chaotic Kozai mechanism. The mass function of these carried stars is extremely top-heavy within 10''. Th...

  12. Hydrodynamic modelling of accretion impacts in classical T Tauri stars: radiative heating of the pre-shock plasma

    Science.gov (United States)

    Costa, G.; Orlando, S.; Peres, G.; Argiroffi, C.; Bonito, R.

    2017-01-01

    Context. It is generally accepted that, in classical T Tauri stars, the plasma from the circumstellar disc accretes onto the stellar surface with free-fall velocity and the impact generates a shock. The impact region is expected to contribute to emission in different spectral bands; many studies have confirmed that the X-rays arise from the post-shock plasma but, otherwise, there are no studies in the literature investigating the origin of the observed UV emission which is apparently correlated to accretion. Aims: We investigated the effect of radiative heating of the infalling material by the post-shock plasma at the base of the accretion stream, with the aim to identify in which region a significant part of the UV emission originates. Methods: We developed a one-dimensional hydrodynamic model describing the impact of an accretion stream onto the stellar surface; the model takes into account the gravity, the radiative cooling of an optically thin plasma, the thermal conduction, and the heating due to absorption of X-ray radiation. The latter term represents the heating of the infalling plasma due to the absorption of X-rays emitted from the post-shock region. Results: We found that the radiative heating of the pre-shock plasma plays a non-negligible role in the accretion phenomenon. In particular, the dense and cold plasma of the pre-shock accretion column is gradually heated up to a few 105K due to irradiation of X-rays arising from the shocked plasma at the impact region. This heating mechanism does not affect significantly the dynamics of the post-shock plasma. On the other hand, a region of radiatively heated gas (that we consider a precursor) forms in the unshocked accretion column and contributes significantly to UV emission. Our model naturally reproduces the luminosity of UV emission lines correlated to accretion and shows that most of the UV emission originates from the precursor.

  13. Accretion/jet activity and narrow [O III] kinematics in young radio galaxies

    Institute of Scientific and Technical Information of China (English)

    Andrew; HUMPHREY

    2010-01-01

    We estimate black hole masses and Edenton ratios for a sample of 81 young radio galaxies,which includes 42 compact steep-spectrum(CSS) and 39 gigahertz-peaked spectrum(GPS) sources.We find that the average black hole(BH) mass of these young radio galaxies is〈log Mbh〉-8.3,which is less than that of radio loud QSOs and low redshift radio galaxies(〈 log Mbh〉-9.0).The CSS/GPS sources have relatively high Eddington ratios,with an average value of〈log Lbol/LEdd〉=-0.75,which are similar to those of narrow line Seyfert 1 galaxies(NLS1s).This suggests that young radio galaxies may not only be in the early stages of their radio activity,but also in the early stage of their accretion activity.We find that the young radio galaxies,as a class,systematically deviate from the Mbh-σ relation defined by nearby inactive galaxies,when using σ[O III] as a surrogate for stellar velocity dispersion σ.We also find that the deviation of the [O III] line width,Δσ =σ[O III]-σ[pred],is correlated with the Eddington ratio;sources with Lbol/LEdd-1 have the largest deviations,which are similar to those of radio quiet QSOs/NLS1s(i.e.,sources in which the radio jets are absent or weak),and where σ[pred] is calculated from the Tremaine et al.relation using our estimated BH masses.A similar result has been obtained for 9 linear radio Seyfert galaxies.On the basis of these results,we suggest that,in addition to the possible jet-gas interactions,accretion activities may also play an important role in shaping the kinematics of the narrow [O III] line in young radio galaxies.

  14. NuSTAR discovery of a cyclotron line in the accreting X-ray pulsar IGR J16393-4643

    CERN Document Server

    Bodaghee, Arash; Fornasini, Francesca A; Krivonos, Roman; Stern, Daniel; Mori, Kaya; Rahoui, Farid; Boggs, Steven E; Christensen, Finn E; Craig, William W; Hailey, Charles J; Harrison, Fiona A; Zhang, William W

    2016-01-01

    The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was observed by NuSTAR in the 3-79 keV energy band for a net exposure time of 50 ks. We present the results of this observation which enabled the discovery of a cyclotron resonant scattering feature with a centroid energy of 29.3(+1.1/-1.3) keV. This allowed us to measure the magnetic field strength of the neutron star for the first time: B = (2.5+/-0.1)e12 G. The known pulsation period is now observed at 904.0+/-0.1 s. Since 2006, the neutron star has undergone a long-term spin-up trend at a rate of P' = -2e-8 s/s (-0.6 s per year, or a frequency derivative of nu' = 3e-14 Hz/s ). In the power density spectrum, a break appears at the pulse frequency which separates the zero slope at low frequency from the steeper slope at high frequency. This addition of angular momentum to the neutron star could be due to the accretion of a quasi-spherical wind, or it could be caused by the transient appearance of a prograde accretion disk that is nearly i...

  15. STELLAR FEEDBACK AND THE EVOLUTION OF YOUNG EMBEDDED STAR CLUSTERS

    Directory of Open Access Journals (Sweden)

    C. Weidner

    2009-01-01

    Full Text Available In this contribution we present the rst results of an theoretical attempt to address the question of the evacuation time-scale but also the star-formation efficiency and the in uence of feedback on the most-massive star in a star cluster. With the use of stellar evolution models, known descriptions for the evolution of an ionised sphere around a star and further models to describe stellar winds, the e ects of young stellar populations of di erent sizes are studied.

  16. 3D Gray Radiative Properties of Accretion Shocks in Young Stellar Objects

    Directory of Open Access Journals (Sweden)

    Ibgui L.

    2014-01-01

    Full Text Available We address the problem of the contribution of radiation to the structure and dynamics of accretion shocks on Young Stellar Objects. Solving the 3D RTE (radiative transfer equation under our “gray LTE approach”, i.e., using appropriate mean opacities computed in local thermodynamic equilibrium, we post-process the 3D MHD (magnetohydrodynamic structure of an accretion stream impacting the stellar chromosphere. We find a radiation flux of ten orders of magnitude larger than the accreting energy rate, which is due to a large overestimation of the radiative cooling. A gray LTE radiative transfer approximation is therefore not consistent with the given MHD structure of the shock. Further investigations are required to clarify the role of radiation, by relaxing both the gray and LTE approximations in RHD (radiation hydrodynamics simulations. Post-processing the obtained structures through the resolution of the non-LTE monochromatic RTE will provide reference radiation quantities against which RHD approximate solutions will be compared.

  17. Strong Variable Ultraviolet Emission from Y Gem: Accretion Activity in an AGB Star with a Binary Companion?

    CERN Document Server

    Sahai, Raghvendra; de Paz, Armando Gil; Contreras, Carmen Sánchez

    2011-01-01

    Binarity is believed to dramatically affect the history and geometry of mass loss in AGB and post-AGB stars, but observational evidence of binarity is sorely lacking. As part of a project to look for hot binary companions to cool AGB stars using the GALEX archive, we have discovered a late-M star, Y Gem, to be a source of strong and variable UV emission. Y Gem is a prime example of the success of our technique of UV imaging of AGB stars in order to search for binary companions. Y Gem's large and variable UV flux makes it one of the most prominent examples of a late AGB star with a mass accreting binary companion. The UV emission is most likely due to emission associated with accretion activity and a disk around a main-sequence companion star. The physical mechanism generating the UV emission is extremely energetic, with an integrated luminosity of a few L(sun) at its peak. We also find weak CO J=2-1 emission from Y Gem with a very narrow line profile (FWHM of 3.4 km/s). Such a narrow line is unlikely to arise...

  18. NGC 1980 Is Not a Foreground Population of Orion: Spectroscopic Survey of Young Stars with Low Extinction in Orion A

    Science.gov (United States)

    Fang, Min; Kim, Jinyoung Serena; Pascucci, Ilaria; Apai, Dániel; Zhang, Lan; Sicilia-Aguilar, Aurora; Alonso-Martínez, Miguel; Eiroa, Carlos; Wang, Hongchi

    2017-04-01

    We perform a spectroscopic survey of the foreground population in Orion A with MMT/Hectospec. We use these data, along with archival spectroscopic data and photometric data, to derive spectral types, extinction values, and masses for 691 stars. Using the Spitzer Space Telescope data, we characterize the disk properties of these sources. We identify 37 new transition disk (TD) objects, 1 globally depleted disk candidate, and 7 probable young debris disks. We discover an object with a mass of less than 0.018-0.030 M ⊙, which harbors a flaring disk. Using the Hα emission line, we characterize the accretion activity of the sources with disks, and confirm that the fraction of accreting TDs is lower than that of optically thick disks (46% ± 7% versus 73% ± 9%, respectively). Using kinematic data from the Sloan Digital Sky Survey and APOGEE INfrared Spectroscopy of the Young Nebulous Clusters program (IN-SYNC), we confirm that the foreground population shows similar kinematics to their local molecular clouds and other young stars in the same regions. Using the isochronal ages, we find that the foreground population has a median age of around 1-2 Myr, which is similar to that of other young stars in Orion A. Therefore, our results argue against the presence of a large and old foreground cluster in front of Orion A.

  19. Young Stellar Object Variability (YSOVAR): Mid Infrared Clues to Accretion Disk Physics and Protostar Rotational Evolution

    Science.gov (United States)

    Stauffer, John; Akeson, Rachel; Allen, Lori; Ardila, David; Barrado, David; Bayo, Amelia; Bouvier, Jerome; Calvet, Nuria; Carey, Sean; Carpenter, John; Ciardi, David; Covey, Kevin; Favata, Fabio; Flaherty, Kevin; Forbrich, Jan; Guieu, Sylvain; Gutermuth, Rob; Hartmann, Lee; Hillenbrand, Lynne; Hora, Joe; McCaughrean, Mark; Megeath, Tom; Morales-Calderon, Maria; Muzerolle, James; Plavchan, Peter; Rebull, Luisa; Skrutskie, Mike; Smith, Howard; Song, Inseok; Stapelfeldt, Karl; Sung, Hwankyung; Terebey, Susan; Vrba, Fred; Werner, Mike; Whitney, Barbara; Winston, Elaine; Wood, Kenny

    2008-12-01

    Spitzer/IRAC in the warm mission is the only facility now existing or planned capable of carrying out an extensive, accurate time series photometric monitoring survey of star-forming regions in the thermal infrared. The demonstrated sensitivity and stability of IRAC allows measurement of the relative fluxes of YSO's down to the substellar mass limit to 1-2% accuracy in star-forming regions out to >500 pc. We propose a time series monitoring exploration science survey of the Orion Nebula Cluster and 11 very young, populous embedded star-forming cores which will provide >D 80 epochs of data for > 1500 YSO's. We will complement these observations with contemporaneous optical and near-IR monitoring data in order to allow comparison of the phase, amplitude and light-curve shape as a function of wavelength. These data will allow us to: (a) provide otherwise unobtainable constraints on the structure of the inner disks in Class I and II YSOs - and hence, perhaps, provide clues to the formation and migration of planets at young ages; (b) measure the short and long-term stability of hot spots on the surfaces of YSO's of all evolutionary stages; and (c) determine rotational periods for the largest sample to date of Class I YSO's and hence obtain the best measure of the initial angular momentum distribution of young stars.

  20. X-ray Emission from Young Stars in the TW Hya Association

    CERN Document Server

    Brown, Alexander; Ayres, Thomas R; France, Kevin; Brown, Joanna M

    2014-01-01

    The 9 Myr old TW Hya Association (TWA) is the nearest group (typical distances of $\\sim$50 pc) of pre-main-sequence (PMS) stars with ages less than 10 Myr and contains stars with both actively accreting disks and debris disks. We have studied the coronal X-ray emission from a group of low mass TWA common proper motion binaries using the {\\it{Chandra}} and {\\it{Swift}} satellites. Our aim is to understand better their coronal properties and how high energy photons affect the conditions around young stars and their role in photo-exciting atoms, molecules and dust grains in circumstellar disks and lower density circumstellar gas. Once planet formation is underway, this emission influences protoplanetary evolution and the atmospheric conditions of the newly-formed planets. The X-ray properties for 7 individual stars (TWA 13A, TWA 13B, TWA 9A, TWA 9B, TWA 8A, TWA 8B, and TWA 7) and 2 combined binary systems (TWA 3AB and TWA 2AB) have been measured. All the stars with sufficient signal require two-component fits to...

  1. Increased Understanding of Accretion in Massive YSOs

    Science.gov (United States)

    De Wit, Willem-Jan; Caratti, A.; Kraus, S.

    2017-06-01

    That massive stars up to 20Msol form by disk accretion is by now reasonably well established. We will present the latest observational results for the formation of single and multiple massive YSOs. By means of optical interferometry using the newly commissioned instrument Gravity at the VLT-I, we show the discovery of a young, embedded, 170AU-wide binary and is the most massive and most compact accreting young binary to date. We will also present the results of a multi-site multi-epoch follow-up campaign of the first well studied accretion outburst in a massive YSO.

  2. Inner disk radius, accretion and the propeller effect in the spin-down phase of neutron stars

    CERN Document Server

    Ertan, Unal

    2015-01-01

    We have investigated the critical conditions required for an efficient steady propeller mechanism in the spin-down phases of magnetized neutron stars with optically thick accretion disks. We have shown through simple analytical calculations that: (1) the strength of the dipole field at the Alfven radius is not sufficient to sustain an efficient mass-outflow even when the magnetic dipole field lines rotate much faster than the escape speed, (2) in the spin-down phase, mass accretion onto the star could persist above a minimum disk mass-flow rate that is orders of magnitude lower than the rate corresponding to the transition between the spin-up and the spin-down states, (3) below this critical mass-flow rate, a steady propeller state could be established with a maximum inner disk radius about 25 times smaller than the Alfven radius. Our results indicate that only for spherical accretion, the inner disk radius is likely to approach the Alfven radius, and for all realistic cases, the accretion-propeller transitio...

  3. On Self-Ignition and the Propagation of Flame Fronts on the Surfaces of Accreting Neutron Stars

    Science.gov (United States)

    Bayliss, A.; Sandquist, E. L.; Taam, R. E.

    1998-12-01

    The behavior of nuclear burning in the accreted layer of a neutron star is investigated for helium and hydrogen-helium mixtures. Attention is focused on the propagation of a thermal wave due to electron conduction or radiative diffusion in the lateral direction. The fully time-dependent calculations reveal that a steady state flame front is not necessarily applicable at high mass accretion rates (dM/dt > (dM/dt)Edd). In particular, there are parameter regimes in which a steady state structure is never attained within physically relevant timescales because the gas ahead of a front self-ignites. Hence, a thermonuclear flash may take place on a timescale unrelated to the timescale for a steady state front to propagate over a homogeneous region. The existence of irregular burst activity in highly luminous neutron star X-ray binary systems may provide some observational support for this theoretical picture.

  4. Precessional Density Wave as a Reason of Turbulence in Accretion Disks of Non-magnetic Close Binary Stars

    Science.gov (United States)

    Bisikalo, D. V.; Kurbatov, E. P.; Kaygorodov, P. V.

    2015-10-01

    3D numerical simulations demonstrate the formation of precessional spiral density waves in accretion disks of close binary stars. The precesional wave occurs in the Keplerian disk as a result of gravitational action of the donor-star. The wave causes the appearance of strong density and velocity gradients in the disk. Linear stability analysis shows that the presence of a radial velocity gradient leads to the instability of radial modes. The perturbation becomes unstable if the radial velocity variations are of the same order or greater than the sound speed on the characteristic wave scale of the perturbations. The unstable perturbations rapidly grow with time and give rise to the emergence and growth of turbulence in the accretion disk. The obtained viscosity (0.01 in terms of Shakura-Sunyaev parameter) is in agreement with observations.

  5. The accretion regimes of a highly magnetised NS: the unique case of NuSTAR J095551+6940.8

    CERN Document Server

    Dall'Osso, Simone; Papitto, Alessandro; Bozzo, Enrico; Stella, Luigi

    2015-01-01

    We analyze archival Chandra HRC observations of the ultra luminous accreting pulsar M82-X2 (NuSTAR J095551+6940.8), and determine an upper limit of $ r_{co}, and the source luminosity is expected to drop by a large factor. We conclude that a magnetically threaded, radiation pressure-dominated disk, around a highly magnetized NS (B~10^{13} G) offers the best intepretation for all the currently observed properties of NuSTAR J095551+6940.8. This source offers an unprecedented opportunity to study the disk-magnetosphere interaction in a new regime of supercritical accretion, and across the transition between-radiation pressure and gas-pressure dominance inside the disk.

  6. RCW 108: Massive Young Stars Trigger Stellar Birth

    Science.gov (United States)

    2008-01-01

    RCW 108 is a region where stars are actively forming within the Milky Way galaxy about 4,000 light years from Earth. This is a complicated region that contains young star clusters, including one that is deeply embedded in a cloud of molecular hydrogen. By using data from different telescopes, astronomers determined that star birth in this region is being triggered by the effect of nearby, massive young stars. This image is a composite of X-ray data from NASA's Chandra X-ray Observatory (blue) and infrared emission detected by NASA's Spitzer Space Telescope (red and orange). More than 400 X-ray sources were identified in Chandra's observations of RCW 108. About 90 percent of these X-ray sources are thought to be part of the cluster and not stars that lie in the field-of-view either behind or in front of it. Many of the stars in RCW 108 are experiencing the violent flaring seen in other young star-forming regions such as the Orion nebula. Gas and dust blocks much of the X-rays from the juvenile stars located in the center of the image, explaining the relative dearth of Chandra sources in this part of the image. The Spitzer data show the location of the embedded star cluster, which appears as the bright knot of red and orange just to the left of the center of the image. Some stars from a larger cluster, known as NGC 6193, are also visible on the left side of the image. Astronomers think that the dense clouds within RCW 108 are in the process of being destroyed by intense radiation emanating from hot and massive stars in NGC 6193. Taken together, the Chandra and Spitzer data indicate that there are more massive star candidates than expected in several areas of this image. This suggests that pockets within RCW 108 underwent localized episodes of star formation. Scientists predict that this type of star formation is triggered by the effects of radiation from bright, massive stars such as those in NGC 6193. This radiation may cause the interior of gas clouds in RCW 108 to

  7. RCW 108: Massive Young Stars Trigger Stellar Birth

    Science.gov (United States)

    2008-01-01

    RCW 108 is a region where stars are actively forming within the Milky Way galaxy about 4,000 light years from Earth. This is a complicated region that contains young star clusters, including one that is deeply embedded in a cloud of molecular hydrogen. By using data from different telescopes, astronomers determined that star birth in this region is being triggered by the effect of nearby, massive young stars. This image is a composite of X-ray data from NASA's Chandra X-ray Observatory (blue) and infrared emission detected by NASA's Spitzer Space Telescope (red and orange). More than 400 X-ray sources were identified in Chandra's observations of RCW 108. About 90 percent of these X-ray sources are thought to be part of the cluster and not stars that lie in the field-of-view either behind or in front of it. Many of the stars in RCW 108 are experiencing the violent flaring seen in other young star-forming regions such as the Orion nebula. Gas and dust blocks much of the X-rays from the juvenile stars located in the center of the image, explaining the relative dearth of Chandra sources in this part of the image. The Spitzer data show the location of the embedded star cluster, which appears as the bright knot of red and orange just to the left of the center of the image. Some stars from a larger cluster, known as NGC 6193, are also visible on the left side of the image. Astronomers think that the dense clouds within RCW 108 are in the process of being destroyed by intense radiation emanating from hot and massive stars in NGC 6193. Taken together, the Chandra and Spitzer data indicate that there are more massive star candidates than expected in several areas of this image. This suggests that pockets within RCW 108 underwent localized episodes of star formation. Scientists predict that this type of star formation is triggered by the effects of radiation from bright, massive stars such as those in NGC 6193. This radiation may cause the interior of gas clouds in RCW 108 to

  8. Nuclear star formation activity and black hole accretion in nearby Seyfert galaxies

    Energy Technology Data Exchange (ETDEWEB)

    Esquej, P. [Centro de Astrobiología, INTA-CSIC, Villafranca del Castillo, E-28850, Madrid (Spain); Alonso-Herrero, A.; Hernán-Caballero, A. [Instituto de Física de Cantabria, CSIC-Universidad de Cantabria, E-39005 Santander (Spain); González-Martín, O.; Ramos Almeida, C.; Rodríguez Espinosa, J. M. [Instituto de Astrofísica de Canarias (IAC), C/Vía Láctea, E-38205, La Laguna (Spain); Hönig, S. F. [UCSB Department of Physics, Broida Hall 2015H, Santa Barbara, CA (United States); Roche, P. [Department of Physics, University of Oxford, Oxford OX1 3RH (United Kingdom); Mason, R. E. [Gemini Observatory, Northern Operations Center, 670 North A' ohoku, HI 96720 (United States); Díaz-Santos, T. [Spitzer Science Center, 1200 East California Boulevard, Pasadena, CA 91125 (United States); Levenson, N. A. [Gemini Observatory, Casilla 603, La Serena (Chile); Aretxaga, I. [Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Aptdo. Postal 51 y 216, 72000 Puebla (Mexico); Packham, C. [Department of Physics and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249 (United States)

    2014-01-01

    Recent theoretical and observational works indicate the presence of a correlation between the star-formation rate (SFR) and active galactic nucleus (AGN) luminosity (and, therefore, the black hole accretion rate, M-dot {sub BH}) of Seyfert galaxies. This suggests a physical connection between the gas-forming stars on kpc scales and the gas on sub-pc scales that is feeding the black hole. We compiled the largest sample of Seyfert galaxies to date with high angular resolution (∼0.''4-0.''8) mid-infrared (8-13 μm) spectroscopy. The sample includes 29 Seyfert galaxies drawn from the AGN Revised Shapley-Ames catalog. At a median distance of 33 Mpc, our data allow us to probe nuclear regions on scales of ∼65 pc (median value). We found no general evidence of suppression of the 11.3 μm polycyclic aromatic hydrocarbon (PAH) emission in the vicinity of these AGN, and we used this feature as a proxy for the SFR. We detected the 11.3 μm PAH feature in the nuclear spectra of 45% of our sample. The derived nuclear SFRs are, on average, five times lower than those measured in circumnuclear regions of 600 pc in size (median value). However, the projected nuclear SFR densities (median value of 22 M {sub ☉} yr{sup –1} kpc{sup –2}) are a factor of 20 higher than those measured on circumnuclear scales. This indicates that the SF activity per unit area in the central ∼65 pc region of Seyfert galaxies is much higher than at larger distances from their nuclei. We studied the connection between the nuclear SFR and M-dot {sub BH} and showed that numerical simulations reproduce our observed relation fairly well.

