WorldWideScience
 
 
1

Stellar laboratories II. New Zn IV and Zn V oscillator strengths and their validation in the hot white dwarfs G191-B2B and RE0503-289  

CERN Multimedia

For the spectral analysis of high-resolution and high-signal-to-noise spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model-atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. In a recent analysis of the ultraviolet (UV) spectrum of the DA-type white dwarf G191-B2B, 21 Zn IV lines were newly identified. Because of the lack of Zn IV data, transition probabilities of the isoelectronic Ge VI were adapted for a first, coarse determination of the photospheric Zn abundance. We performed new calculations of Zn IV and Zn V oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of the Zn IV - V spectrum exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE0503-289. In the UV spectrum of G191-B2B, we identify 31 Zn IV and 16 Zn V lines. Most of these are identified for the first time in an...

Rauch, T; Quinet, P; Kruk, J W

2014-01-01

2

Stellar laboratories. III. New Ba v, Ba vi, and Ba vii oscillator strengths and the barium abundance in the hot white dwarfs G191-B2B and RE 0503-289  

Science.gov (United States)

Context. For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims: Reliable Ba v-vii oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE 0503-289 and to determine their photospheric Ba abundances. Methods: We newly calculated Ba v-vii oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Ba lines exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE 0503-289. Results: For the first time, we identified highly ionized Ba in the spectra of hot white dwarfs. We detected Ba vi and Ba vii lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE 0503-289. The Ba vi/Ba vii ionization equilibrium is well reproduced with the previously determined effective temperature of 70 000 K and surface gravity of log g = 7.5. The Ba abundance is 3.5 ± 0.5 × 10-4 (mass fraction, about 23 000 times the solar value). In the FUSE spectrum of G191-B2B, we identified the strongest Ba vii line (at 993.41 Å) only, and determined a Ba abundance of 4.0 ± 0.5 × 10-6 (about 265 times solar). Conclusions: Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Ba vi-vii line profiles in two white dwarfs' (G191-B2B and RE 0503-289) far-ultraviolet spectra were well reproduced with our newly calculated oscillator strengths. This allowed to determine the photospheric Ba abundance of these two stars precisely. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26666.Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer.Tables 1-3 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/566/A10

Rauch, T.; Werner, K.; Quinet, P.; Kruk, J. W.

2014-06-01

3

Stellar laboratories III. New Ba V, Ba VI, and Ba VII oscillator strengths and the barium abundance in the hot white dwarfs G191-B2B and RE0503-289  

CERN Document Server

For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Reliable Ba V - VII oscillator strengths are used to identify Ba lines in the spectra of the DA-type white dwarf G191-B2B and the DO-type white dwarf RE0503-289 and to determine their photospheric Ba abundances. We newly calculated Ba V - VII oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Ba lines exhibited in high-resolution and high-S/N UV observations of G191-B2B and RE0503-289. For the first time, we identified highly ionized Ba in the spectra of hot white dwarfs. We detected Ba VI and Ba VII lines in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE0503-289. The Ba VI / Ba VII ionization equil...

Rauch, T; Quinet, P; Kruk, J W

2014-01-01

4

Deuterium abundance toward G191-B2B Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission  

CERN Document Server

High-resolution spectra of the hot white dwarf G191-B2B, covering the wavelength region 905-1187A, were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). This data was used in conjunction with existing high-resolution Hubble Space Telescope STIS observations to evaluate the total HI, DI, OI and NI column densities along the line of sight. Previous determinations of N(DI) based upon GHRS and STIS observations were controversial due to the saturated strength of the DI Lyman-alpha line. In the present analysis the column density of DI has been measured using only the unsaturated Lyman-beta and Lyman-gamma lines observed by FUSE. A careful inspection of possible systematic uncertainties tied to the modeling of the stellar continuum or to the uncertainties in the FUSE instrumental characteristics has been performed. The column densities derived are: log N(DI) = 13.40 +/-0.07, log N(OI) = 14.86 +/-0.07, and log N(NI) = 13.87 +/-0.07 quoted with 2-sigma uncertainties. The measurement of the HI column ...

Lemoine, M; Hébrard, G; Désert, J M; Ferlet, R; Etangs, A L; Howk, J C; André, M; Blair, W P; Friedman, S D; Kruk, J W; Lacour, S; Moos, H W; Sembach, K R; Chayer, P; Jenkins, E B; Köster, D; Linsky, J L; Wood, B E; Oegerle, W R; Sonneborn, G; York, D G

2001-01-01

5

Towards a standardised line list for G191-B2B, and other DA type objects  

CERN Document Server

We present a comprehensive analysis of the far UV spectrum of G191-B2B over the range of 900-1700{\\AA} using co-added data from the FUSE and STIS archives. While previous identifications made by Holberg et al. (2003) are reaffirmed in this work, it is found that many previously unidentified lines can now be attributed to Fe, Ni, and a few lighter metals. Future work includes extending this detailed analysis to a wider range of DA objects, in the expectation that a more complete analysis of their atmospheres can be realised.

Preval, Simon P; Holberg, Jay B; Dickinson, N J

2012-01-01

6

The D/H Ratio in Interstellar Gas Towards G191-B2B  

CERN Document Server

We reinvestigate the question of spatial variation of the local D/H abundance, using both archival GHRS spectra, and new echelle spectra of G191-B2B obtained with the Space Telescope Imaging Spectrograph (STIS) aboard HST. Our analysis uses stratified line-blanketed non-LTE model atmosphere calculations to determine the shape of the intrinsic WD Lyman-alpha profile and estimate the WD photospheric contamination of the interstellar lines. Although three velocity components were reported previously towards G191-B2B, we detect only two velocity components. The first component is at V(hel) ~ 8.6 km/s and the second at V(hel) ~ 19.3 km/s, which we identify with the Local Interstellar Cloud (LIC). From the STIS data we derive D/H = 1.60(+0.39,-0.27)X10^-5 for the LIC component, and D/H > 1.26X10^-5 for the 8.6 km/s component (uncertainties denote 2-sigma or 95% confidence limits). The STIS data provide no evidence for local or component-to-component variation in the D/H ratio. Despite using two velocity components ...

Sahu, M S; Bruhweiler, F C; Gull, T R; Bowers, C A; Lindler, D; Feggans, K; Barstow, M A; Hubeny, I; Holberg, J B

1999-01-01

7

Uv spectra of nearby white dwarfs and the nature of the local interstellar medium  

International Nuclear Information System (INIS)

We have investigated the local interstellar medium in the directions of four white dwarfs, G191-B2B, W1346, HD 149499B, and Sirius B. All the observational data were obtained at the high-resolution mode (lambda/?lambdaroughly-equal104) in the spectral range from about 1150 to 3200 A with the International Ultraviolet Explorer (IUE). Interstellar absorption lines of several elements in various stages of ionization are seen against the continuum of the white dwarfs. Low average hydrogen number densities (n-bar/sub HtsI/) are found. They range from n-bar/sub HtsI/ = 0.08 cm-3 for Sirius B, the nearest white dwarf (2.7 pc), to n-bar/sub HtsI/ = 0.006 cm-3 for G191-B2B, the most distant white dwarf (48 pc) studied. The results show, when combined with other recent ultraviolet, EUV, and diffuse X-ray observations, that: (a) the Sun is located inside a low-density (n-bar/sub HtsI/roughly-equal0.1 cm-3) cloud; (b) beyond 2--3 pc from the Sun, this cloud is surrounded, at least in most directions, by an extended region of hot (Troughly-equal10/sup 5en-dash6/ K) thin (nroughly-equal10-2 to 10-3 cm-3) interstellar plasma with no evidence for additional clouds in the lines of sight studied; (c) the elemental depletions of C, N, O, Si, Mg, and possibly Fe are low in the solar vicinity as previously found toward ? Vir, (d) the Sun is moving through this cloud at a relative velocity of about 20 km s-1; and (e) the current results, which are quite consistent with previous ultraviolet, EUV, and diffuse X-ray observations, have significant bearings on the theoretical modeling of the interstellar medium. Subject headings: interstellar: abundances: interstellar: matter: stars: white dwarfs: ultraviolet: spectra

1982-08-01

8

NLTE Line Blanketed Model Atmospheres for Hot, Metal-rich White Dwarfs  

Science.gov (United States)

Recent observations of some hot DA white dwarfs (Feige 24, G191 B2B - Sion et al. 1992, Ap.J. 391, L29; Vennes et al. 1992, Ap.J. 392, L27) and subsequent analyses have demonstrated that their atmospheres are contaminated with heavy metal species with appreciable abundances. So far, modeling was mostly limited to calculating synthetic spectra (usually in LTE), based on previously calculated model atmospheres that were constructed assuming a simplified chemical composition. However, in reality the metal lines may also significantly influence the temperature structure, and consequently the ionization balance of some important species. Since the effective temperature is rather high (55000 to 60000 K), the NLTE effects may be important despite the high gravity. Therefore, in order to settle the question on metal abundances of hot DA white dwarfs, we need to calculate NLTE fully line blanketed model atmospheres. Using our previously developed hybrid complete linearization/accelerated lambda iteration method, we have calculated a set of NLTE models including H, He, C, N, O, and Fe, with some 13000 lines of Fe IV, Fe V and Fe VI taken into account explicitly in model construction. The NLTE departure coefficients for all levels of Fe IV, Fe V, and Fe VI are then employed in the spectrum synthesis program SYNSPEC. In the first part of this study, we present a theoretical comparison between models computed with adding more and more opacity sources, and show how the temperature structure and synthetic spectra develop. We also discuss possible errors arising from using an inconsistent spectrum synthesis (the metals considered only in the spectrum synthesis, not in the model construction). In the second part, we compare the computed UV spectra with with available observations of Feige 24 and G191 B2B, and deduce limits for the iron abundance of these stars.

Hubeny, I.; Lanz, T.

1993-05-01

9

Carbon in white dwarfs  

International Nuclear Information System (INIS)

The problem of obtaining a considerable carbon enhancement in nova envelopes to account for the occurrence of fast novae is examined in detail. No mechanism seems completely satisfactory from the point of view of stellar evolution and white dwarf structure, even considering evidence from single white dwarf observations

1980-05-15

10

White dwarfs - fossil stars  

Energy Technology Data Exchange (ETDEWEB)

The structure and properties of white dwarfs are analyzed. The physical structure of white dwarf interiors is balanced by a quantum-mechanical effect known as electron degeneracy pressure. Besides preventing gravitational collapse, the degenerate electrons also control the thermal structure of the stars. The transport of energy in the interior and near the surface of the stars is discussed. The surface composition of the white dwarfs is examined. It is observed that the surface of the stars contains only one element; 80 percent of the stars contain only H and the remaining stars contain He. It is determined that the intense gravity of the stars leads to a layering arrangement of material within the star. The origin and evolution of white dwarfs, in particular their crystal stage, are described.

Kawaler, S.D.; Winget, D.E.

1987-08-01

11

White dwarf pulsations  

International Nuclear Information System (INIS)

The DA white dwarfs are those which show only the Stark-broadened lines of hydrogen in their spectra. They comprise about 80% of the total white dwarf population. A subset of the DA dwarfs, the ZZ Ceti stars, form a highly homogeneous class of nonradially pulsating variable stars. In this paper we shall review the observations from which both the physical properties of the stars and the characteristics of the pulsations have been derived. Data obtained since the last review of these variables (Robinson 1979) is stressed, as these data are forcing a somewhat revised understanding of the ZZ Ceti stars and their relationship to investigations of white dwarfs and to pulsating variable stars, in general. (orig.)

1980-01-01

12

Brown Dwarf Companions to White Dwarfs  

CERN Document Server

Brown dwarf companions to white dwarfs are rare, but recent infra-red surveys are slowly reveal- ing examples. We present new observations of the post-common envelope binary WD0137-349, which reveals the effects of irradiation on the ~ 0.05M* secondary, and new observations of GD 1400 which show that it too is a close, post-comon envelope system. We also present the lat- est results in a near-infrared photometric search for unresolved ultracool companions and to white dwarfs with UKIDSS. Twenty five DA white dwarfs were identified as having photometric excesses indicative of a low mass companion, with 8-10 of these having a predicted mass in the range asso- ciated with brown dwarfs. The results of this survey show that the unresolved (< 2") brown dwarf companion fraction to DA white dwarfs is 0.3 \\leq fWD+BD \\leq 1.3%.

Burleigh, M R; Dobbie, P D; Farihi, J; Napiwotzki, R; Maxted, P F L; Barstow, M A; Jameson, R F; Casewell, S L; Gänsicke, B T; Marsh, T R

2011-01-01

13

White Dwarf Stars  

Science.gov (United States)

This NASA site provides information about white dwarfs, produced when stars like our Sun exhaust their nuclear fuel and blow off much of their mass. The site contains an explanation of their properties and composition. Additional links include an introductory article, online quiz, cool facts, FAQ, and other resources.

2007-01-26

14

Stark Broadening and White Dwarfs  

CERN Multimedia

White dwarf and pre-white dwarf atmospheres are one of the best examples for the application of Stark broadening research results in astrophysics, due to plasma conditions very favorable for this line broadening mechanism. For example in hot hydrogen-deficient (pre-) white dwarf stars Teff = 75 000 K - 180 000 K and log g = 5.5-8 [cgs]. Even for much cooler DA and DB white dwarfs with typical effective temperatures of 10 000 K - 20 000 K, Stark broadening is usually the dominant broadening mechanism. In this review, Stark broadening in white dwarf spectra is considered and the attention is drawn to the STARK-B database (http://stark-b.obspm.fr/), containing Stark broadening parameters needed for white dwarf spectra analysis and synthesis, as well as to the new search facilities which will provide the collective effort to develop Virtual Atomic and Molecular Data Center (VAMDC - http://vamdc.org/).

Dimitrijevic, Milan S; Simic, Zoran; Sahal-Brechot, Sylvie

2012-01-01

15

The Lowest Mass White Dwarf  

CERN Multimedia

Extremely low mass white dwarfs are very rare objects likely formed in compact binary systems. We present MMT optical spectroscopy of 42 low mass white dwarf candidates serendipitously discovered in a survey for hypervelocity B-type stars. One of these objects, SDSS J0917+46, has Teff= 11,288 \\pm 72 K and log g = 5.48 \\pm 0.03; with an estimated mass of 0.17 M_sun, it is the lowest gravity/mass white dwarf currently known. However, 40 of the low mass candidates are normal DA white dwarfs with apparently inaccurate SDSS g magnitudes. We revisit the identification of low mass white dwarf candidates previously found in the SDSS, and conclude that four objects have M < 0.2 M_sun. None of these white dwarfs show excess emission from a binary companion, and radial velocity searches will be necessary to constrain the nature of the unseen companions.

Kilic, M; Brown, W R; Köster, D; Kilic, Mukremin; Prieto, Carlos Allende; Brown, Warren R.

2006-01-01

16

White Dwarfs in the Galaxy  

Science.gov (United States)

Studies of white dwarf stars contribute to our understanding of star formation and evolution in our Galaxy. We are exploring the properties of a distant white dwarf population in the Galactic disk (V >18) uncovered in the 2dF Quasar Redshift Survey (2QZ). We complemented the original 2QZ spectra obtained at the Anglo Australian Telescope with optical (Sloan) and ultraviolet (GALEX) data. Based on a model atmosphere analysis, we determine detailed parameters (effective temperature, surface gravity and composition) for nearly 1,000 catalogued stars. The selected white dwarfs are relatively young, owing to the blueand UV-excess selection criteria, and belong to a thin-disk population. Their effective temperatures exceed 10,000K corresponding to cooling ages below 1 Gyr. Based on these data, we present a study of the white dwarf luminosity function and Galactic scale-height distribution using various absolute luminosity indicators, and explore the relative luminosity functions of hydrogen-rich (DA) and non-DA white dwarfs. We also discuss the properties of peculiar objects (low-mass, magnetic, high-mass white dwarfs) and examine the incidence of late-type companionship to white dwarfs. This research is supported by the Grant Agency of the Czech Republic (205/05/P186) and by a NASA/GALEX grant NNG05GL42G, and by the College of Science at the Florida Institute of Technology.

Vennes, Stephane; Kawka, A.

2006-12-01

17

Gaia photometry for white dwarfs  

Science.gov (United States)

Context. White dwarfs can be used to study the structure and evolution of the Galaxy by analysing their luminosity function and initial mass function. Among them, the very cool white dwarfs provide the information for the early ages of each population. Because white dwarfs are intrinsically faint only the nearby (~ 20 pc) sample is reasonably complete. The Gaia space mission will drastically increase the sample of known white dwarfs through its 5-6 years survey of the whole sky up to magnitude V = 20-25. Aims: We provide a characterisation of Gaia photometry for white dwarfs to better prepare for the analysis of the scientific output of the mission. Transformations between some of the most common photometric systems and Gaia passbands are derived. We also give estimates of the number of white dwarfs of the different galactic populations that will be observed. Methods: Using synthetic spectral energy distributions and the most recent Gaia transmission curves, we computed colours of three different types of white dwarfs (pure hydrogen, pure helium, and mixed composition with H/He = 0.1). With these colours we derived transformations to other common photometric systems (Johnson-Cousins, Sloan Digital Sky Survey, and 2MASS). We also present numbers of white dwarfs predicted to be observed by Gaia. Results: We provide relationships and colour-colour diagrams among different photometric systems to allow the prediction and/or study of the Gaia white dwarf colours. We also include estimates of the number of sources expected in every galactic population and with a maximum parallax error. Gaia will increase the sample of known white dwarfs tenfold to about 200 000. Gaia will be able to observe thousands of very cool white dwarfs for the first time, which will greatly improve our understanding of these stars and early phases of star formation in our Galaxy. Tables 6 and 7 are available in electronic form at http://www.aanda.orgFull Tables 3-5 are available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/565/A11

Carrasco, J. M.; Catalán, S.; Jordi, C.; Tremblay, P.-E.; Napiwotzki, R.; Luri, X.; Robin, A. C.; Kowalski, P. M.

2014-05-01

18

Singing and dancing white dwarfs  

International Nuclear Information System (INIS)

Accreting white dwarfs have recently been shown to exhibit non-radial pulsations similar to their non-interacting counterparts. This allows us to probe the interior of the accreting white dwarf using seismology, and may be the only way to determine masses for non-eclipsing cataclysmic variables. Improving our understanding of accreting white dwarfs will have implications for models of supernovae Type Ia. Pulsating white dwarfs in cataclysmic variables are also useful in establishing the effects of accretion on pulsations. A search for nonradial pulsations among suitable candidates has led to the discovery of twelve such systems known to date. With the goal of establishing an instability strip (or strips) for these pulsating accretors, we acquired HST ultra-violet time-series spectroscopy of six pulsating white dwarfs in cataclysmic variables in 2007 and 2008. This approach enables us to measure the effective temperature of the white dwarf using the co-added spectrum, and to simultaneously characterize the pulsations. We also intended to constrain the pulsation mode identification by comparing the ultra-violet amplitudes to those from near-simultaneous ground-based photometry. Our preliminary results indicate a broad instability strip in the temperature range of 10500-15400 K.

2009-06-01

19

Astrophysics of white dwarf binaries  

Science.gov (United States)

White dwarf binaries are the most common compact binaries in the Universe and are especially important for low-frequency gravitational wave detectors such as LISA. There are a number of open questions about binary evolution and the Galactic population of white dwarf binaries that can be solved using gravitational wave data and at the same time, our ever improving knowledge about these binaries will help to predict the signals that can be expected for LISA. In addition a number of white dwarf binaries will serve as verification sources for the instrument. I will discuss these issues and report recent, surprising, developments in this field. Finally I report calculations about the feasibility of complementary electro-magnetic observations which unfortunately cannot reproduce the optimistic results of Cooray et al..

Nelemans, Gijs

2006-11-01

20

White dwarf constraints on a varying $G$  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A secular variation of $G$ modifies the structure and evolutionary time scales of white dwarfs. Using an state-of-the-art stellar evolutionary code, an up-to-date pulsational code, and a detailed population synthesis code we demonstrate that the effects of a running $G$ are obvious both in the properties of individual white dwarfs, and in those of the white dwarf populations in clusters. Specifically, we show that the white dwarf evolutionary sequences depend on both the val...

Garci?a-berro, Enrique; Torres, Santiago; Althaus, Leandro G.; Co?rsico, Alejandro H.; Lore?n-aguilar, Pablo; Romero, Alejandra D.; Isern, Jordi

2013-01-01

 
 
 
 
21

The evolution of iron white dwarf stars  

Directory of Open Access Journals (Sweden)

Full Text Available Recent measurements by Hipparcos provide strong observational evidence supporting the existence of white dwarf stars with iron-rich core composition. Here we examine the evolution of iron-rich white dwarfs, for which the cooling is substancially accelerated as compared with the standard carbon-oxigen white dwarfs.

J. A. Panei

2001-01-01

22

Gravitational Radiation from Strongly Magnetized White Dwarfs  

CERN Document Server

The magnetic fields of white dwarfs distort their shape generating an anisotropic moment of inertia. A magnetized white dwarf which rotates obliquely relative to the symmetry axis has a mass quadrupole moment which varies in time, so it will emit gravitational radiation. LISA may be able to detect the gravitational waves from two nearby, quickly rotating white dwarfs.

Heyl, J S

2000-01-01

23

White Dwarf Kinematics vs Mass  

CERN Document Server

We have investigated the relationship between the kinematics and mass of young (0.95Msun) are consistent with the expected relative contributions of single star evolution and mergers. We find that the kinematics are consistent with the majority of high-mass white dwarfs being formed through single star evolution.

Wegg, Christopher

2012-01-01

24

White dwarfs - the once and future suns  

International Nuclear Information System (INIS)

The history and properties of white dwarfs (Bessel's conclusion that Sirius and Procyon have invisible companions, Clark's discovery of Sirius B, Adams and Russell's study of white dwarf spectra, Chandrasekhar's explanation of white dwarf structure by equations incorporating quantum mechanics and relativity) are treated. Formation of white dwarfs, degeneracy, binary white dwarfs (and novae and supernovae) are explained. A mystery nearly 50 years old regarding the spectrum of the star Greenwich +70 degrees-8247 has been solved: it involves a stationary line phenomenon and a magnetic field of 300-500 million gauss. Processes being studied in white dwarfs and white dwarf models include gravitational settling, accretion, dredge-up, radiation pressure, and diffusive hydrogen burning

1986-01-01

25

G-dwarfs, white dwarfs and the local dark matter  

Energy Technology Data Exchange (ETDEWEB)

The observed paucity of low metalicity stars (the G-dwarf problem), the observed paucity of low luminosity white dwarfs and the apparent overabundance of matter in the solar neighborhood are discussed in a single context. In particular, bimodal models of star formation with enhanced cooling of low luminosity white dwarfs may be able to simply solve the G-dwarf problem as well as the local dark matter problem without the overproduction of observable white dwarfs. Constraints from galactic chemical evolution are also discussed. 55 refs., 5 figs.

Olive, K.A.

1986-01-01

26

Spitzer White Dwarf Planet Limits  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present preliminary limits on the presence of planets around white dwarf stars using the IRAC photometer on the Spitzer space telescope. Planets emit strongly in the mid-infrared which allows their presence to be detected as an excess at these wavelengths. We place limits of $5 M_J$ for 8 stars assuming ages of $1 Gyr$, and $10 M_J$ for 23 stars.We describe our survey, present our results and comment on approaches to improve our methodology.

Mullally, F.; Von Hippel, Ted; Winget, D. E.

2006-01-01

27

Acoustic fluxes in white dwarfs  

International Nuclear Information System (INIS)

Estimates of the acoustic flux generated in white dwarf convection zones are given. Wide ranges of mass, effective temperature, and convective efficiency have been considered. Homogeneous hydrogen- and helium-rich convective envelope models were computed as well as inhomogeneous models representing DA white dwarfs with thin layers of hydrogen surrounding helium-dominated regions. The calculations of the transition zone that separates the hydrogen- and helium-rich layers of these models is extended to take into account the fact that both hydrogen and helium are non-trace species. Only very thin layers of hydrogen are allowed if an underlyign helium convection zone is to produce acoustic noise in these inhomogeneous models. It is suggested that such layers are unstable against mixing. Using the input acoustic flux of the homogeneous envelopes, the soft X-ray luminosities of acoustically heated white dwarf coronae are estimated according to Hearn's coronal model. If such coronae exist, they could be detectable by present day techniques

1980-12-15

28

Gravitational radiation from differentially rotating white dwarfs  

International Nuclear Information System (INIS)

We examine the possibility of gravitational radiation from white dwarfs undergoing self-similar oscillations which are fed by the energy of the differential rotation of the white dwarf. We consider two typical cases of angular momentum distribution. Assuming the energy of the self-similar oscillations causing gravitational wave emission is about 1% of the energy dissipated in the differentially rotating white dwarf, the strain amplitudes are found to be less than 10-27 for a white dwarf at ?50 pc. Combined with the mechanism of gravitational radiation based on deformation energy from magnetic white dwarfs, the gravitational radiation from differentially rotating white dwarfs may produce a confusion limited foreground above the stochastic cosmological background for proposed advanced detectors in the decihertz frequency band

2004-12-07

29

Merging White Dwarfs and Thermonuclear Supernovae  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Thermonuclear supernovae result when interaction with a companion reignites nuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway, a catastrophic gain in pressure, and the disintegration of the whole white dwarf. It is usually thought that fusion is reignited in near-pycnonuclear conditions when the white dwarf approaches the Chandrasekhar mass. I briefly describe two long-standing problems faced by this scenario, and our suggestion that these supern...

Kerkwijk, Marten H.

2012-01-01

30

The galactic population of white dwarfs  

Energy Technology Data Exchange (ETDEWEB)

The contribution of white dwarfs of the different Galactic populations to the stellar content of our Galaxy is only poorly known. Some authors claim a vast population of halo white dwarfs, which would be in accordance with some investigations of the early phases of Galaxy formation claiming a top-heavy initial- mass- function. Here, I present a model of the population of white dwarfs in the Milky Way based on observations of the local white dwarf sample and a standard model of Galactic structure. This model will be used to estimate the space densities of thin disc, thick disc and halo white dwarfs and their contribution to the baryonic mass budget of the Milky Way. One result of this investigation is that white dwarfs of the halo population contribute a large fraction of the Galactic white dwarf number count, but they are not responsible for the lion's share of stellar mass in the Milky Way. Another important result is the substantial contribution of the - often neglected - population of thick disc white dwarfs. Misclassification of thick disc white dwarfs is responsible for overestimates of the halo population in previous investigations.

Napiwotzki, Ralf, E-mail: r.napiwotzki@herts.ac.u [Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB (United Kingdom)

2009-06-01

31

The galactic population of white dwarfs  

International Nuclear Information System (INIS)

The contribution of white dwarfs of the different Galactic populations to the stellar content of our Galaxy is only poorly known. Some authors claim a vast population of halo white dwarfs, which would be in accordance with some investigations of the early phases of Galaxy formation claiming a top-heavy initial- mass- function. Here, I present a model of the population of white dwarfs in the Milky Way based on observations of the local white dwarf sample and a standard model of Galactic structure. This model will be used to estimate the space densities of thin disc, thick disc and halo white dwarfs and their contribution to the baryonic mass budget of the Milky Way. One result of this investigation is that white dwarfs of the halo population contribute a large fraction of the Galactic white dwarf number count, but they are not responsible for the lion's share of stellar mass in the Milky Way. Another important result is the substantial contribution of the - often neglected - population of thick disc white dwarfs. Misclassification of thick disc white dwarfs is responsible for overestimates of the halo population in previous investigations.

2009-06-01

32

Gaia - A White Dwarf Discovery Machine  

CERN Document Server

The Gaia data will help to improve the construction of a luminosity function for the disk and the halo and will provide a more accurate determination of the age of our solar neighborhood. Moreover, reliable stellar dynamical investigations of the disk and halo components will be possible. For the first time it will be possible to test the mass-radius relation of white dwarfs in great detail. Moreover, more accurate masses of magnetic and cool white dwarfs can be expected. Gaia is also expected to discover many new pulsating white dwarfs. The Gaia measurements can also complement the measurements of gravitational waves from close white dwarf binaries with Lisa.

Jordan, S

2006-01-01

33

Hot white dwarfs and the UV delay in dwarf novae  

CERN Document Server

We calculate the effect of illumination of dwarf nova accretion discs by radiation from a hot, central, white dwarf. We show that only for very hot white dwarfs (Teff ~ 40 000$ K) the inner region of quiescent dwarf nova discs are partially depleted so that the delay between the rise to outburst of the optical and UV fluxes would be increased as suggested recently by King (1997). This depletion, however, must create several small outbursts between main outbursts, contrary to observations. Lower white dwarf temperatures may cause the outburts to be of the `inside-out' type removing the UV delay. We conclude that white dwarf irradiation of dwarf nova discs is not very efficient for example because the UV radiation from the hot white dwarf does not penetrate deep enough in the disc atmosphere. The total ablation of the inner disc by e.g. evaporation (possibly related to illumination) appears to be a very promising possibility, accounting for both the EUV delay and the general lightcurves properties.