  9. Enriched haloes at redshift z = 2 with no star formation: implications for accretion and wind scenarios

    Science.gov (United States)

    Bouché, N.; Murphy, M. T.; Péroux, C.; Contini, T.; Martin, C. L.; Forster Schreiber, N. M.; Genzel, R.; Lutz, D.; Gillessen, S.; Tacconi, L.; Davies, R.; Eisenhauer, F.

    2012-01-01

    In order to understand which process (e.g. galactic winds, cold accretion) is responsible for the cool (T ˜ 104 K) halo gas around galaxies, we embarked on a programme to study the star formation properties of galaxies selected by their Mg II absorption signature in quasar spectra. Specifically, we searched for the Hα line emission from galaxies near very strong z ≃ 2 Mg II absorbers (with rest-frame equivalent width ? Å) because these could be the signposts of outflows or inflows. Surprisingly, we detect Hα from only four hosts out of 20 sightlines (and two out of the 19 H I-selected sightlines), despite reaching a star formation rate (SFR) sensitivity limit of 2.9 M⊙ yr-1 (5σ) for a Chabrier initial mass function. This low success rate (4/20) is in contrast with our z ≃ 1 survey where we detected 66 per cent (14/21) of the Mg II hosts (down to 0.6 M⊙ yr-1; 5σ). Taking into account the difference in sensitivity between the two surveys, we should have been able to detect ≥11.4 (≥7.6) of the 20 z ≃ 2 hosts - assuming that SFR evolves as ∝(1 + z)γ with γ= 2.5 (or γ= 0) respectively - whereas we found only four galaxies. Interestingly, all the z = 2 detected hosts have observed SFRs ≳ 9 M⊙ yr-1, well above our sensitivity limit, while at z = 1 they all have SFR Prochaska et al. (2005); (4) from Ledoux et al. (2006) (5) from Ryabinkov, Kaminker & Varshalovich (2003); (6) from Srianand et al. (2008)]; (6) FWHM of the seeing PSF; (7) exposure time; (8) observing run ID; (9) dates of observations. 1Source common to both samples.

  10. Impacts of fragmented accretion streams onto Classical T Tauri Stars: UV and X-ray emission lines

    CERN Document Server

    Colombo, Salvatore; Peres, Giovanni; Argiroffi, Costanza; Reale, Fabio

    2016-01-01

    Context. The accretion process in Classical T Tauri Stars (CTTSs) can be studied through the analysis of some UV and X-ray emission lines which trace hot gas flows and act as diagnostics of the post-shock downfalling plasma. In the UV band, where higher spectral resolution is available, these lines are characterized by rather complex profiles whose origin is still not clear. Aims. We investigate the origin of UV and X-ray emission at impact regions of density structured (fragmented) accretion streams.We study if and how the stream fragmentation and the resulting structure of the post-shock region determine the observed profiles of UV and X-ray emission lines. Methods. We model the impact of an accretion stream consisting of a series of dense blobs onto the chromosphere of a CTTS through 2D MHD simulations. We explore different levels of stream fragmentation and accretion rates. From the model results, we synthesize C IV (1550 {\\AA}) and OVIII (18.97 {\\AA}) line profiles. Results. The impacts of accreting blob...

  11. An Ultraluminous X-ray Source Powered by An Accreting Neutron Star

    CERN Document Server

    Bachetti, M; Walton, D J; Grefenstette, B W; Chakrabarty, D; Fürst, F; Barret, D; Beloborodov, A; Boggs, S E; Christensen, F E; Craig, W W; Fabian, A C; Hailey, C J; Hornschemeier, A; Kaspi, V; Kulkarni, S R; Maccarone, T; Miller, J M; Rana, V; Stern, D; Tendulkar, S P; Tomsick, J; Webb, N A; Zhang, W W

    2014-01-01

    Ultraluminous X-ray sources (ULX) are off-nuclear point sources in nearby galaxies whose X-ray luminosity exceeds the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes. Their luminosity ranges from $10^{40}$ erg s$^{-1} $10^{40}$ erg s$^{-1}$), which require black hole masses MBH >50 solar masses and/or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries. Here we report broadband X-ray observations of the nuclear region of the galaxy M82, which contains two bright ULXs. The observations reveal pulsations of average period 1.37 s with a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to $L_X$(3 - 30 keV) = $4.9 \\times 10^{39}$ erg s$^{-1}$. The pulsating source is spatially coincident with a variable ULX which can reach $L_X$ (0.3 - 10 keV) = $1.8 \\times 10^{40}$ erg s$^{-1}$. This ...

  12. Numerical Experiments for Nuclear Flashes toward Superbursts in an Accreting Neutron Star

    Directory of Open Access Journals (Sweden)

    Masa-aki Hashimoto

    2014-01-01

    Full Text Available We show that the superburst would be originated from thermonuclear burning ignited by accumulated fuels in the deep layers compared to normal X-ray bursts. Two cases are investigated for models related to superbursts by following thermal evolution of a realistic neutron star: helium flash and carbon flash accompanied with many normal bursts. For a helium flash, the burst shows the long duration when the accretion rate is low compared with the observation. The flash could become a superburst if the burning develops to the deflagration and/or detonation. For a carbon flash accompanied with many normal bursts, after successive 2786 normal bursts during 1.81 × 109 s, the temperature reaches the deflagration temperature. This is due to the produced carbon which amount reaches to ≈0.1 in the mass fraction. The flash will develop to dynamical phenomena of the deflagration and/or detonation, which may lead to a superburst.

  13. Growing black holes and galaxies: black hole accretion versus star formation rate

    CERN Document Server

    Volonteri, Marta; Netzer, Hagai; Bellovary, Jillian; Dotti, Massimo; Governato, Fabio

    2015-01-01

    We present a new suite of hydrodynamical simulations and use it to study, in detail, black hole and galaxy properties. The high time, spatial and mass resolution, and realistic orbits and mass ratios, down to 1:6 and 1:10, enable us to meaningfully compare star formation rate (SFR) and BH accretion rate (BHAR) timescales, temporal behaviour and relative magnitude. We find that (i) BHAR and galaxy-wide SFR are typically temporally uncorrelated, and have different variability timescales, except during the merger proper, lasting ~0.2-0.3 Gyr. BHAR and nuclear (<100 pc) SFR are better correlated, and their variability are similar. Averaging over time, the merger phase leads typically to an increase by a factor of a few in the BHAR/SFR ratio. (ii) BHAR and nuclear SFR are intrinsically proportional, but the correlation lessens if the long-term SFR is measured. (iii) Galaxies in the remnant phase are the ones most likely to be selected as systems dominated by an active galactic nucleus (AGN), because of the long...

  14. Abbott Wave-Triggered Runaway in Line-Driven Winds from Stars and Accretion Disks

    CERN Document Server

    Feldmeier, A; Feldmeier, Achim; Shlosman, Isaac

    2001-01-01

    Line-driven winds from stars and accretion disks are accelerated by scattering in numerous line transitions. The wind is believed to adopt a unique critical solution, out of the infinite variety of shallow and steep solutions. We study the inherent dynamics of the transition towards the critical wind. A new runaway wind mechanism is analyzed in terms of radiative-acoustic (Abbott) waves which are responsible for shaping the wind velocity law and fixing the mass loss. Three different flow types result, depending on the location of perturbations. First, if the shallow solution is perturbed sufficiently far downstream, a single critical point forms in the flow, which is a barrier for Abbott waves, and the solution tends to the critical one. Second, if the shallow solution is perturbed upstream from this critical point, mass overloading results, and the critical point is shifted inwards. This wind exhibits a broad, stationary region of decelerating flow and its velocity law has kinks. Third, for perturbations eve...

  15. Sustaining star formation rates in spiral galaxies - Supernova-driven turbulent accretion disk models applied to THINGS galaxies

    CERN Document Server

    Vollmer, B

    2010-01-01

    Gas disks of spiral galaxies can be described as clumpy accretion disks without a coupling of viscosity to the actual thermal state of the gas. The model description of a turbulent disk consisting of emerging and spreading clumps (Vollmer & Beckert 2003) contains free parameters, which can be constrained by observations of molecular gas, atomic gas and the star formation rate for individual galaxies. Radial profiles of 18 nearby spiral galaxies from THINGS, HERACLES, SINGS, and GALEX data are used to compare the observed star formation efficiency, molecular fraction, and velocity dispersion to the model. The observed radially decreasing velocity dispersion can be reproduced by the model. In the framework of this model the decrease in the inner disk is due to the stellar mass distribution which dominates the gravitational potential. Introducing a radial break in the star formation efficiency into the model improves the fits significantly. This change in star formation regime is realized by replacing the fr...

  16. Pulsed Accretion in a Variable Protostar

    CERN Document Server

    Muzerolle, James; Flaherty, Kevin; Balog, Zoltan; Gutermuth, Robert

    2013-01-01

    Periodic increases in luminosity arising from variable accretion rates have been predicted for some close pre-main sequence binary stars as they grow from circumbinary disks. The phenomenon is known as "pulsed accretion" and can affect the orbital evolution and mass distribution of young binaries, as well as the potential for planet formation in the circumbinary environment. Accretion variability is a common feature of young stars, with a large range of amplitudes and timescales as measured from multi-epoch observations at optical and infrared wavelengths. Periodic variations consistent with pulsed accretion have been seen in only a few young binaries via optical accretion tracers, albeit intermittently with accretion luminosity variations ranging from 0-50 percent from orbit to orbit. Here we report on a young protostar (age ~10^5 yr) that exhibits periodic variability in which the infrared luminosity increases by a factor of 10 in roughly one week every 25.34 days. We attribute this to pulsed accretion asso...

  17. Probing the Structure of the Accretion Region in a Sample of Magnetic Herbig Ae/Be Stars

    Science.gov (United States)

    Pogodin, M. A.; Cahuasqui, J. A.; Drake, N. A.; Hubrig, S.; Schöller, M.; Petr-Gotzens, M.; Franco, G. A. P.; Lopes, D. F.; Kozlova, O. V.; Wolff, B.; González, J. F.; Carroll, T. A.; Mysore, S.

    2015-04-01

    We present the results of a study of the temporal behavior of several diagnostic lines formed in the region of the accretion-disk/star interaction in the three magnetic Herbig Ae stars HD 101412, HD 104237, and HD 190073. More than 100 spectra acquired with the ISAAC, X-shooter, and CRIRES spectrographs installed at the 8-m VLT telescope (ESO, Chile), as well as at other observatories (OHP, Crimean AO) were analyzed. The spectroscopic data were obtained in the He I λ10 830, Paγ, and He I λ5876 lines. We found that the temporal behavior of the diagnostic lines in the spectra of all program stars can be widely explained by a rotational modulation of the line profiles, generated by a local accretion flow. This result is in good agreement with the predictions of the magnetospheric accretion model. For the first time, the rotation period of HD 104237 (Prot = 5.37±0.03 days) as well as the inclination angle (i = 21°±4°) were determined. Additional analysis of the HARPSpol spectra of HD 104237 and HD 190073, taken from the ESO archive, with the use of the SVD method shows that the magnetic field structure of HD 190073 is likely more complex than a simple dipole and contains a circumstellar component. For the first time, the magnetic field of the secondary component of the binary system HD 104237 was also detected ((Bz) = 128±10 G).

  18. Probing the Structure of the Accretion Region in a Sample of Magnetic Herbig Ae/Be Stars

    CERN Document Server

    Pogodin, M A; Drake, N A; Hubrig, S; Schoeller, M; Petr-Gotzens, M; Franco, G A P; Lopes, D F; Kozlova, O V; Wolff, B; Gonzalez, J F; Carroll, T A; Mysore, S

    2014-01-01

    We present the results of a study of the temporal behaviour of several diagnostic lines formed in the region of the accretion-disk/star interaction in the three magnetic Herbig Ae stars HD101412, HD104237, and HD190073. More than 100 spectra acquired with the ISAAC, X-shooter, and CRIRES spectrographs installed at the VLT-8m telescope (ESO, Chile), as well as at other observatories (OHP, Crimean AO) were analyzed. The spectroscopic data were obtained in the He I lambda10830, Pa gamma and He I lambda5876 lines. We found that the temporal behaviour of the diagnostic lines in the spectra of all program stars can be widely explained by a rotational modulation of the line profiles generated by a local accretion flow. This result is in good agreement with the predictions of the magnetospheric accretion model. For the first time, the rotation period of HD104237 (P_rot = 5.37+-0.03 days), as well as the inclination angle (i = 21+-4deg) were determined. Additional analysis of the HARPSpol spectra of HD104237 and HD190...

  19. Search for OB stars running away from young star clusters. II. The NGC 6357 star-forming region

    Science.gov (United States)

    Gvaramadze, V. V.; Kniazev, A. Y.; Kroupa, P.; Oh, S.

    2011-11-01

    Dynamical few-body encounters in the dense cores of young massive star clusters are responsible for the loss of a significant fraction of their massive stellar content. Some of the escaping (runaway) stars move through the ambient medium supersonically and can be revealed via detection of their bow shocks (visible in the infrared, optical or radio). In this paper, which is the second of a series of papers devoted to the search for OB stars running away from young ( ≲ several Myr) Galactic clusters and OB associations, we present the results of the search for bow shocks around the star-forming region NGC 6357. Using the archival data of the Midcourse Space Experiment (MSX) satellite and the Spitzer Space Telescope, and the preliminary data release of the Wide-Field Infrared Survey Explorer (WISE), we discovered seven bow shocks, whose geometry is consistent with the possibility that they are generated by stars expelled from the young (~1-2 Myr) star clusters, Pismis 24 and AH03 J1725-34.4, associated with NGC 6357. Two of the seven bow shocks are driven by the already known OB stars, HD 319881 and [N78] 34. Follow-up spectroscopy of three other bow-shock-producing stars showed that they are massive (O-type) stars as well, while the 2MASS photometry of the remaining two stars suggests that they could be B0 V stars, provided that both are located at the same distance as NGC 6357. Detection of numerous massive stars ejected from the very young clusters is consistent with the theoretical expectation that star clusters can effectively lose massive stars at the very beginning of their dynamical evolution (long before the second mechanism for production of runaway stars, based on a supernova explosion in a massive tight binary system, begins to operate) and lends strong support to the idea that probably all field OB stars have been dynamically ejected from their birth clusters. A by-product of our search for bow shocks around NGC 6357 is the detection of three circular

  20. The formation and dynamical evolution of young star clusters

    CERN Document Server

    Fujii, Michiko

    2015-01-01

    Recent observations have revealed a variety of young star clusters, including embedded systems, young massive clusters, and associations. We study the formation and dynamical evolution of these clusters using a combination of simulations and theoretical models. Our simulations start with a turbulent molecular cloud that collapses under its own gravity. The stars are assumed to form in the densest regions in the collapsing cloud after an initial free-fall times of the molecular cloud. The dynamical evolution of these stellar distributions are continued by means of direct $N$-body simulations. The molecular clouds typical for the Milky Way Galaxy tend to form embedded clusters which evolve to resemble open clusters. The associations were initially considerably more clumpy, but lost their irregularity in about a dynamical time scale due to the relaxation process. The densest molecular clouds, which are absent in the Milky Way but are typical in starburst galaxies, form massive young star clusters. They indeed ar...

  1. Accretion/Jet Activity and Narrow [O III] Kinematics in Young Radio Galaxies

    CERN Document Server

    Wu, Qingwen; Humphrey, Andrew

    2009-01-01

    We estimate black hole masses and Eddington ratios for a sample of 81 young radio galaxies (42 CSS +39 GPS). We find that the average black hole (BH) mass of these young radio galaxies is ~8.3, which is less than that of radio loud QSOs and low redshift radio galaxies. The CSS/GPS sources have relatively high Eddington ratios, with an average value of =-0.75, which are similar to those of narrow line Seyfert 1 galaxies (NLS1s). This suggests that young radio galaxies may not only be in the early stages of their radio activity, but also in the early stage of their accretion activity. We find that the young radio galaxies as a class deviate systematically from M_bh-\\sigma relation defined by nearby inactive galaxies, when using [O III] as a surrogate for stellar velocity dispersion, \\sigma_* . We also find that the deviation of the [O III] line width is correlated with the Eddington ratio and sources with Lbol/LEdd~1 have the largest deviations, which are similar to those of radio quiet QSOs/NLS1s (radio jets i...

  2. The magnetic field evolution of ULX NuSTAR J095551+6940.8 in M82 - a legacy of accreting magnetar

    Science.gov (United States)

    Pan, Y. Y.; Song, L. M.; Zhang, C. M.; Tong, H.

    2016-09-01

    Ultraluminous X-ray sources are usually believed to be black holes with mass about 102-3 M⊙. However, the recent discovery of NuSTAR J095551+6940.8 in M82 by Bachetti et al. shows that it holds the spin period P = 1.37 s and period derivative dot{P}≈ -2× 10^{-10} s s^{-1}, which provides a strong evidence that some ultraluminous X-ray sources could be neutron stars. We obtain that the source may be an evolved magnetar according to our simulation by employing the model of accretion induced the polar magnetic field decay and standard spin-up torque of an accreting neutron star. The results show that NuSTAR J095551+6940.8 is still in the spin-up process, and the polar magnetic field decays to about 4.5 × 1012 G after accreting ˜10-2.5 M⊙, while the strong magnetic field exists in the out-polar region, which could be responsible for the observed low field magnetar. The ultra luminosity of the source can be explained by the beaming effect and two kinds of accretion-radial random accretion and disc accretion. Since the birth rate of magnetars is about ten per cent of the normal neutron stars, we guess that several ultraluminous X-ray sources should share the similar properties to that of NuSTAR J095551+6940.8.

  3. Time-monitoring Observations of Br$\\gamma$ Emission from Young Stars

    CERN Document Server

    Eisner, J A; Rieke, M J; Flaherty, K M; Stone, Jordan M; Arnold, T J; Cortes, S R; Cox, E; Hawkins, C; Cole, A; Zajac, S; Rudolph, A L

    2014-01-01

    We present multiple epochs of near-IR spectroscopy for a sample of 25 young stars, including T Tauri, Herbig Ae/Be, and FU Ori objects. Using the FSPEC instrument on the Bok 90-inch telescope, we obtained K-band spectra of the BrGamma transition of hydrogen, with a resolution of ~3500. Epochs were taken over a span of >1 year, sampling time-spacings of roughly one day, one month, and one year. The majority of our targets show BrGamma emission, and in some cases these are the first published detections. Time-variability is seen in approximately half of the targets showing BrGamma emission. We compare the observed variability with expectations for rotationally-modulated accretion onto the central stars and time-variable continuum emission or extinction from matter in the inner disk. Our observations are not entirely consistent with models of rotationally-modulated magnetospheric accretion. Further monitoring, over a larger number of epochs, will facilitate more quantitative constraints on variability timescales...

  4. Dynamical ejections of massive stars from young star clusters under diverse initial conditions

    Science.gov (United States)

    Oh, Seungkyung; Kroupa, Pavel

    2016-05-01

    We study the effects that initial conditions of star clusters and their massive star population have on dynamical ejections of massive stars from star clusters up to an age of 3 Myr. We use a large set of direct N-body calculations for moderately massive star clusters (Mecl ≈ 103.5 M⊙). We vary the initial conditions of the calculations, such as the initial half-mass radius of the clusters, initial binary populations for massive stars and initial mass segregation. We find that the initial density is the most influential parameter for the ejection fraction of the massive systems. The clusters with an initial half-mass radius rh(0) of 0.1 (0.3) pc can eject up to 50% (30)% of their O-star systems on average, while initially larger (rh(0) = 0.8 pc) clusters, that is, lower density clusters, eject hardly any OB stars (at most ≈ 4.5%). When the binaries are composed of two stars of similar mass, the ejections are most effective. Most of the models show that the average ejection fraction decreases with decreasing stellar mass. For clusters that are efficient at ejecting O stars, the mass function of the ejected stars is top-heavy compared to the given initial mass function (IMF), while the mass function of stars that remain in the cluster becomes slightly steeper (top-light) than the IMF. The top-light mass functions of stars in 3 Myr old clusters in our N-body models agree well with the mean mass function of young intermediate-mass clusters in M 31, as reported previously. This implies that the IMF of the observed young clusters is the canonical IMF. We show that the multiplicity fraction of the ejected massive stars can be as high as ≈ 60%, that massive high-order multiple systems can be dynamically ejected, and that high-order multiples become common especially in the cluster. We also discuss binary populations of the ejected massive systems. Clusters that are initially not mass-segregated begin ejecting massive stars after a time delay that is caused by mass

  5. Spatially Extended Brackett Gamma Emission in the Environments of Young Stars

    CERN Document Server

    Beck, Tracy L; McGregor, Peter J

    2010-01-01

    The majority of atomic hydrogen Br{\\gamma} emission detected in the spectra of young stellar objects (YSOs) is believed to arise from the recombination regions associated with the magnetospheric accretion of circumstellar disk material onto the forming star. In this paper, we present the results of a K-band IFU spectroscopic study of Br{\\gamma} emission in eight young protostars: CW Tau, DG Tau, Haro 6-10, HL Tau, HV Tau C, RW Aur, T Tau and XZ Tau. We spatially resolve Br{\\gamma} emission structures in half of these young stars and find that most of the extended emission is consistent with the location and velocities of the known Herbig-Haro flows associated with these systems. At some velocities through the Br{\\gamma} line profile, the spatially extended emission comprises 20% or more of the integrated flux in that spectral channel. However, the total spatially extended Br{\\gamma} is typically less than ~10% of the flux integrated over the full emission profile. For DG Tau and Haro 6-10 S, we estimate the m...

  6. The low-mass stellar population in the young cluster Tr37: Disk evolution, accretion, and environment

    CERN Document Server

    Sicilia-Aguilar, Aurora; Sobolev, Andrej; Getman, Konstantin; Henning, Thomas; Fang, Min

    2013-01-01

    We present a study of accretion and protoplanetary disks around M-type stars in the 4 Myr-old cluster Tr37. With a well-studied solar-type population, Tr37 is a benchmark for disk evolution. We used low-resolution spectroscopy to identify 141 members (78 new) and 64 probable members, mostly M-type stars. H\\alpha\\ emission provides information about accretion. Optical, 2MASS, Spitzer, and WISE data are used to trace the SEDs. We construct radiative transfer models to explore the structures of full-disks, pre-transition, transition, and dust-depleted disks. Including the new and previously known members, we confirm that a substantial fraction (~2/5) of disks show signs of evolution, either as radial dust evolution (transition/pre-transition disks) or as a more global evolution (low small-dust masses, dust settling, and weak/absent accretion signatures). Accretion is strongly dependent on the SED type. About half of the transition objects are consistent with no accretion, and dust-depleted disks have weak (or un...

  7. Testing the Young Neutron Star Scenario with Persistent Radio Emission Associated with FRB 121102

    Science.gov (United States)

    Kashiyama, Kazumi; Murase, Kohta

    2017-04-01

    Recently a repeating fast radio burst (FRB) 121102 has been confirmed to be an extragalactic event and a persistent radio counterpart has been identified. While other possibilities are not ruled out, the emission properties are broadly consistent with Murase et al. that theoretically proposed quasi-steady radio emission as a counterpart of both FRBs and pulsar-driven supernovae. Here, we constrain the model parameters of such a young neutron star scenario for FRB 121102. If the associated supernova has a conventional ejecta mass of M ej ≳ a few M ⊙, a neutron star with an age of t age ∼ 10–100 years, an initial spin period of P i ≲ a few ms, and a dipole magnetic field of B dip ≲ a few × 1013 G can be compatible with the observations. However, in this case, the magnetically powered scenario may be favored as an FRB energy source because of the efficiency problem in the rotation-powered scenario. On the other hand, if the associated supernova is an ultra-stripped one or the neutron star is born by the accretion-induced collapse with M ej ∼ 0.1 M ⊙, a younger neutron star with t age ∼ 1–10 years can be the persistent radio source and might produce FRBs with the spin-down power. These possibilities can be distinguished by the decline rate of the quasi-steady radio counterpart.

  8. The drop in the cosmic star formation rate below redshift 2 is caused by a change in the mode of gas accretion and by AGN feedback

    CERN Document Server

    van de Voort, Freeke; Booth, C M; Vecchia, Claudio Dalla

    2011-01-01

    The cosmic star formation rate is observed to drop sharply after redshift z=2. We use a large, cosmological, smoothed particle hydrodynamics simulation to investigate how this decline is related to the evolution of gas accretion and to outflows driven by active galactic nuclei (AGN). We find that the drop in the star formation rate follows a corresponding decline in the global cold-mode accretion rate density onto haloes, but with a delay of order the gas consumption time scale in the interstellar medium. Here we define cold-mode (hot-mode) accretion as gas that is accreted and whose temperature has never exceeded (did exceed) 10^5.5 K. In contrast to cold-mode accretion, which peaks at z~3, the hot mode continues to increase to z~1 and remains roughly constant thereafter. By the present time, the hot mode strongly dominates the global accretion rate onto haloes. Star formation does not track hot-mode halo accretion because most of the hot halo gas never accretes onto galaxies. AGN feedback plays a crucial ro...

  9. Accretion-ejection connection in the young brown dwarf candidate ISO-Cha1 217

    CERN Document Server

    Whelan, E T; Bacciotti, F; Nisini, B; Bonito, R; Antoniucci, S; Stelzer, B; Biazzo, K; D'Elia, V; Ray, T P

    2014-01-01

    As the number of observed brown dwarf outflows is growing it is important to investigate how these outflows compare to the well studied jets from young stellar objects. A key point of comparison is the relationship between outflow and accretion activity and in particular the ratio between the mass outflow and accretion rates ($\\dot{M}_{out}$/$\\dot{M}_{acc}$). The brown dwarf candidate ISO-ChaI 217 was discovered by our group, as part of a spectro-astrometric study of brown dwarfs, to be driving an asymmetric outflow with the blue-shifted lobe having a position angle of $\\sim$ 20$^{\\circ}$. The aim here is to further investigate the properties of ISO-ChaI 217, the morphology and kinematics of its outflow, and to better constrain ($\\dot{M}_{out}$/$\\dot{M}_{acc}$). The outflow is spatially resolved in the $[SII]\\lambda \\lambda 6716,6731$ lines and is detected out to $\\sim$ 1\\farcs6 in the blue-shifted lobe and ~ 1" in the red-shifted lobe. The asymmetry between the two lobes is confirmed although the velocity as...

  10. The bound fraction of young star clusters

    CERN Document Server

    Brinkmann, N; Motwani, B; Kroupa, P

    2016-01-01

    The residual gas within newly formed star clusters is expelled through stellar feedback on timescales ~ 1 Myr. The subsequent expansion of the cluster results in an unbinding of a fraction of stars, before the remaining cluster members can re-virialize and form a surviving cluster. We investigate the bound fraction after gas expulsion as a function of initial cluster mass in stars and gauge the influence of primordial mass segregation, stellar evolution and the tidal field at solar distance. We also assess the impact of the star-formation efficiency and gas expulsion velocity. We perform N-body simulations using Sverre Aarseth's NBODY7 code, starting with compact clusters in their embedded phase and approximate the gas expulsion by means of an exponentially depleting external gravitational field. We follow the process of re-virialization through detailed monitoring of different Lagrange radii over several Myr, examining initial half-mass radii of 0.1 pc, 0.3 pc and 0.5 pc and masses usually ranging from 5x10^...

  11. DISCOVERY AND OBSERVATIONS OF ASASSN-13db, AN EX LUPI-TYPE ACCRETION EVENT ON A LOW-MASS T TAURI STAR

    Energy Technology Data Exchange (ETDEWEB)

    Holoien, Thomas W.-S.; Stanek, K. Z.; Kochanek, C. S.; Shappee, B. J.; Croxall, K.; Wagner, R. M.; Basu, U.; Beacom, J. F.; Jencson, J. [Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States); Prieto, Jose L.; Zhu, Z. [Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Peyton Hall, Princeton, NJ 08544 (United States); Sicilia-Aguilar, A. [Departamento de Física Teórica, Facultad de Ciencias, Universidad Autónoma de Madrid, E-28049 Cantoblanco, Madrid (Spain); Grupe, D. [Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802 (United States); Adams, J. J.; Simon, J. D. [Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101 (United States); Morrell, N. [Carnegie Observatories, Las Campanas Observatory, Colina El Pino, Casilla 601 (Chile); McGraw, S. M. [Department of Physics and Astronomy, Ohio University, 251B Clippinger Labs, Athens, OH 45701 (United States); Bersier, D. [Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF (United Kingdom); Brimacombe, J. [Coral Towers Observatory, Cairns, Queensland 4870 (Australia); Pojmanski, G., E-mail: tholoien@astronomy.ohio-state.edu [Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, 00-478 Warsaw (Poland); and others

    2014-04-20

    We discuss ASASSN-13db, an EX Lupi-type ({sup E}Xor{sup )} accretion event on the young stellar object (YSO) SDSS J051011.01–032826.2 (hereafter SDSSJ0510) discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN). Using archival photometric data of SDSSJ0510 we construct a pre-outburst spectral energy distribution and find that it is consistent with a low-mass class II YSO near the Orion star forming region (d ∼ 420 pc). We present follow-up photometric and spectroscopic observations of the source after the ΔV ∼ –5.4 mag outburst that began in 2013 September and ended in early 2014. These data indicate an increase in temperature and luminosity consistent with an accretion rate of ∼10{sup –7} M {sub ☉} yr{sup –1}, three or more orders of magnitude greater than in quiescence. Spectroscopic observations show a forest of narrow emission lines dominated by neutral metallic lines from Fe I and some low-ionization lines. The properties of ASASSN-13db are similar to those of the EXor prototype EX Lupi during its strongest observed outburst in late 2008.

  12. Quasi-periodic accretion and gravitational waves from oscillating "toroidal neutron stars" around a Schwarzschild black hole

    CERN Document Server

    Zanotti, O; Font, J A

    2003-01-01

    We present general relativistic hydrodynamics simulations of constant specific angular momentum tori orbiting a Schwarzschild black hole. These tori are expected to form as a result of stellar gravitational collapse, binary neutron star merger or disruption, can reach very high rest-mass densities and behave effectively as neutron stars but with a toroidal topology (i.e. ``toroidal neutron stars''). Our attention is here focussed on the dynamical response of these objects to axisymmetric perturbations. We show that, upon the introduction of perturbations, these systems either become unstable to the runaway instability or exhibit a regular oscillatory behaviour resulting in a quasi-periodic variation of the accretion rate as well as of the mass quadrupole. The latter, in particular, is responsible for the emission of intense gravitational radiation whose signal-to-noise ratio at the detector is comparable or larger than the typical one expected in stellar-core collapse, making these new sources of gravitationa...

  13. Production of all $r$-process nuclides by black hole accretion disk outflows from neutron star mergers

    CERN Document Server

    Wu, Meng-Ru; Martínez-Pinedo, Gabriel; Metzger, Brian D

    2016-01-01

    We consider $r$-process nucleosynthesis in outflows from black hole accretion disks formed in double neutron star and neutron star - black hole mergers. These outflows, powered by angular momentum transport processes and nuclear recombination, represent an important -- and in some cases dominant -- contribution to the total mass ejected by the merger. Here we calculate the nucleosynthesis yields from disk outflows using thermodynamic trajectories from hydrodynamic simulations, coupled to a nuclear reaction network. We find that outflows produce a robust abundance pattern around the second $r$-process peak (mass number $A \\sim 130$), independent of model parameters, with significant production of $A < 130$ nuclei. This implies that dynamical ejecta with high electron fraction may not be required to explain the observed abundances of $r$-process elements in metal poor stars. Disk outflows reach the third peak ($ A \\sim 195$) in most of our simulations, although the amounts produced depend sensitively on the ...

  14. GRB060602B = Swift J1749.4-2807: an unusual transiently accreting neutron-star X-ray binary

    CERN Document Server

    Wijnands, Rudy; Cackett, Ed M; Starling, Rhaana L C; Remillard, Ron A

    2007-01-01

    We present an analysis of the Swift BAT and XRT data of GRB060602B, which is most likely an accreting neutron star in a binary system and not a gamma-ray burst. Our analysis shows that the BAT burst spectrum is consistent with a thermonuclear flash (type-I X-ray burst) from the surface of an accreting neutron star in a binary system. The X-ray binary nature is further confirmed by the report of a detection of a faint point source at the position of the XRT counterpart of the burst in archival XMM-Newton data approximately 6 years before the burst and in more recent XMM-Newton data obtained at the end of September 2006 (nearly 4 months after the burst). Since the source is very likely not a gamma-ray burst, we rename the source Swift J1749.4-2807, based on the Swift/BAT discovery coordinates. Using the BAT data of the type-I X-ray burst we determined that the source is at most at a distance of 6.7+-1.3 kpc. For a transiently accreting X-ray binary its soft X-ray behaviour is atypical: its 2-10 keV X-ray lumino...

  15. New Candidate Eruptive Young Stars in Lynds 1340

    CERN Document Server

    Kun, M; O'Linger-Luscusk, J; Moór, A; Stecklum, B; Szegedi-Elek, E; Wolf-Chase, G

    2014-01-01

    We report on the discovery of three candidate eruptive young stars, found during our comprehensive multi-wavelength study of the young stellar population of the dark cloud L1340. These stars are as follows. (1) IRAS~02224+7227 (2MASS 02270555+7241167, HH 487 S) exhibited FUor-like spectrum in our low-resolution optical spectra. The available photometric data restrict its luminosity to 23 L_sun < L_bol < 59 L_sun. (2) 2MASS 02263797+7304575, identified as a classical T Tauri star during our H alpha survey, exhibited an EXor type brightening in 2005 November, at the time of the SDSS observations of the region. (3) 2MASS 02325605+7246055, a low-mass embedded young star, associated with a fan-shaped infrared nebula, underwent an outburst between the DSS1 and DSS2 surveys, leading to the appearance of a faint optical nebula. Our [SII] and H alpha images, as well as the Spitzer IRAC 4.5 micron images revealed Herbig-Haro objects associated with this star. Our results suggest that amplitudes and time scales of...

  16. Hydrodynamic Modeling of Accretion Impacts in Classical T Tauri Stars: Radiative Heating of the Pre-shock Plasma

    CERN Document Server

    Costa, G; Peres, G; Argiroffi, C; Bonito, R

    2016-01-01

    Context. It is generally accepted that, in Classical T Tauri Stars, the plasma from the circumstellar disc accretes onto the stellar surface with free fall velocity, and the impact generates a shock. The impact region is expected to contribute to emission in different spectral bands; many studies have confirmed that the X-rays arise from the post-shock plasma but, otherwise, there are no studies in the literature investigating the origin of the observed UV emission which is apparently correlated to accretion. Aims. We investigated the effect of radiative heating of the infalling material by the post-shock plasma at the base of the accretion stream with the aim to identify in which region a significant part of the UV emission originates. Methods. We developed a 1D hydrodynamic model describing the impact of an accretion stream onto the stellar surface; the model takes into account the gravity, the radiative cooling of an optically thin plasma, the thermal conduction, and the heating due to absorption of X-ray ...

  17. Rapid Variability as a Diagnostic of Accretion and Nuclear Burning in Symbiotic Stars and Supersoft X-ray Sources

    CERN Document Server

    Sokoloski, J L

    2002-01-01

    Accretion disks and nuclear shell burning are present in some symbiotic stars (SS) and probably all supersoft X-ray binaries (SSXBs). Both the disk and burning shell may be involved in the production of dramatic outbursts and, in some cases, collimated jets. A strong magnetic field may also affect the accretion flow and activity in some systems. Rapid-variability studies can probe the interesting region close to the accreting white dwarf (WD) in both SS and SSXBs. I describe fast photometric observations of several individual systems in detail, and review the results of a photometric variability survey of 35 SS. These timing studies reveal the first clearly magnetic SS (Z And), and suggest that an accretion disk is involved in jet production in CH Cyg as well as in the outbursts of both CH Cyg and Z And. They also support the notion that the fundamental power source in most SS is nuclear burning on the surface of a WD, and raise questions about the structure of disks in the SSXBs. Finally, spectroscopic obser...

  18. Magnetohydrodynamic modeling of the accretion shocks in classical T Tauri stars: the role of local absorption on the X-ray emission

    CERN Document Server

    Bonito, R; Argiroffi, C; Miceli, M; Peres, G; Matsakos, T; Stehle, C; Ibgui, L

    2014-01-01

    We investigate the properties of X-ray emission from accretion shocks in classical T Tauri stars (CTTSs), generated where the infalling material impacts the stellar surface. Both observations and models of the accretion process reveal several aspects that are still unclear: the observed X-ray luminosity in accretion shocks is below the predicted value, and the density versus temperature structure of the shocked plasma, with increasing densities at higher temperature, deduced from the observations, is at odds with that proposed in the current picture of accretion shocks. To address these open issues we investigate whether a correct treatment of the local absorption by the surrounding medium is crucial to explain the observations. To this end, we describe the impact of an accretion stream on a CTTS by considering a magnetohydrodynamic model. From the model results we synthesize the X-ray emission from the accretion shock by producing maps and spectra. We perform density and temperature diagnostics on the synthe...