Hameury, J M; Dubus, G

1998-01-01

34

Photometric Variability of Magnetic White Dwarfs  

Science.gov (United States)

We present our survey results searching for photometric variability and rotation periods of 77 magnetic white dwarfs (MWDs). The sample consists of 24 bright (V MWD. Where periods of variability have been determined, the magnetic field strength, temperature, mass and age of the white dwarf have been compared to determine whether any physically interesting correlations emerge.

Lawrie, K. A.; Burleigh, M. R.; Brinkworth, C. S.; Marsh, T. R.; Hodgkin, S. T.; Baker, D. E. A.; Cossins, P.; Littlejohns, O. M.; Scott, A. E.; Steele, P. R.

2013-01-01

35

The white dwarf binary background and LISA  

International Nuclear Information System (INIS)

The assumptions used in developing the canonical Galacitic white dwarf binary background level for LISA are investigated. The differences between several models of the white dwarf binary population are described and a technique for comparing the onset of the confusion limit between different population models is introduced

2006-03-02

36

Old White Dwarfs as a Microlensing Population  

CERN Document Server

A popular interpretation of recent microlensing studies of the line of sight towards the Large Magellanic Cloud invokes a population of old white dwarf stars in the Galactic halo. Below I review the basic properties of old white dwarf stars and the ongoing efforts to detect this population directly.

Hansen, B M S

2000-01-01

37

White Dwarfs Cosmological and Galactic Probes  

CERN Multimedia

The emphasis on white dwarf stars and cosmology arises from the most recent advances in cosmological and galactic structure research in which white dwarf stars are playing a very prominent role. Examples are Type Ia supernovae (i.e. white dwarf supernovae), the origin and evolution of the universe, the age of the galactic disk, cosmochronology using white dwarfs in globular clusters and galactic clusters, and the physics of accretion onto compact (very dense) stars. As an assisting guide to the reader, we have included, by invitation, comprehensive review articles in each of the four major areas of the book, white dwarf supernovae, cosmology, accretion physics and galactic structure. The reviews include introductory material that they build upon. The book is suitable and most useful to advanced undergraduates, graduate students and scientific professionals (e.g. astronomers, astrophysicists, cosmologists, physicists).

Sion, Edward M; Vennes, Stéphane

2005-01-01

38

Discovery of a T dwarf + white dwarf binary system  

CERN Multimedia

We present the discovery of the first T dwarf + white dwarf binary system LSPM 1459+0857AB, confirmed through common proper motion and spectroscopy. The white dwarf is a high proper motion object from the LSPM catalogue that we confirm spectroscopically to be a relatively cool (Teff=5535+\\-45K) and magnetic (B~2MG) hydrogen-rich white dwarf, with an age of at least 4.8Gyrs. The T dwarf is a recent discovery from the UKIRT Infrared Deep Sky Survey (ULAS 1459+0857), and has a spectral type of T4.5+\\-0.5 and a distance in the range 43-69pc. With an age constraint (inferred from the white dwarf) of >4.8Gyrs we estimate Teff=1200-1500K and logg=5.4-5.5 for ULAS 1459+0857, making it a benchmark T dwarf with well constrained surface gravity. We also compare the T dwarf spectra with the latest LYON group atmospheric model predictions, which despite some shortcomings are in general agreement with the observed properties of ULAS 1459+0857. The separation of the binary components (16,500-26,500AU, or 365 arcseconds on t...

Day-Jones, A C; Ruiz, M T; Beaumont, H; Burningham, B; Gallardo, J; Gianninas, A; Bergeron, P; Napiwotzki, R; Jenkins, J S; Zhang, Z H; Murray, D; Catalan, S; Gomes, J

2010-01-01

39

Heating and cooling of accreting white dwarfs  

Science.gov (United States)

In cataclysmic variables (CVs), a white dwarf accretes matter from a main-sequence secondary star which fills its Roche-lobe. The mass accretion affects the temperature of the white dwarfs in these systems by several physical mechanisms, including irradiation and compression. The consequences are an inhomogeneous temperature distribution over the white dwarf surface, short-term heating and cooling of the white dwarf envelope in response to changes in the accretion rate and a retarded core cooling compared to non-accreting white dwarfs. I have analysed these effects in several CVs using ultraviolet spectroscopy obtained with the International Ultraviolet Explorer and with the Hubble Space Telescope. The systems included in this analysis belong to two different subclasses of CVs, polars and dwarf novae. I find that a large polar cap which covers 3-10% of the white dwarf surface and which is heated to ~10.000K above the mean white dwarf temperature is a common feature in the polars V834 Cen, AM Her, DP Leo, QQ Vul and RXJ1313-32. In AM Her, the best-studied case, this polar cap is most likely heated by irradiation with cyclotron emission or thermal bremsstrahlung from a rather high standing shock. The luminosity of this heated pole cap proves to be an important, but hitherto neglected ingredient in the energy balance of the accretion process. The white dwarf temperatures derived in this work for seven magnetic cataclysmic variables show a trend to lower temperatures at shorter orbital periods, which can be understood in the general picture of CV evolution where the systems evolve towards shorter periods. Hence, the orbital period can be considered as a clue to the age of the system. However, one long-period system, RXJ1313-32, is found to have a remarkably low temperature. Viable hypotheses for this low temperature are that RXJ1313-32 presently undergoes a prolonged episode of low accretion activity or that the system became a semi-detached binary only ``recently'', and that the white dwarf had sufficient time to cool during the pre-CV period. In dwarf novae, the white dwarf envelope is heated on a short timescale during dwarf nova outbursts, e.g. by irradiation from the luminous disc-star interface or by compression by the accreted mass. I could show for the first time that in VW Hyi the decrease of the observed ultraviolet flux following an outburst is due to a decrease of the photospheric temperature of the white dwarf. Furthermore, I could show that the white dwarf responds differently to the two types of outburst that the system undergoes. The declining luminosities and temperatures are in general agreement with models based on radiative or compressional heating of the outer layers of the white dwarf. However, from the present data it is not possible to unequivocally identify the heating mechanism. It is possible that the equatorial region of the white dwarf never reaches an equilibrium state due to the frequent repetitive heating. A dwarf nova very similar to VW Hyi, but with a much longer outburst cycle is EK TrA. This system, even though fainter than VW Hyi, may be better suited to study the thermal response of the white dwarf to dwarf nova outbursts. I present an analysis based on ultraviolet and optical spectroscopy of EK TrA which yields a temperature estimate for the white dwarf photosphere. In addition, the optical data show emission from a cool accretion disc (or a corona situated on top of a colder disc), possibly extending over much of the Roche radius of the primary.

Gänsicke, B. T.

40

PREFACE: 16th European White Dwarfs Workshop  

Science.gov (United States)

The 16th European Workshop on White Dwarfs was held in Barcelona, Spain, from 30 June to 4 July 2008 at the premises of the UPC. Almost 120 participants from Europe (France, Germany, United Kingdom, Italy, and several others), America (USA, Canada, Argentina, Brazil, and Chile), and other continents (Australia, South Africa, . . . ) attended the workshop. Among these participants were the most relevant specialists in the field. The topics covered by the conference were: White dwarf structure and evolution Progenitors and Planetary Nebulae White dwarfs in binaries: cataclysmic variables, double degenerates and other binaries White dwarfs, dust disks and planetary systems Atmospheres, chemical composition, magnetic fields Variable white dwarfs White dwarfs in stellar clusters and the halo White Dwarfs as SNIa progenitors The programme included 54 talks, and 45 posters. The oral presentations were distributed into the following sessions: Luminosity function, mass function and populations White dwarf structure and evolution White dwarf ages White dwarf catalogs and surveys Central stars of planetary nebulae Supernovae progenitors White dwarfs in novae and CVs Physical processes in white dwarfs and magnetic white dwarfs Disks, dust and planets around white dwarfs Pulsating white dwarfs Additionally we had a special open session about Spitzer and white dwarfs. The Proceedings of the 16th European Workshop on White Dwarfs are representative of the current state-of-the-art of the research field and include new and exciting results. We acknowledge the very positive attitude of the attendants to the workshop, which stimulated very fruitful discussions that took place in all the sessions and after the official schedule. Also, the meeting allowed new collaborations tp start that will undoubtedly result in significant advances in the research field. We also acknowledge the willingness of the participants to deliver their contributions before the final deadline. We sincerely thank them. The white dwarf community has been steadily growing since the first white dwarf workshop, held in Kiel (Germany) in 1974. Some of the participants in the first colloquium have already effectively retired; others - although officially retired - continue to attend successive workshops, Professor Weidemann, one of the first organizers, being a leading example. We hope we will be able to continue counting on them for many years. A very graphical view of the evolution of the field can be found in the homepage of Professor Detlev Koester, who has collected pictures of almost all the previous workshops:. Additionally, several astronomers coming from related fields have joined our (not so) small community. Most importantly, several generations of young scientists gave their first talks in these workshops. In summary our community is an active one, and we have close, durable and solid ties of friendship. We are optimistic and we foresee that the spirit of the previous workshops will continue in future editions. We would like to express our deepest gratitude to our sponsors: The Universitat Politècnica de Catalunya (UPC), the Institut de Ciències de l'Espai (CSIC), the Institute for Space Studies of Catalonia (IEEC), the Spanish Ministry of Education and Science, the Generalitat de Catalunya, the Ajuntament de Barcelona, the School of Civil Engineering of Barcelona and UPCnet. Finally, the IEEC staff and our graduate students have enthusiastically supported the organization of the workshop in every single detail; without them we would have not succeeded. We thank them especially. Also, we acknowledge the task of the Scientific Organizing Committee, which gave their full support in all the scientific tasks. Enrique García-Berro, UPC Margarida Hernanz, ICE (CSIC) Jordi Isern, ICE (CSIC) Santiago Torres, UPC Editors Conference photograph

Garcia-Berro, Enrique; Hernanz, Margarita; Isern, Jordi; Torres, Santiago

2009-07-01

 
 
 
 
41

Seismology of Accreting White Dwarfs  

CERN Multimedia

Pulsation modes have recently been observed in a handful of white dwarf (WD) primaries of cataclysmic variables, allowing an interesting new probe into the structure of accreting WD's. We briefly discuss the seismology of these objects, how stellar properties may be inferred from the observed mode frequencies, and mode driving mechanisms. For one pulsator, GW Lib, we have shown that a WD mass $M=1.05 M_\\odot$ and accreted envelope mass $M_{\\rm env}=0.4\\times 10^{-4} M_\\odot$ give the best match to the observed pulsation periods. A first exploration of mode driving favors $T_{\\rm eff} = 14 000 K$ and a massive WD, but more work is necessary.

Arras, P; Bildsten, L; Arras, Phil; Townsley, Dean; Bildsten, Lars

2004-01-01

42

White dwarf constraints on a varying $G$  

CERN Document Server

A secular variation of $G$ modifies the structure and evolutionary time scales of white dwarfs. Using an state-of-the-art stellar evolutionary code, an up-to-date pulsational code, and a detailed population synthesis code we demonstrate that the effects of a running $G$ are obvious both in the properties of individual white dwarfs, and in those of the white dwarf populations in clusters. Specifically, we show that the white dwarf evolutionary sequences depend on both the value of $\\dot G/G$, and on the value of $G$ when the white dwarf was born. We show as well that the pulsational properties of variable white dwarfs can be used to constrain $\\dot G/G$. Finally, we also show that the ensemble properties of of white dwarfs in clusters can also be used to set upper bounds to $\\dot G/G$. Precisely, the tightest bound --- $\\dot G/G \\sim -1.8 10^{-12}$ yr$^{-1}$ --- is obtained studying the population of the old, metal-rich, well populated, open cluster NGC 6791. Less stringent upper limits can be obtained compari...

García-Berro, Enrique; Althaus, Leandro G; Córsico, Alejandro H; Lorén-Aguilar, Pablo; Romero, Alejandra D; Isern, Jordi

2013-01-01

43

SDSS DR7 WHITE DWARF CATALOG  

Energy Technology Data Exchange (ETDEWEB)

We present a new catalog of spectroscopically confirmed white dwarf stars from the Sloan Digital Sky Survey (SDSS) Data Release 7 spectroscopic catalog. We find 20,407 white dwarf spectra, representing 19,712 stars, and provide atmospheric model fits to 14,120 DA and 1011 DB white dwarf spectra from 12,843 and 923 stars, respectively. These numbers represent more than a factor of two increase in the total number of white dwarf stars from the previous SDSS white dwarf catalogs based on DR4 data. Our distribution of subtypes varies from previous catalogs due to our more conservative, manual classifications of each star in our catalog, supplementing our automatic fits. In particular, we find a large number of magnetic white dwarf stars whose small Zeeman splittings mimic increased Stark broadening that would otherwise result in an overestimated log g if fit as a non-magnetic white dwarf. We calculate mean DA and DB masses for our clean, non-magnetic sample and find the DB mean mass is statistically larger than that for the DAs.

Kleinman, S. J.; Nitta, A. [Gemini Observatory, 670 North A' ohoku Place, Hilo, HI 96720 (United States); Kepler, S. O.; Pelisoli, Ingrid; Pecanha, Viviane; Costa, J. E. S. [Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS (Brazil); Koester, D. [Institut fuer Theoretische Physik und Astrophysik, Universitaet Kiel, D-24098 Kiel (Germany); Krzesinski, J. [Mt. Suhora Observatory, Pedagogical University of Cracow, ul. Podchorazych 2, 30-084 Cracow (Poland); Dufour, P.; Lachapelle, F.-R.; Bergeron, P. [Departement de Physique, Universite de Montreal, C. P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada); Yip, Ching-Wa [Department of Physics and Astronomy, The Johns Hopkins University, 3701 San Martin Drive, Baltimore, MD 21218 (United States); Harris, Hugh C. [United States Naval Observatory, Flagstaff Station, 10391 West Naval Observatory Road, Flagstaff, AZ 86001-8521 (United States); Eisenstein, Daniel J. [Harvard Smithsonian Center for Astrophysics, 60 Garden Street, MS 20, Cambridge, MA 02138 (United States); Althaus, L.; Corsico, A., E-mail: hch@nofs.navy.mil [Facultad de Ciencias Astronomicas y Geofisicas, Paseo del Bosque S/N, (1900) La Plata (Argentina)

2013-01-15

44

An overview of white dwarf stars  

Directory of Open Access Journals (Sweden)

Full Text Available We present a brief summary of what is currently known about white dwarf stars, with an emphasis on their evolutionary and internal properties. As is well known, white dwarfs represent the end products of stellar evolution for the vast majority of stars and, as such, bear the signatures of past events (such as mass loss, mixing phases, loss and redistribution of angular momentum, and thermonuclear burning that are of essential importance in the evolution of stars in general. In addition, white dwarf stars represent ideal testbeds for our understanding of matter under extreme conditions, and work on their constitutive physics (neutrino production rates, conductive and radiative opacities, interior liquid/solid equations of state, partially ionized and partially degenerate envelope equations of state, diffusion coefficients, line broadening mechanisms is still being actively pursued. Given a set of constitutive physics, cooling white dwarfs can be used advantageously as cosmochronometers. Moreover, the field has been blessed by the existence of four distinct families of pulsating white dwarfs, each mapping a different evolutionary phase, and this allows the application of the asteroseismological method to probe and test their internal structure and evolutionary state. We set the stage for the reviews that follow on cooling white dwarfs as cosmochronometers and physics laboratories, as well as on the properties of pulsating white dwarfs and the asteroseismological results that can be inferred.

Charpinet S.

2013-03-01

45

Surface compositions of magnetic white dwarfs  

International Nuclear Information System (INIS)

Magnetic white dwarfs have mixed spectra (of hydrogen and helium) much more frequently than nonmagnetic white dwarfs. Many authors have suggested that this is caused by the slowing down of gravitational settling by the magnetic field. This is shown not to be the case. The magnetic field can effectively stop diffusion only at very small optical depths (tau< or approx. =10"-"3), not in the usual line forming region. Instead it is suggested that the magnetic field stops the selective wind of hydrogen postulated by Michaud and Fontaine to explain the near complete absence of hydrogen from nonmagnetic, non-DA white dwarfs

1982-02-01

46

DE CVn: A bright, eclipsing red dwarf - white dwarf binary  

CERN Document Server

Close white dwarf - red dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarf - red dwarf binary with a relatively short (~8.7 hours) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study. From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters which we discuss in the frame work of common-envelope evolution. Photometric observations of the eclipses are used to determine an accurate ephemeris. From a model fit to an average low-resolution spectrum of DE CVn we constrain the temperature of the white dwarf and the spectral type of the red dwarf. The eclipse light curve is analysed and combined with the radial velocity curve of the red dwarf determined from time-resolved spectroscopy to derive constraints on the inclination and the masses of the components in the system. The derived ephemeris is HJD_min = 2452784.5533(1) + 0.3641394(2) x E. The red d...

Van den Besselaar, E J M; Morales-Rueda, L; Nelemans, G; Thorstensen, J R; Marsh, T R; Dhillon, V S; Robb, R M; Balam, D; Guenther, E W; Kemp, J; Augusteijn, T; Groot, P J

2007-01-01

47

Magnetic white dwarfs with debris disks  

CERN Multimedia

It has long been accepted that a possible mechanism for explaining the existence of magnetic white dwarfs is the merger of a binary white dwarf system, as there are viable mechanisms for producing sustainable magnetism within the merger product. However, the lack of rapid rotators in the magnetic white dwarf population has been always considered a problematic issue of this scenario. In order to explain this discrepancy we build a model in which the interaction between the magnetosphere of the star and the disk induces angular momentum transfer. Our model predicts that the magnetospheric interaction of magnetic white dwarfs with their disks results in a significant spin down, and we show that the observed rotation period of REJ 0317-853, which is suggested to be a product of a double degenerate merger, can be reproduced.

Külebi, Baybars; Lorén-Aguilar, Pablo; Isern, Jordi; García-Berro, Enrique

2012-01-01

48

Orbital evolution with white-dwarf kicks  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Recent observations of white dwarfs in globular clusters indicate that these stars may get a velocity kick during their time as giants. This velocity kick could originate naturally if the mass loss while on the asymptotic giant branch is slightly asymmetric. If white dwarfs get a kick comparable to the orbital velocity of the binary, the initial Runge-Lenz vector (eccentricity vector) of the orbit is damped to be replaced by a component pointing toward the cross product of t...

Heyl, Jeremy S.

2007-01-01

49

Gaia - A White Dwarf Discovery Machine  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The Gaia data will help to improve the construction of a luminosity function for the disk and the halo and will provide a more accurate determination of the age of our solar neighborhood. Moreover, reliable stellar dynamical investigations of the disk and halo components will be possible. For the first time it will be possible to test the mass-radius relation of white dwarfs in great detail. Moreover, more accurate masses of magnetic and cool white dwarfs can be expected. Ga...

Jordan, Stefan

2006-01-01

50

Comparison of theoretical white dwarf cooling timescales  

CERN Document Server

An accurate assessment of white dwarf cooling times is paramount to place white dwarf cosmochronology of Galactic populations on more solid grounds. This issue is particularly relevant in view of the enhanced observational capabilities provided by the next generation of Extremely Large Telescopes, that will offer more avenues to employ white dwarfs as probes of Galactic evolution and test-beds of fundamental physics. We estimate for the first time the consistency of results obtained from two independent and widely used evolutionary codes (BaSTI and LPCODE) for white dwarf models with fixed mass and chemical stratification, when the same input physics is employed in both codes. We considered 0.55Msun white dwarf models with both pure carbon and uniform carbon-oxygen (50/50 mass fractions) core. We have assessed for the first time the maximum possible accuracy in the current estimates of white dwarf cooling times, resulting only from the different implementations of the stellar evolution equations and homogeneo...

Salaris, Maurizio; García-Berro, Enrique

2013-01-01

51

General Relativistic Calculations for White Dwarf Stars  

CERN Document Server

Properties of white dwarf stars composed of relativistic degenerate electron gas are studied in general relativity. Mass-Radius relation for white dwarf stars is investigated by solving the Tolman-Oppenheimer-Volkoff equation for hydrostatic equilibrium considering the electron gas as non-interacting. Chandrasekhar's limiting mass of $1.452 M_\\odot$ is slightly modified to $1.397 M_\\odot$ for $^4_2$He (and $^{12}_{\\ 6}$C) white dwarf stars. The critical mass for iron white dwarf stars is also calculated using general relativity, which is found to be $1.204 M_\\odot$. The critical densities for gravitational instability is compared with the neutronization threshold. It is found that $^4_2$He and $^{12}_{\\ 6}$C white dwarf stars are stable aganist neutronization with the critical value of $1.397 M_\\odot$ whereas the critical masses for stability are smaller due to neutronization for $^{16}_{\\ 8}$O, $^{20}_{10}$Ne, $^{24}_{12}$Mg, $^{28}_{14}$Si, $^{32}_{16}$S and $^{56}_{26}$Fe white dwarf stars. The maximum sta...

Mathew, Arun

2014-01-01

52

Thirteenth Marcel Grossmann Meeting, Summary of the session, White Dwarf Pulsars and Rotating White Dwarf Theory  

CERN Document Server

This is the summary of the parallel session entitled "White Dwarf Pulsars and Rotating White Dwarf Theory", chaired by Yukikatsu Terada in Thirteenth Marcel Grossmann Meeting. The origin of cosmic rays remains a mystery, even over 100 years since their discovery. Neutron stars (NSs) are considered textbook cases of particle acceleration sites in our Galaxy, but many unresolved numerical problems remain. Searches for new acceleration sites are crucial for astrophysics. The magnetized white dwarfs (MWDs) have the same kind of rotating magnetosphere as NSs, and may be the source of up to 10% of galactic cosmic ray electrons. In the parallel session of the "white dwarf pulsars and rotating white dwarf theory", we focus on the current observational results on white dwarf pulsars, related theories of the radiation process both in white dwarfs and neutron stars, and the origin and rule of white dwarf pulsars, as well as surveying on the current theories of the internal structure and the equation of state of white dw...

Terada, Yukikatsu

2013-01-01

53

Maximum gravitational redshift of white dwarfs  

International Nuclear Information System (INIS)

The stability of uniformly rotating, cold white dwarfs is examined in the framework of the Parametrized Post-Newtonian (PPN) formalism of Will and Nordtvedt. The maximum central density and gravitational redshift of a white dwarf are determined as functions of five of the nine PPN parameters (?, ?, zeta2, zeta3, and zeta4), the total angular momentum J, and the composition of the star. General relativity predicts that the maximum redshifts is 571 km s-1 for nonrotating carbon and helium dwarfs, but is lower for stars composed of heavier nuclei. Uniform rotation can increase the maximum redshift to 647 km s-1 for carbon stars (the neutronization limit) and to 893 km s-1 for helium stars (the uniform rotation limit). The redshift distribution of a larger sample of white dwarfs may help determine the composition of their cores

1976-02-01

54

Detonations in white dwarf dynamical interactions  

Science.gov (United States)

In old, dense stellar systems collisions of white dwarfs are a rather frequent phenomenon. Here, we present the results of a comprehensive set of smoothed particle hydrodynamics simulations of close encounters of white dwarfs aimed to explore the outcome of the interaction and the nature of the final remnants for different initial conditions. Depending on the initial conditions and the white dwarf masses, three different outcomes are possible. Specifically, the outcome of the interaction can be either a direct or a lateral collision or the interaction can result in the formation of an eccentric binary system. In those cases in which a collision occurs, the infalling material is compressed and heated such that the physical conditions for a detonation may be reached during the most violent phases of the merger. While we find that detonations occur in a significant number of our simulations, in some of them the temperature increase in the shocked region rapidly lifts degeneracy, leading to the quenching of the burning. We thus characterize under which circumstances a detonation is likely to occur as a result of the impact of the disrupted star on the surface of the more massive white dwarf. Finally, we also study which interactions result in bound systems, and in which ones the more massive white dwarf is also disrupted as a consequence of the dynamical interaction. The sizeable number of simulations performed in this work allows us to find how the outcome of the interaction depends on the distance at closest approach, and on the masses of the colliding white dwarfs, and which is the chemical pattern of the nuclearly processed material. Finally, we also discuss the influence of the masses and core chemical compositions of the interacting white dwarfs and the different kinds of impact in the properties of the remnants.

Aznar-Siguán, G.; García-Berro, E.; Lorén-Aguilar, P.; José, J.; Isern, J.

2013-09-01

55

THE MASSES OF POPULATION II WHITE DWARFS  

International Nuclear Information System (INIS)

Globular star clusters are among the first stellar populations to have formed in the Milky Way, and thus only a small sliver of their initial spectrum of stellar types are still burning hydrogen on the main sequence today. Almost all of the stars born with more mass than 0.8 Msun have evolved to form the white dwarf cooling sequence of these systems, and the distribution and properties of these remnants uniquely holds clues related to the nature of the now evolved progenitor stars. With ultra-deep Hubble Space Telescope imaging observations, rich white dwarf populations of four nearby Milky Way globular clusters have recently been uncovered, and are found to extend impressive 5-8 mag in the faint-blue region of the Hertzsprung-Russell diagram. In this paper, we characterize the properties of these population II remnants by presenting the first direct mass measurements of individual white dwarfs near the tip of the cooling sequence in the nearest of the Milky Way globulars, M4. Based on Gemini/GMOS and Keck/LRIS multiobject spectroscopic observations, our results indicate that 0.8 Msun population II main-sequence stars evolving today form 0.53 ± 0.01 Msun white dwarfs. We discuss the implications of this result as it relates to our understanding of stellar structure and evolution of population II stars and for the age of the Galactic halo, as measured with white dwarf cooling theory.

2009-11-01

56

Magnetic white dwarfs in binary systems  

International Nuclear Information System (INIS)

The cataclysmic variables are close binary systems consisting of a late type star and a collapsed star, usually a white dwarf, undergoing mass exchange. According to the standard model, the late type star (the secondary) fills its Roche lobe and material escaping from the inner Lagrangian point is transferred to the primary by means of a mass transfer stream and an accretion disc. Recently a new class of cataclysmic variables has been discovered with distinctive optical properties that are inexplicable in terms of the standard model. These systems known as the AM Herculis type variables have provided the first direct evidence for the presence of strong magnetic fields in the white dwarfs of some cataclysmic variables. A review is presented of some of the important properties of these variables with emphasis on the unique system VV Puppis which has provided the first unequivocal evidence for high harmonic cyclotron radiation from white dwarfs

1982-01-01

57

White dwarf stars with chemically stratified atmospheres  

International Nuclear Information System (INIS)

Recent observations and theory suggest that some white dwarfs may have chemically stratified atmospheres: thin layers of hydrogen lying above helium-rich envelopes. Models of such atmospheres show that a discontinuous temperature inversion can occur at the boundary between the layer. Model spectra for layered atmospheres at 30,000 K and 50,000 K tend to have smaller decrements at 912 A, 504 A, and 228 A than uniform atmospheres would have. On the basis of their continuous extreme ultraviolet spectra, it is possible to distinguish observationally between uniform and layered atmospheres for hot white dwarfs

1982-08-15

58

White dwarf atmospheres and circumstellar environments  

CERN Document Server

Written by selected astronomers at the forefront of their fields, this timely and novel book compiles the latest results from research on white dwarf stars, complementing existing literature by focusing on fascinating new developments in our understanding of the atmospheric and circumstellar environments of these stellar remnants. Complete with a thorough refresher on the observational characteristics and physical basis for white dwarf classification, this is a must-have resource for researchers interested in the late stages of stellar evolution, circumstellar dust and nebulae, and the future

Hoard, Donald W

2012-01-01

59

FUSE observations of hot white dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We have analyzed FUSE observations of six hot white dwarf stars: four DA white dwarfs with T_eff >= 45000K, the DAO Feige55 (T_eff = 55000K), and the DA CD -38 10980 (T_eff = 24000K). Photospheric lines from SiIV, PV, and SVI can be observed in the majority of the five hotter objects. Feige55 shows also several other heavier elements. The measured abundances agree only partly with the predictions of the radiative levitation theory. We attribute this to current limitations of...

Wolff, B.; Kruk, J. W.; Koester, D.; Allard, N. F.; Ferlet, R.; Vidal-madjar, A.

2001-01-01

60

First VLT spectra of white dwarfs in a globular cluster  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present the first spectra obtained with the Very Large Telescope for white dwarfs in a globular cluster. Estimates of atmospheric parameters are obtained and compared to evolutionary tracks. We discuss possible implications for the distance scale of globular clusters and white dwarf evolution and demonstrate how white dwarfs might be used to establish an independent distance scale to globular clusters.

Moehler, S.; Heber, U.; Napiwotzki, R.; Koester, D.; Renzini, A.

2000-01-01

 
 
 
 
61

Hot Accretion onto White Dwarfs in Quiescent Dwarf Novae  

CERN Document Server

We present dynamically consistent solutions for hot accretion onto unmagnetized, rotating white dwarfs (WDs) in five quiescent dwarf novae. The measured WD rotation rates (and other system parameters) in RX And, SS Cyg, U Gem, VW Hyi and WZ Sge imply spindown of the WD by an extended hot flow emitting most of its X-rays in the vicinity of the stellar surface. In general, energy advection is absent and the flow is stable to convection and hydrodynamical outflows. In rapidly rotating systems, the X-ray luminosity provides only an upper limit on the quiescent accretion rate because of substantial stellar spindown luminosity. We suggest that the presence of hot flows in quiescent dwarf novae may limit the long-term WD rotation rates to significantly sub-Keplerian values.