  19. The Brightest Young Star Clusters in NGC 5253

    NARCIS (Netherlands)

    Calzetti, D.; Johnson, K.E.; Adamo, A.; Gallagher III, J.S.; Andrews, J.E.; Smith, L.J.; Clayton, G.C.; Lee, J.C.; Sabbi, E.; Ubeda, L.; Kim, H.; Ryon, J.E.; Thilker, D.; Bright, S.N.; Zackrisson, E.; Kennicutt, R.C.; de Mink, S.E.; Whitmore, B.C.; Aloisi, A.; Chandar, R.; Cignoni, M.; Cook, D.; Dale, D.A.; Elmegreen, B.G.; Elmegreen, D.M.; Evans, A.S.; Fumagalli, M.; Gouliermis, D.A.; Grasha, K.; Grebel, E.K.; Krumholz, M.R.; Waterbos, R.; Wofford, A.; Brown, T.M.; Christian, C.; Dobbs, C.; Herrero, A.; Kahre, L.; Messa, M.; Nair, P.; Nota, A.; Östlin, G.; Pellerin, A.; Sacchi, E.; Schaerer, D.; Tosi, M.

    2015-01-01

    The nearby dwarf starburst galaxy NGC 5253 hosts a number of young, massive star clusters, the two youngest of which are centrally concentrated and surrounded by thermal radio emission (the "radio nebula"). To investigate the role of these clusters in the starburst energetics, we combine new and

  20. The Brightest Young Star Clusters in NGC 5253

    NARCIS (Netherlands)

    Calzetti, D.; Johnson, K.E.; Adamo, A.; Gallagher III, J.S.; Andrews, J.E.; Smith, L.J.; Clayton, G.C.; Lee, J.C.; Sabbi, E.; Ubeda, L.; Kim, H.; Ryon, J.E.; Thilker, D.; Bright, S.N.; Zackrisson, E.; Kennicutt, R.C.; de Mink, S.E.; Whitmore, B.C.; Aloisi, A.; Chandar, R.; Cignoni, M.; Cook, D.; Dale, D.A.; Elmegreen, B.G.; Elmegreen, D.M.; Evans, A.S.; Fumagalli, M.; Gouliermis, D.A.; Grasha, K.; Grebel, E.K.; Krumholz, M.R.; Waterbos, R.; Wofford, A.; Brown, T.M.; Christian, C.; Dobbs, C.; Herrero, A.; Kahre, L.; Messa, M.; Nair, P.; Nota, A.; Östlin, G.; Pellerin, A.; Sacchi, E.; Schaerer, D.; Tosi, M.

    2015-01-01

    The nearby dwarf starburst galaxy NGC 5253 hosts a number of young, massive star clusters, the two youngest of which are centrally concentrated and surrounded by thermal radio emission (the "radio nebula"). To investigate the role of these clusters in the starburst energetics, we combine new and arc

  1. Multiwavelength study of the low-luminosity outbursting young star HBC 722

    Science.gov (United States)

    Kóspál, Á.; Ábrahám, P.; Acosta-Pulido, J. A.; Dunham, M. M.; García-Álvarez, D.; Hogerheijde, M. R.; Kun, M.; Moór, A.; Farkas, A.; Hajdu, G.; Hodosán, G.; Kovács, T.; Kriskovics, L.; Marton, G.; Molnár, L.; Pál, A.; Sárneczky, K.; Sódor, Á.; Szakáts, R.; Szalai, T.; Szegedi-Elek, E.; Szing, A.; Tóth, I.; Vida, K.; Vinkó, J.

    2016-11-01

    Context. HBC 722 (V2493 Cyg) is a young eruptive star in outburst since 2010. Spectroscopic evidence suggests that the source is an FU Orionis-type object, with an atypically low outburst luminosity. Aims: Because it was well characterized in the pre-outburst phase, HBC 722 is one of the few FUors from which we can learn about the physical changes and processes associated with the eruption, including the role of the circumstellar environment. Methods: We monitored the source in the BVRIJHKS bands from the ground and at 3.6 and 4.5 μm from space with the Spitzer Space Telescope. We analyzed the light curves and studied the evolving spectral energy distribution by fitting a series of steady accretion disk models at many epochs covering the outburst. We also analyzed the spectral properties of the source based on our new optical and infrared spectra, comparing our line inventory with those published in the literature for other epochs. We also mapped HBC 722 and its surroundings at millimeter wavelengths. Results: From the light-curve analysis we conclude that the first peak of the outburst in 2010 September was mainly due to an abrupt increase in the accretion rate in the innermost part of the system. This was followed after a few months by a long-term process, when the brightening of the source was mainly due to a gradual increase in the accretion rate and the emitting area. Our new observations show that the source is currently in a constant plateau phase. We found that the optical spectrum was similar in the first peak and following periods, but around the peak the continuum was bluer and the Hα profile changed significantly between 2012 and 2013. The source was not detected in the millimeter continuum, but we discovered a flattened molecular gas structure with a diameter of 1700 au and mass of 0.3 M⊙ centered on HBC 722. Conclusions: While the first brightness peak might be interpreted as a rapid fall of piled-up material from the inner disk onto the star, the

  2. Anomalous Spectral Types and Intrinsic Colors of Young Stars

    CERN Document Server

    Pecaut, Mark J

    2016-01-01

    We highlight differences in spectral types and intrinsic colors observed in pre-main sequence (pre-MS) stars. Spectral types of pre-MS stars are wavelength-dependent, with near-infrared spectra being 3-5 spectral sub-classes later than the spectral types determined from optical spectra. In addition, the intrinsic colors of young stars differ from that of main-sequence stars at a given spectral type. We caution observers to adopt optical spectral types over near-infrared types, since Hertzsprung-Russell (H-R) diagram positions derived from optical spectral types provide consistency between dynamical masses and theoretical evolutionary tracks. We also urge observers to deredden pre-MS stars with tabulations of intrinsic colors specifically constructed for young stars, since their unreddened colors differ from that of main sequence dwarfs. Otherwise, V-band extinctions as much as ~0.6 mag erroneously higher than the true extinction may result, which would introduce systematic errors in the H-R diagram positions ...

  3. Anomalous Spectral Types and Intrinsic Colors of Young Stars

    Science.gov (United States)

    Pecaut, Mark J.

    2016-01-01

    We highlight differences in spectral types and intrinsic colors observed in pre-main sequence (pre-MS) stars. Spectral types of pre-MS stars are wavelength-dependent, with near-infrared spectra being 3-5 spectral sub-classes later than the spectral types determined from optical spectra. In addition, the intrinsic colors of young stars differ from that of main-sequence stars at a given spectral type. We caution observers to adopt optical spectral types over near-infrared types, since Hertzsprung-Russell (H-R) diagram positions derived from optical spectral types provide consistency between dynamical masses and theoretical evolutionary tracks. We also urge observers to deredden pre-MS stars with tabulations of intrinsic colors specifically constructed for young stars, since their unreddened colors differ from that of main sequence dwarfs. Otherwise, V-band extinctions as much as ~0.6 mag erroneously higher than the true extinction may result, which would introduce systematic errors in the H-R diagram positions and thus bias the inferred ages.

  4. X-ray Emission from Young Stars in the TW Hya Association

    Science.gov (United States)

    Brown, Alexander; Herczeg, Gregory J.; Ayres, Thomas R.; France, Kevin; Brown, Joanna M.

    2015-01-01

    The 9 Myr old TW Hya Association (TWA) is the nearest group (typical distances of ˜50 pc) of pre-main-sequence (PMS) stars with ages less than 10 Myr and contains stars with both actively accreting disks and debris disks. We have studied the coronal X-ray emission from a group of low mass TWA common proper motion binaries using the Chandra and Swift satellites. Our aim is to understand better their coronal properties and how high energy photons affect the conditions around young stars and their role in photo-exciting atoms, molecules and dust grains in circumstellar disks and lower density circumstellar gas. Once planet formation is underway, this emission influences protoplanetary evolution and the atmospheric conditions of the newly-formed planets. The X-ray properties for 7 individual stars (TWA 13A, TWA 13B, TWA 9A, TWA 9B, TWA 8A, TWA 8B, and TWA 7) and 2 combined binary systems (TWA 3AB and TWA 2AB) have been measured. All the stars with sufficient signal require two-component fits to their CCD-resolution X-ray spectra, typically with a dominant hot (~2 kev (25 MK)) component and a cooler component at ~0.4 keV (4 MK). The brighter sources all show significant X-ray variability (at a level of 50-100% of quiescence) over the course of 5-15 ksec observations due to flares. We present the X-ray properties for each of the stars and find that the coronal emission is in the super-saturated rotational domain.

  5. X-ray Properties of Young Stars and Stellar Clusters

    CERN Document Server

    Feigelson, E; Güdel, M; Stassun, K G; Feigelson, Eric; Townsley, Leisa; Gudel, Manuel

    2006-01-01

    Although the environments of star and planet formation are thermodynamically cold, substantial X-ray emission from 10-100 MK plasmas is present. In low mass pre-main sequence stars, X-rays are produced by violent magnetic reconnection flares. In high mass O stars, they are produced by wind shocks on both stellar and parsec scales. The recent Chandra Orion Ultradeep Project, XMM-Newton Extended Survey of Taurus, and Chandra studies of more distant high-mass star forming regions reveal a wealth of X-ray phenomenology and astrophysics. X-ray flares mostly resemble solar-like magnetic activity from multipolar surface fields, although extreme flares may arise in field lines extending to the protoplanetary disk. Accretion plays a secondary role. Fluorescent iron line emission and absorption in inclined disks demonstrate that X-rays can efficiently illuminate disk material. The consequent ionization of disk gas and irradiation of disk solids addresses a variety of important astrophysical issues of disk dynamics, pla...

  6. Panchromatic Hubble Andromeda Treasury XVI. Star Cluster Formation Efficiency and the Clustered Fraction of Young Stars

    CERN Document Server

    Johnson, L Clifton; Dalcanton, Julianne J; Beerman, Lori C; Fouesneau, Morgan; Lewis, Alexia R; Weisz, Daniel R; Williams, Benjamin F; Bell, Eric F; Dolphin, Andrew E; Larsen, Søren S; Sandstrom, Karin; Skillman, Evan D

    2016-01-01

    We use the Panchromatic Hubble Andromeda Treasury (PHAT) survey dataset to perform spatially resolved measurements of star cluster formation efficiency ($\\Gamma$), the fraction of stellar mass formed in long-lived star clusters. We use robust star formation history and cluster parameter constraints, obtained through color-magnitude diagram analysis of resolved stellar populations, to study Andromeda's cluster and field populations over the last $\\sim$300 Myr. We measure $\\Gamma$ of 4-8% for young, 10-100 Myr old populations in M31. We find that cluster formation efficiency varies systematically across the M31 disk, consistent with variations in mid-plane pressure. These $\\Gamma$ measurements expand the range of well-studied galactic environments, providing precise constraints in an HI-dominated, low intensity star formation environment. Spatially resolved results from M31 are broadly consistent with previous trends observed on galaxy-integrated scales, where $\\Gamma$ increases with increasing star formation r...

  7. Southern near-infrared photometric monitoring of Galactic young star clusters (NIP of Stars)

    CERN Document Server

    Barbá, R; Gunthardt, G; Robledo, S Torres; Jaque, M; Soto, M; Ferrero, G; Arias, J; Román-Lópes, A; Gamen, R; Hormazabal, J Astudillo

    2012-01-01

    We have performed a near-infrared photometric monitoring of 39 galactic young star clusters and star-forming regions, known as {\\em NIP of Stars}, between the years 2009--2011, using the Swope telescope at Las Campanas Observatory (Chile) and the RetroCam camera. The primary objective of the campaign is to perform a census of photometric variability of such clusters and to discover massive eclipsing binary stars. In this work, we describe the general idea, the implementation of the survey, and the first preliminary results of some of the observed clusters. This monitoring program is complementary to the Vista Variables in the V\\'ia L\\'actea (VVV), as the brightest sources observed in NIP of Stars are saturated in VVV.

  8. Southern near-infrared photometric monitoring of Galactic young star clusters (NIP of Stars)

    Science.gov (United States)

    Barbá, R.; Morrell, N. I.; Gunthardt, G.; Torres Robledo, S.; Jaque, M.; Soto, M.; Ferrero, G.; Arias, J. I.; Roman-Lopes, A.; Gamen, R. C.; Astudillo Hormazabal, J.

    We have performed a near-infrared photometric monitoring of 39 galactic young star clusters and star-forming regions, known as NIP of Stars, be- tween the years 2009-2011, using the Swope telescope at Las Campanas Observatory (Chile) and the RetroCam camera. The primary objective of the campaign is to perform a census of photometric variability of such clus- ters and to discover massive eclipsing binary stars. In this work, we describe the general idea, the implementation of the survey, and the first preliminary results of some of the observed clusters. This monitoring program is com- plementary to the Vista Variables in the Vía Láctea (VVV), as the brightest sources observed in NIP of Stars are saturated in VVV.

  9. Young stars in the periphery of the Large Magellanic Cloud

    Science.gov (United States)

    Moni Bidin, C.; Casetti-Dinescu, D. I.; Girard, T. M.; Zhang, L.; Méndez, R. A.; Vieira, K.; Korchagin, V. I.; van Altena, W. F.

    2017-04-01

    Despite their close proximity, the complex interplay between the two Magellanic Clouds, the Milky Way and the resulting tidal features, is still poorly understood. Recent studies have shown that the Large Magellanic Cloud (LMC) has a very extended disc strikingly perturbed in its outskirts. We search for recent star formation in the far outskirts of the LMC, out to ˜ 30° from its centre. We have collected intermediate-resolution spectra of 31 young star candidates in the periphery of the LMC and measured their radial velocity, stellar parameters, distance and age. Our measurements confirm membership to the LMC of six targets, for which the radial velocity and distance values match well with those of the Cloud. These objects are all young (10-50 Myr), main-sequence stars, projected between 7° and 13° from the centre of the parent galaxy. We compare the velocities of our stars with those of a disc model, and find that our stars have low to moderate velocity differences with the disc model predictions, indicating that they were formed in situ. Our study demonstrates that recent star formation occurred in the far periphery of the LMC, where thus far only old objects were known. The spatial configuration of these newly formed stars appears ring-like with a radius of 12 kpc and a displacement of 2.6 kpc from the LMC's centre. This structure, if real, would be suggestive of a star formation episode triggered by an off-centre collision between the Small Magellanic Cloud and the LMC's disc.

  10. On the new braking index of PSR B0540-69: further support for magnetic field growth of neutron stars following submergence by fallback accretion

    Science.gov (United States)

    Ekşi, K. Yavuz

    2017-08-01

    The magnetic fields of the nascent neutron stars could be submerged in the crust by rapid fallback accretion and could diffuse to the surface later in life. According to this field burial scenario young pulsars may have growing magnetic fields, a process known to result with less-than-three braking indices; larger braking indices implying longer field-growth time-scales. A nascent neutron star with a larger kick velocity would accrete less amount of matter leading to a shallower burial of its field and a more rapid field growth. Such an inverse relation between the field-growth time-scale inferred from the braking indices and space velocity of pulsars was claimed in the past as a prediction of the field burial scenario. With a braking index of n ˜ 2 and large space velocity PSR B0540-69 was then an outlier in the claimed relation. The object recently made a transition to a rapid spin-down state accompanied by a low braking index. This new braking index implies a much shorter time-scale for the field growth which is consistent with the high space velocity of the object, in better agreement with the claimed relation. This observation lends support to the field burial scenario and implies that the growth of the magnetic field does not proceed at a constant pace but is slowed or completely halted at times. The slow spin-down state associated with the high braking index before 2011 which lasted for at least about 30 yr was then such an episode of slow field growth.

  11. The evolutionary tracks of young massive star clusters

    CERN Document Server

    Pfalzner, S; Steinhausen, M; Vincke, K; Menten, K

    2014-01-01

    Stars mostly form in groups consisting of a few dozen to several ten thousand members. For 30 years, theoretical models provide a basic concept of how such star clusters form and develop: they originate from the gas and dust of collapsing molecular clouds. The conversion from gas to stars being incomplete, the left over gas is expelled, leading to cluster expansion and stars becoming unbound. Observationally, a direct confirmation of this process has proved elusive, which is attributed to the diversity of the properties of forming clusters. Here we take into account that the true cluster masses and sizes are masked, initially by the surface density of the background and later by the still present unbound stars. Based on the recent observational finding that in a given star-forming region the star formation efficiency depends on the local density of the gas, we use an analytical approach combined with \\mbox{N-body simulations, to reveal} evolutionary tracks for young massive clusters covering the first 10 Myr....

  12. Impacts of fragmented accretion streams onto classical T Tauri stars: UV and X-ray emission lines

    Science.gov (United States)

    Colombo, S.; Orlando, S.; Peres, G.; Argiroffi, C.; Reale, F.

    2016-10-01

    Context. The accretion process in classical T Tauri stars (CTTSs) can be studied through the analysis of some UV and X-ray emission lines which trace hot gas flows and act as diagnostics of the post-shock downfalling plasma. In the UV-band, where higher spectral resolution is available, these lines are characterized by rather complex profiles whose origin is still not clear. Aims: We investigate the origin of UV and X-ray emission at impact regions of density structured (fragmented) accretion streams. We study if and how the stream fragmentation and the resulting structure of the post-shock region determine the observed profiles of UV and X-ray emission lines. Methods: We modeled the impact of an accretion stream consisting of a series of dense blobs onto the chromosphere of a CTTS through two-dimensional (2D) magnetohydrodynamic (MHD) simulations. We explored different levels of stream fragmentation and accretion rates. From the model results, we synthesize C IV (1550 Å) and O VIII (18.97 Å) line profiles. Results: The impacts of accreting blobs onto the stellar chromosphere produce reverse shocks propagating through the blobs and shocked upflows. These upflows, in turn, hit and shock the subsequent downfalling fragments. As a result, several plasma components differing for the downfalling velocity, density, and temperature are present altoghether. The profiles of C IV doublet are characterized by two main components: one narrow and redshifted to speed ≈ 50 km s-1 and the other broader and consisting of subcomponents with redshift to speed in the range 200-400 km s-1. The profiles of O VIII lines appear more symmetric than C IV and are redshifted to speed ≈ 150 km s-1. Conclusions: Our model predicts profiles of C IV line remarkably similar to those observed and explains their origin in a natural way as due to stream fragmentation. Movies are available at http://www.aanda.org

  13. Exploring the circumstellar environment of the young eruptive star V2492 Cyg

    CERN Document Server

    Kóspál, Á; Acosta-Pulido, J A; Morales, M J Arévalo; Balog, Z; Carnerero, M I; Szegedi-Elek, E; Farkas, A; Henning, Th; Kelemen, J; Kovács, T; Kun, M; Marton, G; Mészáros, Sz; Moór, A; Pál, A; Sárneczky, K; Szakáts, R; Szalai, N; Szing, A; Tóth, I; Turner, N J; Vida, K

    2013-01-01

    Context. V2492 Cyg is a young eruptive star that went into outburst in 2010. The near-infrared color changes observed since the outburst peak suggest that the source belongs to a newly defined sub-class of young eruptive stars where time-dependent accretion and variable line-of-sight extinction play a combined role in the flux changes. Aims. In order to learn about the origin of the light variations and to explore the circumstellar and interstellar environment of V2492 Cyg, we monitored the source at ten different wavelengths, between 0.55 \\mu m and 2.2 \\mu m from the ground, and between 3.6 \\mu m and 160 \\mu m from space. Methods. We analyze the light curves and study the color-color diagrams via comparison with the standard reddening path. We examine the structure of the molecular cloud hosting V2492 Cyg by computing temperature and optical depth maps from the far-infrared data. Results. We find that the shapes of the light curves at different wavelengths are strictly self-similar, and the observed variabil...

  14. Protoplanetary disk shadowing by gas infalling onto the young star AK Sco

    CERN Document Server

    de Castro, Ana I Gomez; France, Kevin; Sytov, Alexey; Bisikalo, Dmitry

    2016-01-01

    Young solar-type stars grow through the accretion of material from the circumstellar disk during pre-main sequence (PMS) evolution. The ultraviolet radiation generated in this process plays a key role in the chemistry and evolution of young planetary disks. In particular, the hydrogen Lyman-alpha line (Lya) etches the disk surface by driving photoevaporative flows that control disk evolution. Using the Hubble Space Telescope, we have monitored the PMS binary star AK Sco during the periastron passage and have detected a drop of the H2 flux by up to 10% lasting 5.9 hours. We show that the decrease of the H2 flux can be produced by the occultation of the stellar Lya photons by a gas stream in free fall from 3 R*. Given the high optical depth of the Lya line, a very low gas column of NH > 5e17 cm-2 suffices to block the Lya radiation without producing noticeable effects in the rest of the stellar spectral tracers.

  15. The Correlation Dimension of Young Stars in Dwarf Galaxies

    CERN Document Server

    Odekon, M C

    2006-01-01

    We present the correlation dimension of resolved young stars in four actively star-forming dwarf galaxies that are sufficiently resolved and transparent to be modeled as projections of three-dimensional point distributions. We use data in the Hubble Space Telescope archive; photometry for one of them, UGCA 292, is presented here for the first time. We find that there are statistically distinguishable differences in the nature of stellar clustering among the sample galaxies. The young stars of VII Zw 403, the brightest galaxy in the sample, have the highest value for the correlation dimension and also the most dramatic decrease with logarithmic scale, falling from $1.68\\pm0.14$ to $0.10\\pm0.05$ over less than a factor of ten in $r$. This decrease is consistent with the edge effect produced by a projected Poisson distribution within a 2:2:1 ellipsoid. The young stars in UGC 4483, the faintest galaxy in the sample, exhibit very different behavior, with a constant value of about 0.5 over this same range in $r$, e...

  16. Targeting Young Stars with Kepler: Planet Formation, Migration Mechanisms and the Early History of Planetary Systems

    CERN Document Server

    Lloyd, James P; Mamajek, Eric; Spiegel, David S; Covey, Kevin R; Shkolnik, Evgenya L; Walkowicz, Lucianne; Chavez, Miguel; Bertone, Emanuele; Aguilar, Jose Manuel Olmedo

    2013-01-01

    This white paper discusses a repurposed mission for the Kepler spacecraft that focusses on solving outstanding problems in planet formation and evolution by targeting the study of the hot Jupiter population of young stars. This mission can solve the question of the mode of migration of hot Jupiters, address the problem of whether Jupiters form by hot-start (gravitational instability) or cold-start (core accretion) mechanisms, and provide a wealth of data on the early stages of planetary system evolution during the active phases of stars which impact planetary habitability. In one year of observations of three weeks dwell time per field, Kepler would increase by more than an order of magnitude the number of known hot Jupiters, which can be followed up with fast cadence observations to to search for transit timing variations and to perform asteroseismological characterization of the host stars. This mission scenario continues to operate Kepler in the photometric monitoring mode for which it was designed, and is...

  17. Young stars in the galactic center: one or two disks?

    Energy Technology Data Exchange (ETDEWEB)

    Bartko, H; Eisenhauer, F; Fritz, T; Genzel, R; Gillessen, S; Ott, T; Pfuhl, O; Trippe, S [Max-Planck-Institute for Extraterrestrial Physics, Garching (Germany); Martins, F [GRAAL, Universit Montpellier II (France); Paumard, T [LESIA, Observatoire de Paris, CNRS, UPMC, Universit Paris Direrot, Meudon (France)], E-mail: hbartko@mpe.mpg.de

    2008-10-15

    The central pc around the super-massive black hole in the Galactic Center hosts a population of young and massive stars. The majority of these stars (outside the central 1') have been found to reside in disks. Here we develop a detailed statistical analysis method of the properties of these disks including a robust test of the significance of these disks versus an isotropic stellar population. We apply this method to the data set obtained with the AO assisted integral field spectrograph SINFONI on the ESO/VLT as of the end of 2007 and present preliminary results.

  18. An in-depth study of a neutron star accreting at low Eddington rate: on the possibility of a truncated disc and an outflow

    Science.gov (United States)

    Degenaar, N.; Pinto, C.; Miller, J. M.; Wijnands, R.; Altamirano, D.; Paerels, F.; Fabian, A. C.; Chakrabarty, D.

    2017-01-01

    Due to observational challenges, our knowledge of low-level accretion flows around neutron stars is limited. We present NuSTAR, Swift and Chandra observations of the low-mass X-ray binary IGR J17062-6143, which has been persistently accreting at ≃0.1 per cent of the Eddington limit since 2006. Our simultaneous NuSTAR/Swift observations show that the 0.5-79 keV spectrum can be described by a combination of a power law with a photon index of Γ ≃ 2, a blackbody with a temperature of kTbb ≃ 0.5 keV (presumably arising from the neutron star surface) and disc reflection. Modelling the reflection spectrum suggests that the inner accretion disc was located at Rin ≳ 100 GM/c2 (≳225 km) from the neutron star. The apparent truncation may be due to evaporation of the inner disc into a radiatively-inefficient accretion flow, or due to the pressure of the neutron star magnetic field. Our Chandra gratings data reveal possible narrow emission lines near 1 keV that can be modelled as reflection or collisionally ionized gas, and possible low-energy absorption features that could point to the presence of an outflow. We consider a scenario in which this neutron star has been able to sustain its low accretion rate through magnetic inhibition of the accretion flow, which gives some constraints on its magnetic field strength and spin period. In this configuration, IGR J17062-6143 could exhibit a strong radio jet as well as a (propeller-driven) wind-like outflow.

  19. Impact of initial models and variable accretion rates on the pre-main-sequence evolution of massive and intermediate-mass stars and the early evolution of HII regions

    CERN Document Server

    Haemmerlé, Lionel

    2016-01-01

    Massive star formation requires the accretion of gas at high rate while the star is already bright. Its actual luminosity depends sensitively on the stellar structure. We compute pre-main-sequence tracks for massive and intermediate-mass stars with variable accretion rates and study the evolution of stellar radius, effective temperature and ionizing luminosity, starting at $2\\,M_\\odot$ with convective or radiative structures. The radiative case shows a much stronger swelling of the protostar for high accretion rates than the convective case. For radiative structures, the star is very sensitive to the accretion rate and reacts quickly to accretion bursts, leading to considerable changes in photospheric properties on timescales as short as 100 - 1000 yr. The evolution for convective structures is much less influenced by the instantaneous accretion rate, and produces a monotonically increasing ionizing flux that can be many orders of magnitude smaller than in the radiative case. For massive stars, it results in ...

  20. Supersaturation and Activity-Rotation Relation in PMS stars: the case of the Young Cluster h Per

    CERN Document Server

    Argiroffi, C; Micela, G; Sciortino, S; Moraux, E; Bouvier, J; Flaccomio, E

    2016-01-01

    The magnetic activity of late-type MS stars is characterized by different regimes, and their activity levels are well described by Ro, the ratio between P_rot and the convective turnover time. Very young PMS stars show, similarly to MS stars, intense magnetic activity. However they do not show clear activity-rotation trends, and it still debated which stellar parameters determine their magnetic activity levels. To bridge the gap between MS and PMS stars, we studied the activity-rotation relation in the young cluster h Per, a ~13 Myr old cluster, that contains both fast and slow rotators, whose members have ended their accretion phase and have already developed a radiative core. It offers us the opportunity to study the activity level of intermediate-age PMS stars with different rotational velocities, excluding any interactions with the circumstellar environment. We constrained the magnetic activity levels of h Per members measuring their X-ray emission from a Chandra observation, while P_rot were obtained by ...

  1. The galactocentric radius dependent upper mass limit of young star clusters: stochastic star formation ruled out

    CERN Document Server

    Pflamm-Altenburg, Jan; Kroupa, Pavel

    2013-01-01

    It is widely accepted that the distribution function of the masses of young star clusters is universal and can be purely interpreted as a probability density distribution function with a constant upper mass limit. As a result of this picture the masses of the most-massive objects are exclusively determined by the size of the sample. Here we show, with very high confidence, that the masses of the most-massive young star clusters in M33 decrease with increasing galactocentric radius in contradiction to the expectations from a model of a randomly sampled constant cluster mass function with a constant upper mass limit. Pure stochastic star formation is thereby ruled out. We use this example to elucidate how naive analysis of data can lead to unphysical conclusions.

  2. Stellar Collisions in Young Clusters: Formation of (Very) Massive Stars?

    CERN Document Server

    Freitag, Marc

    2007-01-01

    In young star clusters, the density can be high enough and the velocity dispersion low enough for stars to collide and merge with a significant probability. This has been suggested as a possible way to build up the high-mass portion of the stellar mass function and as a mechanism leading to the formation of one or two very massive stars (M > 150 Msun) through a collisional runaway. I quickly review the standard theory of stellar collisions, covering both the stellar dynamics of dense clusters and the hydrodynamics of encounters between stars. The conditions for collisions to take place at a significant rate are relatively well understood for idealised spherical cluster models without initial mass segregation, devoid of gas and composed of main-sequence (MS) stars. In this simplified situation, 2-body relaxation drives core collapse through mass segregation and a collisional phase ensues if the core collapse time is shorter than the MS lifetime of the most massive stars initially present. The outcome of this p...

  3. GRACES observations of young [alpha/Fe]-rich stars

    CERN Document Server

    Yong, David; Venn, Kim A; Chene, Andre-Nicolas; Keown, Jared; Malo, Lison; Martioli, Eder; Alves-Brito, Alan; Asplund, Martin; Dotter, Aaron; Martell, Sarah L; Melendez, Jorge; Schlesinger, Katharine J

    2016-01-01

    We measure chemical abundance ratios and radial velocities in four massive (i.e., young) [alpha/Fe]-rich red giant stars using high-resolution high-S/N spectra from ESPaDOnS fed by Gemini-GRACES. Our differential analysis ensures that our chemical abundances are on the same scale as the Alves-Brito et al. (2010) study of bulge, thin and thick disk red giants. We confirm that the program stars have enhanced [alpha/Fe] ratios and are slightly metal poor. Aside from lithium enrichment in one object, the program stars exhibit no chemical abundance anomalies when compared to giant stars of similar metallicity throughout the Galaxy. This includes the elements Li, O, Si, Ca, Ti, Cr, Ni, Cu, Ba, La, and Eu. Therefore, there are no obvious chemical signatures that can help to reveal the origin of these unusual stars. While our new observations show that only one star (not the Li-rich object) exhibits a radial velocity variation, simulations indicate that we cannot exclude the possibility that all four could be binarie...

  4. Young alpha-enriched giant stars in the solar neighbourhood

    CERN Document Server

    Martig, Marie; Aguirre, Victor Silva; Hekker, Saskia; Mosser, Benoit; Elsworth, Yvonne; Bovy, Jo; Stello, Dennis; Anders, Friedrich; García, Rafael A; Tayar, Jamie; Rodrigues, Thaíse S; Basu, Sarbani; Carrera, Ricardo; Ceillier, Tugdual; Chaplin, William J; Chiappini, Cristina; Frinchaboy, Peter M; García-Hernández, D A; Hearty, Fred R; Holtzman, Jon; Johnson, Jennifer A; Mathur, Savita; Mészáros, Szabolcs; Miglio, Andrea; Nidever, David; Pinsonneault, Marc; Schiavon, Ricardo P; Schneider, Donald P; Serenelli, Aldo; Shetrone, Matthew; Zamora, Olga

    2014-01-01

    We derive age constraints for 1639 red giants in the APOKASC sample for which seismic parameters from Kepler, as well as effective temperatures, metallicities and [{\\alpha}/Fe] values from APOGEE DR12 are available. We investigate the relation between age and chemical abundances for these stars, using a simple and robust approach to obtain ages. We first derive stellar masses using standard seismic scaling relations, then determine the maximum possible age for each star as function of its mass and metallicity, independently of its evolutionary stage. While the overall trend between maximum age and chemical abundances is a declining fraction of young stars with increasing [{\\alpha}/Fe], at least 14 out of 241 stars with [{\\alpha}/Fe]>0.13 are younger than 6 Gyr. Five stars with [{\\alpha}/Fe]>0.2 have ages below 4 Gyr. We examine the effect of modifications in the standard seismic scaling relations, as well as the effect of very low helium fractions, but these changes are not enough to make these stars as old a...

  5. Multi-wavelength study of the low-luminosity outbursting young star HBC 722

    CERN Document Server

    Kóspál, Á; Acosta-Pulido, J A; Dunham, M M; García-Álvarez, D; Hogerheijde, M R; Kun, M; Moór, A; Farkas, A; Hajdu, G; Hodosán, G; Kovács, T; Kriskovics, L; Marton, G; Molnár, L; Pál, A; Sárneczky, K; Sódor, Á; Szakáts, R; Szalai, T; Szegedi-Elek, E; Szing, A; Tóth, I; Vida, K; Vinkó, J

    2016-01-01

    HBC 722 (V2493 Cyg) is a young eruptive star in outburst since 2010. It is an FU Orionis-type object with an atypically low outburst luminosity. Because it was well characterized in the pre-outburst phase, HBC 722 is one of the few FUors where we can learn about the physical changes and processes associated with the eruption. We monitored the source in the BVRIJHKs bands from the ground, and at 3.6 and 4.5 $\\mu$m from space with the Spitzer Space Telescope. We analyzed the light curves and the spectral energy distribution by fitting a series of steady accretion disk models at many epochs. We also analyzed the spectral properties of the source based on new optical and infrared spectra. We also mapped HBC 722 and its surroundings at millimeter wavelengths. From the light curve analysis we concluded that the first peak of the outburst in 2010 September was due to an abrupt increase of the accretion rate in the innermost part of the system. This was followed by a long term process, when the brightening was mainly...

  6. NuSTAR and Suzaku X-ray Spectroscopy of NGC 4151: Evidence for Reflection from the Inner Accretion Disk

    CERN Document Server

    Keck, M L; Ballantyne, D R; Bauer, F; Boggs, S E; Christensen, F E; Craig, W W; Dauser, T; Elvis, M; Fabian, A C; Fuerst, F; García, J; Grefenstette, B W; Hailey, C J; Harrison, F A; Madejski, G; Marinucci, A; Matt, G; Reynolds, C S; Stern, D; Walton, D J; Zoghbi, A

    2015-01-01

    We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spin a>0.9 accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coron...

  7. The mass donor star and the accretion disc of the dwarf nova V2051 Ophiuchi in the infrared

    Science.gov (United States)

    Wojcikiewicz, Eduardo; Baptista, Raymundo; Ribeiro, Tiago

    2016-07-01

    We report the analysis of infrared JHK_s high speed photometry of the dwarf nova V2051 Oph in quiescence. We model the ellipsoidal variations in the light curve to measure the fluxes of the mass donor star. Its colors are consistent with an M8 ± 1 spectral type with an equivalent blackbody temperature of T_{bb}= (2700± 300) K, in agreement with spectroscopic measurements and with theoretical expectation for donor stars at the same orbital period. We use the mass donor star fluxes and the Barnes & Evans relation to find a photometric parallax distance of (102 ± 16) pc to the binary. At this distance the outbursts of V2051 Oph occur at disc temperatures everywhere lower than the minimum/critical temperature predicted by the disc instability model, underscoring previous suggestions that they are powered by mass transfer bursts. We subtract the contribution of the mass donor star and apply eclipse mapping techniques to the remaining light curve in order to investigate the structure and emission of its accretion disc. The infrared accretion disc is bright and 'blue' in the inner regions and becomes progressively fainter and redder with increasing radii, indicating that the disc temperature decreases with radius. Bulges in the eclipse shape, more prominent in the H and K_s bands, lead to asymmetric arcs in the eclipse maps reminiscent of the spiral arms found in disc maps of outbursting dwarf novae. The arcs show an azimuthal extent of ˜90^o, extend from the intermediate to the outer disc regions (0.3-0.4 R_{L1}, where R_{L1} is the distance from disc center to the inner lagrangian point), and account for ≃ 30 per cent of the total flux in the H and K_s bands.

  8. Jet formation from massive young stars: Magnetohydrodynamics versus radiation pressure

    CERN Document Server

    Vaidya, Bhargav; Beuther, Henrik; Porth, Oliver

    2011-01-01

    Observations indicate that outflows from massive young stars are more collimated during their early evolution compared to later stages. Our paper investigates various physical processes that impacts the outflow dynamics, i.e. its acceleration and collimation. We perform axisymmetric MHD simulations particularly considering the radiation pressure exerted by the star and the disk. We have modified the PLUTO code to include radiative forces in the line-driving approximation. We launch the outflow from the innermost disk region (r < 50 AU) by magneto-centrifugal acceleration. In order to disentangle MHD effects from radiative forces, we start the simulation in pure MHD, and later switch on the radiation force. We perform a parameter study considering different stellar masses (thus luminosity), magnetic flux, and line-force strength. For our reference simulation - assuming a 30 Msun star, we find substantial de-collimation of 35 % due to radiation forces. The opening angle increases from 20 deg to 32 deg for st...

  9. The Magnetic Field Evolution of ULX NuSTAR J095551+6940.8 in M82--A Legacy of Accreting Magnetar

    CERN Document Server

    Pan, Y Y; Zhang, C M; Tong, H

    2015-01-01

    Ultra luminous X-ray sources (ULXs) are usually believed to be black holes with mass about 10^{2--3}M_{sun}. However, the recent discovery of ULX NuSTAR J095551+6940.8 in M82 with the spin period P=1.37s and period derivation P_{dot}=-2*10^{-10} ss^{-1} provides a strong evidence that some ULXs are accreting neutron stars (NSs). To investigate such a particular accreting neutron star, we ascribe it as an evolved magnetar in the accretion binary system. By means of the model of accretion induced the NS magnetic evolution and standard spinup torque, we calculate the magnetic field decay and spin-up of M82 X-2, and show that its magnetic field is now 4.5*10^{12} G, which is evolved from a magnetar in a high mass Xray binary system (HMXB) with the initial values of magnetic field B~10^{14.5} G and spin period P~100 s by accreting ~10^{-3}M_{sun}, while the mass accretion rate for spin-up is set as 5.0*10^{18} gs^{-1}. The evolutionary track of magnetic field and spin period of M82 X-2 is simulated and plotted in ...

  10. The Young Cluster and Star Forming Region NGC 2264

    OpenAIRE

    Dahm, S. E.

    2008-01-01

    NGC 2264 is a young Galactic cluster and the dominant component of the Mon OB1 association lying approximately 760 pc distant within the local spiral arm. The cluster is hierarchically structured, with subclusters of suspected members spread across several parsecs. Associated with the cluster is an extensive molecular cloud complex spanning more than two degrees on the sky. Star formation is ongoing within the region as evidenced by the presence of numerous embedded clusters of protostars, mo...

  11. NuSTAR reveals the extreme properties of the super-Eddington accreting SMBH in PG 1247+267

    CERN Document Server

    Lanzuisi, G; Comastri, A; Cappi, M; Dadina, M; Marinucci, A; Masini, A; Matt, G; Vagnetti, F; Vignali, C; Ballantyne, D R; Bauer, F E; Boggs, S E; Brandt, W N; Brusa, M; Christensen, F E; Craig, W W; Fabian, A C; Farrah, D; Hailey, C J; Harrison, F A; Luo, B; Piconcelli, E; Puccetti, S; Ricci, C; Saez, C; Stern, D; Walton, D J; Zhang, W W

    2016-01-01

    PG1247+267 is one of the most luminous known quasars at $z\\sim2$ and is a strongly super-Eddington accreting SMBH candidate. We obtained NuSTAR data of this intriguing source in December 2014 with the aim of studying its high-energy emission, leveraging the broad band covered by the new NuSTAR and the archival XMM-Newton data. Several measurements are in agreement with the super-Eddington scenario for PG1247+267: the soft power law ($\\Gamma=2.3\\pm0.1$); the weak ionized Fe emission line and a hint of the presence of outflowing ionized gas surrounding the SMBH. The presence of an extreme reflection component is instead at odds with the high accretion rate proposed for this quasar. This can be explained with three different scenarios; all of them are in good agreement with the existing data, but imply very different conclusions: i) a variable primary power law observed in a low state, superimposed on a reflection component echoing a past, higher flux state; ii) a power law continuum obscured by an ionized, Comp...