Medvedev, M V; Medvedev, Mikhail V.; Menou, Kristen

2001-01-01

62

The Observational Signature of Old White Dwarfs  

CERN Document Server

The recent discovery of microlensing of stars in the Large Magellanic Cloud has excited much interest in the nature of the lensing population. Detailed analyses indicate that the mass of these objects ranges from 0.3-0.8 solar masses, suggesting that they might be white dwarfs, the faint remnants of stellar evolution. The confirmation of such an hypothesis would yield profound insights into the early history of our galaxy and the early generations of stars in the universe. Previous attempts have been made to place theoretical constraints on this scenario, but were unduly pessimistic because they relied on inadequate evolutionary models. Here we present the first results from detailed evolutionary models appropriate for the study of white dwarfs of truly cosmological vintage. We find that the commonly held notion that old white dwarfs are red to hold only for helium atmosphere dwarfs and that hydrogen atmosphere dwarfs will be blue, with colours similar to those of the faint point sources found in the Hubble D...

Hansen, B M S

1998-01-01

63

Binary White Dwarfs in the Galactic Halo  

Science.gov (United States)

We use the stellar population synthesis code SeBa (Portegies Zwart & Verbunt (1996), Toonen, Nelemans & Portegies Zwart (2012)) to study the halo white dwarf population. Here we assume a Kroupa initial mass function and compare 4 models, varying two parameters: the star formation (SF) history of the halo (either continuous SF during 2.5 Gyr, which started 13.2 Gyr ago, or a SF burst during 360 Myr, which started 12.9 Gyr ago - see the left panel of the figure) and the binary fraction of the halo (either 100% single stars, or 100% binaries). White dwarf cooling models (Althaus et al. (2009) and Renedo et al. (2010)) allow us to plot the halo white dwarf luminosity function for these 4 models, as is done in the right panel of the figure. Combined with an assumption about the density distribution of halo stars, we will study which of these white dwarfs Gaia can see, and what that can tell us about the initial parameter distributions in the halo. In the near future, we plan to use the Munich-Groningen semi-analytical galaxy formation model (Starkenburg et al. (2013)), to obtain key ingredients for the population synthesis modeling, such as a realistic star formation history (see the left panel of the figure).

van Oirschot, Pim; Nelemans, Gijs; Helmi, Amina; Starkenburg, Else; Pols, Onno; Brown, Anthony G. A.

2014-01-01

64

Magnetic white dwarfs with debris discs  

CERN Document Server

It has long been accepted that a possible mechanism for explaining the existence of magnetic white dwarfs is the merger of a binary white dwarf system, as there are viable mechanisms for producing sustainable magnetic fields within the merger product. However, the lack of rapid rotators in the magnetic white dwarf population has been always considered a problematic issue of this scenario. Smoothed Particle Hydrodynamics simulations show that in mergers in which the two white dwarfs have different masses a disc around the central compact object is formed. If the central object is magnetized it can interact with the disc through its magnetosphere. The torque applied by the disc changes the spin of the star, whereas the transferred angular momentum from the star to the disc determines the properties of the disc. In this work we build a model for the disc evolution under the effect of magnetic accretion, and for the angular momentum evolution of the star, which can be compared with the observations. Our model pre...

Külebi, Baybars; Lorén-Aguilar, Pablo; Isern, Jordi; García-Berro, Enrique

2013-01-01

65

Secular evolution of accreting white dwarfs and type I supernovae  

International Nuclear Information System (INIS)

The growth of a carbon-oxygen white dwarf is investigated within the framework of the general shell-flash theory of accreting white dwarfs. Account is taken of the envelope restructing in response to the shell flash and to possible mass loss from the white dwarf. Estimates are given for the amount of helium accumulated between successive hydrogen shell flashes. Significant helium accumulation is possible for mass-accretion rates (which depend on the white-dwarf mass) greater than 10-9 M/sub sun/ yr-1. The results are discussed in the context of exploding white-dwarf models of Type I supernovae

1982-09-01

66

Circumstellar Disks at White Dwarfs: Observations  

CERN Document Server

I present a thorough review of existing observational studies of circumstellar disks at white dwarf stars. This is done from a historical perspective beginning just prior to the ground-based discovery of dust at G29-38 nearly 25 years ago, and following the many scientific breakthroughs made with the Spitzer Space Telescope. The chapter is ideal for new or advanced students of this field and includes significant material on the observational properties of metal-polluted white dwarf atmospheres, which are physically connected to their circumstellar environments. Included are all published observations circa mid-2010, with an emphasis on the nascent field of exoterrestrial planetary (bulk) chemistry via disk-polluted atmospheric spectroscopy.

Farihi, J

2011-01-01

67

Lessons for Asteroseismology from White Dwarf Stars  

CERN Document Server

The interpretation of pulsation data for Sun-like stars is currently facing challenges quite similar to those faced by white dwarf modelers ten years ago. The observational requirements for uninterrupted long-term monitoring are beginning to be satisfied by successful multi-site campaigns and dedicated satellite missions. But exploration of the most important physical parameters in theoretical models has been fairly limited, making it difficult to establish a detailed best-fit model for a particular set of oscillation frequencies. I review the past development and the current state of white dwarf asteroseismology, with an emphasis on what this can tell us about the road to success for asteroseismology of other types of stars.

Metcalfe, T S

2005-01-01

68

Orbital evolution with white-dwarf kicks  

CERN Multimedia

Recent observations of white dwarfs in globular clusters indicate that these stars may get a velocity kick during their time as giants. This velocity kick could originate naturally if the mass loss while on the asymptotic giant granch is slightly asymmetric. If white dwarfs get a kick comparable to the orbital velocity of the binary, the initial Runge-Lenz vector (eccentricity vector) of the orbit is damped to be replaced by a component pointing toward the cross product of the initial angular momentum and the force. The final eccentricity may be of order unity and if the kick is sufficiently large, the system may be disrupted. These results may have important ramifications for the evolution of binary stars and planetary systems.

Heyl, Jeremy S

2007-01-01

69

LTE model atmospheres for accreting white dwarfs  

International Nuclear Information System (INIS)

The results are presented of self-consistent atmosphere calculations appropriate for the optically thick regions of accretion flows on to white dwarfs, and which can be used to model the observed soft X-ray -EUV spectra of cataclysmic variables. The calculations take into account irradiation by hard X-rays from other parts of the accretion flow, and the pressure of the accretion flow itself. It is shown that radiation pressure limits the effective temperature Tsub(eff) of stable atmospheres to kTsub(eff) < or approx., 20-40 eV, depending on the white dwarf mass: the approximation that the atmosphere is fully ionized and that the Eddington limit applies is shown to be incorrect. (author)

1987-06-01

70

Hydrogen model atmospheres for white dwarf stars  

CERN Multimedia

We present a detailed calculation of model atmospheres for DA white dwarfs. Our atmosphere code solves the atmosphere structure in local thermodynamic equilibrium with a standard partial linearization technique, which takes into account the energy transfer by radiation and convection. This code incorporates recent improved and extended data base of collision induced absorption by molecular hydrogen. We analyse the thermodynamic structure and emergent flux of atmospheres in a range 2500 < Teff < 60000$ K and 6.5 < log g < 9.0. Bolometric correction and colour indices are provided for a subsample of the model grid. Comparison of the colours is made with published observational material and results of other recent model calculations. Motivated by the increasing interest on helium core white dwarfs, we analyse the photometric characteristics of these stars during their cooling, using evolutionary models recently available. Effective temperatures, surface gravities, masses and ages have been determined...

Rohrmann, R D

2001-01-01

71

The SDSS White Dwarf - M Star Library  

CERN Document Server

The Sloan Digital Sky Survey (SDSS), originally targeted at quasi-stellar objects, has provided us with a wealth of astronomical byproducts through the last decade. Since then, the number of white dwarfs (WDs) with physically bound main-sequence star companions (mostly dM stars) has increased radically, allowing for fundamentally new insights into stellar physics. Different methods for the retrieval and follow-up analysis of SDSS WD-dM binaries have been applied in the literature, leading to a rising number of WD-dM catalogs. Here we present a detailed literature search, coupled with our own hunting for SDSS WD-dMs by color selection, the outcome being named the "SDSS White Dwarf - M Star Library". We also explain improvements of our automated spectral analysis method.

Heller, René; Østensen, Roy H

2011-01-01

72

Cooling Models for Old White Dwarfs addendum  

CERN Document Server

We present new white dwarf cooling models which incorporate an accurate outer boundary condition based on new opacity and detailed radiative transfer calculations. We find that helium atmosphere dwarfs cool considerably faster than has previously been claimed, while old hydrogen atmosphere dwarfs will deviate significantly from black body appearance. We use our new models to derive age limits for the Galactic disk. We find that the Liebert, Dahn & Monet (1988) luminosity function yields an age of only 6 Gyr if it is complete to stated limits. However, age estimates of individual dwarfs and the luminosity function of Oswalt et al (1995) are both consistent with disk ages as large as dark matter in Galactic halos. We find that previous attempts using inadequate cooling models were too severe and that direct detection limits allow a halo that is 11 Gyr old. If the halo is composed solely of helium atmosphere dwarfs, the lower age limit is only 7.5 Gyr. We also demonstrate the importance of studying the cooli...

Hansen, B M S

1999-01-01

73

The Link Between Planetary Systems, Dusty White Dwarfs, and Metal Polluted White Dwarfs  

CERN Document Server

It has long been suspected that metal polluted white dwarfs (types DAZ, DBZ, and DZ) and white dwarfs with dusty disks possess planetary systems, but a specific physical mechanism by which planetesimals are perturbed close to a white dwarf has not yet been fully posited. In this paper we demonstrate that mass loss from a central star during post main sequence evolution can sweep planetesimals into interior mean motion resonances with a single giant planet. These planetesimals are slowly removed through chaotic excursions of eccentricity that in time create radial orbits capable of tidally disrupting the planetesimal. Numerical N-body simulations of the Solar System show that a sufficient number of planetesimals are perturbed to explain white dwarfs with both dust and metal pollution, provided other white dwarfs have more massive relic asteroid belts. Our scenario requires only one Jupiter-sized planet and a sufficient number of asteroids near its 2:1 interior mean motion resonance. Finally, we show that once ...

Debes, J; Stark, C

2012-01-01

74

Dark-matter admixed white dwarfs  

CERN Document Server

We study the equilibrium structures of white dwarfs with dark matter cores formed by non-self-annihilating dark matter DM particles with mass ranging from 1 GeV to 100 GeV, which are assumed to form an ideal degenerate Fermi gas inside the stars. For DM particles of mass 10 GeV and 100 GeV, we find that stable stellar models exist only if the mass of the DM core inside the star is less than O(10^-3) Msun and O(10^-6) Msun, respectively. The global properties of these stars, and in particular the corresponding Chandrasekhar mass limits, are essentially the same as those of traditional white dwarf models without DM. Nevertheless, in the 10 GeV case, the gravitational attraction of the DM core is strong enough to squeeze the normal matter in the core region to densities above neutron drip, far above those in traditional white dwarfs. For DM with particle mass 1 GeV, the DM core inside the star can be as massive as around 0.1 Msun and affects the global structure of the star significantly. In this case, the radiu...

Leung, S -C; Lin, L -M; Wong, K -W

2013-01-01

75

Double White Dwarf Mergers with CASTRO  

Science.gov (United States)

Type Ia supernovae are among the brightest explosions in the Universe and are recognized as reliable distance indicators. While the accepted cause of these events is the thermonuclear incineration of white dwarf stars instigated by accretion, the characteristics of the donor stars remain under investigation. Recent observational evidence supports the hypothesis that the progenitors of some events are binary white dwarf systems, and has spurred renewed theoretical interest in a merger scenario. We present preliminary work modeling such a system using the modern, three-dimensional compressible hydrodynamics code CASTRO. CASTRO uses an adaptive Eulerian grid to highly resolve features of interest and has been demonstrated to accurately address typical dynamical problems in astrophysics. We present simulations showing that it is an appropriate tool for the case of white dwarfs orbiting each other under Newtonian gravity. We show the capability of the code to conserve energy and angular momentum over multiple orbits, and we discuss the differences between simulations in inertial and co-rotating reference frames. We also present verification tests of the gravity solver and describe the importance of appropriate boundary conditions. With the success of these verification tests, the next step is to apply CASTRO to the inspiral and onset of mass transfer and determine areas of likely nuclear burning. This work was supported in part by the NSF under award AST-1211563.

Katz, Maximilian P.; Zingale, M.; Calder, A.; Swesty, F. D.

2013-01-01

76

Detonations in white dwarf dynamical interactions  

CERN Multimedia

In old, dense stellar systems collisions of white dwarfs are a rather frequent phenomenon. Here we present the results of a comprehensive set of Smoothed Particle Hydrodynamics simulations of close encounters of white dwarfs aimed to explore the outcome of the interaction and the nature of the final remnants for different initial conditions. Depending on the initial conditions and the white dwarf masses, three different outcomes are possible. Specifically, the outcome of the interaction can be either a direct or a lateral collision or the interaction can result in the formation of an eccentric binary system. In those cases in which a collision occurs, the infalling material is compressed and heated such that the physical conditions for a detonation may be reached during the most violent phases of the merger. While we find that detonations occur in a significant number of our simulations, in some of them the temperature increase in the shocked region rapidly lifts degeneracy, leading to the quenching of the bu...

Aznar-Siguán, Gabriela; Lorén-Aguilar, Pablo; José, Jordi; Isern, Jordi

2013-01-01

77

Discovery of a peculiar DQ white dwarf  

CERN Multimedia

We report the discovery of a new carbon rich white dwarf that was identified during a proper motion survey for cool white dwarfs based on photographic material used for the construction of the Guide Star Catalog II. Its large proper motion (0.48 arcsec/yr) and faint apparent magnitude (V = 18.7) suggest a nearby object of low luminosity. A low-resolution spectrum taken with the William Herschel Telescope clearly shows strong C2 Deslandres-d'Azambuja and Swan bands, which identify the star as a DQ white dwarf. The strength of the Deslandres-d'Azambuja bands and the depression of the continuum in the Swan-band region are signs of enhanced carbon abundance for the given Teff. Comparison of our spectrophotometric data to published synthetic spectra suggests 6000 K < Teff < 8000 K although further analysis with specialized synthetic models appear necessary to derive both Teff and chemical composition. Finally, the range of spatial velocity estimated for this object makes it a likely member of the halo or thi...

Carollo, D; Spagna, A; Smart, R L; Lattanzi, M G; McLean, B J; Pinfield, D J

2002-01-01

78

White Dwarf Mass Distribution in the SDSS  

CERN Multimedia

We determined masses for the 7167 DA and 507 DB white dwarf stars classified as single and non-magnetic in data release four of the Sloan Digital Sky Survey (SDSS). We obtained revised Teff and log g determinations for the most massive stars by fitting the SDSS optical spectra with a synthetic spectra grid derived from model atmospheres extending to log g=10.0. We also calculate radii from evolutionary models and create volume-corrected mass distributions for our DA and DB samples. The mean mass for the DA stars brighter than g=19 and hotter than Teff=12000K is M(DA)= 0.593+/-0.016M(Sun). For the 150 DBs brighter than g=19 and hotter than Teff=16000K, we find M(DB)=0.711+/-0.009 M(Sun). It appears the mean mass for DB white dwarf stars may be significantly larger than that for DAs. We also report the highest mass white dwarf stars ever found, up to 1.33 M(Sun).

Kepler, S O; Nitta, A; Köster, D; Castanheira, B G; Giovannini, O; Costa, A F M; Althaus, L

2006-01-01

79

Anderson and Stoner Published White Dwarf Mass Limits Before Chandrasekhar  

CERN Multimedia

In their engaging recountals of Chandrasekhar's extraordinary career (Physics Today, vol 63, Issue 12, Dec 2010), neither Dyson nor Wali mention that Chandrasekhar was the third person not the first, to publish a white dwarf mass limit incorporating a relativistic treatment of degenerate electrons. As it has become a common misconception that Chandrasekhar was the first, a clarifying reminder on this historical point is warranted. In short, the white dwarf mass limit widely attributed to Chandrasekhar (1931) should be the specific white dwarf mass limit calculated for a polytrope. The insight that a relativistic treatment of degeneracy leads to the existence of a white dwarf mass limit first appeared in papers of W. Anderson (1929) and E.C. Stoner (1930) for a uniform density star. Accordingly, Chandrasekhar (1931) cites Stoner (1930) and points out that the polytrope white dwarf mass limit is less than Stoner's uniform density white dwarf mass limit by about 20%.

Blackman, Eric G

2011-01-01

80

Constraining white-dwarf kicks in globular clusters  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The wind of an asymptotic-giant-branch stars is sufficiently strong that if it is slightly asymmetric, it can propel the star outside of the open cluster of its birth or significantly alter its trajectory through a globular cluster; therefore, if these stellar winds are asymmetric, one would expect a deficit of white dwarfs of all ages in open clusters and for young white dwarfs to be less radially concentrated than either their progenitors or older white dwarfs in globular ...

Heyl, Jeremy S.

2007-01-01

 
 
 
 
81

MACHOs, White Dwarfs, and the Age of the Universe  

Digital Repository Infrastructure Vision for European Research (DRIVER)

(Abridged Abstract) A favored interpretation of recent microlensing measurements towards the Large Magellanic Cloud implies that a large fraction (i.e. 10--50%) of the mass of the galactic halo is composed of white dwarfs. We compare model white dwarf luminosity functions to the data from the observational surveys in order to determine a lower bound on the age of any substantial white dwarf halo population (and hence possibly on the age of the Universe). We compare various t...

Graff, David S.; Laughlin, Gregory; Freese, Katherine

1997-01-01

82

Formation and appearance of pulsar-like white dwarfs  

CERN Document Server

Accretion-driven spin-up of a magnetized white dwarf in a close binary system is discussed. We address a situation in which the magnetic field of the white dwarf is screening during the accretion phase and re-generating due to the field diffusion through the accreted material after it. We find this scenario to be effective for a formation of massive pulsar-like white dwarfs.

Ikhsanov, Nazar

2014-01-01

83

Spectroscopic Identification of Cool White Dwarfs in the Solar Neighbourhood  

CERN Document Server

The New Luyten Two-Tenths catalog contains a large number of high-proper motion white dwarf candidates that remain to be spectroscopically confirmed. We present new spectroscopic observations as well as SDSS archival spectra of 49 white dwarf candidates which have been selected from the revised NLTT catalog of Salim & Gould 2003. Out of these, 34 are cool DA white dwarfs with temperatures ranging from approximately 5000 K up to 11690 K, and 11 are DC white dwarfs with temperatures ranging from 4300 K (NLTT 18555) up to 11000 K. Three of the DA white dwarfs also display abundances of heavy elements (NLTT 3915, NLTT 44986 and NLTT 43806) and one is a cool magnetic white dwarf (NLTT 44447) with an estimated magnetic field strength of 1.3 MG. We also present a new cool DQ white dwarf (NLTT 31347) with an estimated temperature of 6250 K. We supplement our sample with SDSS ugriz photometry for a fraction of the newly identified white dwarfs. A kinematical study of this sample of white dwarfs, characterized by p...

Kawka, A; Kawka, Adela; Vennes, Stephane

2006-01-01

84

Carbon-oxygen white dwarf presupernova models  

International Nuclear Information System (INIS)

The long-term evolution of accreting C-O white dwarfs as a possible source of neutron stars and/or SNI events is considered. The effects of solidification of the star's core are included, in particular carbon-oxygen separation. Different degrees of chemical differentiation and a wide range of accretion rates are studied. We determine the pre-flash configurations and we show that a variety of results are possible, ranging from total collapse to partial disruption due to off-centre ignition of carbon. (orig.)

1981-07-10

85

Oxygen neutronization in accreting white dwarfs  

International Nuclear Information System (INIS)

Solid carbon-oxygen white dwarf cores have been shown to be likely initial configurations for collapse to neutron star densities. Solidification seems to entail carbon/oxygen separation, with oxygen settling at the star's center and carbon being confined to more external, lower-density layers. Electron captures on 16O are then the triggering mechanism for collapse. Here we elucidate the outcome of the complete reaction network started by those captures and we derive simple expressions for accurately describing the approach to nuclear statistical equilibrium along this hitherto neglected evolutionary path. (orig.)

1983-07-01

86

The white dwarf masses in cataclysmic variables  

Science.gov (United States)

The mechanism proposed by Hameury et al. (1988) for the 114-min periodicity in 6 of the 13 known AM Her magnetic cataclysmic variables (MCVs) is examined in the light of published observational data on EXO 033319-2554.2, a newly discovered 126-min-period MCV (Giommi et al., 1987; Beuermann and Thomas, 1987). The techniques used to estimate the masses of the white-dwarf primaries in MCVs are discussed, and it is concluded that the mass of EXO 033319-2554.2 is significantly greater (by 0.25-0.60 solar mass) than that in the MCVs with 114-min periodicity.

Hameury, J. M.; Lasota, J. P.; King, A. R.

1988-04-01

87

Simulating Gaia performances on white dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

One of the most promising space missions of ESA is the astrometric satellite Gaia, which will provide very precise astrometry and multicolour photometry, for all 1.3 billion objects to V~20, and radial velocities with accuracies of a few km/s for most stars brighter than V ~ 17. Consequently, full homogeneous six-dimensional phase-space information for a huge number of stars will become available. Our Monte Carlo simulator has been used to estimate the number of white dwarfs...

Torres, S.; Garcia-berro, E.; Isern, J.; Figueras, F.

2005-01-01

88

White dwarfs in common proper motion binaries.  

Czech Academy of Sciences Publication Activity Database

. San Francisco : Astronomical Society of the Pacific, 2010 - (Prša, A.), s. 147-148 ISBN 978-1-58381-750-6. - (ASP Conference Series. 435).[Binaries - Key to Comprehension of the Universe. Brno (CZ), 08.06.2009-12.06.2009]Grant CEP: GA ?R GD205/08/H005; GA AV ?R(CZ) IAA300030908; GA MŠk(CZ) LC06014Grant ostatní: GA AV ?R(CZ) IAA301630901Výzkumný zám?r: CEZ:AV0Z10030501Klí?ová slova: white dwarfsKód oboru RIV: BN - Astronomie a nebeská mechanika, astrofyzika

Arazimová, Eva; Kawka, Adela; Vennes, Stephane

89

White-dwarf-white-dwarf galactic background in the LISA data  

International Nuclear Information System (INIS)

LISA (Laser Interferometer Space Antenna) is a proposed space mission, which will use coherent laser beams exchanged between three remote spacecraft to detect and study low-frequency cosmic gravitational radiation. In the low part of its frequency band, the LISA strain sensitivity will be dominated by the incoherent superposition of hundreds of millions of gravitational wave signals radiated by inspiraling white-dwarf binaries present in our own Galaxy. In order to estimate the magnitude of the LISA response to this background, we have simulated a synthesized population that recently appeared in the literature. Our approach relies on entirely analytic expressions of the LISA time-delay interferometric responses to the gravitational radiation emitted by such systems, which allows us to implement a computationally efficient and accurate simulation of the background in the LISA data. We find the amplitude of the galactic white-dwarf binary background in the LISA data to be modulated in time, reaching a minimum equal to about twice that of the LISA noise for a period of about two months around the time when the Sun-LISA direction is roughly oriented towards the Autumn equinox. This suggests that, during this time period, LISA could search for other gravitational wave signals incoming from directions that are away from the galactic plane. Since the galactic white-dwarf background will be observed by LISA not as a stationary but rather as a cyclostationary random process with a period of 1 yr, we summarize the theory of cyclostationary random processes, present the corresponding generalized spectral method needed to characterize such process, and make a comparison between our analytic results and those obtained by applying our method to the simulated data. We find that, by measuring the generalized spectral components of the white-dwarf background, LISA will be able to infer properties of the distribution of the white-dwarf binary systems present in our Galaxy

2005-06-15

90

The mass of the white dwarf in GW Libra  

CERN Multimedia

We report a mass and rotational broadening (vsini) for the pulsating white dwarf component of the WZ Sge type Dwarf Nova GW Lib based on high-resolution VLT spectroscopy that resolves the MgII 4481A absorption feature. Its gravitational redshift combined with white dwarf mass-radius models, provides us with a direct measurement of the white dwarf mass of M_1 = 0.84 pm 0.02 M_sun. The line is clearly resolved and if associated with rotational broadening gives vsini=87.0 pm 3.4 km/s, equivalent to a spin period of 97 pm 12s.

van Spaandonk, L; Marsh, T R; Parsons, S G

2010-01-01

91

Pure hydrogen atmosphere for very cool white dwarfs  

CERN Multimedia

Microlensing events observed in the line of sight toward the LMC indicate that a significant fraction of the mass of the dark halo of the Galaxy is probably composed of white dwarfs. In addition, white dwarf sequences have now be observed in the HR diagrams of several globular clusters. Because of the unavailability of white dwarf atmospheres for Teff < 4000K, cooling time scales for white dwarfs older than ~ 10 Gyr are very uncertain. Moreover, the identification of a MACHO white dwarf population by direct observation depends on a knowledge of the colors and bolometric corrections of very-cool white dwarfs. In this paper we present the first detailed model atmospheres and spectra of very cool hydrogen white dwarfs for Teff < 4000K. We include the latest description of the opacities of hydrogen and significantly, we introduce a non-ideal equation of state in the atmosphere calculation. We find that due to strong absorption from H_2 in the infrared, very old white dwarfs are brightest in the V, R, and I ...

Saumon, D S

1999-01-01

92

Column accretion on to white dwarfs  

International Nuclear Information System (INIS)

An analysis is made of the global behaviour of all accretion column solutions in which the accreting matter reaches equipartition (Tsub(e)=Tsub(i)) before landing on the white dwarf. It is shown that all such columns remain very optically thin, and can be divided into two classes corresponding to the thermal conductive flux into the star becoming large (type 1) or always remaining small (type 2). The unique equipartition solution which can be matched to a perturbed white dwarf atmosphere is one in which the conductive flux into the star vanishes, thus establishing that equipartition flows cannot account for the very large soft X-ray excesses of the AM Herculis stars. It is argued that instead the accretion process in these objects involves transport by energetic 'non-local' electrons which are not in equipartition with the ions. In consequence the hot, optically thin post-shock region of the accretion flow must be limited in size by the equipartition length. This in turn limits the hard X-ray luminosity to a small fraction of the total accretion luminosity, in full agreement with observations of AM Herculis. (author)

1983-01-01

93

VW Hyi - The white dwarf revealed  

Science.gov (United States)

Nonsimultaneous IUE, optical, and near-IR observations of VW Hyi at quiescence are presented. Using these and UV data from other investigations, a broad feature in the ultraviolet is identified with L-alpha absorption. The presence and width of the line imply that (1) the white dwarf in VW Hyi is directly visible in the UV and (2) the effective temperature of this star is approximately 18,000 + or - 2000 K for log g = 8. The continuum observations, combined with the J and K photometry of Sherrington et al., (1980), can be fit with a combination of this relatively cool white dwarf and a steady-state disk model with an accretion rate of 10 to the -11th solar masses/yr. Additional observations of the hump in the optical light curve can be reasonably fit by a 12,000-K blackbody. Such a source is consistent with the hump being a minor contribution to the system's overall continuum distribution shortward of 2000 A and longward of about 1 micron.

Mateo, M.; Szkody, P.

1984-01-01

94

Accretion on to Magnetic White Dwarfs  

Directory of Open Access Journals (Sweden)

The polars have no counterparts in neutron star systems and their study provides unique insights into the complex nature of the magnetospheric boundary. The observed properties of accretion shocks at the white dwarf surface such as the anomalous soft-X-ray excess and its time variability provide strong support for the hypothesis that under certain circumstances the field channelled funnel flow is “blobby”. This has been attributed to interchange instabilities such as the Magnetic Rayleigh-Taylor instability in the shocked gas at the stream-magnetosphere boundary where the stream fragments into discrete clumps of gas. As the clumps penetrate into the magnetosphere, they are shredded into smaller mass blobs via the Kelvin-Helmholtz instability that then couple on to field lines over an extended inner transition region in the orbital plane. The more massive blobs penetrate deep into the photosphere of the white dwarf releasing their energy as a reprocessed soft-X-ray black body component. Although similar instabilities are expected in the inner transition region in disced accretion albeit on a different scale there has been no direct observational evidence for blobby accretion in the generally lower field and disced IPs.

Wickramasinghe Dayal

2014-01-01

95

Spectrophotometric study of some cool white dwarfs  

Energy Technology Data Exchange (ETDEWEB)

Spectrophotometry of four cool (approx. 7000 K) white dwarfs with large parallaxes is reported, from the Hale reflector and the IUE. Almost the entire flux is measured, and the bolometric luminosity, temperature and radius. Stein 2051 is a DC, with an astrometric binary companion of type dM4. The helium-dominated atmosphere of the DC shows no metallic lines in the ultraviolet. The radius 0.011 Rsun gives, from the mass-radius relation, a mass of 0.68 Msun (light-element core) or 0.47 Msun (iron core); astrometric solutions correspond either to 0.50 or 0.72 Msun. Ross 627, a DA, shows no metals in the ultraviolet. The radius is between 0.010 and 0.013 Rsun, and its mass is in the range 0.55 to 0.75 Rsun. The star L870-2, a DA, has the best established, larger than average radius for a white dwarf (unless it is itself an unresolved binary). The radius, in the range 0.016 to 0.020 Rsun, corresponds to a maximum 0.36 Msun, and a minimum 0.20 Msun, for a helium core. An 8-hr exposure to study the near-ultraviolet spectrum of vMa2 proved only partially successful.

Greenstein, J.L. (Palomar Observatory, Pasadena, CA (USA))

1983-06-01

96

Freak waves in white dwarfs and magnetars  

International Nuclear Information System (INIS)

We report properties of ion acoustic freak waves that propagate in a plasma composed of warm ions and ultrarelativistic electrons and positrons. The dynamics of the nonlinear freak waves is governed by the nonlinear Schrödinger equation. The possible region for the freak waves to exist is defined precisely for typical parameters of white dwarfs and magnetars corona. It is found that for low wave number, the nonlinear ion-acoustic wave packets are structurally stable in magnetars corona than in white dwarfs. However, for large wave numbers the situation is opposite. The critical wave number threshold (kc), which indicates where the modulational instability sets in, is defined for both applications. It is seen that near to kc the freak wave amplitude becomes high, but it decreases whenever we stepped away from kc. For the wave numbers close to kc, the increase of the unperturbed density ratio of positrons-to-electrons (?) would lead to increase the freak wave amplitude, but for larger wave numbers the amplitude decreases with the increase of ?.

2012-12-01

97

Freak waves in white dwarfs and magnetars  

Energy Technology Data Exchange (ETDEWEB)

We report properties of ion acoustic freak waves that propagate in a plasma composed of warm ions and ultrarelativistic electrons and positrons. The dynamics of the nonlinear freak waves is governed by the nonlinear Schroedinger equation. The possible region for the freak waves to exist is defined precisely for typical parameters of white dwarfs and magnetars corona. It is found that for low wave number, the nonlinear ion-acoustic wave packets are structurally stable in magnetars corona than in white dwarfs. However, for large wave numbers the situation is opposite. The critical wave number threshold (k{sub c}), which indicates where the modulational instability sets in, is defined for both applications. It is seen that near to k{sub c} the freak wave amplitude becomes high, but it decreases whenever we stepped away from k{sub c}. For the wave numbers close to k{sub c}, the increase of the unperturbed density ratio of positrons-to-electrons ({beta}) would lead to increase the freak wave amplitude, but for larger wave numbers the amplitude decreases with the increase of {beta}.

Sabry, R. [Theoretical Physics Group, Physics Department, Faculty of Science, Damietta University, New Damietta 34517 (Egypt); Department of Physics, College of Science and Humanitarian Studies, Salman bin Abdulaziz University, Alkharj (Saudi Arabia); International Centre for Advanced Studies in Physical Sciences, Faculty of Physics and Astronomy, Ruhr University Bochum, D-44780 Bochum (Germany); Moslem, W. M. [International Centre for Advanced Studies in Physical Sciences, Faculty of Physics and Astronomy, Ruhr University Bochum, D-44780 Bochum (Germany); Department of Physics, Faculty of Science, Port Said University, Port Said (Egypt); Centre for Theoretical Physics, The British University in Egypt (BUE), El-Shorouk City, Cairo (Egypt); Shukla, P. K. [International Centre for Advanced Studies in Physical Sciences, Faculty of Physics and Astronomy, Ruhr University Bochum, D-44780 Bochum (Germany); Department of Mechanical and Aerospace Engineering and Center for Energy Research, University of California, San Diego, La Jolla, California 92093 (United States)

2012-12-15

98

White dwarfs in the Capodimonte deep field  

CERN Multimedia

In this article we describe the search for white dwarfs (WDs) in the multi-band photometric data of the Capodimonte deep field survey. The WD candidates were selected through the V-R_C vs B-V color-color diagram. For two bright objects, the WD nature has been confirmed spectroscopically, and the atmospheric parameters (Teff and logg) have been determined. We have computed synthetic stellar population models for the observed field and the expected number of white dwarfs agrees with the observations. The possible contamination by turn-off and horizontal branch halo stars has been estimated. The quasar (QSO) contamination has been determined by comparing the number of WD candidates in different color bins with state-of-the-art models and previous observations. The WD space density is measured at different distances from the Sun. The total contamination (non-degenerate stars + QSOs) in our sample is estimated to be around 30%. This work should be considered a small experiment in view of more ambitious projects to...

Silvotti, R; Cignoni, M; Alcalá, J M; Capaccioli, M; Grado, A; Pannella, M

2008-01-01

99

Fate of accreting white dwarfs: Type I supernovae vs collapse  

International Nuclear Information System (INIS)

The final fate of accreting C + O white dwarfs is either thermonuclear explosion or collapse, if the white dwarf mass grows to the Chandrasekhar mass. We discuss how the fate depends on the initial mass, age, composition of the white dwarf and the mass accretion rate. Relatively fast accretion leads to a carbon deflagration at low central density that gives rise to a Type Ia supernova. Slower accretion induces a helium detonation that could be observed as a Type Ib supernova. If the initial mass of the C + O white dwarf is larger than 1.2 Msub solar, a carbon deflagration starts at high central density and induces a collapse of the white dwarf to form a neutron star. We examine the critical condition for which a carbon deflagration leads to collapse, not explosion. For the case of explosion, we discuss to what extent the nucleosynthesis models are consistent with spectra of Type Ia and Ib supernovae. 61 refs., 18 figs

1986-04-03

100

Hard X-Ray Emission Associated with White Dwarfs  

Science.gov (United States)

Inspired by the hard X-ray emission from WD 2226-210, the central star of the Helix Nebula, we have made a systematic search for similar sources by correlating the white dwarf catalog of McCook & Sion and the ROSAT Position Senstive Proportional Counter (PSPC) point-source catalog of White, Giommi, & Angelini. We find 76 white dwarfs coincident with X-ray sources at a high level of confidence. Among these sources, 17 show significant hard X-ray emission at energies greater than 0.5 keV. Twelve of these white dwarfs with hard X-ray emission are in known binary systems, in two of which the accretion of the close companion's material onto the white dwarf produces hard X-ray emission, and in the other 10 the late-type companions' coronal activity emits hard X-rays. One apparently single white dwarf is projected near an active galactic nucleus that is responsible for the hard X-ray emission. The remaining four white dwarfs and two additional white dwarfs with hard X-ray emission appear single. The lack of near-IR excess from the apparently single white dwarfs suggests that either X-ray observations are more effective than near-IR photometry in diagnosing faint companions, or a different emission mechanism is needed. It is intriguing that 50% of the six apparently single white dwarfs with hard X-ray emission are among the hottest white dwarfs. We have compared X-ray properties of 11 hot white dwarfs with different spectral types and conclude that stellar pulsation and fast stellar winds are not likely the origin of the hard X-ray emission, but a leakage of the high-energy Wien tail of emission from deep in the stellar atmosphere remains a tantalizing source of hard X-ray emission from hot DO and DQZO white dwarfs. A complete survey using the entire ROSAT PSPC archive is needed to enlarge the sample of white dwarfs with hard X-ray emission. Follow-up near-IR photometric observations are needed to verify the existence of late-type companions, and high-resolution deep X-ray observations are needed to verify the positional coincidence and to study the X-ray spectral properties in order to determine the origin and nature of the hard X-ray emission.

O'Dwyer, Ian J.; Chu, You-Hua; Gruendl, Robert A.; Guerrero, Martín A.; Webbink, Ronald F.

2003-04-01

 
 
 
 
101

An unsuccessful search for brown dwarf companions to white dwarf stars  

International Nuclear Information System (INIS)

A search was carried out for brown dwarf stars in the solar neighbourhood, using the IRAS data base, in connection with the 'missing mass' theory. The paper was presented at the workshop on 'Astrophysics of brown dwarfs', Virginia, USA, 1985. No brown dwarf companions to white dwarf stars were discovered. However it is suggested that a similar search with a more sensitive survey instrument may yield some detections. (U.K.)

1985-10-14

102

White Dwarf - Red Dwarf Systems Resolved with the Hubble Space Telescope: I. First Results  

CERN Document Server

First results are presented for a Hubble Space Telescope Advanced Camera for Surveys snapshot study of white dwarfs with likely red dwarf companions. Of 48 targets observed and analyzed so far, 27 are totally or partially resolved into two or more components, while an additional 15 systems are almost certainly unresolved binaries. These results provide the first direct empirical evidence for a bimodal distribution of orbital separations among binary systems containing at least one white dwarf.

Farihi, J; Wachter, S

2006-01-01

103

Ages of White Dwarf-Red Subdwarf Systems  

CERN Document Server

We provide the first age estimates for two recently discovered white dwarf-red subdwarf systems, LHS 193AB and LHS 300AB. These unusual systems provide a new opportunity for linking the reliable age estimates for the white dwarfs to the (measurable) metallicities of the red subdwarfs. We have obtained precise photometry in the $V_{J}R_{KC}I_{KC}JH$ bands and spectroscopy covering from 6000\\AA to 9000\\AA for the two new systems, as well as for a comparison white dwarf-main sequence red dwarf system, GJ 283 AB. Using model grids available in the literature, we estimate the cooling age as well as temperature, surface gravity, mass, progenitor mass and {\\it total} lifetimes of the white dwarfs. The results indicate that the two new systems are probably ancient thick disk objects with ages of at least 6-9 Gyr. We also conduct searches of red dwarf and white dwarf compendia from SDSS data and the L{\\'e}pine Shara Proper Motion (LSPM) catalog for additional common proper motion white dwarf-red subdwarf systems. Only...

Monteiro, H; Henry, T; Subasavage, J; Beaulieu, T; Monteiro, Hektor; Jao, Wei-Chun; Henry, Todd; Subasavage, John; Beaulieu, Thom

2006-01-01

104

Magnetic White Dwarfs in the Infrared  

Science.gov (United States)

Magnetic cataclysmic variables (mCV) form from binary systems containing a magnetic white dwarf (mWD) and a main sequence companion. These mCV systems are divided into two classes, polars and intermediate polars, based on magnetic field strength. The distributions of orbital periods for known polar and intermediate polar systems are very different. Polars are clumped around 1.5 hours and intermediate polars are more evenly distributed over a wider range. At near infrared wavelengths the low mass companions in the mCV progenitors dominate the colors of these WD + red dwarf binary stars. We have extracted the near-infrared photometry from 2MASS of known mWDs to search for undetected low mass companions. We are able to recover several known mWD + low mass companion binary systems and gain insight into the source populations for the two mCV classes. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, and is based on work supported by the National Aeronautics and Space Administration under an ADP grant issued through the Office of Space Science. This work uses data products from 2MASS, a joint project of the U. of Massachusetts and IPAC/Caltech, funded by NASA and the NSF.

Wellhouse, J. W.; Hoard, D. W.; Howell, S. B.; Wachter, S.

2004-12-01

105

Collapse of carbon-oxygen white dwarfs  

International Nuclear Information System (INIS)

Carbon-oxygen white dwarfs formed in close binary systems may become unstable by mass accretion. Recent results concerning carbon-oxygen separation at the freezing point during the phase of cooling may have very important consequences for the problem of neutron star formation. The central, high-density regions of the star are then made of pure oxygen, the carbon being rejected to lower-density layers. When the star is compressed, carbon ignition can only happen after neutronization of the central (oxygen) regions. We show that, in this case, the chances of collapse to a neutron star are independent from the rate of mass accretion, in contrast with previous studies. A likely mechanism for neutron star formation emerges from this picture

1980-10-01

106

Remnants of Binary White Dwarf Mergers  

Science.gov (United States)

We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

Raskin, Cody; Scannapieco, Evan; Fryer, Chris; Rockefeller, Gabriel; Timmes, F. X.

2012-02-01

107

Complexity and white-dwarf structure  

International Nuclear Information System (INIS)

From the low-mass non-relativistic case to the extreme relativistic limit, the density profile of a white dwarf is used to evaluate the CLMC complexity measure [R. Lopez-Ruiz, H.L. Mancini, X. Calbet, Phys. Lett. A 209 (1995) 321]. Similarly to the recently reported atomic case where, by averaging shell effects, complexity grows with the atomic number [C.P. Panos, K.Ch. Chatzisavvas, Ch.C. Moustakidis, E.G. Kyrkou, Phys. Lett. A 363 (2007) 78; A. Borgoo, F. De Proft, P. Geerlings, K.D. Sen, Chem. Phys. Lett. 444 (2007) 186; J. Sanudo, R. Lopez-Ruiz, Int. Rev. Phys. 2 (2008) 223], here complexity grows as a function of the star mass reaching a maximum finite value in the Chandrasekhar limit

2009-02-23

108

Remnants of Binary White Dwarf Mergers  

CERN Multimedia

We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

Raskin, Cody; Fryer, Chris; Rockefeller, Gabriel; Timmes, F X

2011-01-01

109

Binary white dwarfs in the halo of the Milky Way  

CERN Document Server

Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the evolution of low-metallicity halo stars at distances up to ~ 3 kpc using the binary population synthesis code SeBa. We use two different white dwarf cooling models to predict the present-day luminosities of halo white dwarfs. We determine the white dwarf luminosity functions (WDLFs) for eight different halo models and compare these with the observed halo WDLF of white dwarfs in the SuperCOSMOS Sky Survey. Furthermore, we predict the properties of binary white dwarfs in the halo and determine the number of halo white dwarfs that is expected to be observed with the Gaia satellite. Results: By comparing the WDLFs, we find that a standard IMF matches the observations more accurately than a top-heavy one, but the difference w...

van Oirschot, Pim; Toonen, Silvia; Pols, Onno; Brown, Anthony G A; Helmi, Amina; Zwart, Simon Portegies

2014-01-01

110

Gravitational Settling of 22Ne and White Dwarf Evolution  

Science.gov (United States)

We study the effects of the sedimentation of the trace element 22Ne in the cooling of white dwarfs. In contrast with previous studies-which adopted a simplified treatment of the effects of 22Ne sedimentation-this is done self-consistently for the first time, using an up-to-date stellar evolutionary code in which the diffusion equation is coupled with the full set of equations of stellar evolution. Due the large neutron excess of 22Ne, this isotope rapidly sediments in the interior of the white dwarf. Although we explore a wide range of parameters, we find that when using the most reasonable assumptions concerning the diffusion coefficient and the physical state of the white dwarf interior, the delay introduced by the ensuing chemical differentiation is minor for a typical 0.6 Msolar white dwarf. For more massive white dwarfs, say MWD~1.0 Msolar, the delay turns out to be considerably larger. These results are in qualitatively good accord with those obtained in previous studies, but we find that the magnitude of the delay introduced by 22Ne sedimentation was underestimated by a factor of ~2. We also perform a preliminary study of the impact of 22Ne sedimentation on the white dwarf luminosity function. Finally, we hypothesize on the possibility of detecting the sedimentation of 22Ne using pulsating white dwarfs in the appropriate effective temperature range with accurately determined rates of change of the observed periods.

García-Berro, E.; Althaus, L. G.; Córsico, A. H.; Isern, J.

2008-04-01

111

The white dwarf companions of 56 Per and HR 3643  

CERN Document Server

We have obtained low-dispersion IUE spectra of the stars 56 Persei (F4 V) and HR 3643 (F7 II), as part of a survey of late-type stars with a 1565 A flux excess in the TD-1 ultraviolet sky survey. The IUE spectrum of each star reveals the presence of a hot white dwarf companion. We fit the Ly alpha profile and ultraviolet continuum using pure hydrogen models, but the distance of the primary star is also needed to uniquely constrain the white dwarf parameters. We derive Teff = 16,420 +- 420 K, logg = 8.46 +- 0.2 for the white dwarf companion to 56 Per, using the photometric distance of 30.1 +- 2.8 pc. The implied white dwarf mass is 0.90 +- 0.12 Msun, considerably above the median mass (~0.6 Msun) of single white dwarfs. The parameters of the white dwarf in HR 3643 are not well constrained, mainly due to a large uncertainty in the distance. By assuming a reasonable range of gravity for the white dwarf (7.3 < log g < 9.0), we derive -1.4 < Mv < 0.6 for the F7 II star, and 28,970 < Teff < 35, 99...

Landsman, W B; Bergeron, P; Landsman, Wayne; Simon, Theodore; Bergeron, Pierre

1995-01-01

112

RX J2130.6+4710 - an eclipsing white dwarf - M-dwarf binary star  

CERN Multimedia

(Abridged) We report the detection of eclipses in the close white-dwarf - M-dwarf binary star RXJ2130.6+4710. We estimate that the spectral type of the M-dwarf is M3.5Ve or M4Ve. We estimate that the effective temperature of the white dwarf is Teff = 18000K +- 1000K. We have used the width of the primary eclipse and duration of totality measured precisely from Ultracam u' data combined with the amplitude of the ellipsoidal effect in the I band and the semi-amplitudes of the spectroscopic orbits to derive masses and radii for the M-dwarf and white dwarf. The M-dwarf has a mass of 0.555 +- 0.023 solar masses and a radius of 0.534 +- 0.053 solar radii, which is a typical radius for stars of this mass. The mass of the white dwarf is 0.554 +- 0.017 solar masses and its radius is 0.0137 +- 0.0014 solar radii, which is the radius expected for a carbon-oxygen white dwarf of this mass and effective temperature RXJ2130.6+4710 is a rare example of a pre-cataclysmic variable star which will start mass transfer at a perio...

Maxted, P F L; Morales-Rueda, L; Barstow, M A; Dobbie, P D; Schreiber, M; Dhillon, V S; Brinkworth, C S

2004-01-01

113

Time-dependent accretion onto magnetized white dwarfs  

International Nuclear Information System (INIS)

We consider time-dependent accretion onto magnetized white dwarfs. A detailed description of a numerical method of solution to the hydrodynamical equations is given. The postshock flow is cooled by optically thin bremsstrahlung and is thermally unstable. As a result the shock height undergoes periodic oscillations. We consider the properties of this oscillation as a function of the accretion rate and of the mass and radius of the white dwarf. The structure of the accretion flow depends on a single scaling parameter. Below a critical accretion rate, which depends on the particular white dwarf, the nature of the accretion flow changes and the shock propagates up the accretion column indefinitely

1982-07-01

114

The WFCAM transit survey and cool white dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present results from our search for cool white dwarfs in the WTS (WFCAM Transit Survey). Repeat observations starting in 2007 allowed to produce deep stacked images in J and measure proper motions. We combine this with deep optical imaging to select cool white dwarf candidates (Teff < 5000?K). About 27 cool white dwarf candidates with proper motions above 0.10 arcsec/yr were identified in one of the fields representing 1/8th of the survey area. Follow-up spectroscopy with the 10.2?m GT...

Catalan S.; Napiwotzki R.; Hodgkin S.; Cristobal, Hornillos D.; Pinfield D.

2013-01-01

115

White Dwarf Stars as a Polytropic Gas Spheres  

CERN Document Server

Due to the highly degeneracy of electrons in white dwarf stars, we expect that the relativistic effects play very important role in these stars. In the present article, we study the properties of the condensed matter in white dwarfs using Newtonian and relativistic polytropic fluid sphere. Two polytropic indices (namely n=3 and n=1.5) are proposed to investigate the physical characteristics of the models. We solve the Lane-Emden equations numerically.. The results show that the relativistic effect is small in white dwarf stars.

Nouh, M I; Elkhateeb, M M; Korany, B

2014-01-01

116

White Dwarfs in NGC 6791: Avoiding the Helium Flash  

CERN Document Server

We propose that the anomalously bright white dwarf luminosity function observed in NGC 6791 (Bedin et al 2005) is the consequence of the formation of 0.5 Msun white dwarfs with Helium cores instead of Carbon cores. This may happen if mass loss during the ascent of the Red Giant Branch is strong enough to prevent a star from reaching the Helium flash. Such a model can explain the slower white dwarf cooling (relative to standard models) and fits naturally with scenarios advanced to explain Extreme Horizontal Branch stars, a population of which are also found in this cluster.

Hansen, B

2005-01-01

117

Discovery of a widely separated ultracool dwarf-white dwarf binary  

Science.gov (United States)

We present the discovery of the widest known ultracool dwarf-white dwarf binary. This binary is the first spectroscopically confirmed widely separated system from our target sample. We have used the Two-Micron All-Sky Survey (2MASS) and SuperCOSMOS archives in the southern hemisphere, searching for very widely separated ultracool dwarf-white dwarf binaries, and find one common proper motion system, with a separation of 3650-5250au at an estimated distance of 41-59pc, making it the widest known system of this type. Spectroscopy reveals 2MASS J0030-3740 is a DA white dwarf with Teff = 7600 +/- 100K, log(g) = 7.79-8.09 and MWD = 0.48-0.65Msolar. We spectroscopically type the ultracool dwarf companion (2MASS J0030-3739) as M9+/-1 and estimate a mass of 0.07-0.08Msolar,Teff = 2000-2400K and log(g) = 5.30-5.35, placing it near the mass limit for brown dwarfs. We estimate the age of the system to be >1.94Gyr (from the white dwarf cooling age and the likely length of the main-sequence lifetime of the progenitor) and suggest that this system and other such wide binaries can be used as benchmark ultracool dwarfs.

Day-Jones, A. C.; Pinfield, D. J.; Napiwotzki, R.; Burningham, B.; Jenkins, J. S.; Jones, H. R. A.; Folkes, S. L.; Weights, D. J.; Clarke, J. R. A.

2008-08-01

118

NLTT 5306: the shortest period detached white dwarf+brown dwarf binary  

Science.gov (United States)

We have spectroscopically confirmed a brown dwarf mass companion to the hydrogen atmosphere white dwarf NLTT 5306. The white dwarf's atmospheric parameters were measured using the Sloan Digital Sky Survey and X-shooter spectroscopy as Teff = 7756 ± 35 K and log(g) = 7.68 ± 0.08, giving a mass for the primary of MWD = 0.44 ± 0.04 M? at a distance of 71 ± 4 pc with a cooling age of 710 ± 50 Myr. The existence of the brown dwarf secondary was confirmed through the near-infrared arm of the X-shooter data and a spectral type of dL4-dL7 was estimated using standard spectral indices. Combined radial velocity measurements from the Sloan Digital Sky Survey, X-shooter and the Hobby-Eberly Telescope's High Resolution Spectrograph of the white dwarf give a minimum mass of 56 ± 3 MJup for the secondary, confirming the substellar nature. The period of the binary was measured as 101.88 ± 0.02 min using both the radial velocity data and i'-band variability detected with the Isaac Newton Telescope. This variability indicates `day' side heating of the brown dwarf companion. We also observe H? emission in our higher resolution data in phase with the white dwarf radial velocity, indicating that this system is in a low level of accretion, most likely via a stellar wind. This system represents the shortest period white dwarf+brown dwarf binary and the secondary has survived a stage of common envelope evolution, much like its longer period counterpart, WD 0137-349. Both systems likely represent bona fide progenitors of cataclysmic variables with a low-mass white dwarf and a brown dwarf donor.

Steele, P. R.; Saglia, R. P.; Burleigh, M. R.; Marsh, T. R.; Gänsicke, B. T.; Lawrie, K.; Cappetta, M.; Girven, J.; Napiwotzki, R.

2013-03-01

119

Methods for calculating circular polarisation in magnetic white dwarfs  

International Nuclear Information System (INIS)

In this paper the analytical method is compared with two approximate methods which give circular polarisation results directly for flux rather than intensity. Each of these methods has been used in interpreting data from magnetic white dwarfs

1979-01-01

120

Doubling the number of pulsating DB white dwarfs  

International Nuclear Information System (INIS)

We are searching for new pulsating DB white dwarf stars (DBVs) based on the newly found white dwarf stars from the spectra obtained by the Sloan Digital Sky Survey. DBVs pulsate at hotter temperature ranges than their better known cousins, DAVs or ZZ Ceti stars. Since the evolution of white dwarf stars is characterized by cooling, asteroseismological studies of DBVs give us opportunities to study white dwarf structure at a different evolutionary stage than the DAVs. The hottest DBVs are thought to have neutrino luminosities exceeding their photon luminosities (Winget et al. 2004), a quantity measurable through asteroseismology. Therefore, they can also be used to study neutrino physics in the stellar interior. At the time of the meeting, we reported on the nine new DBVs, doubling the number of previously known DBVs. Here we report the new nine pulsators' lightcurves and power spectra.

2009-06-01

 
 
 
 
121

Doubling the number of pulsating DB white dwarfs  

Energy Technology Data Exchange (ETDEWEB)

We are searching for new pulsating DB white dwarf stars (DBVs) based on the newly found white dwarf stars from the spectra obtained by the Sloan Digital Sky Survey. DBVs pulsate at hotter temperature ranges than their better known cousins, DAVs or ZZ Ceti stars. Since the evolution of white dwarf stars is characterized by cooling, asteroseismological studies of DBVs give us opportunities to study white dwarf structure at a different evolutionary stage than the DAVs. The hottest DBVs are thought to have neutrino luminosities exceeding their photon luminosities (Winget et al. 2004), a quantity measurable through asteroseismology. Therefore, they can also be used to study neutrino physics in the stellar interior. At the time of the meeting, we reported on the nine new DBVs, doubling the number of previously known DBVs. Here we report the new nine pulsators' lightcurves and power spectra.

Nitta, Atsuko; Kleinman, S J [Gemini Observatory, 670 N A' ohoku Pl., Hilo, HI 96720 (United States); Krzenski, J [Mount Suhora Observatory, Cracow Pedagogical University, Cracow (Poland); Kepler, S O [Inst de Fsica UFRGS Campus do Vale CP 15051 BR Porto Alegre, RS 91501-970 Brazil (Brazil); Metcalfe, T S [High Altitude Observatory, National Center for, Atmospheric Research, PO Box 3000, Boulder CO 80307 (United States); Mukadam, Anjum S [Department of Astronomy, University of Washington, 3910 15th Ave NE, Seattle WA 98195 (United States); Mullally, F [Deptartment of Astrophysical Sciences, Princeton University, Princeton, NJ, 08544 (United States); Nather, R E; Winget, D E [Department of Astronomy, University of Texas at Austin, Austin, TX (United States); Sullivan, D [School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, NZ (Australia); Thompson, Susan E, E-mail: anitta@gemini.ed [Department of Physics and Astronomy, University of Delaware, Newark, DE (United States)

2009-06-01

122

Ages of white dwarf-red subdwarf systems  

Directory of Open Access Journals (Sweden)

Full Text Available We provide the first age estimates for two recently discovered white dwarf-red subdwarf systems, LHS 193AB and LHS 300AB. These systems provide a new opportunity for linking the reliable age estimates for the white dwarfs to the (measurable metallicities of the red subdwarfs. We have obtained precise photometry in the VJRKCIKCJH bands and spectroscopy covering from 6,000°A to 9,000°A (our spectral coveragefor the two new systems, as well as for a comparison white dwarfmain sequence red dwarf system, GJ 283 AB. Using model grids, we estimate the cooling age as well as temperature, surface gravity, mass, progenitor mass and total lifetimes of the white dwarfs. The results indicate that the two new systems are probably ancient thick disk objects with ages of at least 6-9 gigayears (Gyr.

Thom Beaulieu

2006-01-01

123

Constraining white-dwarf kicks in globular clusters  

CERN Multimedia

The wind of an asymptotic-giant-branch stars is sufficiently strong that if it is slightly asymmetric, it can propel the star outside of the open cluster of its birth or significantly alter its trajectory through a globular cluster; therefore, if these stellar winds are asymmetric, one would expect a deficit of white dwarfs of all ages in open clusters and for young white dwarfs to be less radially concentrated than either their progenitors or older white dwarfs in globular clusters. This latter effect has recently been observed. Hence, detailed studies of the radial distribution of young white dwarfs in globular clusters could provide a unique probe of mass loss on the asymptotic giant branch and during the formation of planetary nebulae both as a function of metallicity and a limited range of stellar mass.

Heyl, Jeremy S

2007-01-01

124

Three eclipsing white dwarf plus main sequence binaries from SDSS  

Science.gov (United States)

We identify SDSS 0110+1326, SDSS 0303+0054 and SDSS 1435+3733 as three eclipsing white dwarf plus main sequence binaries from the Sloan Digital Sky Survey, and report on their follow-up observations. Orbital periods for the three systems are established through multi-season photometry. Time-resolved spectroscopic observations lead to the determination of the radial velocities of the secondary stars. A decomposition technique of the SDSS spectra is used to estimate the surface gravities and effective temperatures of the white dwarfs, as well as the spectral types of the secondaries. By combining the constraints from the spectral decomposition, the radial velocity data and the modeling of the systems' light curves, we determine the physical parameters of the stellar components. Two of the white dwarfs are of low mass (Mwd ~ 0.4 Modot), while the third white dwarf is unusually massive (MWD ~ 0.8-0.9 Modot) for a post-common envelope system.