  12. Pre-main sequence accretion in the low metallicity Galactic star-forming region Sh 2-284

    CERN Document Server

    Kalari, V M

    2014-01-01

    We present optical spectra of pre-main sequence (PMS) candidates around the H$\\alpha$ region taken with the Southern African Large Telescope, SALT, in the low metallicity ($Z$) Galactic region Sh 2-284, which includes the open cluster Dolidze 25 with an atypical low metallicity of $Z$ $\\sim$ 1/5 $Z_{\\odot}$. It has been suggested on the basis of both theory and observations that PMS mass-accretion rates, $\\dot M_{\\rm{acc}}$, are a function of $Z$. We present the first sample of spectroscopic estimates of mass-accretion rates for PMS stars in any low-$Z$ star-forming region. Our data-set was enlarged with literature data of H$\\alpha$ emission in intermediate-resolution R-band spectroscopy. Our total sample includes 24 objects spanning a mass range between 1 - 2 $M_{\\odot}$ and with a median age of approximately 3.5 Myr. The vast majority (21 out of 24) show evidence for a circumstellar disk on the basis of 2MASS and Spitzer infrared photometry. We find $\\dot M_{\\rm{acc}}$ in the 1 - 2 $M_{\\odot}$ interval to d...

  13. Neutron star crust cooling in KS 1731-260: the influence of accretion outburst variability on the crustal temperature evolution

    CERN Document Server

    Ootes, Laura S; Wijnands, Rudy; Degenaar, Nathalie

    2016-01-01

    Using a theoretical model, we track the thermal evolution of a cooling neutron star crust after an accretion induced heating period with the goal of constraining the crustal parameters. We present for the first time a crust cooling model $-\\text{ } NSCool\\text{ } -$ that takes into account detailed variability during the full outburst based on the observed light curve. We apply our model to KS 1731-260. The source was in outburst for $\\sim$12 years during which it was observed to undergo variations on both long (years) and short (days-weeks) timescales. Our results show that KS 1731-260 does not reach a steady state profile during the outburst due to fluctuations in the derived accretion rate. Additionally, long time-scale outburst variability mildly affects the complete crust cooling phase, while variations in the final months of the outburst strongly influence the first $\\sim$40 days of the calculated cooling curve. We discuss the consequences for estimates of the neutron star crust parameters, and argue th...

  14. Contrasting Behaviour from Two Be/X-ray Binary Pulsars: Insights into Differing Neutron Star Accretion Modes

    Science.gov (United States)

    Townsend, L. J.; Drave, S. P.; Hill, A. B.; Coe, M. J.; Corbet, R. H. D.; Bird, A. J.

    2013-01-01

    In this paper we present the identification of two periodic X-ray signals coming from the direction of the Small Magellanic Cloud (SMC). On detection with the Rossi X-ray Timing Explorer (RXTE), the 175.4 s and 85.4 s pulsations were considered to originate from new Be/X-ray binary (BeXRB) pulsars with unknown locations. Using rapid follow-up INTEGRAL and XMM-Newton observations, we show the first pulsar (designated SXP175) to be coincident with a candidate high-mass X-ray binary (HMXB) in the northern bar region of the SMC undergoing a small Type II outburst. The orbital period (87d) and spectral class (B0-B0.5IIIe) of this system are determined and presented here for the first time. The second pulsar is shown not to be new at all, but is consistent with being SXP91.1 - a pulsar discovered at the very beginning of the 13 year long RXTE key monitoring programme of the SMC. Whilst it is theoretically possible for accreting neutron stars to change spin period so dramatically over such a short time, the X-ray and optical data available for this source suggest this spin-up is continuous during long phases of X-ray quiescence, where accretion driven spin-up of the neutron star should be minimal.

  15. Neutron star crust cooling in KS 1731-260: the influence of accretion outburst variability on the crustal temperature evolution

    Science.gov (United States)

    Ootes, Laura S.; Page, Dany; Wijnands, Rudy; Degenaar, Nathalie

    2016-10-01

    Using a theoretical model, we track the thermal evolution of a cooling neutron star crust after an accretion-induced heating period with the goal of constraining the crustal parameters. We present for the first time a crust cooling model - NSCOOL - that takes into account detailed variability during the full outburst based on the observed light curve. We apply our model to KS 1731-260. The source was in outburst for ˜12 yr during which it was observed to undergo variations on both long (years) and short (days-weeks) time-scales. Our results show that KS 1731-260 does not reach a steady state profile during the outburst due to fluctuations in the derived accretion rate. Additionally, long time-scale outburst variability mildly affects the complete crust cooling phase, while variations in the final months of the outburst strongly influence the first ˜40 d of the calculated cooling curve. We discuss the consequences for estimates of the neutron star crust parameters, and argue that detailed modelling of the final phase of the outburst is key to constraining the origin of the shallow heat source.

  16. Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

    CERN Document Server

    Orlando, S; Argiroffi, C; Reale, F; Peres, G; Miceli, M; Matsakos, T; Stehle', C; Ibgui, L; de Sa, L; Chie`ze, J P; Lanz, T

    2013-01-01

    (abridged) AIMS. We investigate the dynamics and stability of post-shock plasma streaming along nonuniform stellar magnetic fields at the impact region of accretion columns. We study how the magnetic field configuration and strength determine the structure, geometry, and location of the shock-heated plasma. METHODS. We model the impact of an accretion stream onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our model takes into account the gravity, the radiative cooling, and the magnetic-field-oriented thermal conduction. RESULTS. The structure, stability, and location of the shocked plasma strongly depend on the configuration and strength of the magnetic field. For weak magnetic fields, a large component of B may develop perpendicular to the stream at the base of the accretion column, limiting the sinking of the shocked plasma into the chromosphere. An envelope of dense and cold chromospheric material may also develop around the shocked column. For strong magnetic fields, th...

  17. Disks, accretion and outflows of brown dwarfs

    CERN Document Server

    Joergens, V; Liu, Y; Pascucci, I; Whelan, E; Alcala, J; Biazzo, K; Costigan, G; Gully-Santiago, M; Henning, Th; Natta, A; Rigliaco, E; Rodriguez-Ledesma, V; Sicilia-Aguilar, A; Tottle, J; Wolf, S

    2012-01-01

    Characterization of the properties of young brown dwarfs are important to constraining the formation of objects at the extreme low-mass end of the IMF. While young brown dwarfs share many properties with solar-mass T Tauri stars, differences may be used as tests of how the physics of accretion/outflow and disk chemistry/dissipation depend on the mass of the central object. This article summarizes the presentations and discussions during the splinter session on 'Disks, accretion and outflows of brown dwarfs' held at the CoolStars17 conference in Barcelona in June 2012. Recent results in the field of brown dwarf disks and outflows include the determination of brown dwarf disk masses and geometries based on Herschel far-IR photometry (70-160 um), accretion properties based on X-Shooter spectra, and new outflow detections in the very low-mass regime.

  18. Time-Variable Accretion in the TW Hya Star/Disk System

    CERN Document Server

    Eisner, J A; Najita, J R; McCarthy, D; Kulesa, C; Swift, B J; Teske, J

    2010-01-01

    We present two epochs of observations of TW Hya from the high-dispersion near-IR spectrograph ARIES at the MMT. We detect strong emission from the Brackett gamma transition of hydrogen, indicating an accretion rate substantially larger than previously estimated using hydrogen line emission. The Brackett gamma line-strength varies across our two observed epochs. We also measure circumstellar-to-stellar flux ratios (i.e., veilings) that appear close to zero in both epochs. These findings suggest that TW Hya experiences episodes of enhanced accretion while the inner disk remains largely devoid of dust. We discuss several physical mechanisms that may explain these observations.

  19. Star Formation & Young Stellar Content in the W3 GMC

    CERN Document Server

    Rivera-Ingraham, A; Polychroni, D; Moore, T J T

    2011-01-01

    In this work we have carried out an in-depth analysis of the young stellar content in the W3 GMC. The YSO population was identified and classified in the IRAC/MIPS color-magnitude space according to the `Class' scheme and compared to other classifications based on intrinsic properties. Class 0/I and II candidates were also compared to low/intermediate-mass pre-main-sequence stars selected through their colors and magnitudes in 2MASS. We find that a reliable color/magnitude selection of low-mass PMS stars in the infrared requires prior knowledge of the protostar population, while intermediate mass objects can be more reliably identified. By means of the MST algorithm and our YSO spatial distribution and age maps we investigated the YSO groups and the star formation history in W3. We find signatures of clustered and distributed star formation in both triggered and quiescent environments. The central/western parts of the GMC are dominated by large scale turbulence likely powered by isolated bursts of star format...

  20. Discovery of solar system-size halos around young stars

    Science.gov (United States)

    Beckwith, S.; Skrutskie, M. F.; Zuckerman, B.; Dyck, H. M.

    1984-01-01

    Near-infrared speckle interferometric observations of five pre-main-sequence stars reveal a core-halo structure around two of these stars: HL Tau and R Mon. The halo light distribution is shown to arise from scattered light from small circumstellar particles. Halo sizes of 320 x 200 AU (alpha x delta FWHM) and 1300 x 1300 AU are deduced for HL Tau and R Mon, respectively, and the halo light is substantially bluer than the stellar light. The minimum mass of small particles in the scattering regions is comparable to the earth's mass in HL Tau and ten times greater in R Mon. Mass loss from the stars is almost certainly insufficient to produce the halo matter. The halos probably consist of relatively slowly moving matter bound gravitationally to the stars. From the size and mass of the circumstellar matter, it appears likely that these halos are in the early stage in the formation of planet-forming disks around the young stars.

  1. The effects of x-rays on star formation and black hole growth in young galaxies

    Science.gov (United States)

    Spaans, Marco; Aykutalp, Aycin; Wise, John H.; Meijerink, Rowin

    2012-09-01

    We investigate the growth of seed black holes in young galaxies and the impact of their X-ray feedback. We have performed two simulations using the adaptive mesh refinement hydrodynamical code Enzo, for the singular collapse scenario in the presence of a UV background radiation field of 105 and 103 J21. We have extended Enzo to include X-ray chemistry driven by a seed black hole with an initial mass of 5×104Msolar. Radiation transfer is performed polychromatically using the module Moray and H2 self-shielding is included. We use two different star formation recipes for the creation of Pop III and PopII/I stars and their feedback effects. We find that in the high UV background radiation case, no Pop III stars are formed until at least z = 10, due to the low H2 fractions (10-8). In the low UV background case the H2 abundances are orders of magnitude higher and pop III star formation is efficient, as is metal enrichment. We find that the production of X-rays drives an H II region that pushes out gas in the high UV background case and leads to a low duty cycle. For the low UV background run black hole accretion enjoys a duty cycle of 50%. We conclude that seed black holes with masses of 5×104Msolar, which are formed through the singular collapse of an atomic cooling halo under the influence of low (high) UV background radiation field, can(not) be the origin of supermassive black holes that we see at z = 6.

  2. Panchromatic Hubble Andromeda Treasury. XVI. Star Cluster Formation Efficiency and the Clustered Fraction of Young Stars

    Science.gov (United States)

    Johnson, L. Clifton; Seth, Anil C.; Dalcanton, Julianne J.; Beerman, Lori C.; Fouesneau, Morgan; Lewis, Alexia R.; Weisz, Daniel R.; Williams, Benjamin F.; Bell, Eric F.; Dolphin, Andrew E.; Larsen, Søren S.; Sandstrom, Karin; Skillman, Evan D.

    2016-08-01

    We use the Panchromatic Hubble Andromeda Treasury survey data set to perform spatially resolved measurements of star cluster formation efficiency (Γ), the fraction of stellar mass formed in long-lived star clusters. We use robust star formation history and cluster parameter constraints, obtained through color-magnitude diagram analysis of resolved stellar populations, to study Andromeda’s cluster and field populations over the last ˜300 Myr. We measure Γ of 4%-8% for young, 10-100 Myr-old populations in M31. We find that cluster formation efficiency varies systematically across the M31 disk, consistent with variations in mid-plane pressure. These Γ measurements expand the range of well-studied galactic environments, providing precise constraints in an H i-dominated, low-intensity star formation environment. Spatially resolved results from M31 are broadly consistent with previous trends observed on galaxy-integrated scales, where Γ increases with increasing star formation rate surface density (ΣSFR). However, we can explain observed scatter in the relation and attain better agreement between observations and theoretical models if we account for environmental variations in gas depletion time (τ dep) when modeling Γ, accounting for the qualitative shift in star formation behavior when transitioning from a H2-dominated to a H i-dominated interstellar medium. We also demonstrate that Γ measurements in high ΣSFR starburst systems are well-explained by τ dep-dependent fiducial Γ models.

  3. Dynamic Young Stars and their Disks: A Temporal View of NGC 2264 with Spitzer and CoRoT*

    Directory of Open Access Journals (Sweden)

    Cody Ann Marie

    2014-01-01

    Full Text Available Variability is a signature feature of young stars. Among the well known light curve phenomena are periodic variations attributed to surface spots and irregular changes associated with accretion or circumstellar disk material. While decades of photometric monitoring have provided a framework for classifying young star variability, we still know surprisingly little about its underlying mechanisms and connections to the surrounding disks. In the past few years, dedicated photometric monitoring campaigns from the ground and space have revolutionized our view of young stars in the time domain. We present a selection of optical and infrared time series from several recent campaigns, highlighting the Coordinated Synoptic Investigation of NGC 2264 (“CSI 2264”– a joint30-day effort with the Spitzer, CoRoT, and MOST telescopes. The extraordinary photometric precision, high cadence, and long time baseline of these observations is now enabling correlation of variability properties at very different wavelengths, corresponding to locations from the stellar surface to the inner 0.1 AU of the disk. We present some results of the CSI 2264 program, including new classes of optical/infrared behavior. Further efforts to tie observed variability features to physical models will provide insights into the inner disk environment at a time when planet formation may be underway.

  4. The Extremely Young Star Cluster Population In Haro 11

    CERN Document Server

    Adamo, Angela; Zackrisson, Erik; Hayes, Matthew

    2009-01-01

    We have performed a deep multi-band photometric analysis of the star cluster population of Haro 11. This starburst galaxy (log L_FUV = 10.3 L_sun) is considered a nearby analogue of Lyman break galaxies (LBGs) at high redshift. The study of the numerous star clusters in the systems is an effective way to investigate the formation and evolution of the starburst phase. In fact, the SED fitting models have revealed a surprisingly young star cluster population, with ages between 0.5 and 40 Myr, and estimated masses between 10^3 and 10^7 solar masses. An independent age estimation has been done with the EW(Halpha) analysis of each cluster. This last analysis has confirmed the young ages of the clusters. We noticed that the clusters with ages between 1 and 10 Myr show a flux excess in H (NIC3/F160W) and/or I (WFPC2/F814W) bands with respect to the evolutionary models. Once more Haro 11 represents a challenge to our understanding.

  5. Structural parameters of young star clusters: fractal analysis

    Science.gov (United States)

    Hetem, A.

    2017-07-01

    A unified view of star formation in the Universe demand detailed and in-depth studies of young star clusters. This work is related to our previous study of fractal statistics estimated for a sample of young stellar clusters (Gregorio-Hetem et al. 2015, MNRAS 448, 2504). The structural properties can lead to significant conclusions about the early stages of cluster formation: 1) virial conditions can be used to distinguish warm collapsed; 2) bound or unbound behaviour can lead to conclusions about expansion; and 3) fractal statistics are correlated to the dynamical evolution and age. The technique of error bars estimation most used in the literature is to adopt inferential methods (like bootstrap) to estimate deviation and variance, which are valid only for an artificially generated cluster. In this paper, we expanded the number of studied clusters, in order to enhance the investigation of the cluster properties and dynamic evolution. The structural parameters were compared with fractal statistics and reveal that the clusters radial density profile show a tendency of the mean separation of the stars increase with the average surface density. The sample can be divided into two groups showing different dynamic behaviour, but they have the same dynamic evolution, since the entire sample was revealed as being expanding objects, for which the substructures do not seem to have been completely erased. These results are in agreement with the simulations adopting low surface densities and supervirial conditions.

  6. ACCRETION-INHIBITED STAR FORMATION IN THE WARM MOLECULAR DISK OF THE GREEN-VALLEY ELLIPTICAL GALAXY NGC 3226?

    Energy Technology Data Exchange (ETDEWEB)

    Appleton, P. N.; Bitsakis, T.; Alatalo, K. [NASAHerschel Science Center, Infrared Processing and Analysis Center, Caltech, 770S Wilson Avenue, Pasadena, CA 91125 (United States); Mundell, C. [Astrophysics Research Institute, John Moores University, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF (United Kingdom); Lacy, M. [NRAO, Charlottesville, VA (United States); Armus, L. [Spitzer NASAHerschel Science Center, 1200 East California Boulevard, Caltech, Pasadena, CA 91125 (United States); Charmandaris, V. [Department of Physics, University of Crete, GR-71003 Heraklion (Greece); Duc, P.-A. [Service d' Astrophysique, Laboratoire AIM, CEA-Saclay, Orme des Merisiers, Bat 709, F-91191 Gif sur Yvette (France); Lisenfeld, U. [Dept. Fisica Teorica y del Cosmos, University of Granada, Edifica Mecenas, Granada (Spain); Ogle, P., E-mail: apple@ipac.caltech.edu [NASA Extragalactic Database, IPAC, Caltech, 1200 East California Boulevard, Caltech, Pasadena, CA 91125 (United States)

    2014-12-20

    We present archival Spitzer photometry and spectroscopy and Herschel photometry of the peculiar ''Green Valley'' elliptical galaxy NGC 3226. The galaxy, which contains a low-luminosity active galactic nucleus (AGN), forms a pair with NGC 3227 and is shown to lie in a complex web of stellar and H I filaments. Imaging at 8 and 16 μm reveals a curved plume structure 3 kpc in extent, embedded within the core of the galaxy and coincident with the termination of a 30 kpc long H I tail. In situ star formation associated with the infrared (IR) plume is identified from narrowband Hubble Space Telescope (HST) imaging. The end of the IR plume coincides with a warm molecular hydrogen disk and dusty ring containing 0.7-1.1 × 10{sup 7} M {sub ☉} detected within the central kiloparsec. Sensitive upper limits to the detection of cold molecular gas may indicate that a large fraction of the H{sub 2} is in a warm state. Photometry derived from the ultraviolet (UV) to the far-IR shows evidence for a low star-formation rate of ∼0.04 M {sub ☉} yr{sup –1} averaged over the last 100 Myr. A mid-IR component to the spectral energy distribution (SED) contributes ∼20% of the IR luminosity of the galaxy, and is consistent with emission associated with the AGN. The current measured star formation rate is insufficient to explain NGC 3226's global UV-optical ''green'' colors via the resurgence of star formation in a ''red and dead'' galaxy. This form of ''cold accretion'' from a tidal stream would appear to be an inefficient way to rejuvenate early-type galaxies and may actually inhibit star formation.

  7. Molecular tracers of photo-evaporating disks around young stars

    Science.gov (United States)

    Nguyen, T. K.; Viti, S.; Williams, D. A.