Pyrzas, S.; Gänsicke, B. T.; Marsh, T. R.; Aungwerojwit, A.; Rebassa-Mansergas, A.; Southworth, J.; Rodríguez-Gil, P.; Schreiber, M. R.; Koester, D.

2009-06-01

125

Calibrating Cosmological Chronometers: White Dwarf Masses via Astrometry.  

Science.gov (United States)

In an effort to increase the number of accurate dynamical masses for white dwarfs (WDs), we have begun an initiative using Hubble Space Telescope's Fine Guidance Sensors (FGS) to resolve suspected binary WDs. With the increasing number of WD trigonometric...

E. P. Nelan H. C. Harris J. P. Subasavage T. J. Henry W. Jao

2009-01-01

126

COS UV Spectroscopy of Pulsating DB White Dwarfs  

Science.gov (United States)

Convection is an important energy transfer process for most stars. Yet convection is poorly understood, and remains one of the largest sources of theoretical uncertainty in stellar modeling. Pulsating white dwarfs are the ideal laboratories to provide to a self consistent description of convection in different environments. We can combine asteroseismology with nonlinear analysis of pulsating white dwarf light curves to provide empirical descriptions of convection across the hydrogen and helium instability strips. However, our ability to interpret the convection turnover timescale and its dependence on effective temperature across the helium white dwarf instability strip is severely limited by the large errors associated with optical spectroscopic temperature determinations for these objects. We present preliminary analysis of COS observations of a sample of pulsating DB white dwarfs. These observations will determine the slope of their energy distributions and determine the presence of trace abundances of H, C, O, and Si, leading to greatly improved effective temperature determinations for these stars.

Provencal, Judith L.; Nitta, A.; Shipman, H. L.; Dalessio, J.; Montgomery, M.; Thompson, S. E.

2014-01-01

127

Halo White Dwarfs, Thick Disks and a Sanity Check  

CERN Document Server

The discovery of a population of high proper motion white dwarfs by Oppenheimer et al (2001) has caused a lot of speculation as to the origin of these stars. I show that the age distribution of the white dwarfs offers a kind of sanity check in these discussions. In particular, this population appears to have a similar age distribution to those in the standard, thin disk white dwarf population. This is not what is expected for either the halo or thick disk, which are thought to be old populations. It may indicate a different dynamical origin entirely, or it may be just be the tail of a larger distribution, implying a significantly larger total density in even older white dwarfs.

Hansen, B M S

2001-01-01

128

Towards an Understanding of the Atmospheres of Cool White Dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Cool white dwarfs with Teff < 6000 K are the remnants of the oldest stars that existed in our Galaxy. Their atmospheres, when properly characterized, can provide valuable information on white dwarf evolution and ultimately star formation through the history of the Milky Way. Understanding the atmospheres of these stars requires joined observational effort and reliable atmosphere modeling. We discuss and analyze recent observations of the near-ultraviolet (UV) and near-infrar...

Kowalski, Piotr M.; Saumon, Didier; Holberg, Jay; Leggett, Sandy

2012-01-01

129

QUIESCENT NUCLEAR BURNING IN LOW-METALLICITY WHITE DWARFS  

International Nuclear Information System (INIS)

We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich (DA) white dwarfs with very low metallicity progenitors (Z = 0.0001). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main-sequence masses, covering the most usual interval of white dwarf masses—from 0.53 M ? to 0.83 M ?. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase in which nuclear reactions are still important and find that nuclear energy sources play a key role during long periods of time, considerably increasing the cooling times from those predicted by standard white dwarf models. In particular, we find that for this metallicity and for white dwarf masses smaller than about 0.6 M ?, nuclear reactions are the main contributor to the stellar luminosity for luminosities as low as log (L/L ?) ? –3.2. This, in turn, should have a noticeable impact in the white dwarf luminosity function of low-metallicity stellar populations

2013-09-20

130

QUIESCENT NUCLEAR BURNING IN LOW-METALLICITY WHITE DWARFS  

Energy Technology Data Exchange (ETDEWEB)

We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich (DA) white dwarfs with very low metallicity progenitors (Z = 0.0001). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main-sequence masses, covering the most usual interval of white dwarf masses—from 0.53 M {sub ?} to 0.83 M {sub ?}. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase in which nuclear reactions are still important and find that nuclear energy sources play a key role during long periods of time, considerably increasing the cooling times from those predicted by standard white dwarf models. In particular, we find that for this metallicity and for white dwarf masses smaller than about 0.6 M {sub ?}, nuclear reactions are the main contributor to the stellar luminosity for luminosities as low as log (L/L {sub ?}) ? –3.2. This, in turn, should have a noticeable impact in the white dwarf luminosity function of low-metallicity stellar populations.

Miller Bertolami, Marcelo M.; Althaus, Leandro G. [Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata (Argentina); García-Berro, Enrique [Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, E-08860 Castelldefels (Spain)

2013-09-20

131

Lyman-alpha wing absorption in cool white dwarf stars  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Kowalski & Saumon (2006) identified the missing absorption mechanism in the observed spectra of cool white dwarf stars as the Ly-alpha red wing formed by the collisions between atomic and molecular hydrogen and successfully explained entire spectra of many cool DA-type white dwarfs. Owing to the important astrophysical implications of this issue, we present here an independent assessment of the process. For this purpose, we compute free-free quasi-molecular absorption in Lym...

Rohrmann, R. D.; Althaus, L. G.; Kepler, S. O.

2010-01-01

132

Gravitational wave radiation from the coalescence of white dwarfs  

CERN Document Server

We compute the emission of gravitational radiation from the merging of a close white dwarf binary system. This is done for a wide range of masses and compositions of the white dwarfs, ranging from mergers involving two He white dwarfs, mergers in which two CO white dwarfs coalesce to mergers in which a massive ONe white dwarf is involved. In doing so we follow the evolution of binary system using a Smoothed Particle Hydrodynamics code. Even though the coalescence process of the white dwarfs involves considerable masses, moving at relatively high velocities with a high degree of asymmetry we find that the signature of the merger is not very strong. In fact, the most prominent feature of the coalescence is that in a relatively small time scale (of the order of the period of the last stable orbit, typically a few minutes) the sources stop emitting gravitational waves. We also discuss the possible implications of our calculations for the detection of the coalescence within the framework of future space-borne inte...

Loren-Aguilar, P; Isern, J; Lobo, J A; García-Berro, E

2004-01-01

133

Theoretical models for asteroseismology of DA white dwarf stars  

Energy Technology Data Exchange (ETDEWEB)

Because white dwarfs are the most common end state of stellar evolution, determining their internal structure will yield many clues about the final stages of stellar evolution and the physics of matter under extreme conditions. We present the results of our parametric survey of evolutionary models of compositionally stratified white dwarfs with hydrogen surface layers (DA white dwarfs) and provide a comprehensive set of theoretical {ital g}-mode pulsation periods for comparison to observations of pulsating DA white dwarfs. This survey complements the previous survey of helium atmosphere (DB) white dwarf periods of Bradley, Winget, & Wood. We show how to use the periods of low-overtone and/or trapped modes to constrain the internal structure of pulsating DA white dwarfs by utilizing their sensitivity to the total stellar mass and the location of the hydrogen/helium transition zone. We use G117-B15A as an example to demonstrate the potential of our models for asteroseismology; we suggest that G117-B15A has a mass of 0.55 {ital M}{sub {circle_dot}} and a hydrogen layer mass of {approx_equal}1.5{times}10{sup {minus}4} {ital M}{sub {asterisk}}. {copyright} {ital 1996 The American Astronomical Society.}

Bradley, P.A. [XTA, MS B220, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

1996-09-01

134

The White Dwarf in EM Cygni: Beyond The Veil  

CERN Document Server

We present a spectral analysis of the FUSE spectra of EM Cygni, a Z Cam DN system. The FUSE spectrum, obtained in quiescence, consists of 4 individual exposures (orbits): two exposures, at orbital phases phi ~ 0.65 and phi ~ 0.90, have a lower flux; and two exposures, at orbital phases phi =0.15 and 0.45, have a relatively higher flux. The change of flux level as a function of the orbital phase is consistent with the stream material (flowing over and below the disk from the hot spot region to smaller radii) partially masking the white dwarf. We carry out a spectral analysis of the FUSE data, obtained at phase 0.45 (when the flux is maximual, using the codes TLUSTY and SYNSPEC. Using a single white dwarf spectral component, we obtain a white dwarf temperature of 40,000K, rotating at 100km/s. The white dwarf, or conceivably, the material overflowing the disk rim, shows suprasolar abundances of silicon, sulphur and possibly nitrogen. Using a white dwarf+disk composite model, we obtain that the white dwarf temper...

Godon, P; Barrett, P E; Linnell, A P

2009-01-01

135

White dwarf models of supernovae and cataclysmic variables  

International Nuclear Information System (INIS)

If the accreting white dwarf increases its mass to the Chandrasekhar mass, it will either explode as a Type I supernova or collapse to form a neutron star. In fact, there is a good agreement between the exploding white dwarf model for Type I supernovae and observations. We describe various types of evolution of accreting white dwarfs as a function of binary parameters (i.e,. composition, mass, and age of the white dwarf, its companion star, and mass accretion rate), and discuss the conditions for the precursors of exploding or collapsing white dwarfs, and their relevance to cataclysmic variables. Particular attention is given to helium star cataclysmics which might be the precursors of some Type I supernovae or ultrashort period x-ray binaries. Finally we present new evolutionary calculations using the updated nuclear reaction rates for the formation of O+Ne+Mg white dwarfs, and discuss the composition structure and their relevance to the model for neon novae. 61 refs., 14 figs

1986-06-16

136

MACHOs, White Dwarfs, and the Age of the Universe  

CERN Multimedia

(Abridged Abstract) A favored interpretation of recent microlensing measurements towards the Large Magellanic Cloud implies that a large fraction (i.e. 10-50%) of the mass of the galactic halo is composed of white dwarfs. We compare model white dwarf luminosity functions to the data from the observational surveys in order to determine a lower bound on the age of any substantial white dwarf halo population (and hence possibly on the age of the Universe). We compare various theoretical white dwarf luminosity functions, in which we vary hese three parameters, with the abovementioned survey results. From this comparison, we conclude that if white dwarfs do indeed constitute more than 10% of the local halo mass density, then the Universe must be at least 10 Gyr old for our most extreme allowed values of the parameters. When we use cooling curves that account for chemical fractionation and more likely values of the IMF and the bolometric correction, we find tighter limits: a white dwarf MACHO fraction of 10% (30%) ...

Graff, D S; Freese, Katherine; Graff, David S.; Laughlin, Gregory; Freese, Katherine

1997-01-01

137

DA white dwarfs observed in LAMOST pilot survey  

CERN Document Server

A total of $\\sim640,000$ objects from LAMOST pilot survey have been publicly released. In this work, we present a catalog of DA white dwarfs from the entire pilot survey. We outline a new algorithm for the selection of white dwarfs by fitting S\\'ersic profiles to the Balmer H$\\beta$, H$\\gamma$ and H$\\delta$ lines of the spectra, and calculating the equivalent width of the CaII K line. 2964 candidates are selected by constraining the fitting parameters and the equivalent width of CaII K line. All the spectra of candidates are visually inspected. We identify 230 (59 of them are already in Villanova and SDSS WD catalog) DA white dwarfs, 20 of which are DA white dwarfs with non-degenerate companions. In addition, 128 candidates are classified as DA white dwarf/subdwarfs, which means the classifications are ambiguous. The result is consistent with the expected DA white dwarf number estimated based on the LEGUE target selection algorithm.

Zhang, Yue-Yang; Liu, Chao; Lépine, Sebastien; Newberg, Heidi Jo; Carlin, Jeffrey L; Carrell, Kenneth; Yang, Fan; Gao, Shuang; Xu, Yan; Li, Jing; Zhang, Hao-Tong; Zhao, Yong-Heng; Luo, A-Li; Bai, Zhong-Rui; Yuan, Hai-Long; Jin, Ge

2013-01-01

138

Follow-up Observations of SPY White Dwarf + M-Dwarf Binaries  

Science.gov (United States)

We present the results of follow-up observations of white-dwarf + M-dwarf binaries identified using spectra obtained as part of the SPY survey. Spectra of the H? region were obtained with the SPIRAL spectrograph on the AAT telescope. Of the eleven stars observed, seven are binaries with periods in the range 2.8 hours to 7.7 days. We also show that one of our targets, WD 0137-349, has a brown dwarf companion.

Maxted, P. F. L.; Napiwotzki, R.; Marsh, T. R.; Burleigh, M. R.; Dobbie, P. D.; Hogan, E.; Nelemans, G.

2007-09-01

139

DISCOVERY OF AN ULTRAMASSIVE PULSATING WHITE DWARF  

Energy Technology Data Exchange (ETDEWEB)

We announce the discovery of the most massive pulsating hydrogen-atmosphere white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12, 030 {+-} 210 K WD with a log g =9.08 {+-} 0.06, which corresponds to a mass of 1.20 {+-} 0.03 M{sub Sun }. Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425 to 595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution, and can possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in turn constrains Type Ia supernovae progenitor systems.

Hermes, J. J.; Castanheira, Barbara G.; Winget, D. E.; Montgomery, M. H.; Harrold, Samuel T. [Department of Astronomy, University of Texas at Austin, Austin, TX 78712 (United States); Kepler, S. O. [Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS (Brazil); Gianninas, A. [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, OK 73019 (United States); Brown, Warren R., E-mail: jjhermes@astro.as.utexas.edu [Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 (United States)

2013-07-01

140

Two New Tidally Distorted White Dwarfs  

CERN Document Server

We identify two new tidally distorted white dwarfs (WDs), SDSS J174140.49+652638.7 and J211921.96-001825.8 (hereafter J1741 and J2119). Both stars are extremely low mass (ELM, < 0.2 Msun) WDs in short-period, detached binary systems. High-speed photometric observations obtained at the McDonald Observatory reveal ellipsoidal variations and Doppler beaming in both systems; J1741, with a minimum companion mass of 1.1 Msun, has one of the strongest Doppler beaming signals ever observed in a binary system (0.59 \\pm 0.06% amplitude). We use the observed ellipsoidal variations to constrain the radius of each WD. For J1741, the star's radius must exceed 0.074 Rsun. For J2119, the radius exceeds 0.10 Rsun. These indirect radius measurements are comparable to the radius measurements for the bloated WD companions to A-stars found by the Kepler spacecraft, and they constitute some of the largest radii inferred for any WD. Surprisingly, J1741 also appears to show a 0.23 \\pm 0.06% reflection effect, and we discuss possi...

Hermes, J J; Brown, Warren R; Montgomery, M H; Winget, D E

2012-01-01

 
 
 
 
141

White Dwarf Cosmochronology in the Solar Neighborhood  

CERN Document Server

The study of the stellar formation history in the solar neighborhood is a powerful technique to recover information about the early stages and evolution of the Milky Way. We present a new method which consists of directly probing the formation history from the nearby stellar remnants. We rely on the volume complete sample of white dwarfs within 20 pc, where accurate cooling ages and masses have been determined. The well characterized initial-final mass relation is employed in order to recover the initial masses (1 < M/Msun < 8) and total ages for the local degenerate sample. We correct for moderate biases that are necessary to transform our results to a global stellar formation rate, which can be compared to similar studies based on the properties of main-sequence stars in the solar neighborhood. Our method provides precise formation rates for all ages except in very recent times, and the results suggest an enhanced formation rate for the solar neighborhood in the last 5 Gyr compared to the range 5 <...

Tremblay, P -E; Soderblom, D R; Cignoni, M; Cummings, J

2014-01-01

142

Discovery of an ultramassive pulsating white dwarf  

CERN Document Server

We announce the discovery of the most massive pulsating hydrogen-atmosphere (DA) white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12,030 +/- 210 K WD with a log(g) = 9.08 +/- 0.06, which corresponds to a mass of 1.20 +/- 0.03 Msun. Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425-595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core ...

Hermes, J J; Castanheira, Barbara G; Gianninas, A; Winget, D E; Montgomery, M H; Brown, Warren R; Harrold, Samuel T

2013-01-01

143

White dwarf cooling and large extra dimensions  

International Nuclear Information System (INIS)

Theories of fundamental interactions with large extra dimensions have recently become very popular. Astrophysical bounds from the Sun, red giants, and SN 1987a have already been derived by other authors for the theory proposed by Arkani-Hamed, Dimopoulos, and Dvali. In this paper we consider the G117-B15A pulsating white dwarf (ZZ Ceti star) for which the secular rate at which the period of its fundamental mode increases has been accurately measured and claimed that this mode of G117-B15A is perhaps the most stable oscillation ever recorded in the optical band. Because an additional channel of energy loss (Kaluza-Klein gravitons) would speed up the cooling rate, one is able to use the aforementioned stability to derive a bound on theories with large extra dimensions. Within the framework of the theory with large extra dimensions proposed by Arkani-Hamed, Dimopoulos, and Dvali we find the lower bound on string compactification scale Ms>14.3 TeV/c2 which is more stringent than solar or red-giant bounds

2002-02-15

144

The Binary White Dwarf LHS 3236  

Science.gov (United States)

The white dwarf LHS 3236 (WD1639+153) is shown to be a double-degenerate binary, with each component having a high mass. Astrometry at the U.S. Naval Observatory gives a parallax and distance of 30.86 ± 0.25 pc and a tangential velocity of 98 km s-1, and reveals binary orbital motion. The orbital parameters are determined from astrometry of the photocenter over more than three orbits of the 4.0 yr period. High-resolution imaging at the Keck Observatory resolves the pair with a separation of 31 and 124 mas at two epochs. Optical and near-IR photometry give a set of possible binary components. Consistency of all data indicates that the binary is a pair of DA stars with temperatures near 8000 and 7400 K and with masses of 0.93 and 0.91 M ? also possible is a DA primary and a helium DC secondary with temperatures near 8800 and 6000 K and with masses of 0.98 and 0.69 M ?. In either case, the cooling ages of the stars are ~3 Gyr and the total ages are Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Harris, Hugh C.; Dahn, Conard C.; Dupuy, Trent J.; Canzian, Blaise; Guetter, Harry H.; Hartkopf, William I.; Ireland, Michael J.; Leggett, S. K.; Levine, Stephen E.; Liu, Michael C.; Luginbuhl, Christian B.; Monet, Alice K. B.; Stone, Ronald C.; Subasavage, John P.; Tilleman, Trudy; Walker, Richard L.

2013-12-01

145

New Pulsating White Dwarfs in Cataclysmic Variables  

CERN Document Server

The number of discovered non-radially pulsating white dwarfs (WDs) in cataclysmic variables (CVs) is increasing rapidly by the aid of the Sloan Digital Sky Survey (SDSS). We performed photometric observations of two additional objects, SDSS J133941.11+484727.5 (SDSS 1339), independently discovered as a pulsator by Gansicke et al. (2006), and SDSS J151413.72+454911.9, which we identified as a CV/ZZ Ceti hybrid. In this letter we present the results of the remote observations of these targets performed with the Nordic Optical Telescope (NOT) during the Nordic-Baltic Research School at Moletai Observatory, and follow-up observations executed by NOT in service mode. We also present 3 candidates we found to be non-pulsating. The results of our observations show that the main pulsation frequencies agree with those found in previous CV/ZZ Ceti hybrids, but specifically for SDSS 1339 the principal period differs slightly between individual observations and also from the recent independent observation by Gansicke et a...

Nilsson, R; Ytre-Eide, M; Solheim, J E; Warner, B

2006-01-01

146

Effective geometry of a white dwarf  

International Nuclear Information System (INIS)

The ''effective geometry'' formalism is used to study the perturbations of a white dwarf described as a self-gravitating fermion gas with a completely degenerate relativistic equation of state of barotropic type. The quantum nature of the system causes an absence of homological properties, manifested instead by polytropic stars, and requires a parametric study of the solutions both at the numerical and analytical level. We have explicitly derived a compact analytical parametric approximate solution of Pade type, which gives density curves and stellar radii in good accordance with already existing numerical results. After validation of this new type of approximate solutions, we use them to construct the effective acoustic metric governing general perturbations following Chebsch's formalism. Even in this quantum case, the stellar surface exhibits a curvature singularity due to the vanishing of density, as already evidenced in past studies on nonquantum self-gravitating polytropic stars. The equations of the theory are finally numerically integrated in the simpler case of irrotational spherical pulsating perturbations, including the effect of backreaction, in order to have a dynamical picture of the process occurring in the acoustic metric.

2011-03-15

147

TWO NEW TIDALLY DISTORTED WHITE DWARFS  

International Nuclear Information System (INIS)

We identify two new tidally distorted white dwarfs (WDs), SDSS J174140.49+652638.7 and J211921.96–001825.8 (hereafter J1741 and J2119). Both stars are extremely low mass (ELM, ? 0.2 M?) WDs in short-period, detached binary systems. High-speed photometric observations obtained at the McDonald Observatory reveal ellipsoidal variations and Doppler beaming in both systems; J1741, with a minimum companion mass of 1.1 M?, has one of the strongest Doppler beaming signals ever observed in a binary system (0.59% ± 0.06% amplitude). We use the observed ellipsoidal variations to constrain the radius of each WD. For J1741, the star's radius must exceed 0.074 R?. For J2119, the radius exceeds 0.10 R?. These indirect radius measurements are comparable to the radius measurements for the bloated WD companions to A-stars found by the Kepler spacecraft, and they constitute some of the largest radii inferred for any WD. Surprisingly, J1741 also appears to show a 0.23% ± 0.06% reflection effect, and we discuss possible sources for this excess heating. Both J1741 and J2119 are strong gravitational wave sources, and the time-of-minimum of the ellipsoidal variations can be used to detect the orbital period decay. This may be possible on a timescale of a decade or less.

2012-04-10

148

Cool DZ white dwarfs in the SDSS  

CERN Multimedia

We report the identification of 26 cool DZ white dwarfs that lie across and below the main sequence in the Sloan Digital Sky Survey (SDSS) u-g vs. g-r two-color diagram; 21 of these stars are new discoveries. The sample was identified by visual inspection of all spectra of objects that fall below the main sequence in the two-color diagram, as well as by an automated search for characteristic spectral features over a large area in color space that included the main sequence. The spectra and photometry provided by the SDSS project are interpreted with model atmospheres, including all relevant metals. Effective temperatures and element abundances are determined, while the surface gravity has to be assumed and was fixed at the canonical value of log g = 8. These stars represent the extension of the well-known DZ sequence towards cooler temperatures and fill the gap around Teff = 6500 K present in a previous study. The metal abundances are similar to those in the hotter DZ, but the lowest abundances are missing, p...

Koester, D; Gänsicke, B T; Dufour, P

2011-01-01

149

Magnetically powered outbursts from white dwarf mergers  

CERN Multimedia

Merger of a white dwarf binary creates a differentially rotating object which is expected to generate strong magnetic fields. Kinetic energy stored in differential rotation is partially dissipated in the magnetically dominated corona, which forms a hot variable outflow with ejection velocity comparable to $10^9$ cm s$^{-1}$. The outflow should carry significant mass and energy for hours to days, creating an expanding fireball with the following features. (i) The fireball is initially opaque and its internal energy is dominated by the trapped thermal radiation. The stored heat is partially converted to kinetic energy of the flow (through adiabatic cooling) and partially radiated away. (ii) Internal shocks develop in the fireball and increase its radiative output. (iii) A significant fraction of the emitted energy is in the optical band. As a result, a bright optical transient with luminosity $L\\sim 10^{41}-10^{42}$ erg s$^{-1}$ and a characteristic peak duration comparable to 1 day may be expected from the mer...

Beloborodov, Andrei M

2013-01-01

150

White Dwarf - Red Dwarf Systems Resolved with the Hubble Space Telescope. II. Full Snapshot Survey Results  

CERN Document Server

{Abrigded} Results are presented for a Hubble Space Telescope Advanced Camera for Surveys high-resolution imaging campaign of 90 white dwarfs with known or suspected low mass stellar and substellar companions. Of the 72 targets which remain candidate and confirmed white dwarfs with near-infrared excess, 43 are spatially resolved into two or more components, and a total of 12 systems are potentially triples. There is a possible, slight deficit of earlier spectral types (bluer colors) among the spatially unresolved companions, exactly the opposite of expectations if significant mass is transferred to the companion during the common envelope phase. Using the best available distance estimates, the low mass companions to white dwarfs exhibit a bimodal distribution in projected separation. This result supports the hypothesis that during the giant phases of the white dwarf progenitor, any unevolved companions either migrate inward to short periods of hours to days, or outward to periods of hundreds to thousands of y...

Farihi, J; Wachter, S

2010-01-01

151

Anomalous cooling of the massive white dwarf in U Geminorum following a narrow dwarf nova outburst  

CERN Document Server

We obtained Hubble GHRS medium resolution (G160M grating) phase-resolved spectroscopic observations of the prototype dwarf nova U Geminorum during dwarf nova quiescence, 13 days and 61 days following the end of a narrow outburst. The spectral wavelength ranges were centered upon three different line regions: N V (1238Å, 1242Å), Si III (1300Å) and He II (1640Å). All of the quiescent spectra at both epochs are dominated by absorption lines and show no emission features. The Si III and He II absorption line velocities versus orbital phase trace the orbital motion of the white dwarf but the N~V absorption velocities appear to deviate from the white dwarf motion. We confirm our previously reported low white dwarf rotational velocity, V sin i= 100 km/s. We obtain a white dwarf orbital velocity semi-amplitude K1=107 km/s. Using the gamma-velocity of Wade (1981) we obtain an Einstein redshift of 80.4 km/s and hence a carbon core white dwarf mass of ~1.1 Msun. We report the first subsolar chemical abundances of C ...

Sion, E M; Szkody, P; Sparks, W B; Gänsicke, B T; Huang, M; Mattei, J A; Sion, Edward M.; Szkody, Paula; Sparks, Warren; Gaensicke, Boris; Huang, Min; Mattei, Janet

1997-01-01

152

A precision study of two eclipsing white dwarf plus M dwarf binaries  

CERN Document Server

We use a combination of X-shooter spectroscopy, ULTRACAM high-speed photometry and SOFI near-infrared photometry to measure the masses and radii of both components of the eclipsing post common envelope binaries SDSS J1212-0123 and GK Vir. For both systems we measure the gravitational redshift of the white dwarf and combine it with light curve model fits to determine the inclinations, masses and radii. For SDSS J1212-0123 we find a white dwarf mass and radius of 0.439 +/- 0.002 Msun and 0.0168 +/- 0.0003 Rsun, and a secondary star mass and radius of 0.273 +/- 0.002 Msun and 0.306 +/- 0.007 Rsun. For GK Vir we find a white dwarf mass and radius of 0.564 +/- 0.014 Msun and 0.0170 +/- 0.0004 Rsun, and a secondary star mass and radius of 0.116 +/- 0.003 Msun and 0.155 +/- 0.003 Rsun. The mass and radius of the white dwarf in GK Vir are consistent with evolutionary models for a 50,000K carbon-oxygen core white dwarf. Although the mass and radius of the white dwarf in SDSS J1212-0123 are consistent with carbon-oxyge...

Parsons, S G; Gänsicke, B T; Rebassa-Mansergas, A; Dhillon, V S; Littlefair, S P; Copperwheat, C M; Hickman, R D G; Burleigh, M R; Kerry, P; Koester, D; Gómez-Morán, A Nebot; Pyrzas, S; Savoury, C D J; Schreiber, M R; Schmidtobreick, L; Schwope, A D; Steele, P R; Tappert, C

2011-01-01

153

A COMPREHENSIVE SPECTROSCOPIC ANALYSIS OF DB WHITE DWARFS  

International Nuclear Information System (INIS)

We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 Msun for our sample, with a dispersion of only 0.09 Msun. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M sun) DB white dwarfs. We derive a luminosity function based on a subset of DB white dwarfs identified in the Palomar-Green Survey. We show that 20% of all white dwarfs in the temperature range of interest are DB stars, although the fraction drops to half this value above Teff ? 20,000 K. We also show that the persistence of DB stars with no hydrogen features at low temperatures is difficult to reconcile with a scenario involving accretion from the interstellar medium, often invoked to account for the observed hydrogen abundances in DBA stars. We present evidence for the existence of two different evolutionary channels that produce DB white dwarfs: the standard model where DA stars are transformed into DB stars through the convective dilution of a thin hydrogen layer and a second channel where DB stars retain a helium atmosphere throughout their evolution. We finally demonstrate that the instability strip of pulsating V777 Her white dwarfs contains no non-variables, if the hydrogen content of these stars is properly accounted for.

2011-08-10

154

Three New Eclipsing White-dwarf - M-dwarf Binaries Discovered in a Search for Transiting Planets Around M-dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present three new eclipsing white-dwarf / M-dwarf binary systems discovered during a search for transiting planets around M-dwarfs. Unlike most known eclipsing systems of this type, the optical and infrared emission is dominated by the M-dwarf components, and the systems have optical colors and discovery light curves consistent with being Jupiter-radius transiting planets around early M-dwarfs. We detail the PTF/M-dwarf transiting planet survey, part of the Palomar Transi...