    2002-06-01

    Disks around massive young stellar objects, and disks around low-mass stars irradiated by nearby OB associations, are eroded by photo-evaporation. In the latter case, this erosion may be an important factor in planet formation. As Johnstone et al. (\\cite{johnstone98a}) have shown, photo-evaporating material is gravitationally retained within a critical radius from the star, and constitutes an envelope similar to a Photon-Dominated Region (PDR) that normally arises at the edge of a molecular cloud irradiated by a massive star. We explore the chemistry of such a PDR/disk system to examine the contribution that it may make to the molecular species that may be observed. The model is in two phases; firstly, a collapse from low density to a high density appropriate for a disk; and, secondly, a 2D calculation of the irradiation of disk material by the radiation field of the central massive star or nearby OB association. The model follows the chemistry self-consistently through both phases. We compute the column densities of species through the PDR/disk system, averaged over the disk. We validate our model by comparing predicted averaged molecular column densities with those of several species detected in the disk around the 10 solar mass star GL 2591, currently the sole example known of this kind of object. Results are in good agreement for a model in which the outer part of the PDR is hot while the inner part is cool, and in which the local ionization rate is comparable with that caused by cosmic rays in the local interstellar medium. We show that in addition to the four detected species, there should be many others also detectable in this system, including HCN, NH3 and CS. Similar conclusions should apply to other disks around massive stars. Disks around low-mass stars are much more common; our models show that when irradiated by a nearby OB association such disks with their attendant PDRs also generate a rich chemistry. No detections of molecules in such objects have

  8. Production of the entire range of r-process nuclides by black hole accretion disc outflows from neutron star mergers

    Science.gov (United States)

    Wu, Meng-Ru; Fernández, Rodrigo; Martínez-Pinedo, Gabriel; Metzger, Brian D.

    2016-12-01

    We consider r-process nucleosynthesis in outflows from black hole accretion discs formed in double neutron star and neutron star-black hole mergers. These outflows, powered by angular momentum transport processes and nuclear recombination, represent an important - and in some cases dominant - contribution to the total mass ejected by the merger. Here we calculate the nucleosynthesis yields from disc outflows using thermodynamic trajectories from hydrodynamic simulations, coupled to a nuclear reaction network. We find that outflows produce a robust abundance pattern around the second r-process peak (mass number A ˜ 130), independent of model parameters, with significant production of A dynamical ejecta with high electron fraction may not be required to explain the observed abundances of r-process elements in metal poor stars. Disc outflows reach the third peak (A ˜ 195) in most of our simulations, although the amounts produced depend sensitively on the disc viscosity, initial mass or entropy of the torus, and nuclear physics inputs. Some of our models produce an abundance spike at A = 132 that is absent in the Solar system r-process distribution. The spike arises from convection in the disc and depends on the treatment of nuclear heating in the simulations. We conclude that disc outflows provide an important - and perhaps dominant - contribution to the r-process yields of compact binary mergers, and hence must be included when assessing the contribution of these systems to the inventory of r-process elements in the Galaxy.

  9. Metallicity inhomogeneities in local star-forming galaxies as sign of recent metal-poor gas accretion

    CERN Document Server

    Almeida, J Sanchez; Munoz-Tunon, C; Elmegreen, D M; Elmegreen, B G; Mendez-Abreu, J

    2014-01-01

    We measure the oxygen metallicity of the ionized gas along the major axis of seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14, show 0.5 dex metallicity decrements in inner regions with enhanced star-formation activity. This behavior is similar to the metallicity drop observed in a number of local tadpole galaxies by Sanchez Almeida et al. (2013) and interpreted as showing early stages of assembling in disk galaxies, with the star formation sustained by external metal-poor gas accretion. The agreement with tadpoles has several implications: (1) it proves that galaxies other than the local tadpoles present the same unusual metallicity pattern. (2) Our metallicity inhomogeneities were inferred using the direct method, thus discarding systematic errors usually attributed to other methods. (3) Taken together with the tadpole data, our findings suggest a threshold around one tenth the solar value for the metallicity drops to show up. Although galaxies with clear metallicity drops are ra...

  10. CBS 78 - A second extremely rare DBZ degenerate star with accreted calcium

    Science.gov (United States)

    Sion, E. M.; Shipman, H. L.; Wagner, R. M.; Starrfield, S. G.; Liebert, J.

    1986-09-01

    The paper reports the discovery of a second, extremely rare DBZ degenerate CBS 78 which reveals photospheric calcium in its nearly pure helium atmosphere. The presence of calcium strongly suggests accretion of interstellar matter as its origin, but hydrogen is not detected. The properties of CBS 78 are compared with other cool helium-rich degenerates with metals. The suggested existence of an empirical correlation between the hydrogen-to-calcium abundance ratio and effective temperature is confirmed. This correlation shows that above a critical temperature of about 11,000 K, a screening mechanism is operative which prevents accretion of interstellar hydrogen onto white dwarfs; this mechanism apparently does not operate at lower temperatures.

  11. Zeeman-Doppler imaging of active young solar type stars

    CERN Document Server

    Hackman, Thomas; Rosén, Lisa; Kochukhov, Oleg; Käpylä, Maarit J

    2015-01-01

    By studying young magnetically active late-type stars, i.e. analogues to the young Sun, one can draw conclusions on the evolution of the solar dynamo. We determine the topology of the surface magnetic field and study the relation between the magnetic field and cool photospheric spots in three young late-type stars. High-resolution spectropolarimetry of the targets were obtained with the HARPSpol instrument mounted at the ESO 3.6 m telescope. The signal-to-noise ratio of the Stokes IV measurements were boosted by combining the signal from a large number of spectroscopic absorption lines through the least squares deconvolution technique. Surface brightness and magnetic field maps were calculated using the Zeeman-Doppler imaging technique. All the three targets show clear signs of both magnetic fields and cool spots. Only one of the targets, namely V1358 Ori, shows evidence of the dominance of non-axisymmetric modes. In two of the targets, the poloidal field is significantly stronger than the toroidal one, indic...

  12. Two Populations of Young Massive Star Clusters in Arp 220

    CERN Document Server

    Wilson, C D; Longden, R; Scoville, N Z; Wilson, Christine D.; Harris, William E.; Longden, Rebecca

    2006-01-01

    We present new optical observations of young massive star clusters in Arp 220, the nearest ultraluminous infrared galaxy, taken in UBVI with the Hubble Space Telescope ACS/HRC camera. We find a total of 206 probable clusters whose spatial distribution is centrally concentrated toward the nucleus of Arp 220. We use model star cluster tracks to determine ages, luminosities, and masses for 14 clusters with complete UBVI indices or previously published near-infrared data. We estimate rough masses for 24 additional clusters with I < 24 mag from BVI indices alone. The clusters with useful ages fall into two distinct groups: a ``young'' population (< 10 Myr) and an intermediate-age population (~300 Myr). There are many clusters with masses clearly above 10^6 Msun and possibly even above 10^7 Msun in the most extreme instances. These masses are high enough that the clusters being formed in the Arp 220 starburst can be considered as genuine young globular clusters. In addition, this study allows us to extend the...

  13. Resolved photometry of extragalactic young massive star clusters

    CERN Document Server

    Larsen, S S; Eldridge, J J; Langer, N; Bastian, N; Seth, A; Smith, L J; Brodie, J; Efremov, Y N

    2011-01-01

    We present colour-magnitude diagrams (CMDs) for a sample of seven young massive clusters in the galaxies NGC 1313, NGC 1569, NGC 1705, NGC 5236 and NGC 7793. The clusters have ages in the range 5-50 million years and masses of 10^5 -10^6 Msun. Although crowding prevents us from obtaining photometry in the central regions of the clusters, we are still able to measure up to 30-100 supergiant stars in each of the richest clusters, along with the brighter main sequence stars. The resulting CMDs and luminosity functions are compared with photometry of artificially generated clusters, designed to reproduce the photometric errors and completeness as realistically as possible. In agreement with previous studies, our CMDs show no clear gap between the H-burning main sequence and the He-burning supergiant stars, contrary to predictions by common stellar isochrones. In general, the isochrones also fail to match the observed number ratios of red-to-blue supergiant stars, although the difficulty of separating blue supergi...

  14. Circumbinary Molecular Rings Around Young Stars in Orion

    CERN Document Server

    Zapata, Luis A; Rodriguez, Luis F; Schilke, Peter; Kurtz, Stan

    2007-01-01

    We present high angular resolution 1.3 mm continuum, methyl cyanide molecular line, and 7 mm continuum observations made with the Submillimeter Array and the Very Large Array, toward the most highly obscured and southern part of the massive star forming region OMC1S located behind the Orion Nebula. We find two flattened and rotating molecular structures with sizes of a few hundred astronomical units suggestive of circumbinary molecular rings produced by the presence of two stars with very compact circumstellar disks with sizes and separations of about 50 AU, associated with the young stellar objects 139-409 and 134-411. Furthermore, these two circumbinary rotating rings are related to two compact and bright {\\it hot molecular cores}. The dynamic mass of the binary systems obtained from our data are $\\geq$ 4 M$_\\odot$ for 139-409 and $\\geq$ 0.5 M$_\\odot$ for 134-411. This result supports the idea that intermediate-mass stars will form through {\\it circumstellar disks} and jets/outflows, as the low mass stars d...

  15. Thermal structures of accreting neutron stars with neutrino losses due to strong pion condensations

    CERN Document Server

    Matsuo, Yasuhide; Hayashida, Koutarou; Liu, Helei; Noda, Tsuneo; Fujimoto, Masayuki Y

    2016-01-01

    Quiescent X-ray luminosities are presented in low mass X-ray binaries with use of evolutionary calculations. The calculated luminosities are compared with observed ones in terms of timeaveraged mass accretion rate. It is shown that neutrino emission by strong pion condensation can explain quiescent X-ray luminosity of SAX J1808.4-3658 and we do not need direct Urca processes concerning nucleons and/or hyperons.

  16. Constraints on the Neutron Star and Inner Accretion Flow in Serpens X-1 Using NuSTAR

    CERN Document Server

    Miller, J M; Fuerst, F; Bachetti, M; Barret, D; Grefenstette, B W; Tendulkar, S; Harrison, F A; Boggs, S E; Chakrabarty, D; Christensen, F E; Craig, W W; Fabian, A C; Hailey, C J; Natalucci, L; Paerels, F; Rana, V; Stern, D K; Tomsick, J A; Zhang, W W

    2013-01-01

    We report on an observation of the neutron star low-mass X-ray binary Serpens X-1, made with NuSTAR. The extraordinary sensitivity afforded by NuSTAR facilitated the detection of a clear, robust, relativistic Fe K emission line from the inner disk. A relativistic profile is required over a single Gaussian line from any charge state of Fe at the 5-sigma level of confidence, and any two Gaussians of equal width at the same confidence. The Compton back-scattering "hump" peaking in the 10-20 keV band is detected for the first time in a neutron star X-ray binary. Fits with relativistically-blurred disk reflection models suggest that the disk likely extends close to the innermost stable circular orbit (ISCO) or stellar surface. The best-fit blurred reflection models constrain the gravitational redshift from the stellar surface to be z > 0.16. The data are broadly compatible with the disk extending to the ISCO; in that case, z > 0.22 and R < 12.6 km (assuming M = 1.4 Msun and a=0, where a = cJ/GM^2). If the star ...

  17. Photometric Determination of the Mass Accretion Rates of Pre-main-sequence Stars. V. Recent Star Formation in the 30 Dor Nebula

    Science.gov (United States)

    De Marchi, Guido; Panagia, Nino; Beccari, Giacomo

    2017-09-01

    We report on the properties of the low-mass stars that recently formed in the central ∼ 2\\buildrel{ \\prime}\\over{.} 7× 2\\buildrel{ \\prime}\\over{.} 7 of 30 Dor, including the R136 cluster. Using the photometric catalog of De Marchi et al., based on observations with the Hubble Space Telescope, and the most recent extinction law for this field, we identify 1035 bona fide pre-main-sequence (PMS) stars showing {{H}}α excess emission at the 4σ level with an {{H}}α equivalent width of 20 Å or more. We find a wide spread in age spanning the range ∼ 0.1{--}50 {Myr}. We also find that the older PMS objects are placed in front of the R136 cluster and are separated from it by a conspicuous amount of absorbing material, indicating that star formation has proceeded from the periphery into the interior of the region. We derive physical parameters for all PMS stars, including masses m, ages t, and mass accretion rates {\\dot{M}}{acc}. To identify reliable correlations between these parameters, which are intertwined, we use a multivariate linear regression fit of the type {log}{\\dot{M}}{acc}=a× {log}t+b× {log}m+c. The values of a and b for 30 Dor are compatible with those found in NGC 346 and NGC 602. We extend the fit to a uniform sample of 1307 PMS stars with 0.5contract NAS5-26555.

  18. Dynamical star-disk interaction in the young stellar system V354 Mon

    CERN Document Server

    Fonseca, N N J; Bouvier, J; Favata, F; Flaccomio, E

    2014-01-01

    The main goal of this work is to characterize the mass accretion and ejection processes of the classical T Tauri star V354 Mon, a member of the young stellar cluster NGC 2264. In March 2008, photometric and spectroscopic observations of V354 Mon were obtained simultaneously with the CoRoT satellite, the 60 cm telescope at the Observat\\'orio Pico dos Dias (LNA - Brazil) equipped with a CCD camera and Johnson/Cousins BVRI filters, and the SOPHIE \\'echelle spectrograph at the Observatoire de Haute-Provence (CNRS - France). The light curve of V354 Mon shows periodical minima (P = 5.26 +/- 0.50 days) that vary in depth and width at each rotational cycle. From the analysis of the photometric and spectroscopic data, it is possible to identify correlations between the emission line variability and the light-curve modulation of the young system, such as the occurrence of pronounced redshifted absorption in the H_alpha line at the epoch of minimum flux. This is evidence that during photometric minima we see the accreti...

  19. Evolutionary Synthesis Modelling of Young Star Clusters in Merging Galaxies

    CERN Document Server

    Anders, P; De Grijs, R; Anders, Peter; Alvensleben, Uta Fritze - v.; Grijs, Richard de

    2003-01-01

    The observational properties of globular cluster systems (GCSs) are vital tools to investigate the violent star formation histories of their host galaxies. This violence is thought to have been triggered by galaxy interactions or mergers. The most basic properties of a GCS are its luminosity function (number of clusters per luminosity bin) and color distributions. A large number of observed GCS show bimodal color distributions, which can be translated into a bimodality in either metallicity and/or age. An additional uncertainty comes into play when one considers extinction. These effects can be disentangled either by obtaining spectroscopic data for the clusters or by imaging observations in at least four passbands. This allows us then to discriminate between various formation scenarios of GCSs, e.g. the merger scenario by Ashman & Zepf, and the multi-phase collapse model by Forbes et. al.. Young and metal-rich star cluster populations are seen to form in interacting and merging galaxies. We analyse multi...

  20. Activity trends in young solar-type stars

    CERN Document Server

    Lehtinen, Jyri; Hackman, Thomas; Kajatkari, Perttu; Henry, Gregory W

    2015-01-01

    We apply the Continuous Period Search (CPS) time series analysis method on Johnson B and V band photometry of 21 young and active solar-type, collected over 16 to 27 years and characterize the behaviour of their activity. Using the CPS method, differential rotation could be estimated from the observed variations of the photometric rotation period. Active longitudes were retrieved by applying a non-parametric period search on the light curve minimum epochs, and activity cycles by applying a secondary period search on the modelled light curve mean and amplitude values. We supplemented the time series results by calculating new $\\log{R'_{\\rm HK}}$ emission indices for the stars from high resolution spectroscopy. The measurements of the photometric rotation period variations point to a trend of increasing differential rotation coefficients towards longer rotation periods but do not reveal any dependence from the effective temperature of the stars. The secondary period searches revealed activity cycles in 18 of th...

  1. Investigating the rotational evolution of very low-mass stars and brown dwarfs in young clusters using Monte Carlo simulations

    CERN Document Server

    Vasconcelos, M J

    2016-01-01

    Context. Very low-mass (VLM) stars and brown dwarfs (BDs) present a different rotational behaviour from their solar mass counter-parts. Aims. We investigate the rotational evolution of young VLM stars and BDs using Monte Carlo simulations under the hypothesis of disk locking and stellar angular momentum conservation. Methods. We built a set of objects with masses ranging from 0.01 Mo to 0.4 Mo and considered models with single- and double- peaked initial period distributions with and without disk locking. An object is considered to be diskless when its mass accretion rate is below a given threshold. Results. Models with initial single-peaked period distributions reproduce the observations well given that BDs rotate faster than VLM stars. We observe a correlation between rotational period and mass when we relax the disk locking hypothesis, but with a shallower slope compared to some observational results. The angular momentum evolution of diskless stars is flatter than it is for stars with a disk which occurs ...

  2. NuSTAR reveals the extreme properties of the super-Eddington accreting supermassive black hole in PG 1247+267

    DEFF Research Database (Denmark)

    Lanzuisi, G.; Perna, M.; Comastri, A.

    2016-01-01

    PG1247+267 is one of the most luminous known quasars at z similar to 2 and is a strongly super-Eddington accreting supermassive black hole (SMBH) candidate. We obtained NuSTAR data of this intriguing source in December 2014 with the aim of studying its high-energy emission, leveraging the broad...

  3. An in-depth study of a neutron star accreting at low Eddington rate: On the possibility of a truncated disk and an outflow

    CERN Document Server

    Degenaar, N; Miller, J M; Wijnands, R; Altamirano, D; Paerels, F; Fabian, A C; Chakrabarty, D

    2016-01-01

    Due to observational challenges our knowledge of low-level accretion flows around neutron stars is limited. We present Nustar, Swift and Chandra observations of the low-mass X-ray binary IGR J17062-6143, which has been persistently accreting at ~0.1 per cent of the Eddington limit since 2006. Our simultaneous Nustar/Swift observations show that the 0.5-79 keV spectrum can be described by a combination of a power law with a photon index of Gamma~2, a black body with a temperature of kT_bb~0.5 keV (presumably arising from the neutron star surface), and disk reflection. Modeling the reflection spectrum suggests that the inner accretion disk was located at R_in>100 GM/c2 (>225 km) from the neutron star. The apparent truncation may be due to evaporation of the inner disk into a radiatively-inefficient accretion flow, or due to the pressure of the neutron star magnetic field. Our Chandra gratings data reveal possible narrow emission lines near 1 keV that can be modeled as reflection or collisionally-ionized gas, an...

  4. GRB060602B = Swift J1749.4−2807: an unusual transiently accreting neutron-star X-ray binary

    NARCIS (Netherlands)

    Wijnands, R.; Rol, E.; Cackett, E.; Starling, R.L.C.; Remillard, R.A.

    2009-01-01

    We present an analysis of the Swift Burst Alert Telescope (BAT) and X-ray telescope (XRT) data of GRB060602B, which is most likely an accreting neutron star in a binary system and not a gamma-ray burst. Our analysis shows that the BAT burst spectrum is consistent with a thermonuclear flash (type I X

  5. Evidence for Accretion High-Resolution X-ray Spectroscopy of the Classical T Tauri Star TW Hydrae

    CERN Document Server

    Kästner, J H; Schulz, N S; Canizares, C R; Weintraub, D A; Kastner, Joel H.; Huenemoerder, David P.; Schulz, Norbert S.; Canizares, Claude R.; Weintraub, David A.

    2002-01-01

    We present high resolution X-ray spectra of the X-ray bright classical T Tauri star, TW Hydrae, covering the wavelength range of 1.5-25 AA. The differential emission measure derived from fluxes of temperature-sensitive emission lines shows a plasma with a sharply peaked temperature distribution, peaking at log T = 6.5. Abundance anomalies are apparent, with iron very deficient relative to oxygen, while neon is enhanced relative to oxygen. Density-sensitive line ratios of Ne IX and O VII indicate densities near log n_e = 13. A flare with rapid (~1 ks) rise time was detected during our 48 ksec observation; however, based on analysis of the emission-line spectrum during quiescent and flaring states, the derived plasma parameters do not appear strongly time-dependent. The inferred plasma temperature distribution and densities are consistent with a model in which the bulk of the X-ray emission from TW Hya is generated via mass accretion from its circumstellar disk. Assuming accretion powers the X-ray emission, our...