Law, Nicholas M.; Kraus, Adam L.; Street, Rachel; Fulton, Benjamin J.; Hillenbrand, Lynne A.; Shporer, Avi; Lister, Tim; Baranec, Christoph; Bloom, Joshua S.; Bui, Khanh; Burse, Mahesh P.; Cenko, S. Bradley; Das, H. K.; Davis, Jack T. C.; Dekany, Richard G.

2011-01-01

155

The formation of massive white dwarfs in cataclysmic binaries  

International Nuclear Information System (INIS)

In contrast to the mass spectrum of single white dwarfs which has a single narrow peak at approximately 0.6 solar mass, the observed mass spectrum of white dwarfs of cataclysmic binaries (CB's) shows a rather uniform distribution of the masses in the range approximately 0.3 solar mass to approximately 1.3 solar mass. The formation of CB's with white dwarfs of less than about 0.8 solar mass can be understood as the result of a binary evolution according to low mass Case B or Case C with a subsequent spiraling-in in a common envelope. On the other hand the formation of massive white dwarfs of M approximately > 1 solar mass can be explained as the result of a massive Case B mass transfer yielding a helium star which subsequently undergoes a second Case B mass transfer (so called Case BB evolution). The ultimate product of such an evolution is either a CO-white dwarf with a mass up to the Chandrasekhar limit or a neutron star. The formation of CB's via Case BB evolution requires the binary to undergo at least one, most probably two separate phases of spiraling-in in a common envelope. (Auth.)

1981-09-03

156

A Multi-Survey Approach to White Dwarf Discovery  

CERN Document Server

By selecting astrometric and photometric data from the Sloan Digital Sky Survey (SDSS), the L{\\'e}pine & Shara Proper Motion North Catalog (LSPM-North), the Two Micron All Sky Survey (2MASS), and the USNO-B1.0 catalog, we use a succession of methods to isolate white dwarf candidates for follow-up spectroscopy. Our methods include: reduced proper motion diagram cuts, color cuts, and atmospheric model adherence. We present spectroscopy of 26 white dwarfs obtained from the CTIO 4m and APO 3.5m telescopes. Additionally, we confirm 28 white dwarfs with spectra available in the SDSS DR7 database but unpublished elsewhere, presenting a total of 54 WDs. We label one of these as a recovered WD while the remaining 53 are new discoveries. We determine physical parameters and estimate distances based on atmospheric model analyses. Three new white dwarfs are modeled to lie within 25 pc. Two additional white dwarfs are confirmed to be metal-polluted (DAZ). Follow-up time series photometry confirms another object to be ...

Sayres, Conor; Bergeron, P; Dufour, P; Davenport, James R A; AlSayyad, Yusra; Tofflemire, Benjamin M

2012-01-01

157

The White Dwarf Cooling Sequence of NGC6397  

CERN Document Server

We present the results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC6397, focussing attention on the cluster's white dwarf cooling sequence. This sequence is shown to extend over 5 magnitudes in depth, with an apparent cutoff at magnitude F814W=27.6. We demonstrate, using both artificial star tests and the detectability of background galaxies at fainter magnitudes, that the cutoff is real and represents the truncation of the white dwarf luminosity function in this cluster. We perform a detailed comparison between cooling models and the observed distribution of white dwarfs in colour and magnitude, taking into account uncertainties in distance, extinction, white dwarf mass, progenitor lifetimes, binarity and cooling model uncertainties. After marginalising over these variables, we obtain values for the cluster distance modulus and age of \\mu_0 = 12.02 \\pm 0.06 and T_c = 11.47 \\pm 0.47Gyr (95% confidence limits). Our inferred distance and white dwarf initial-final mass relat...

Hansen, B M S; Brewer, J; Dotter, A; Fahlman, G G; Hurley, J; King, I; Reitzel, David B; Richer, H B; Rich, R M; Shara, M M; Stetson, P B; Hansen, Brad M. S.; Anderson, Jay; Brewer, James; Dotter, Aaron; Fahlman, Greg. G.; Hurley, Jarrod; King, Ivan; Reitzel, David; Richer, Harvey B.; Shara, Michael M.; Stetson, Peter B.

2007-01-01

158

A Comprehensive Spectroscopic Analysis of DB White Dwarfs  

CERN Document Server

We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 Mo for our sample, with a dispersion of only 0.09 Mo. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M < 0.5 Mo) DB white dwarfs. We derive a luminosity function based on a subset of DB white dwarfs identified in the Palomar-Green survey. We show that 20% of all white dwarfs in the temperature range of interest are DB stars, although the fraction drops to half this value above Teff ~ 20,000 K. We also show that the persistence of DB stars with no hydrogen features at low temperatures is difficult to reconcile with a scenario involving accretion from the interstellar medium, often invoked to account for the observed hydrogen abundances in DBA stars. W...

Bergeron, P; Dufour, Pierre; Beauchamp, A; Hunter, C; Saffer, Rex A; Gianninas, A; Ruiz, M T; Limoges, M -M; Dufour, Patrick; Fontaine, G; Liebert, James

2011-01-01

159

A DEEPLY ECLIPSING DETACHED DOUBLE HELIUM WHITE DWARF BINARY  

International Nuclear Information System (INIS)

Using Liverpool Telescope+RISE photometry we identify the 2.78 hr period binary star CSS 41177 as a detached eclipsing double white dwarf binary with a 21,100 K primary star and a 10,500 K secondary star. This makes CSS 41177 only the second known eclipsing double white dwarf binary after NLTT 11748. The 2 minute long primary eclipse is 40% deep and the secondary eclipse 10% deep. From Gemini+GMOS spectroscopy, we measure the radial velocities of both components of the binary from the H? absorption line cores. These measurements, combined with the light curve information, yield white dwarf masses of M1 = 0.283 ± 0.064 Msun and M2 = 0.274 ± 0.034 Msun, making them both helium core white dwarfs. As an eclipsing, double-lined spectroscopic binary, CSS 41177 is ideally suited to measuring precise, model-independent masses and radii. The two white dwarfs will merge in roughly 1.1 Gyr to form a single sdB star.

2011-07-10

160

Quiescent nuclear burning in low-metallicity white dwarfs  

CERN Multimedia

We discuss the impact of residual nuclear burning in the cooling sequences of hydrogen-rich DA white dwarfs with very low metallicity progenitors ($Z=0.0001$). These cooling sequences are appropriate for the study of very old stellar populations. The results presented here are the product of self-consistent, fully evolutionary calculations. Specifically, we follow the evolution of white dwarf progenitors from the zero-age main sequence through all the evolutionary phases, namely the core hydrogen-burning phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. This is done for the most relevant range of main sequence masses, covering the most usual interval of white dwarf masses --- from $0.53\\, M_{\\sun}$ to $0.83\\, M_{\\sun}$. Due to the low metallicity of the progenitor stars, white dwarfs are born with thicker hydrogen envelopes, leading to more intense hydrogen burning shells as compared with their solar metallicity counterparts. We study the phase ...

Bertolami, Marcelo M Miller; Garcia-Berro, Enrique

2013-01-01

 
 
 
 
161

Searching for benchmark systems containing ultra-cool dwarfs and white dwarfs  

Directory of Open Access Journals (Sweden)

Full Text Available We have used the 2MASS all-sky survey and WISE to look for ultracool dwarfs that are part of multiple systems containing main sequence stars. We cross-matched L dwarf candidates from the surveys with Hipparcos and Gliese stars, finding two new systems. We consider the binary fraction for L dwarfs and main sequence stars, and further assess possible unresolved multiplicity within the full companion sample. This analysis shows that some of the L dwarfs in this sample might actually be unresolved binaries themselves. We have also identified a sample of common proper motion systems in which a main sequence star has a white dwarf as wide companion. These systems can help explore key issues in star evolution theory, as the initial-final mass relationship of white dwarfs, or the chromospheric activity-age relationship for stars still in the main sequence. Spectroscopy for 50 white dwarf candidates, selected from the SuperCOSMOS Science Archive, was obtained. We have also observed 6 of the main sequence star companions, and have estimated their effective temperatures, rotational and microturbulent velocities and metallicities.

Pinfield D.J.

2013-04-01

162

Asteroseismology of pulsating DA white dwarfs with fully evolutionary models  

Directory of Open Access Journals (Sweden)

Full Text Available We present a new approach for asteroseismology of DA white dwarfs that consists in the employment of a large set of non-static, physically sound, fully evolutionary models representative of these stars. We already have applied this approach with success to pulsating PG1159 stars (GW Vir variables. Our white dwarf models, which cover a wide range of stellar masses, effective temperatures, and envelope thicknesses, are the result of fully evolutionary computations that take into account the complete history of the progenitor stars from the ZAMS. In particular, the models are characterized by self-consistent chemical structures from the centre to the surface, a crucial aspect of white dwarf asteroseismology. We apply this approach to an ensemble of 44 bright DAV (ZZ Ceti stars.

Althaus L.G.

2013-03-01

163

Direct Distance Measurement to the Dusty White Dwarf GD 362  

CERN Document Server

We present trigonometric parallax observations of GD 362 obtained over seven epochs using the MDM 2.4m Hiltner Telescope. The existence of a dust disk around this possibly massive white dwarf makes it an interesting target for parallax observations. The measured parallax for GD 362 places it at a distance of 50.6 pc, which implies that its radius and mass are ~ 0.0106 Rsun and 0.71 Msun, respectively. GD 362 is not as massive as initially thought (1.2Msun). Our results are entirely consistent with the distance and mass estimates (52.2 pc and 0.73 Msun) by Zuckerman et al., who demonstrated that GD 362 has a helium dominated atmosphere. Dropping GD 362 from the list of massive white dwarfs, there are no white dwarfs with M > 0.9 Msun that are known to host circumstellar dust disks.

Kilic, Mukremin; Koester, Detlev; 10.1086/595786

2009-01-01

164

Evaporation and Accretion of Extrasolar Comets Following White Dwarf Kicks  

CERN Multimedia

Several lines of observational evidence suggest that white dwarfs receive small birth kicks due to anisotropic mass loss. If other stars possess extrasolar analogues to the Solar Oort cloud, the orbits of comets in such clouds will be scrambled by white dwarf natal kicks. Although most comets will be unbound, some will be placed on low angular momentum orbits vulnerable to sublimation or tidal disruption. The dusty debris from these comets will manifest itself as a debris disk temporarily visible around newborn white dwarfs; examples of such disks may already have been seen in the Helix Nebula, and around several other young WDs. Future observations with the James Webb Space Telescope will distinguish this hypothesis from alternatives such as a dynamically excited Kuiper Belt analogue. If interpreted as indeed being cometary in origin, the observation that >15% of young WDs possess such disks provides indirect evidence that low mass gas giants (thought necessary to produce an Oort cloud) are common in the out...

Stone, Nicholas; Loeb, Abraham

2014-01-01

165

Testing energy non-additivity in white dwarfs  

CERN Multimedia

We consider a particular effect which can be expected in scenarios of deviations from Special Relativity induced by Planckian physics: the loss of additivity in the total energy of a system of particles. We argue about the necessity to introduce a length scale to control the effects of non-additivity for macroscopic objects and consider white dwarfs as an appropriate laboratory to test this kind of new physics. We study the sensitivity of the mass-radius relation of the Chandrasekhar model to these corrections by comparing the output of a simple phenomenological model to observational data of white dwarfs.

Carmona, J M; Gracia-Ruiz, R; Loret, N

2013-01-01

166

Type Ia Supernovae and High Velocity White Dwarfs  

CERN Document Server

I examine the hypothesis that many of the high velocity white dwarfs observed by Oppenheimer et al. (2001) are the remnants of donor stars from binaries that produced type Ia supernovae via the `single degenerate' channel. If this channel is a significant contributor to the Galactic SN Ia supernova rate, then the local density of such remnants with V>100 km/s could be as high as 2.e-4 per cubic parsec, comparable to the densities found by Oppenheimer et al. This white dwarf population differs from others in that it is composed exclusively of single stars.

Hansen, B

2003-01-01

167

White dwarfs as physics laboratories: the case of axions  

CERN Document Server

White dwarfs are almost completely degenerate objects that cannot obtain energy from thermonuclear sources, so their evolution is just a gravothermal cooling process. Recent improvements in the accuracy and precision of the luminosity function and in pulsational data of variable white dwarfs suggest that they are cooling faster than expected from conventional theory. In this contribution we show that the inclusion of an additional cooling term due to axions able to interact with electrons with a coupling constant g_ae ~(2-7)x10^{-13} allows to fit better the observations.

Isern, J; Catalan, S; Corsico, A; Garcia-Berro, E; Salaris, M; Torres, S

2012-01-01

168

Astro-archaeology - The white dwarfs and hot subwarfs  

Science.gov (United States)

By 'astroarcheology' is presently meant the effort to ascertain the Galaxy's past in light of what is found in its most ancient, white dwarf constituents. Attention is given to the controversial role of the hot subdwarfs and the theory of white dwarf spectral evolution, as well as to the concept of the 'Whole Earth Telescope', involving continuous photometric coverage of rapidly varying astronomical sources and thereby eliminating the otherwise troublesome diurnal gaps in data. Much higher resolution of the power spectra of these objects is attainable by these means than any current alternative.

Van Horn, Hugh M.

1991-01-01

169

Hubble Space Telescope high resolution spectroscoy of the exposed white dwarf in the dwarf nova VW Hydri in quiescence: A rapidly rotating white dwarf  

Science.gov (United States)

We obtained a far-ultraviolet spectrum of the dwarf nova VW Hyi in quiescence, with the Hubble Space Telescope Goddard High Resolution Spectrograph covering the region of the Si iv lambda(lambda)1393, 1402 resonance doublet. The broad, shallow Si iv doublet feature is fully resolved, has a total equivalent width of 2.8 A, and is the first metal absorption feature to be clearly detected in the exposed white dwarf. Our synthetic spectral analysis, using a model grid constructed with the code TLUSTY, resulted in a reasonable fit to a white dwarf photosphere with T(sub eff) = 22,000 +/- 2000 K, log g = 8.0 +/- 0.3, an approximately solar Si/H abundance, and a rotational velocity, v sin i approximately equal to 600 km/s. This rotation rate, while not definitive because it is based upon just one line transition, is 20% of the Keplerian (breakup) velocity of the white dwarf and hence does not account for the unexpectedly low boundary-layer luminosity inferred from the soft-X-ray/extreme ultra-violet bands where most of the boundary-layer luminosity should be radiated. The predicted boundary-layer luminosity for a 0.6 solar mass white dwarf accreting at the rate 10(exp -10) solar mass/yr and rotating at 600 km/s, corresponding to VW Hyi in quiescence, is 2 x 10(exp 32) ergs/s when proper account is taken of the rotational kinetic energy going into spinning up the white dwarf. If the boundary-layer area is equal to that of the white dwarf, then T(sub bl) = 24,000 K. This is essentially identical to the photspheric luminosity and temperature determined in far-ultraviolet photospheric analyses. If the boundary-layer area is 10(exp -3) of the white dwarf surface area, then T(sub bl) = 136,000 K.

Sion, Edward M.; Huang, Min; Szkody, Paula; Cheng, Fu-Hua

1995-01-01

170

Constraining white-dwarf kicks in globular clusters : IV. Retarding Core Collapse  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Observations of white dwarfs in the globular clusters NGC 6397 and Omega Centauri indicate that these stars may get a velocity kick during their time as giants. If the mass loss while on the asymptotic giant branch is slightly asymmetric, the resulting white dwarf could be born with such a velocity kick. These energetic white dwarfs will impart their excess energy on other stars as they travel through the cluster. A Monte-Carlo simulation of the white-dwarfs kicks combined...

Heyl, Jeremy S.; Penrice, Matthew

2009-01-01

171

The luminosity function of white dwarfs and M dwarfs, using dark nebulae as opaque outer screens  

CERN Document Server

By using dark nebulae as opaque outer screens, the luminosity function of white dwarfs and M dwarfs has been studied. High-extinction areas towards the Orion A, Serpens and rho Oph cloud complexes were observed, covering a volume corresponding to 464 pc3 in the solar neighbourhood, complete to Mv=16.5. Foreground stars were selected by VRI photometry and photometric parallaxes. The resulting foreground sample consists of 21 M dwarfs and 7 white dwarfs. The derived M-dwarf luminosity function is consistent with previous studies, showing no substantial upturn beyond Mv=16. The 7 white dwarfs imply a local density of 0.013(+-0.005)Msun/pc3 (~15% of the dynamical mass in the solar neighbourhood) supporting other recent results but obtained with a completely different method. For the clouds, foreground-star data were used to derive independent cloud distances, while the background stars and a simple model of the Milky Way gave reliable cloud extinctions.

Festin, L

1998-01-01

172

Physical properties of IP Pegasi: an eclipsing dwarf nova with an unusually cool white dwarf  

CERN Document Server

We present high speed photometric observations of the eclipsing dwarf nova IP Peg taken with the triple-beam camera ULTRACAM mounted on the William Herschel Telescope. The primary eclipse in this system was observed twice in 2004, and then a further sixteen times over a three week period in 2005. Our observations were simultaneous in the Sloan u', g' and r' bands. By phase-folding and averaging our data we make the first significant detection of the white dwarf ingress in this system and find the phase width of the white dwarf eclipse to be 0.0935 +/- 0.0003, significantly higher than the previous best value of between 0.0863 and 0.0918. The mass ratio is found to be q = M2 /M1 = 0.48 +/- 0.01, consistent with previous measurements, but we find the inclination to be 83.8 +/- 0.5 deg, significantly higher than previously reported. We find the radius of the white dwarf to be 0.0063 +/- 0.0003 solar radii, implying a white dwarf mass of 1.16 +/- 0.02 solar masses. The donor mass is 0.55 +/- 0.02 solar masses. Th...

Copperwheat, C M; Dhillon, V S; Littlefair, S P; Hickman, R; Gänsicke, B T; Southworth, J

2009-01-01

173

ULTRACAM observations of two accreting white dwarf pulsators  

CERN Multimedia

In this paper we present high time-resolution observations of GW Librae and SDSS J161033.64-010223.3 -- two cataclysmic variables which have shown periodic variations attributed to non-radial pulsations of the white dwarf. We observed both these systems in their quiescent states and detect the strong pulsations modes reported by previous authors. The identification of further periodicities in GW Lib is limited by the accretion-driven flickering of the source, but in the case of SDSS 1610 we identify several additional low-amplitude periodicities. In the case of SDSS 1610, there is evidence to suggest that the two primary signals have a different colour dependence, suggesting that they may be different spherical harmonic modes. We additionally observed GW Lib during several epochs following its 2007 dwarf nova outburst: the first time a dwarf nova containing a pulsating white dwarf has been observed in such a state. We do not observe any periodicities, suggesting that the heating of the white dwarf had either ...

Copperwheat, C M; Dhillon, V S; Littlefair, S P; Woudt, P A; Warner, B; Steeghs, D; Gänsicke, B T; Southworth, J

2008-01-01

174

Diffusion and hydrogen shell burning on slowly accreting white dwarfs  

International Nuclear Information System (INIS)

The stability of hydrogen shell burning in accreted envelopes on white dwarfs is examined for the domain of low accretion rates. Account is taken of the effects of diffusion of CNO nuclei attributable to sedimentation associated with gravity and temperature gradients. Due to the direct competition with accretion, the effects of such sedimentation of CNO elements become important only for accretion rates smaller than M = 10-11 M/sub sun/ yr-1, for hydrogen shell burning in steady state approximation. Such a steady state is stable, for white dwarfs of mass -12 M/sub sun/ yr-1; this results from the weak temperature dependences of the proton-proton cycles and the relatively weak degeneracy at the burning shell. Reduction of CNO abundance levels due to sedimentation acts to enlarge this accretion rate range by approximately a factor of 2. The stable steady state is realized only when gas is accreted onto a hot white dwarf. For accretion onto cool white dwarfs, the stable regime is bypassed by recurrent shell flashes. In this latter case, the concentration of CNO elements will not be greatly affected by sedimentation because of its low efficiency at the low temperatures characterizing the accretion phase

1982-06-01

175

Mass limits for non-degenerate white dwarfs  

International Nuclear Information System (INIS)

We obtain the dependence of the upper mass limit of white dwarf stars on the specific entropy per electron. For non-degenerate stars significantly larger masses are found. Stellar collapse as an entropy-producing process can therefore have a self-stabilizing influence on the stellar evolution. (orig.)

1981-07-13

176

Rapidly accreting white dwarfs as supernova type Ia progenitors  

CERN Document Server

The nature of the progenitors of type Ia supernovae is still a mystery. While plausible candidates are known for both the single degenerate and double degenerate models, the observed numbers fall significantly short of what is required to reproduce the type Ia supernovae rate. Some of the most promising single-degenerate type Ia progenitors are recurrent novae and super-soft sources (SSS). White dwarfs with higher mass transfer rates can also be type Ia supernova progenitors. For these rapidly accreting white dwarfs (RAWD), more material than is needed for steady burning accretes on the white dwarf, and extends the white dwarf's photosphere. Unlike super-soft sources, such objects will likely not be detectable at soft X-ray energies, but will be bright at longer wavelengths, such as the far ultraviolet (UV). Possible examples include LMC N66 and the V Sagittae stars. We present a survey using multi-object spectrographs looking for RAWD in the central core of the SMC, from objects selected to be bright in the ...

Lepo, Kelly

2013-01-01

177

The frequency of planetary debris around young white dwarfs  

CERN Multimedia

(Abridged) We present the results of the first unbiased survey for metal pollution among H-atmosphere (DA) white dwarfs with cooling ages of 20-200 Myr and 17000K 0.8 Msun is found to be currently accreting, which suggests a large fraction are double-degenerate mergers, and the merger discs do not commonly reform large planetesimals or otherwise pollute the remnant. We reconfirm our previous finding that two white dwarf Hyads are currently accreting rocky debris. At least 27%, and possibly up to ~50%, of all white dwarfs with cooling ages 20-200 Myr are accreting planetary debris. At Teff > 23000K, the luminosity of white dwarfs is likely sufficient to vaporize circumstellar dust, and hence no stars with strong metal-pollution are found. However, planetesimal disruption events should occur in this cooling age and Teff range as well, and likely result in short phases of high mass transfer rates. It appears that the formation of rocky planetary material is common around 2-3 Msun late B- and A-type stars.

Koester, Detlev; Farihi, Jay

2014-01-01

178

11-12 Gyr old White Dwarfs 30 parsecs Away  

CERN Multimedia

We present a detailed model atmosphere analysis of two of the oldest stars known in the solar neighborhood, the high proper motion white dwarfs SDSS J110217.48+411315.4 (hereafter J1102) and WD 0346+246 (hereafter WD0346). We present trigonometric parallax observations of J1102, which places it at a distance of only 33.7 +- 2.0 pc. Based on the state of the art model atmospheres, optical, near-, mid-infrared photometry, and distances, we constrain the temperatures, atmospheric compositions, masses, and ages for both stars. J1102 is an 11 Gyr old (white dwarf plus main-sequence age), 0.62 Msol white dwarf with a pure H atmosphere and Teff = 3830 K. WD0346 is an 11.5 Gyr old, 0.77 Msol white dwarf with a mixed H/He atmosphere and Teff = 3650 K. Both stars display halo kinematics and their ages agree remarkably well with the ages of the nearest globular clusters, M4 and NGC 6397. J1102 and WD0346 are the closest examples of the oldest halo stars that we know of.

Kilic, Mukremin; Kowalski, P M; Andrews, J

2012-01-01

179

A Comprehensive Spectroscopic Analysis of DB White Dwarfs  

Science.gov (United States)

We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 M sun for our sample, with a dispersion of only 0.09 M sun. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M account for the observed hydrogen abundances in DBA stars. We present evidence for the existence of two different evolutionary channels that produce DB white dwarfs: the standard model where DA stars are transformed into DB stars through the convective dilution of a thin hydrogen layer and a second channel where DB stars retain a helium atmosphere throughout their evolution. We finally demonstrate that the instability strip of pulsating V777 Her white dwarfs contains no non-variables, if the hydrogen content of these stars is properly accounted for.

Bergeron, P.; Wesemael, F.; Dufour, Pierre; Beauchamp, A.; Hunter, C.; Saffer, Rex A.; Gianninas, A.; Ruiz, M. T.; Limoges, M.-M.; Dufour, Patrick; Fontaine, G.; Liebert, James

2011-08-01

180

Bose-Einstein condensation in helium white dwarf stars. I  

Energy Technology Data Exchange (ETDEWEB)

The formation of a Bose-Einstein condensate in the interior of helium white dwarfs stars is discussed. Following the proposal made by Gabadadze and Rosen, we have explored the consequences of such a mechanism by calculating the cooling time of the stars. We have found that it is shorter than the value predicted by the standard model.

Mosquera, M.E. [Faculty of Astronomy and Geophysics, University of La Plata, Paseo del Bosque s.n., La Plata (Argentina); Department of Physics, University of La Plata, c.c. 67 1900, La Plata (Argentina); Civitarese, O., E-mail: osvaldo.civitarese@fisica.unlp.edu.a [Department of Physics, University of La Plata, c.c. 67 1900, La Plata (Argentina); Benvenuto, O.G.; De Vito, M.A. [Faculty of Astronomy and Geophysics, University of La Plata, Paseo del Bosque s.n., La Plata (Argentina); Instituto de Astrofisica La Plata, CCT (Argentina)

2010-01-18

 
 
 
 
181

The Chandrasekhar's Equation for Two-Dimensional Hypothetical White Dwarfs  

CERN Document Server

In this article we have extended the original work of Chandrasekhar on the structure of white dwarfs to the two-dimensional case. Although such two-dimensional stellar objects are hypothetical in nature, we strongly believe that the work presented in this article may be prescribed as Master of Science level class problem for the students in physics.

De, Sanchari

2014-01-01

182

A Search for Jovian Planets around Hot White Dwarfs  

CERN Multimedia

Current searches for extrasolar planets have concentrated on observing the reflex Doppler shift of solar-type stars. Little is known, however, about planetary systems around non-solar-type stars. We suggest a new method to extend planetary searches to hot white dwarfs. Near a hot white dwarf, the atmosphere of a Jovian planet will be photoionized and emit hydrogen recombination lines, which may be detected by high- dispersion spectroscopic observations. Multi-epoch monitoring can be used to distinguish between non-LTE stellar emission and planetary emission, and to establish the orbital parameters of the detected planets. In the future, high-precision astrometric measurements of the hot white dwarf will allow the masses of the detected planets to be determined. Searches for Jovian planets around hot white dwarfs will provide invaluable new insight on the development of planetary systems around stars more massive than the Sun and on how stellar evolution affects these systems. We present high-dispersion spectr...

Chu, Y H; Gruendl, R A; Brandner, W

2000-01-01

183

Lithium production in the merging of white dwarf stars  

CERN Multimedia

The origin of R Coronae Borealis stars has been elusive for over 200 years. Currently, two theories for their formation have been presented. These are the Final Flash scenario, in which a dying asymptotic giant branch (AGB) star throws off its atmosphere to reveal the hydrogen poor, heavily processed material underneath, and the double degenerate scenario, in which two white dwarfs merge to produce a new star with renewed vigour. Some theories predict that the temperatures reached during the latter scenario would destroy any lithium originally present in the white dwarfs. The observed lithium content of some R Coronae Borealis stars, therefore, is often interpreted as an indication that the Final Flash scenario best describes their formation. In this paper, it is shown that lithium production can, indeed, occur in the merging of a helium white dwarf with a carbon-oxygen white dwarf if their chemical composition, particularly that of 3He, is fully considered. The production mechanism is described in detail, an...

Longland, Richard; José, Jordi; García-Berro, Enrique; Althaus, Leandro G

2012-01-01

184

Asteroseismic constraints on diffusion in white dwarf envelopes  

Science.gov (United States)

The asteroseismic analysis of white dwarfs allows us to peer below their photospheres and determine their internal structure. At ˜28 000 K EC20058-5234 is the hottest known pulsating helium atmosphere white dwarf. As such, it constitutes an important link in the evolution of white dwarfs down the cooling track. It is also astrophysically interesting because it is at a temperature where white dwarfs are expected to cool mainly through the emission of plasmon neutrinos. In the present work, we perform an asteroseismic analysis of EC20058-5234 and place the results in the context of stellar evolution and time-dependent diffusion calculations. We use a parallel genetic algorithm complemented with targeted grid searches to find the models that fit the observed periods best. Comparing our results with similar modelling of EC20058-5234's cooler cousin CBS114, we find a helium envelope thickness consistent with time-dependent diffusion calculations and obtain a precise mode identification for EC20058-5234.

Bischoff-Kim, A.; Metcalfe, T. S.