  6. Improved estimate of the detectability of gravitational radiation from a magnetically confined mountain on an accreting neutron star

    CERN Document Server

    Vigelius, M

    2009-01-01

    We give an improved estimate of the detectability of gravitational waves from magnetically confined mountains on accreting neutron stars. The improved estimate includes the following effects for the first time: three-dimensional hydromagnetic ("fast") relaxation, three-dimensional resistive ("slow") relaxation, realistic accreted masses $M_a \\la 2 \\times 10^{-3} M_\\odot$, (where the mountain is grown ab initio by injection), and verification of the curvature rescaling transformation employed in previous work. Typically, a mountain does not relax appreciably over the lifetime of a low-mass X-ray binary. The ellipticity reaches $\\epsilon \\approx 2 \\times 10^{-5}$ for $M_a=2\\times 10^{-3} M_\\odot$. The gravitational wave spectrum for triaxial equilibria contains an additional line, which, although weak, provides valuable information about the mountain shape. We evaluate the detectability of magnetic mountains with Initial and Advanced LIGO. For a standard, coherent matched filter search, we find a signal-to-nois...

  7. NuSTAR and XMM-Newton Observations of NGC 1365: Extreme Absorption Variability and a Constant Inner Accretion Disk

    CERN Document Server

    Walton, D J; Harrison, F A; Fabian, A C; Miller, J M; Arevalo, P; Ballantyne, D R; Boggs, S E; Brenneman, L W; Christensen, F E; Craig, W W; Elvis, M; Fuerst, F; Gandhi, P; Grefenstette, B W; Hailey, C J; Kara, E; Luo, B; Madsen, K K; Marinucci, A; Matt, G; Parker, M L; Reynolds, C S; Rivers, E; Ross, R R; Stern, D; Zhang, W W

    2014-01-01

    We present a spectral analysis of four coordinated NuSTAR+XMM-Newton observations of the Seyfert galaxy NGC 1365. These exhibit an extreme level of spectral variability, which is primarily due to variable line-of-sight absorption, revealing relatively unobscured states in this source for the first time. Despite the diverse range of absorption states, each of the observations displays the same characteristic signatures of relativistic reflection from the inner accretion disk. Through time-resolved spectroscopy we find that the strength of the relativistic iron line and the Compton reflection hump relative to the intrinsic continuum are well correlated, as expected if they are two aspects of the same broadband reflection spectrum. We apply self-consistent disk reflection models to these time-resolved spectra in order to constrain the inner disk parameters, allowing for variable, partially covering absorption to account for the vastly different absorption states observed. Each of the four observations is treated...

  8. MAGNETOHYDRODYNAMIC MODELING OF THE ACCRETION SHOCKS IN CLASSICAL T TAURI STARS: THE ROLE OF LOCAL ABSORPTION IN THE X-RAY EMISSION

    Energy Technology Data Exchange (ETDEWEB)

    Bonito, R.; Argiroffi, C.; Peres, G. [Dip. di Fisica e Chimica, Università di Palermo, P.zza del Parlamento 1, I-90134 Palermo (Italy); Orlando, S.; Miceli, M.; Ibgui, L. [INAF-Osservatorio Astronomico di Palermo, P.zza del Parlamento 1, I-90134 Palermo (Italy); Matsakos, T. [Department of Astronomy and Astrophysics, The University of Chicago, Chicago, IL 60637 (United States); Stehle, C., E-mail: sbonito@astropa.unipa.it [LERMA, Observatoire de Paris, Université Pierre et Marie Curie, Ecole Normale Superieure, Universite Cergy-Pontoise, CNRS, F-75014 Paris (France)

    2014-11-10

    We investigate the properties of X-ray emission from accretion shocks in classical T Tauri stars (CTTSs), generated where the infalling material impacts the stellar surface. Both observations and models of the accretion process reveal several aspects that are still unclear: the observed X-ray luminosity in accretion shocks is below the predicted value, and the density versus temperature structure of the shocked plasma, with increasing densities at higher temperature, deduced from the observations, is at odds with that proposed in the current picture of accretion shocks. To address these open issues, we investigate whether a correct treatment of the local absorption by the surrounding medium is crucial to explain the observations. To this end, we describe the impact of an accretion stream on a CTTS by considering a magnetohydrodynamic model. From the model results, we synthesize the X-ray emission from the accretion shock by producing maps and spectra. We perform density and temperature diagnostics on the synthetic spectra, and we directly compare the results with observations. Our model shows that the X-ray fluxes inferred from the emerging spectra are lower than expected because of the complex local absorption by the optically thick material of the chromosphere and of the unperturbed stream. Moreover, our model, including the effects of local absorption, explains in a natural way the apparently puzzling pattern of density versus temperature observed in the X-ray emission from accretion shocks.

  9. Stable and unstable accretion in the classical T Tauri stars IM Lup and RU Lup as observed by MOST

    Science.gov (United States)

    Siwak, Michal; Ogloza, Waldemar; Rucinski, Slavek M.; Moffat, Anthony F. J.; Matthews, Jaymie M.; Cameron, Chris; Guenther, David B.; Kuschnig, Rainer; Rowe, Jason F.; Sasselov, Dimitar; Weiss, Werner W.

    2016-03-01

    Results of the time variability monitoring of the two classical T Tauri stars, RU Lup and IM Lup, are presented. Three photometric data sets were utilized: (1) simultaneous (same field) MOST satellite observations over four weeks in each of the years 2012 and 2013, (2) multicolour observations at the South African Astronomical Observatory in April-May of 2013, (3) archival V-filter All Sky Automated Survey (ASAS) data for nine seasons, 2001-2009. They were augmented by an analysis of high-resolution, public-domain VLT-UT2 Ultraviolet Visual Echelle Spectrograph spectra from the years 2000 to 2012. From the MOST observations, we infer that irregular light variations of RU Lup are caused by stochastic variability of hotspots induced by unstable accretion. In contrast, the MOST light curves of IM Lup are fairly regular and modulated with a period of about 7.19-7.58 d, which is in accord with ASAS observations showing a well-defined 7.247 ± 0.026 d periodicity. We propose that this is the rotational period of IM Lup and is due to the changing visibility of two antipodal hotspots created near the stellar magnetic poles during the stable process of accretion. Re-analysis of RU Lup high-resolution spectra with the broadening function approach reveals signs of a large polar coldspot, which is fairly stable over 13 years. As the star rotates, the spot-induced depression of intensity in the broadening function profiles changes cyclically with period 3.710 58 d, which was previously found by the spectral cross-correlation method.

  10. MAGNETIC BRAKING AND FIELD DISSIPATION IN THE PROTOSTELLAR ACCRETION PHASE

    Directory of Open Access Journals (Sweden)

    D. Galli

    2009-01-01

    Full Text Available We summarize recent theoretical work addressing the role of magnetic elds in the process of star formation. First, we concentrate on the efficiency of magnetic braking during cloud collapse and its consequences on the formation of centrifugally supported disks around young stars. Then, we relate this issue to the well-known magnetic ux problem of star formation, and we show that the introduction of non-ideal MHD e ects is a necessary step toward the development of self-consistent models for the collapse of molecular clouds and the formation and evolution of accretion disks around young stars.

  11. Possible evidence for metal accretion onto the surfaces of metal-poor main-sequence stars

    Energy Technology Data Exchange (ETDEWEB)

    Hattori, Kohei; Yoshii, Yuzuru [Institute of Astronomy, School of Science, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015 (Japan); Beers, Timothy C. [National Optical Astronomy Observatories, Tucson, AZ 85719 (United States); Carollo, Daniela [Department of Physics and Astronomy, Macquarie University, Sydney, 2109 NSW (Australia); Lee, Young Sun, E-mail: khattori@ioa.s.u-tokyo.ac.jp [Department of Astronomy, New Mexico State University, Las Cruces, NM 88003 (United States)

    2014-04-01

    The entire evolution of the Milky Way, including its mass-assembly and star-formation history, is imprinted onto the chemo-dynamical distribution function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase space spanned by position, velocity, and elemental abundance ratios. In particular, the chemo-dynamical distribution functions for low-mass stars (e.g., G- or K-type dwarfs) are precious tracers of the earliest stages of the Milky Way's formation, since their main-sequence lifetimes approach or exceed the age of the universe. A basic tenet of essentially all previous analyses is that the stellar metallicity, usually parameterized as [Fe/H], is conserved over time for main-sequence stars (at least those that have not been polluted due to mass transfer from binary companions). If this holds true, any correlations between metallicity and kinematics for long-lived main-sequence stars of different masses, effective temperatures, or spectral types must strictly be the same, since they reflect the same mass-assembly and star-formation histories. By analyzing a sample of nearby metal-poor halo and thick-disk stars on the main sequence, taken from Data Release 8 of the Sloan Digital Sky Survey, we find that the median metallicity of G-type dwarfs is systematically higher (by about 0.2 dex) than that of K-type dwarfs having the same median rotational velocity about the Galactic center. If it can be confirmed, this finding may invalidate the long-accepted assumption that the atmospheric metallicities of long-lived stars are conserved over time.

  12. Young stars in ɛ Chamaleontis and their disks: disk evolution in sparse associations

    Science.gov (United States)

    Fang, M.; van Boekel, R.; Bouwman, J.; Henning, Th.; Lawson, W. A.; Sicilia-Aguilar, A.

    2013-01-01

    ɛ Cha members with 10 μm features detected in their IRS spectra. We combine the dust properties derived in the ɛ Cha sample with those found using identical or similar methods in the MBM 12, Coronet, η Cha associations, and in the cores-to-disks legacy program. We find that disks around low-mass young stars show a negative radial gradient in the mass-averaged grain size and mass fraction of crystalline silicates. A positive correlation exists between the mass-averaged grain sizes of amorphous silicates and the accretion rates if the latter is above ~10-9 M⊙ yr-1, possibly indicating that those disks are sufficiently turbulent to prevent grains of several microns in size to sink into the disk interior. Based on observations performed at ESO's La Silla-Paranal observatory under programme 076.C-0470.

  13. Young Star May Be Belching Spheres of Gas, Astronomers Say

    Science.gov (United States)

    2001-05-01

    A young star more than 2,000 light-years away in the constellation Cepheus may be belching out spheres of gas, say astronomers who observed it with the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope. Not only is the star ejecting spheres of gas, the researchers say, but it also may be ejecting them repeatedly, phenomena not predicted by current theories of how young stars shed matter. Cepheus A star-forming region with blowups of detail In order to remain stable while accumulating matter, young stars have to throw off some of the infalling material to avoid "spinning up" so fast they would break apart, according to current theories. Infalling matter forms a thin spinning disk around the core of the new star, and material is ejected in twin "jets" perpendicular to the plane of the disk. "Twin jets have been seen emerging from many young stars, so we are quite surprised to see evidence that this object may be ejecting not jets, but spheres of gas," said Paul T.P. Ho, an astronomer at the Harvard-Smithsonian Center for Astrophysics. The research is reported in the May 17 edition of the scientific journal Nature. The astronomers observed a complex star-forming region in Cepheus and found an arc of water molecules that act like giant celestial amplifiers to boost the strength of radio signals at a frequency of 22 GHz. Such radio-wave amplifiers, called masers, show up as bright spots readily observed with radio telescopes. "With the great ability of the VLBA to show fine detail, we could track the motions of these maser spots over a period of weeks, and saw that this arc of water molecules is expanding at nearly 20,000 miles per hour," said Ho. "This was possible because we could detect detail equivalent to seeing Lincoln's nose on a penny in Los Angeles from the distance of New York," Ho added. "These observations pushed the tremendous capabilities of the VLBA and of modern computing power to their limits. This is an extremely complex

  14. Possible Evidence for Metal Accretion onto the Surfaces of Metal-Poor Main-Sequence Stars

    CERN Document Server

    Hattori, Kohei; Beers, Timothy C; Carollo, Daniela; Lee, Young Sun

    2014-01-01

    The entire evolution of the Milky Way, including its mass-assembly and star-formation history, is imprinted onto the chemo-dynamical distribution function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase space spanned by position, velocity, and elemental abundance ratios. In particular, the chemo-dynamical distribution functions for low-mass stars (e.g., G- or K-type dwarfs) are precious tracers of the earliest stages of the Milky Way's formation, since their main-sequence lifetimes approach or exceed the age of the universe. A basic tenet of essentially all previous analyses is that the stellar metallicity, usually parametrized as [Fe/H], is conserved over time for main-sequence stars (at least those that have not been polluted due to mass transfer from binary companions). If this holds true, any correlations between metallicity and kinematics for long-lived main-sequence stars of different masses, effective temperatures, or spectral types must strictly be the same, since they reflect the ...

  15. The SW Sex-type star 2MASS J01074282+4845188: an unusual bright accretion disk with non-steady emission and a hot white dwarf

    CERN Document Server

    Khruzina, T; Kjurkchieva, D; 10.1051/0004-6361/201220385

    2013-01-01

    We present new photometric and spectral observations of the newly discovered nova-like eclipsing star 2MASS J01074282+4845188. To obtain a light curve solution we used model of a nova-like star whose emission sources are a white dwarf surrounded by an accretion disk, a secondary star filling its Roche lobe, a hot spot and a hot line. 2MASS J01074282+4845188 shows the deepest permanent eclipse among the known nova-like stars. It is reproduced by covering the very bright accretion disk by the secondary component. The luminosity of the disk is much bigger than that of the rest light sources. The determined high temperature of the disk is typical for that observed during the outbursts of CVs. The primary of 2MASS J01074282+4845188 is one of the hottest white dwarfs in CVs. The temperature of 5090 K of its secondary is also quite high and more appropriate for a long-period SW Sex star. It might be explained by the intense heating from the hot white dwarf and the hot accretion disk of the target. The high mass accr...

  16. The young active star SAO 51891 (V383 Lac)

    CERN Document Server

    Biazzo, Katia; Marilli, Ettore; Covino, Elvira; Alcala', Juan M; Cakirli, Omur; Klutsch, Alexis; Meyer, Michael R

    2009-01-01

    Our aim is investigating surface inhomogeneities of the young late-type star SAO51891, from photosphere to upper chromosphere, analyzing contemporaneous high-resolution spectra and broad-band photometry. The FOCES@CAHA spectral range is used to determine spectral classification and derive vsini and Vrad. The Li abundance is measured to estimate the age. The BVRIJHKs bands are used to construct the SED. The variations of our BV fluxes and Teff are used to infer the presence of photospheric spots and observe their behavior over time. The chromospheric activity is studied applying the spectral subtraction technique to Halpha, CaII H&K, Heps, and CaII IRT lines. We find SAO51891 to be a young K0-1V star with Li abundance close to the Pleiades upper envelope, confirming its youth (~100 Myr), also inferred from its kinematical membership to the Local Association. We detect no IR excess from SED analysis, and rotational modulation of luminosity, Teff, CaII, and Heps total fluxes. A spot model with two active reg...

  17. Young Stars and Protostellar Cores near NGC 2023

    CERN Document Server

    Mookerjea, B; Jarrett, T H; McMullin, J P

    2009-01-01

    We investigate the young (proto)stellar population in NGC 2023 and the L 1630 molecular cloud bordering the HII region IC 434, using Spitzer IRAC and MIPS archive data, JCMT SCUBA imaging and spectroscopy as well as targeted BIMA observations of one of the Class 0 protostars, NGC 2023 MM1. We have performed photometry of all IRAC and MIPS images, and used color-color diagrams to identify and classify all young stars seen within a 22'x26' field along the boundary between IC 434 and L 1630. For some stars, which have sufficient optical, IR, and/or sub-millimeter data we have also used the online SED fitting tool for a large 2D archive of axisymmetric radiative transfer models to perform more detailed modeling of the observed SEDs. We identify 5 sub-millimeter cores in our 850 and 450 micron SCUBA images, two of which have embedded class 0 or I protostars. Observations with BIMA are used to refine the position and characteristics of the Class 0 source NGC 2023 MM 1. These observations show that it is embedded in...

  18. Multiple jets from the young star IRAS 21334 + 5039

    Science.gov (United States)

    Smith, Howard A.; Fischer, Jacqueline

    1992-01-01

    The source IRAS 21334 + 5039, a young stellar object (YSO) with broad CO outflow velocity profiles, was imaged in the 1-2 micron region with broad-band J, H, and K filters, and with a Fabry-Perot set to the molecular hydorgen v = 1-0 S(1) line, the hydrogen Br-gamma recombination line, and the neighboring continua. At 2 microns the source has an elongated continuum emission structure centered on the star. The structure has very blue tips, with a weak VLA 6 cm continuum source coincident with one of them. Strong molecular hydrogen emission appears as bow-shaped arcs oriented along an axis perpendicular to the continuum emission, and as a weaker structure aligned with it. Both the continuum and the molecular hydrogen emission are most likely produced by jets, possibly emanating in multiple directions from the central source at large angles to each other. While jets are a common feature of young stars, this source provides solid evidence for multiple jet structures.

  19. The Brightest Young Star Clusters in NGC 5253

    CERN Document Server

    Calzetti, D; Adamo, A; Gallagher, J S; Andrews, J E; Smith, L J; Clayton, G C; Lee, J C; Sabbi, E; Ubeda, L; Kim, H; Ryon, J E; Thilker, D; Bright, S N; Zackrisson, E; Kennicutt, R C; de Mink, S E; Whitmore, B C; Aloisi, A; Chandar, R; Cignoni, M; Cook, D; Dale, D A; Elmegreen, B G; Elmegreen, D M; Evans, A S; Fumagalli, M; Gouliermis, D A; Grasha, K; Grebel, E K; Krumholz, M R; Walterbos, R; Wofford, A; Brown, T M; Christian, C; Dobbs, C; Herrero, A; Kahre, L; Messa, M; Nair, P; Nota, A; Oestlin, G; Pellerin, A; Sacchi, E; Schaerer, D; Tosi, M

    2015-01-01

    The nearby dwarf starburst galaxy NGC5253 hosts a number of young, massive star clusters, the two youngest of which are centrally concentrated and surrounded by thermal radio emission (the `radio nebula'). To investigate the role of these clusters in the starburst energetics, we combine new and archival Hubble Space Telescope images of NGC5253 with wavelength coverage from 1500 Ang to 1.9 micron in 13 filters. These include H-alpha, P-beta, and P-alpha, and the imaging from the Hubble Treasury Program LEGUS (Legacy Extragalactic UV Survey). The extraordinarily well-sampled spectral energy distributions enable modeling with unprecedented accuracy the ages, masses, and extinctions of the 9 optically brightest clusters (M_V < -8.8) and the two young radio nebula clusters. The clusters have ages ~1-15 Myr and masses ~1x10^4 - 2.5x10^5 M_sun. The clusters' spatial location and ages indicate that star formation has become more concentrated towards the radio nebula over the last ~15 Myr. The most massive cluster ...

  20. Photometric monitoring of the young star Par 1724 in Orion

    Science.gov (United States)

    Neuhäuser, R.; Koeltzsch, A.; Raetz, St.; Schmidt, T. O. B.; Mugrauer, M.; Young, N.; Bertoldi, F.; Roell, T.; Eisenbeiss, T.; Hohle, M. M.; Vaňko, M.; Ginski, C.; Rammo, W.; Moualla, M.; Broeg, C.

    2009-05-01

    We report new photometric observations of the ˜ 200 000 year old naked weak-line run-away T Tauri star Par 1724, located north of the Trapezium cluster in Orion. We observed in the broad band filters B, V, R, and I using the 90 cm Dutch telescope on La Silla, the 80 cm Wendelstein telescope, and a 25 cm telescope of the University Observatory Jena in Großschwabhausen near Jena. The photometric data in V and R are consistent with a ˜ 5.7 day rotation period due to spots, as observed before between 1960ies and 2000. Also, for the first time, we present evidence for a long-term 9 or 17.5 year cycle in photometric data (V band) of such a young star, a cycle similar to that to of the Sun and other active stars. Based on observations obtained with telescopes of the University Observatory Jena, which is operated by the Astrophysical Institute of the Friedrich-Schiller-University; a telescope of the University Observatory Munich on Mount Wendelstein, the 0.9m ESO-Dutch telescope on La Silla, Chile, and with the All Sky Automated Survey (ASAS) project (www.astrouw.edu.pl/asas).