2011-06-01

185

Binary Star Origin of High Field Magnetic White Dwarfs  

CERN Document Server

White dwarfs with surface magnetic fields in excess of $1 $MG are found as isolated single stars and relatively more often in magnetic cataclysmic variables. Some 1,253 white dwarfs with a detached low-mass main-sequence companion are identified in the Sloan Digital Sky Survey but none of these is observed to show evidence for Zeeman splitting of hydrogen lines associated with a magnetic field in excess of 1MG. If such high magnetic fields on white dwarfs result from the isolated evolution of a single star then there should be the same fraction of high field white dwarfs among this SDSS binary sample as among single stars. Thus we deduce that the origin of such high magnetic fields must be intimately tied to the formation of cataclysmic variables. CVs emerge from common envelope evolution as very close but detached binary stars that are then brought together by magnetic braking or gravitational radiation. We propose that the smaller the orbital separation at the end of the common envelope phase, the stronger ...

Tout, C A; Liebert, J; Ferrario, L; Pringle, J E

2008-01-01

186

The ELM Survey. IV. 24 White Dwarf Merger Systems  

CERN Multimedia

We present new radial velocity and X-ray observations of extremely low-mass (ELM, 0.2 Msol) white dwarf candidates in the Sloan Digital Sky Survey (SDSS) Data Release 7 area. We identify seven new binary systems with 1-18 h orbital periods. Five of the systems will merge due to gravitational wave radiation within 10 Gyr, bringing the total number of merger systems found in the ELM Survey to 24. The ELM Survey has now quintupled the known merger white dwarf population. It has also discovered the eight shortest period detached binary white dwarf systems currently known. We discuss the characteristics of the merger and non-merger systems observed in the ELM Survey, including their future evolution. About half of the systems have extreme mass ratios. These are the progenitors of the AM Canum Venaticorum systems and supernovae .Ia. The remaining targets will lead to the formation of extreme helium stars, subdwarfs, or massive white dwarfs. We identify three targets that are excellent gravitational wave sources. Th...

Kilic, Mukremin; Prieto, Carlos Allende; Kenyon, S J; Heinke, Craig O; Agueros, M A; Kleinman, S J

2012-01-01

187

The brightest pure-H ultracool white dwarf  

CERN Multimedia

We report the identification of LSR J0745+2627 in the United Kingdom InfraRed Telescope Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) as a cool white dwarf with kinematics and age compatible with the thick-disk/halo population. LSR J0745+2627 has a high proper motion (890 mas/yr) and a high reduced proper motion value in the J band (H_J=21.87). We show how the infrared-reduced proper motion diagram is useful for selecting a sample of cool white dwarfs with low contamination. LSR J0745+2627 is also detected in the Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Explorer (WISE). We have spectroscopically confirmed this object as a cool white dwarf using X-Shooter on the Very Large Telescope. A detailed analysis of its spectral energy distribution reveals that its atmosphere is compatible with a pure-H composition model with an effective temperature of 3880+-90 K. This object is the brightest pure-H ultracool white dwarf (Teff<4000 K) ever identified. We have constrained the dis...

Catalan, S; Pinfield, D J; Smith, L C; Zhang, Z H; Napiwotzki, R; Marocco, F; Day-Jones, A C; Gomes, J; Forde, K P; Lucas, P W; Jones, H R A

2012-01-01

188

A Critical Examination of Halo White Dwarf Candidates  

CERN Multimedia

A detailed analysis of halo white dwarf candidates is presented, which is based on model atmosphere fits to observed energy distributions built from photoelectric or photographic magnitudes. Most of the candidates identified in reduced proper motion diagrams are shown to be too warm (Teff > 5000 K) and most likely too young to be members of the galactic halo, while the tangential velocities of the cooler and thus older white dwarfs are shown to be entirely consistent with the disk population. The results suggest that some white dwarf stars born in the young disk may have high velocities with respect to the local standard of rest. Such objects could represent the remnants of donor stars from close mass-transfer binaries that produced type Ia supernovae via the single degenerate channel, or other scenarios suggested in the literature. Ongoing surveys that rely solely on reduced proper motion diagrams are likely to identify more of these high velocity young degenerates, rather than to unveil the old white dwarf ...

Bergeron, P

2003-01-01

189

The frequency of planetary debris around young white dwarfs  

Science.gov (United States)

Context. Heavy metals in the atmospheres of white dwarfs are thought in many cases to be accreted from a circumstellar debris disk, which was formed by the tidal disruption of a rocky planetary body within the Roche radius of the star. The abundance analysis of photospheric elements and conclusions about the chemical composition of the accreted matter are a new and promising method of studying the composition of extrasolar planetary systems. However, ground-based searches for metal-polluted white dwarfs that rely primarily on the detection of the Ca ii K line become insensitive at Teff > 15 000 K because this ionization state depopulates. Aims: We present the results of the first unbiased survey for metal pollution among hydrogen-atmosphere (DA type) white dwarfs with cooling ages in the range 20-200 Myr and 17 000 K 23 000 K, in excellent agreement with the absence of infrared excess from dust around these warmer stars. The median, main sequence progenitor of our sample corresponds to an A-type star of ?2 M?, and we find 13 of 23 white dwarfs descending from main sequence 2-3 M?, late B- and A-type stars to be currently accreting. Only one of 14 targets with Mwd > 0.8 M? is found to be currently accreting, which suggests a large fraction of these stars result from double-degenerate mergers, and the merger disks do not commonly reform large planetesimals or otherwise pollute the remnant. We reconfirm our previous finding that two 625 Myr Hyades white dwarfs are currently accreting rocky planetary debris. Conclusions: At least 27% of all white dwarfs with cooling ages 20-200 Myr are accreting planetary debris, but that fraction could be as high as ?50%. At Teff > 23 000 K, the luminosity of white dwarfs is probably sufficient to vaporize circumstellar dust grains, so no stars with strong metal-pollution are found. Planetesimal disruption events should occur in this cooling age and temperature range as well, and they are likely to result in short phases of high mass-transfer rates. It appears that the formation of rocky planetary material is common around 2-3 M? late B- and A-type stars. Table 1 is available in electronic form at http://www.aanda.org

Koester, D.; Gänsicke, B. T.; Farihi, J.

2014-06-01

190

Searching for white dwarfs candidates in Sloan Digital Sky Survey Data  

CERN Document Server

Large amount of observational spectroscopic data are recently available from different observational projects, like Sloan Digital Sky Survey. It's become more urgent to identify white dwarfs stars based on data itself i.e. without modelling white dwarf atmospheres. In particular, existing methods of white dwarfs identification presented in Kleinman et al. (2004) and in Eisenstein et al. (2006) did not allow to find all the white dwarfs in examined data. We intend to test various criteria of searching for white dwarf candidates, based on photometric and spectral features.

zyty, MirosÅ?aw Nale\\; Ciechanowska, Anna; Madej, Jerzy

2008-01-01

191

Detached white-dwarf close-binary stars CV's extended family  

CERN Document Server

I review detached binaries consisting of white dwarfs with either other white dwarfs or low mass main-sequence stars in tight orbits around them. Orbital periods have been measured for 15 white dwarf/white dwarf systems and 22 white dwarf/M dwarf systems. While small compared to the number of periods known for CVs (>300), I argue that each variety of detached system has a space density an order of magnitude higher that of CVs. While theory matches the observed distribution of orbital periods of the white dwarf/white dwarf binaries, it predicts white dwarfs of much lower mass than observed. Amongst both types of binary are clear examples of helium core white dwarfs, as opposed to the usual CO composition; similar systems must exist amongst the CVs. White dwarf/M dwarf binaries suffer from selection effects which diminish the numbers seen at long and short periods. They are useful for the study of irradiation; I discuss evidence to suggest that Balmer emission is broadened by optical depth effects to an extent ...

Marsh, T R

1999-01-01

192

Detached white-dwarf close-binary stars - CV's extended family  

Science.gov (United States)

I review detached binaries consisting of white dwarfs with either other white dwarfs or low mass main-sequence stars in tight orbits around them. Orbital periods have been measured for 15 white dwarf/white dwarf systems and 22 white dwarf/M dwarf systems. While small compared to the number of periods known for CVs (>300), I argue that each variety of detached system has a space density an order of magnitude higher that of CVs. While theory matches the observed distribution of orbital periods of the white dwarf/white dwarf binaries, it predicts white dwarfs of much lower mass than observed. Amongst both types of binary are clear examples of helium core white dwarfs, as opposed to the usual CO composition; similar systems must exist amongst the CVs. White dwarf/M dwarf binaries suffer from selection effects which diminish the numbers seen at long and short periods. They are useful for the study of irradiation; I discuss evidence to suggest that Balmer emission is broadened by optical depth effects to an extent which limits its usefulness for imaging the secondary stars in CVs.

Marsh, T. R.

2000-04-01

193

Hot accreting white dwarfs in the quasi-static approximation  

International Nuclear Information System (INIS)

Properties of white dwarfs which are accreting hydrogen-rich matter at rates in the range 1.5 x 10-9 to 2.5 x 10-7 M/sub sun/ yr-1 are investigated in several approximations. Steady-burning models, in which matter is processed through nuclear-burning shells as rapidly as it is accreted, provide a framework for understanding the properties of models in which thermal pulses induced by hydrogen burning and helium burning are allowed to occur. In these latter models, the underlying carbon-oxygen core is chosen to be in a cycle-averaged steady state with regard to compressional heating and neutrino losses. Several of these models are evolved in the quasi-static approximation. Combining results obtained in the steady-burning approximation with those obtained in the quasi-static approximation, expressions are obtained for estimating, as functions of accretion rate and white dwarf mass, the thermal pulse recurrence period and the duration of hydrogen-burning phases. The time spent by an accreting model burning hydrogen as a large star of giant dimensions versus time spent burning hydrogen as a hot dwarf is also estimated as a function of model mass and accretion rate. Finally, suggestions for detecting observational counterparts of the theoretical models and suggestions for further theoretical investigations are offered. Subject headings: stars: accretion: stars: interiors: stars: novae: stars: symbiotic: stars: white dwarfs

1982-08-01

194

A Third Hot White Dwarf Companion Detected by Kepler  

Science.gov (United States)

We have found a system listed in the Kepler Binary Catalog (P orb = 3.273 days) that we have determined is comprised of a low-mass, thermally bloated, hot white dwarf orbiting an A star of about 2.3 M sun. In this work, we designate the object, KIC 10657664, simply as "KHWD3" (Kepler Hot White Dwarf 3). We use the transit depth of ~0.66%, the eclipse depth of ~1.9%, and regular smooth periodic variations at the orbital frequency and twice the orbital frequency to analyze the system parameters. The smooth periodic variations are identified with the classical ellipsoidal light variation (ELV) and illumination (ILL) effects, and the newly utilized Doppler boosting (DB) effect. Given the measured values of R/a and inclination angle of the binary, both the ELV and DB effects are mostly sensitive to the mass ratio, q = M 2/M 1, of the binary. The two effects yield values of q which are somewhat inconsistent—presumably due to unidentified systematic effects—but which nonetheless provide a quite useful set of possibilities for the mass of the white dwarf (either 0.26 ± 0.04 M sun or 0.37 ± 0.08 M sun). All of the other system parameters are determined fairly robustly. In particular, we show that the white dwarf has a radius of 0.15 ± 0.01 R sun, which is extremely bloated over the radius it would have as a fully degenerate object, and an effective temperature T effsime14,500 K. Binary evolution scenarios and models for this system are discussed. We suggest that the progenitor binary was comprised of a primary of mass ~2.2 M sun (the progenitor of the current hot white dwarf) and a secondary of mass ~1.4 M sun (the progenitor of the current A star in the system). We compare this new system with three other white dwarfs in binaries that likely were formed via stable Roche-lobe overflow (KOI-74, KOI-81, and the inner Regulus binary).

Carter, Joshua A.; Rappaport, Saul; Fabrycky, Daniel

2011-02-01

195

A THIRD HOT WHITE DWARF COMPANION DETECTED BY KEPLER  

International Nuclear Information System (INIS)

We have found a system listed in the Kepler Binary Catalog (Porb = 3.273 days) that we have determined is comprised of a low-mass, thermally bloated, hot white dwarf orbiting an A star of about 2.3 Msun. In this work, we designate the object, KIC 10657664, simply as 'KHWD3' (Kepler Hot White Dwarf 3). We use the transit depth of ?0.66%, the eclipse depth of ?1.9%, and regular smooth periodic variations at the orbital frequency and twice the orbital frequency to analyze the system parameters. The smooth periodic variations are identified with the classical ellipsoidal light variation (ELV) and illumination (ILL) effects, and the newly utilized Doppler boosting (DB) effect. Given the measured values of R/a and inclination angle of the binary, both the ELV and DB effects are mostly sensitive to the mass ratio, q = M2/M1, of the binary. The two effects yield values of q which are somewhat inconsistent-presumably due to unidentified systematic effects-but which nonetheless provide a quite useful set of possibilities for the mass of the white dwarf (either 0.26 ± 0.04 Msun or 0.37 ± 0.08 Msun). All of the other system parameters are determined fairly robustly. In particular, we show that the white dwarf has a radius of 0.15 ± 0.01 Rsun, which is extremely bloated over the radius it would have as a fully degenerate object, and an effective temperature Teff?14,500 K. Binary evolution scenarios and models for this system are discussed. We suggest that the progenitor binary was comprised of a primary of mass ?2.2 Msun (the progenitor of the current hot white dwarf) and a secondary of mass ?1.4 Msun (the progenitor of the current A star in the system). We compare this new system with three other white dwarfs in binaries that likely were formed via stable Roche-lobe overflow (KOI-74, KOI-81, and the inner Regulus binary).

2011-02-20

196

WHITE DWARF-RED DWARF SYSTEMS RESOLVED WITH THE HUBBLE SPACE TELESCOPE. II. FULL SNAPSHOT SURVEY RESULTS  

International Nuclear Information System (INIS)

Results are presented for a Hubble Space Telescope Advanced Camera for Surveys high-resolution imaging campaign of 90 white dwarfs with known or suspected low-mass stellar and substellar companions. Of the 72 targets that remain candidate and confirmed white dwarfs with near-infrared excess, 43 are spatially resolved into two or more components, and a total of 12 systems are potentially triples. For 68 systems where a comparison is possible, 50% have significant photometric distance mismatches between their white dwarf and M dwarf components, suggesting that white dwarf parameters derived spectroscopically are often biased due to the cool companion. Interestingly, 9 of the 30 binaries known to have emission lines are found to be visual pairs and hence widely separated, indicating an intrinsically active cool star and not irradiation from the white dwarf. There is a possible, slight deficit of earlier spectral types (bluer colors) among the spatially unresolved companions, exactly the opposite of expectations if significant mass is transferred to the companion during the common envelope phase. Using the best available distance estimates, the low-mass companions to white dwarfs exhibit a bimodal distribution in projected separation. This result supports the hypothesis that during the giant phases of the white dwarf progenitor, any unevolved companions either migrate inward to short periods of hours to days, or outward to periods of hundreds to thousands of years. No intermediate projected separations of a few to several AU are found among these pairs. However, a few double M dwarfs (within triples) are spatially resolved in this range, empirically demonstrating that such separations were readily detectable among the binaries with white dwarfs. A straightforward and testable prediction emerges: all spatially unresolved, low-mass stellar and substellar companions to white dwarfs should be in short-period orbits. This result has implications for substellar companion and planetary orbital evolution during the post-main-sequence lifetime of their stellar hosts.

2010-10-01

197

The contribution of red dwarfs and white dwarfs to the halo dark matter  

CERN Multimedia

The nature of the several microlensing events observed by the MACHO team towards the LMC still remains controversial. Low-mass substellar objects and stars with masses larger than ~1 M_{sun} have been ruled out as major components of a MACHO galactic halo, while stars of half solar masses are the most probable candidates. In this paper we assess jointly the relative contributions of both red dwarfs and white dwarfs to the mass budget of the galactic halo. In doing so we use a Monte Carlo simulator which incorporates up-to-date evolutionary sequences of both red dwarfs and white dwarfs as well as detailed descriptions of our Galaxy and of the LMC. We explore the complete mass range between 0.08 and 1 M_{sun} as possible microlensing candidates and we compare the synthetic populations obtained with our simulator with the results obtained by the MACHO and EROS experiments. Our results indicate that, despite that the contribution of the red dwarf population increases by a factor of 2 the value of the optical dept...

Torres, S; Isern, J; García-Berro, E

2008-01-01

198

Low heat conduction in white dwarf boundary layers?  

CERN Document Server

X-ray spectra of dwarf novae in quiescence observed by Chandra and XMM-Newton provide new information on the boundary layers of their accreting white dwarfs. Comparison of observations and models allows us to extract estimates for the thermal conductivity in the accretion layer and reach conclusions on the relevant physical processes. We calculate the structure of the dense thermal boundary layer that forms under gravity and cooling at the white dwarf surface on accretion of gas from a hot tenuous ADAF-type coronal inflow. The distribution of density and temperature obtained allows us to calculate the strength and spectrum of the emitted X-ray radiation. They depend strongly on the values of thermal conductivity and mass accretion rate. We apply our model to the dwarf nova system VW Hyi and compare the spectra predicted for different values of the thermal conductivity with the observed spectrum. We find a significant deviation for all values of thermal conductivity that are a sizable fraction of the Spitzer c...

Liu, F K; Meyer-Hofmeister, E; Burwitz, V

2008-01-01

199

On the origin of high-field magnetic white dwarfs  

CERN Document Server

High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mechanism that produces the magnetic field are still unknown. Here we show that the hot, convective, differentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores is able to produce magnetic fields of the required strength that do not decay for long timescales. We also show, using an state-of-the-art Monte Carlo simulator, that the expected number of high-field magnetic white dwarfs produced in this way is consistent with that found in the solar neighborhood.

García-Berro, E; Lorén-Aguilar, P; Aznar-Siguán, G; Camacho, J; Külebi, B; Isern, J; Althaus, L G; Córsico, A H

2012-01-01

200

The White Dwarf Luminosity Function from SDSS Imaging Data  

CERN Document Server

A sample of white dwarfs is selected from SDSS DR3 imaging data using their reduced proper motions, based on improved proper motions from SDSS plus USNO-B combined data. Numerous SDSS and followup spectra (Kilic et al. 2005) are used to quantify completeness and contamination of the sample; kinematic models are used to understand and correct for velocity-dependent selection biases. A luminosity function is constructed covering the range 7 < M_bol < 16, and its sensitivity to various assumptions and selection limits is discussed. The white dwarf luminosity function based on 6000 stars is remarkably smooth, and rises nearly monotonically to M_bol = 15.3. It then drops abruptly, although the small number of low-luminosity stars in the sample and their unknown atmospheric composition prevent quantitative conclusions about this decline. Stars are identified that may have high tangential velocities, and a preliminary luminosity function is constructed for them.

Harris, H C; Kilic, M; Liebert, J; Williams, K A; Von Hippel, T; Levine, S E; Monet, D G; Eisenstein, D J; Kleinman, S J; Metcalfe, T S; Nitta, A; Winget, D E; Brinkmann, J; Fukugita, M; Knapp, G R; Lupton, R H; Smith, J A; Schneider, D P; Harris, Hugh C.; Munn, Jeffrey A.; Kilic, Mukremin; Liebert, James; Williams, Kurtis A.; Hippel, Ted von; Levine, Stephen E.; Monet, David G.; Eisenstein, Daniel J.; Nitta, Atsuko; Fukugita, Masataka; Lupton, Robert H.; Schneider, Donald P.

2005-01-01

 
 
 
 
201

Helium shell flashes and evolution of accreting white dwarfs  

International Nuclear Information System (INIS)

In a close binary system or in a dense cloud, gas may be accreted onto a carbon-oxygen white dwarf and will be processed into helium by hydrogen burning in an accreted envelope. As a result, a helium zone grows in mass, and a helium shell flash takes place just as in cores of red giant stars. Properties of such helium shell flashes are investigated both by a generalized theory of shell flash and numerical computations. It is found that the shell flash grows up to a strength of supernova explosion when the mass of the helium zone is large enough on a massive white dwarf (> or approx. =0.7 M/sub sun/). Otherwise, shell flashes are relatively weak: Even then protons in the envelope are mixed into a helium convective zone, and it becomes a site of s-process nucleosynthesis

1982-06-01

202

Time resolved circular polarimetry of white dwarf pulsars  

International Nuclear Information System (INIS)

There are two recognized subsets of cataclysmic binaries that contain white dwarfs: the AM Her stars, whose white dwarf components are sufficiently magnetized to ensure synchronous rotation with the orbital period, and the DQ Her stars, which do not maintain synchronism and probably have been spun up by mass accretion. A pilot survey has shown that for all objects but V533 Her the amplitude of the periodic component of the circular polarization is not greater than 0.10-0.15% in the blue band. In the case of V533 Her approximate 0.5% sinusoidal changes in a circular polarization have been detected on two nights. This might mean that in DQ Her stars the dilution by an unpolarized background is very large, and furthermore, that this dilution is variable. (Auth.)

1981-09-03

203

Generalized theory of shell flash and accreting white dwarfs  

International Nuclear Information System (INIS)

Shell flashes take place both in deep interior of red giant stars and near surface of accreting white dwarfs. Theories of shell flashes have been thus far presented piece by piece in different papers. It is the purpose of the present review to construct and generalize them in order to reach better understanding. A non-linear yet almost analytical theory is presented which treats the development of the shell flash in finite amplitude. Recurrence of the shell flashes is also shown to be well understood as a non-linear oscillation in dissipative system which tends to be its limit cycle. As a result strength of the peak energy-generation and recurrence time of the shell flashes are related with mass of the accreting white dwarfs, accretion rate, etc. (Auth.)

1980-07-25

204

Thermogalvanomagnetic phenomena in neutron stars and white dwarf stars  

International Nuclear Information System (INIS)

The electron thermal and electrical conductivities and thermoelectric coefficient are found and the induction and heat transfer equations are presented for the degenerate cores of white dwarfs and degenerate layers of the envelopes of neutron stars with the density rho11 g/cm3 in the presence of nonquantizing magnetic field. The scattering of electrons on ions and on phonons and impurities is taken into account. The thermogalvanomagnetic phenomena which can take place im neutron stars and white dwarfs are discussed: the drift of the magnetic field towards the surface or into the interior by the heat flow emerging from the star, various mechanisms of nonuniform heating connected with the magnetic field. It is pointed out that in degenerate stars some mechanisms of inhomogeneous heat release cause by magnetic field can take place

1980-01-01

205

Infrared Signatures of Disrupted Minor Planets at White Dwarfs  

CERN Multimedia

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T 3e8 g/s display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 1e6 g/s in the solar system zodiacal cloud and 1e10 g/s often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

Farihi, J; Zuckerman, B

2009-01-01

206

Weak magnetic fields in white dwarfs and their direct progenitors?  

CERN Document Server

We have carried out a re-analysis of polarimetric data of central stars of planetary nebulae, hot subdwarfs, and white dwarfs taken with FORS1 (FOcal Reducer and low dispersion Spectrograph) on the VLT (Very Large Telescope), and added a large number of new observations in order to increase the sample. A careful analysis of the observations using only one wavelength calibration for the polarimetrically analysed spectra and for all positions of the retarder plate of the spectrograph is crucial in order to avoid spurious signals. We find that the previous detections of magnetic fields in subdwarfs and central stars could not be confirmed while about 10% of the observed white dwarfs have magnetic fields at the kilogauss level.

Jordan, S; Landstreet, J; Fossati, L; Valyanin, G G; Monin, D; Wade, G A; Werner, K; O'Toole, S J

2012-01-01

207

Mock LISA data challenge for the Galactic white dwarf binaries  

International Nuclear Information System (INIS)

We present data analysis methods used in the detection and estimation of parameters of gravitational-wave signals from the white dwarf binaries in the mock LISA data challenge. Our main focus is on the analysis of challenge 3.1, where the gravitational-wave signals from more than 6x107 Galactic binaries were added to the simulated Gaussian instrumental noise. The majority of the signals at low frequencies are not resolved individually. The confusion between the signals is strongly reduced at frequencies above 5 mHz. Our basic data analysis procedure is the maximum likelihood detection method. We filter the data through the template bank at the first step of the search, then we refine parameters using the Nelder-Mead algorithm, we remove the strongest signal found and we repeat the procedure. We detect reliably and estimate parameters accurately of more than ten thousand signals from white dwarf binaries.

2010-03-15

208

A Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs  

CERN Document Server

We present an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations. In particular, we revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz, and discuss the astrophysical implications on the temperature scale and mean mass, as well as the chemical evolution of these stars. We also analyze 40 new DQ stars discovered in the first data release of the Sloan Digital Sky Survey. Our analysis confirms that effective temperatures derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models. Similarly the mean mass of the trigonometric parallax sample, = 0.62 Mo, is significantly lower than that obtained from pure helium models, = 0.73 Mo, and more consistent with the spectroscopic mean mass of DB stars, = 0.59 Mo, the most likely progenitors of DQ white dwarfs. We find that DQ stars form a remarkably well defined sequence in a ...

Dufour, P; Fontaine, G

2005-01-01

209

The metal content of hot DA white dwarf spectra  

CERN Multimedia

A study of high ion metal absorption features present in the spectra of hot DA white dwarfs is presented. An analysis of three DAs is performed, where previous studies came to conflicting conclusions as to the stars' nitrogen configurations. The nitrogen abundances were found to be in keeping with DAs of higher Teff, with a homogeneous distribution. A search for circumstellar gas discs was performed on eight stars, where circumstellar pollution may explain the differences between predicted and observed metal abundances. No positive detections were made. Already the subject of previous studies, the circumstellar absorption features seen at many hot DAs were again analysed, using a more advanced technique than those implemented in previous studies. This allowed, for the first time, column density measurements for all non-photospheric absorbing material. The derived column density measurements are consistent with those predicted to exist in white dwarf Stromgren Spheres, and the velocities of the absorbing mater...

Dickinson, Nathan

2012-01-01

210

Spectral modeling of gaseous metal disks around DAZ white dwarfs  

CERN Document Server

We report on our attempt for the first non-LTE modeling of gaseous metal disks around single DAZ white dwarfs recently discovered by Gaensicke et al. and thought to originate from a disrupted asteroid. We assume a Keplerian rotating viscous disk ring composed of calcium and hydrogen and compute the detailed vertical structure and emergent spectrum. We find that the observed infrared CaII emission triplet can be modeled with a hydrogen-deficient gas ring located at R=1.2 R_sun, inside of the tidal disruption radius, with Teff about 6000 K and a low surface mass density of about 0.3 g/cm**2. A disk having this density and reaching from the central white dwarf out to R=1.2 R_sun would have a total mass of 7 10**21 g, corresponding to an asteroid with about 160 km diameter.

Werner, K; Rauch, T

2008-01-01

211

Zero Impact Parameter White Dwarf Collisions in FLASH  

CERN Multimedia

We systematically explore zero impact parameter collisions of white dwarfs with the Eulerian adaptive grid code FLASH for 0.64+0.64 M$_{\\odot}$ and 0.81+0.81 M$_{\\odot}$ mass pairings. Our models span a range of effective linear spatial resolutions from 5.2$\\times10^{7}$ to 1.2$\\times10^{7}$ cm. However, even the highest resolution models do not quite achieve strict numerical convergence, due to the challenge of properly resolving small-scale burning and energy transport. The lack of strict numerical convergence from these idealized configurations suggest that quantitative predictions of the ejected elemental abundances that are generated by binary white dwarf collision and merger simulations should be viewed with caution. Nevertheless, the convergence trends do allow some patterns to be discerned. We find that the 0.64+0.64 M$_{\\odot}$ head-on collision model produces 0.32 M$_{\\odot}$ of \

Hawley, Wendy; Timmes, Francis

2012-01-01

212

Spitzer Surveys of IR Excesses of White Dwarfs  

CERN Document Server

IR excesses of white dwarfs (WDs) can be used to diagnose the presence of low-mass companions, planets, and circumstellar dust. Using different combinations of wavelengths and WD temperatures, circumstellar dust at different radial distances can be surveyed. The Spitzer Space Telescope has been used to search for IR excesses of white dwarfs. Two types of circumstellar dust disks have been found: (1) small disks around cool WDs with Teff 100,000 K. The small dust disks are within the Roche limit, and are commonly accepted to have originated from tidally crushed asteroids. The large dust disks, at tens of AU from the central WDs, have been suggested to be produced by increased collisions among Kuiper Belt-like objects. In this paper, we discuss Spitzer IRAC surveys of small dust disks around cool WDs, a MIPS survey of large dust disks around hot WDs, and an archival Spitzer survey of IR excesses of WDs.

Chu, Y -H; Bilíkovà, J; Riddle, A; Su, K Y -L

2010-01-01

213

A population synthesis study of the luminosity function of hot white dwarfs  

CERN Multimedia

We present a coherent and detailed Monte Carlo simulation of the population of hot white dwarfs. We assess the statistical significance of the hot end of the white dwarf luminosity function and the role played by the bolometric corrections of hydrogen-rich white dwarfs at high effective temperatures. We use the most up-to-date stellar evolutionary models and implement a full description of the observational selection biases to obtain realistic simulations of the observed white dwarf population. Our theoretical results are compared with the luminosity function of hot white dwarfs obtained from the Sloan Digital Sky Survey (SDSS), for both DA and non-DA white dwarfs. We find that the theoretical results are in excellent agreement with the observational data for the population of white dwarfs with hydrogen deficient atmospheres (non-DA white dwarfs). For the population of white dwarfs with hydrogen-rich atmospheres (white dwarfs of the DA class), our simulations show some discrepancies with the observations for ...

Torres, S; Krzesinski, J; Kleinman, S J

2014-01-01

214

Magneto-optical effects in magnetic white dwarfs  

International Nuclear Information System (INIS)

The impact of magnetic birefringence or magneto-optical effects on the line spectra of magnetic white dwarfs is investigated using model calculations. It is shown that at fields at which the line splitting resembles that of an overlapping Zeeman triplet, the central depth of the p component can be increased by as much as 50 per cent. Changes are also found in polarization near the central component. (author)

1981-01-01

215

Diffusion processes in Ap-stars and white dwarfs  

International Nuclear Information System (INIS)

Diffusion processes in a completely ionized multicomponent plasma are studied. The transport phenomena for ions as well : as for electrons are considered. The transport coefficients are derived by two-polynomial Chapman-Enskog technique. The effect of the magnetic field on transport phenomena is taken into account. Detailed comparison of the obtained results with those of other authors is made. Some possible astrophysical consequences of the theory (surface layers of Ap-stars and white dwarfs) are briefly discussed in conclusion

1982-01-01

216

Model atmospheres of DA white dwarfs. I. Calculation method  

Science.gov (United States)

The method used to calculate DA-type white dwarf model atmospheres is described. For solution of the equation of radiative transfer Feautrier's method is used. Temperature corrections are determined by Rybicki's method. Besides, the method of integral equations proposed by Böhm-Vitense is discussed. The method of taking into account convection and the way to determine an appropriate starting convective model are described.

Galdikas, A.

217

Weak magnetic fields in white dwarfs and their direct progenitors?  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We have carried out a re-analysis of polarimetric data of central stars of planetary nebulae, hot subdwarfs, and white dwarfs taken with FORS1 (FOcal Reducer and low dispersion Spectrograph) on the VLT (Very Large Telescope), and added a large number of new observations in order to increase the sample. A careful analysis of the observations using only one wavelength calibration for the polarimetrically analysed spectra and for all positions of the retarder plate of the spect...

Jordan, S.; Bagnulo, S.; Landstreet, J.; Fossati, L.; Valyanin, G. G.; Monin, D.; Wade, G. A.; Werner, K.; O Toole, S. J.

2012-01-01

218

Luminosity limits on white dwarfs in a Galactic shroud  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We place observational constraints on a recently proposed Galactic population, dubbed the shroud (Gyuk & Gates 1999, Gates & Gyuk 2001). The shroud would be a very thick Galactic disk of low luminosity objects, most likely old white dwarfs, proposed to explain the optical depth seen in microlensing surveys towards the Magellanic clouds. The shroud is a simple alternative to the lenses being distributed in a classical, near-spherical dark halo; the advantage of the shroud is ...

Holopainen, Janne; Flynn, Chris

2004-01-01

219

GD 154: White dwarf with multi- and monoperiodic pulsation  

Directory of Open Access Journals (Sweden)

Full Text Available We present the white dwarf GD 154 as an example where either monoperiodic or multiperiodic pulsation were found at different epochs. The mono-multi-monoperiodic stage seems to alternate. Many questions have been raised. Is this behaviour connected to the evolution of DAV stars? How often does it happen? Is there any regularity in this change of the pulsational behaviour or is it irregular?

Bognár Zs.

2013-03-01

220

White Dwarf based evaluation of the GALEX absolute calibration  

CERN Document Server

This paper describes a revised photometric calibration of the \\emph{Galaxy Evolution Explorer} magnitudes, based on measurements of DA white dwarfs. The photometric magnitudes of white dwarfs measured by \\emph{GALEX} are compared to predicted magnitudes based on independent spectroscopic data (108 stars) and alternately to \\emph{IUE} UV fluxes of the white dwarfs (218 stars). The results demonstrate a significant non-linear correlation and small offset between archived \\emph{GALEX} fluxes and observed and predicted UV fluxes for our sample. The primary source of non-linearity may be due to detector dead time corrections for brighter stars, but it should be noted that there was a predicted non-linearity in the fainter stars as well. Sample expressions are derived which 'correct' observed \\emph{GALEX} magnitudes to an absolute magnitude scale that is linear with respect, and directly related, to the \\emph{Hubble Space Telescope} photometric scale. These corrections should be valid for stars dimmer than magnitud...

Camarota, L

2013-01-01

 
 
 
 
221

The double-layered chemical structure in DB white dwarfs  

CERN Multimedia

We study the structure and evolution of white dwarf stars with helium-rich atmospheres (DB) in a self-consistent way with the predictions of time-dependent element diffusion. Our treatment of diffusion includes gravitational settling and chemical and thermal diffusion. OPAL radiative opacities for arbitrary metallicity and carbon-and oxygen-rich compositions are employed. Emphasis is placed on the evolution of the diffusion-modeled double-layered chemical structure. This structure, which is characterized by a pure helium envelope atop an intermediate remnant shell rich in helium, carbon and oxygen, is expected for pulsating DB white dwarfs, assuming that they are descendants of hydrogen-deficient PG1159 post-AGB stars. We find that, depending on the stellar mass, if DB white dwarf progenitors are formed with a helium content smaller than \\approx 10^-3 M_*, a single-layered configuration is expected to emerge during the DB pulsation instability strip. We also explore the consequences of diffusively evolving ch...

Althaus, L G

2004-01-01

222

Long-term evolution of accreting carbon white dwarfs  

International Nuclear Information System (INIS)

The long-term evolution of an accreting carbon white dwarf of 1.2 M/sub sun/ is investigated. It is found that for a mass accretion rate of 10-9 M/sub sun/ yr-1 a degenerate-helium-shell flash develops, whereas for an accretion rate of 5 x 1010 M/sub sun/ yr-1 a central carbon flash develops. An upper bound to the mass accretion rate for which a strong helium-shell flash occurs is estimated to be 3 x 10-8 M/sub sun/ yr-1, a value which is nearly independent of the initial mass of the carbon white dwarf. A weak helium-shell flash is indicated for mass accretion rates in excess of about 3 x 10-8 M/sub sun/ yr-1, while carbon ignition at high density (rhoapprox. =1010 g cm-3), perhaps, leading to a strong helium-shell flash is indicated for high-mass white dwarfs accreting at rates below about 5 x 10-10 M/sub sun/ yr-1. The results are discussed in the context of the nova and Type I supernova phenomena

1980-12-01

223

Theoretical radial pulsation analyses of DA white dwarfs  

Energy Technology Data Exchange (ETDEWEB)

A linear, nonadiabatic, radial pulsation analysis of DA white dwarfs in the range of effective temperatures: 7 X 10/sup 3/white dwarfs and suggest that these oscillations should be observable in white dwarfs just to the blue and just to the red of the observational blue and red edges of the ZZ Ceti nonradial variables.

Starrfield, S. (Los Alamos National Lab., NM); Cox, A.N.; Hodson, S.W.; Clancy, S.P.

1983-06-15

224

Discovery of two new accreting pulsating white dwarf stars  

CERN Document Server

We report the discovery of two new accreting pulsating white dwarf stars amongst the cataclysmic variables of the Sloan Digital Sky Survey: SDSSJ074531.91+453829.5 and SDSSJ091945.10+085710.0. We observe high amplitude non-sinusoidal variations of 4.5-7% at a period close to 1230s in the optical light curves of SDSSJ074531.91+453829.5 and a low amplitude variation of 0.7-1.6% near 260s in the light curves of SDSSJ091945.10+085710.0. We infer that these optical variations are a consequence of nonradial g-mode pulsations in the accreting primary white dwarfs of these cataclysmic variables. However we cannot rule out the remote possibility that the 260s period could be the spin period of the accreting white dwarf SDSSJ091945.10+085710.0. We also uncovered a non-variable SDSSJ171145.08+301320.0 during our search; our two observing runs exclude any pulsation related periodicities in the range of 85-1400s with an amplitude greater than or equal to 0.5%. This discovery paper brings the total number of known accretin...

Mukadam, Anjum S; Szkody, P; Aungwerojwit, A; Howell, Steve B; Fraser, O J; Silvestri, N M

2007-01-01

225

White Dwarfs as a Source of Constraints on Exotic Physics  

International Nuclear Information System (INIS)

In this paper we briefly review main ideas underlying the constraints on exotic physics coming from Astrophysics already used by the others. Next we present a new bound coming from the White Dwarf cooling. Such stringent bound is possible due to accurate measurements offered by astroseismology . Specifically we consider the G117-B15A pulsating white dwarf (ZZ Ceti star) for which the speed of the period increase has been accurately measured for its fundamental oscillation mode. It has been claimed that this mode detected in G117-B15A is perhaps the most stable oscillation ever recorded in the optical band. Then we review our result concerning the bounds on compactification scale in the theory with large extra dimensions according to Arkani-Hamed, Dimopoulos and Dvali. Because an additional channel of energy loss (Kaluza-Klein gravitons) would speed up the cooling rate, one is able to use the aforementioned stability to derive a bound on compactification scale. We find the lower bound on compactification scale to be Ms > 14.3 TeV/c2 which is more stringent than solar or red-giant bounds, as well as the bound coming from LEP. In final section we point out that pulsating hot ''pre-White Dwarf'' PG 1159-035 (GW Virginis) whose oscillation period increases at the rate of the order of magnitude larger than predicted could be a promising object for further investigations. (author)

2001-11-01

226

Constraining the neutrino magnetic dipole moment from white dwarf pulsations  

CERN Document Server

Pulsating white dwarf stars can be used as astrophysical laboratories to constrain the properties of weakly interacting particles. Comparing the cooling rates of these stars with the expected values from theoretical models allows us to search for additional sources of cooling due to the emission of axions, neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive an upper bound to the neutrino magnetic dipole moment using an estimate of the rate of period change of the pulsating DB white dwarf star PG 1351+489. By comparing the theoretical rate of change of period expected for this star with the rate of change of period with time of PG 1351+489, we assess the possible existence of additional cooling by neutrinos with magnetic dipole moment. Our models suggest the existence of some additional cooling in this pulsating DB white dwarf, consistent with a non-zero magnetic dipole moment. Our upper limit for the neutrino magnetic dipole moment is somewhat less restrictive than, but still compat...

Córsico, Alejandro H; Bertolami, Marcelo M Miller; Kepler, S O; García-Berro, Enrique

2014-01-01

227

Evidence for Terrestrial Planetary System Remnants at White Dwarfs  

CERN Multimedia

The last several years have brought about a dynamic shift in the view of exoplanetary systems in the post-main sequence, perhaps epitomized by the evidence for surviving rocky planetary bodies at white dwarfs. Coinciding with the launch of the Spitzer Space Telescope, both space- and ground-based data have supported a picture whereby asteroid analogs persist at a significant fraction of cool white dwarfs, and are prone to tidal disruption when passing close to the compact stellar remnant. The ensuing debris can produce a detectable infrared excess, and the material gradually falls onto the star, polluting the atmosphere with heavy elements that can be used to determine the bulk composition of the destroyed planetary body. Based on the observations to date, the parent bodies inferred at white dwarfs are best described as asteroids, and have a distinctly rocky composition similar to material found in the inner Solar System. Their minimum masses are typical of large asteroids, and can approach or exceed the mass...

Farihi, J

2010-01-01

228

Towards an Understanding of the Atmospheres of Cool White Dwarfs  

CERN Document Server

Cool white dwarfs with Teff < 6000 K are the remnants of the oldest stars that existed in our Galaxy. Their atmospheres, when properly characterized, can provide valuable information on white dwarf evolution and ultimately star formation through the history of the Milky Way. Understanding the atmospheres of these stars requires joined observational effort and reliable atmosphere modeling. We discuss and analyze recent observations of the near-ultraviolet (UV) and near-infrared (IR) spectrum of several cool white dwarfs including DQ/DQp stars showing carbon in their spectra. We present fits to the entire spectral energy distribution (SED) of selected cool stars, showing that the current pure-hydrogen atmosphere models are quite reliable, especially in the near-UV spectral region. Recently, we also performed an analysis of the coolest known DQ/DQp stars investigating further the origin of the C2 Swan bands-like spectral features that characterize the DQp stars. We show that the carbon abundances derived for ...

Kowalski, Piotr M; Holberg, Jay; Leggett, Sandy

2012-01-01

229

White-dwarf-based evaluation of the GALEX absolute calibration  

Science.gov (United States)

This paper describes a revised photometric calibration of the Galaxy Evolution Explorer (GALEX) magnitudes, based on measurements of DA white dwarfs. The photometric magnitudes of white dwarfs measured by GALEX are compared to predicted magnitudes based on independent spectroscopic data (108 stars) and alternately to International Ultraviolet Explorer (IUE) ultraviolet (UV) fluxes of the white dwarfs (218 stars). The results demonstrate a significant non-linear correlation and small offset between archived GALEX fluxes and observed and predicted UV fluxes for our sample. The primary source of non-linearity may be due to detector dead time corrections for brighter stars, but it should be noted that there was a predicted non-linearity in the fainter stars as well. Sample expressions are derived which `correct' observed GALEX magnitudes to an absolute magnitude scale that is linear with respect, and directly related, to the Hubble Space Telescope photometric scale. These corrections should be valid for stars dimmer than magnitudes 9.3 and 10.5 in the near-UV and far-UV, respectively, and brighter than magnitude 17.5 in both.

Camarota, L.; Holberg, J. B.

2014-03-01

230

Observations and Origins of the Hot DQ White Dwarf Stars  

Science.gov (United States)

Of the two hundred or so white dwarf stars showing spectroscopic signatures of carbon (the DQs), the hottest dozen are unique in having atmospheres actually dominated by carbon and oxygen. Furthermore, many of these hot DQ white dwarfs are photometrically variable, and a large fraction show Zeeman splitting in their spectral lines, revealing the presence of magnetic fields. Based on theoretical pulsational stability studies, pulsations have been proposed to explain the observed brightness variations. However, based on our observations of the hot DQs with the SOAR telescope and using archival data, I will present evidence that the hot DQs are the result of CO-core white dwarf mergers that were not massive enough to explode as Type Ia supernovae. In this scenario, the variable hot DQs are magnetic rotators whose magnetic fields, fast rotation rates, and atmospheric composition are all explained by the merger. As such the masses of the hot DQs will place lower limits on the mass of the proposed channel of sub-Chandrasekhar double-degenerate Type Ia SNe. And their field strengths and spin periods will provide observational constraints for models of double-degenerate mergers and magnetic field generation.

Dunlap, Bart H.

2014-01-01

231

Atmospheric parameters and carbon abundance for hot DB white dwarfs  

CERN Document Server

Atmospheric parameters for hot DB (helium atmosphere) white dwarfs near effective temperatures of 25000K are extremely difficult to determine from optical spectroscopy. This is particularly unfortunate, because this is the range of variable DBV or V777 Her stars. Accurate atmospheric parameters are needed to help or confirm the asteroseismic analysis of these objects. Another important aspect is the new class of white dwarfs - the hot DQ - detected by Dufour et al. (2007), with spectra dominated by carbon lines. The analysis shows that their atmospheres are pure carbon. The origin of these stars is not yet understood, but they may have an evolutionary link with the hotter DBs as studied here. Our aim is to determine accurate atmospheric parameters and element abundances and study the implications for the evolution white dwarfs of spectral classes DB and hot DQ. High resolution UV spectra of five DBs are studied with model atmospheres. We determine stellar parameters and abundances or upper limits of C and Si....

Koester, Detlev; Gänsicke, Boris T

2014-01-01

232

Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid  

CERN Multimedia

3D model atmospheres for giants, dwarfs, and white dwarfs, computed with the CO5BOLD code and part of the CIFIST grid, have been used for spectroscopic and asteroseismic studies. Unlike existing plane-parallel 1D structures, these simulations predict the spatially and temporally resolved emergent intensity so that granulation can be analysed, which provides insights on how convective energy transfer operates in stars. The wide range of atmospheric parameters of the CIFIST 3D simulations (3600 < Teff (K) < 13,000 and 1 < log g < 9) allows the comparison of convective processes in significantly different environments. We show that the relative intensity contrast is correlated with both the Mach and Peclet numbers in the photosphere. The horizontal size of granules varies between 3 and 10 times the local pressure scale height, with a tight correlation between the factor and the Mach number of the flow. Given that convective giants, dwarfs, and white dwarfs cover the same range of Mach and Peclet numb...

Tremblay, P -E; Freytag, B; Steffen, M; Caffau, E

2013-01-01

233

Detection of a white dwarf companion to the white dwarf binary SDSSJ125733.63+542850.5  

CERN Document Server

SDSSJ125733.63+542850.5 (hereafter SDSSJ1257+5428) is a white dwarf from the Sloan Digital Sky Survey recently shown to exhibit high-amplitude radial velocity variations on a period of 4.56 hours suggesting that it has either a neutron star or black-hole binary companion. At a distance of only 48 pc, this would make it the closest remnant of a supernova known and imply that such systems are common in our Galaxy and others. Here we present optical spectroscopy that shows that the companion star in SDSSJ1257+5428 is in fact another white dwarf. SDSSJ1257+5428's spectrum is thus a composite, with narrow line cores from a cool, low mass white dwarf (7000 K; 0.2 Msun), and broad wings from its hotter, high-mass companion (10,000 K; > 1 Msun). We present evidence that suggests that the high-mass star is rapidly rotating with v sin i = 500 to 1000 km/s. This suggests that the most recent phase of mass transfer was long-lasting and stable as against the usually-assumed common envelope phase. Within the constraints se...

Marsh, T R; Steeghs, D; Southworth, J; Koester, D; Harris, V; Merry, L

2010-01-01

234

Revisiting the Impact of Axions in the Cooling of White Dwarfs  

CERN Document Server

It has been shown that the shape of the luminosity function of white dwarfs can be a powerful tool to check for the possible existence of DFSZ-axions. In particular, Isern et al. (2008) showed that, if the axion mass is of the order of a few meV, then the white dwarf luminosity function is sensitive enough to detect their existence. For axion masses of about $m_a > 5$ meV the axion emission can be a primary cooling mechanism for the white dwarf and the feedback of the axion emission into the thermal structure of the white dwarf needs to be considered. Here we present computations of white dwarf cooling sequences that take into account the effect of axion emission in a self consistent way by means of full stellar evolution computations. Then, we study and discuss the impact of the axion emission in the white dwarf luminosity function.

Melendez, Brenda; Althaus, Leandro

2012-01-01

235

Constraining white-dwarf kicks in globular clusters : IV. Retarding Core Collapse  

CERN Document Server

Observations of white dwarfs in the globular clusters NGC 6397 and Omega Centauri indicate that these stars may get a velocity kick during their time as giants. This velocity kick could originate naturally if the mass loss while on the asymptotic giant branch is slightly asymmetric. The kicks may be large enough to dramatically change the radial distribution of young white dwarfs, giving them larger energies than other stars in the cluster. As these energetic white dwarfs travel through the cluster they can impart their excess energy on the other stars in the cluster. A Monte-Carlo simualtion of the white-dwarfs kicks combined with estimate of the phase-space diffusion of the white dwarfs reveals that as the white dwarfs equilibrate, they lose most of their energy in the central region of the cluster. They could possibly mimic the effect of binaries, puffing up the cluster and delaying core collapse.

Heyl, Jeremy S

2009-01-01

236

Detonations in Sub-Chandrasekhar Mass C+O White Dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Explosions of sub-Chandrasekhar-mass white dwarfs are one alternative to the standard Chandrasekhar-mass model of Type Ia supernovae. They are interesting since binary systems with sub-Chandrasekhar-mass primary white dwarfs should be common and this scenario would suggest a simple physical parameter which determines the explosion brightness, namely the mass of the exploding white dwarf. Here we perform one-dimensional hydrodynamical simulations, associated post-processing n...

Sim, S. A.; Roepke, F. K.; Hillebrandt, W.; Kromer, M.; Pakmor, R.; Fink, M.; Ruiter, A. J.; Seitenzahl, I. R.

2010-01-01

237

Constraining white-dwarf kicks in globular clusters : II. Observational Significance  

Digital Repository Infrastructure Vision for European Research (DRIVER)

If the winds of an asymptotic-giant-branch stars are sufficiently strong are slightly asymmetric, they can alter the star's trajectory through a globular cluster; therefore, if these winds are asymmetric, one would expect young white dwarfs to be less radially concentrated than either their progenitors or older white dwarfs in globular clusters. This latter effect has recently been observed. Additionally the young white dwarfs should have larger typical velocities than their...

Heyl, Jeremy S.

2007-01-01

238

Constraining white-dwarf kicks in globular clusters : III. Cluster Heating  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Recent observations of white dwarfs in globular clusters indicate that these stars may get a velocity kick during their time as giants. This velocity kick could originate naturally if the mass loss while on the asymptotic giant branch is slightly asymmetric. The kicks may be large enough to dramatically change the radial distribution of young white dwarfs, giving them larger energies than other stars in the cluster. As these energetic white dwarfs travel through the cluster ...

Heyl, Jeremy S.

2008-01-01

239

A new kind of pulsator in the DB valley of white dwarf stars  

Science.gov (United States)

Recent systematic surveys providing massive datasets of white dwarf stars show that there is still a deficit of a factor of 2.5 in the ratio of white dwarfs showing helium absorption lines to those with hydrogen absorption lines in the temperature range 30 000 K Teff < 45 000 K. Since all white dwarfs have to evolve through this temperature range along similar sequences in the color-magnitude diagram, this implies that most of the helium atmosphere white dwarfs evolve into hydrogen atmosphere hot DA stars in that temperature range and then back again to helium atmosphere white dwarfs. I describe possible scenarios for this chameleon-like disguises of white dwarfs with helium dominant atmospheres and a new kind of pulsating white dwarfs predicted to appear near the lower end of the temperature range 45 000-30 000 K from these scenarios. Recently searches have discovered a pulsating DA white dwarf, WD1017-138, in the predicted temperature range. This discovery opens the possibility of a direct test of the explanation for the deficit of helium atmosphere white dwarfs at these temperatures by asteroseismic probing of the atmospheric layers of the hot DAV stars.

Shibahashi, H.

2013-12-01

240

Mass Radius Relation of Strongly Magnetized White Dwarfs and the Effects of Landau Quantization  

CERN Multimedia

We compute static equilibria of White Dwarf stars containing strong poloidal magnetic field, and present the modification of White Dwarf mass-radius relation caused by the magnetic field. We find that a maximum White Dwarf mass of $\\sim 1.9 M_{\\odot}$ may be supported if the interior field is as strong as $\\sim10^{10}$ T. This mass is over 30 % larger than the traditional Chandrasekhar Limit. The equation of state of electron degenerate matter can be strongly modified due to Landau quantization at such high magnetic fields. We find, however, that this does not significantly affect the structure of the White Dwarf.

Bera, Prasanta

2014-01-01

 
 
 
 
241

GASEOUS MATERIAL ORBITING THE POLLUTED, DUSTY WHITE DWARF HE 1349–2305  

International Nuclear Information System (INIS)

We present new spectroscopic observations of the polluted, dusty, helium-dominated atmosphere white dwarf star HE 1349–2305. Optical spectroscopy reveals weak Ca II infrared triplet emission indicating that metallic gas debris orbits and is accreted by the white dwarf. Atmospheric abundances are measured for magnesium and silicon while upper limits for iron and oxygen are derived from the available optical spectroscopy. HE 1349–2305 is the first gas disk-hosting white dwarf star identified among previously known polluted white dwarfs. Further characterization of the parent body polluting this star will require ultraviolet spectroscopy.

2012-05-20

242

GASEOUS MATERIAL ORBITING THE POLLUTED, DUSTY WHITE DWARF HE 1349-2305  

Energy Technology Data Exchange (ETDEWEB)

We present new spectroscopic observations of the polluted, dusty, helium-dominated atmosphere white dwarf star HE 1349-2305. Optical spectroscopy reveals weak Ca II infrared triplet emission indicating that metallic gas debris orbits and is accreted by the white dwarf. Atmospheric abundances are measured for magnesium and silicon while upper limits for iron and oxygen are derived from the available optical spectroscopy. HE 1349-2305 is the first gas disk-hosting white dwarf star identified among previously known polluted white dwarfs. Further characterization of the parent body polluting this star will require ultraviolet spectroscopy.

Melis, Carl; Burgasser, Adam J. [Center for Astrophysics and Space Sciences, University of California, San Diego, CA 92093-0424 (United States); Dufour, P. [Departement de Physique, Universite de Montreal, Montreal, QC H3C 3J7 (Canada); Farihi, J. [Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH (United Kingdom); Bochanski, J. [Massachusetts Institute of Technology, Kavli Institute for Astrophysics and Space Research, Building 37, Room 664B, 77 Massachusetts Avenue, Cambridge, MA 02139 (United States); Parsons, S. G.; Gaensicke, B. T. [Department of Physics, University of Warwick, Coventry CV4 7AL (United Kingdom); Koester, D. [Institut fuer Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel (Germany); Swift, Brandon J., E-mail: cmelis@ucsd.edu [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States)

2012-05-20

243

Magnetic white dwarfs in the Early Data Release of the Sloan Digital Sky Survey  

CERN Document Server

We have identified 7 new magnetic DA white dwarfs in the Early Data Release of the Sloan Digital Sky Survey. Our selection strategy has also recovered all the previously known magnetic white dwarfs contained in the SDSS EDR, KUV03292+0035 and HE0330-0002. Analysing the SDSS fibre spectroscopy of the magnetic DA white dwarfs with our state-of-the-art model spectra, we find dipole field strengths 1.5<=B_d<=63MG and effective temperatures 8500<=Teff<=39000K. As a conservative estimate, we expect that the complete SDSS will increase the number of known magnetic white dwarfs by a factor 3.

Gänsicke, B T; Jordan, S

2002-01-01

244

Gaseous Material Orbiting the Polluted, Dusty White Dwarf HE1349-2305  

CERN Multimedia

We present new spectroscopic observations of the polluted, dusty, helium-dominated atmosphere white dwarf star HE1349-2305. Optical spectroscopy reveals weak CaII infrared triplet emission indicating that metallic gas debris orbits and is accreted by the white dwarf. Atmospheric abundances are measured for magnesium and silicon while upper limits for iron and oxygen are derived from the available optical spectroscopy. HE1349-2305 is the first gas disk-hosting white dwarf star identified amongst previously known polluted white dwarfs. Further characterization of the parent body polluting this star will require ultraviolet spectroscopy.

Melis, Carl; Farihi, J; Bochanski, J; Burgasser, Adam J; Parsons, S; Gaensicke, B; Koester, D; Swift, Brandon

2012-01-01

245

3D-Kinematics of White Dwarfs from the SPY-Project  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a progress report on the kinematical analysis of the entire SPY (ESO SN Ia Progenitor surveY) sample of about one thousand white dwarfs and hot subdwarfs. In a previous study (Pauli et al. 2003, 2006) 398 DA white dwarfs have been analysed already. Here we extend the study to 634 DA white dwarfs. We discuss kinematic criteria for a distinction of thin disk, thick disk and halo populations. This is the largest homogeneous sample of white dwarfs for which accurate 3...

Richter, R.; Heber, U.; Napiwotzki, R.

2006-01-01

246

Absolute Proper Motions to B~22.5 IV. Faint, Low Velocity White Dwarfs and the White Dwarf Population Density Law  

CERN Document Server

The reduced proper motion diagram (RPMD) for a complete sample of faint stars with high accuracy proper motions in the North Galactic Pole field SA57 is investigated. Eight stars with very large reduced proper motions are identified as faint white dwarf candidates. We discriminate these white dwarf candidates from the several times more numerous QSOs based on proper motion and variability. We discuss the implausibility that these stars could be any kind of survey contaminant. If {\\it bona fide} white dwarfs, the eight candidates found here represent a portion of the white dwarf population hitherto uninvestigated by previous surveys by virtue of the faint magnitudes and low proper motions. The newly discovered stars suggest a disk white dwarf scaleheight larger than the values of 250-350 pc typically assumed in assessments of the local white dwarf density. Both a and a more complex maximum likelihood analysis of the spatial distribution of our likely thin disk white dwarfs yield scaleheights of 400-600 pc whi...

Majewski, S R

2001-01-01

247

Pulsation properties of DA white dwarfs: radial mode instabilities  

International Nuclear Information System (INIS)

We have solved the equations describing linear, nonadiabatic, radial pulsations for models of compositionally stratified, evolving, DA white dwarfs. We find a kappa-mechanism, radial mode, instability strip that is caused by the development of a hydrogen partial ionization zone during the evolutionary cooling of our models. Instabilities occur for radial modes with periods, product, in the range 4> or approx. =product> or approx. =0.2 s, with e-folding growth times, tau/sub e/, in the range 2 x 109> or approx. =tau/sub e/ > or approx. =8 x 102 s; the minimum growth times occur in the shortest period unstable modes. Comparison with our previous calculations for nonradial pulsations indicates that the blue edge for the radial instability strip is approx.1600 K hotter than the blue edge of the theoretical ZZ Ceti instability strip, and further, that the maximum instability occurs at temperatures approx.600 K hotter than the blue edge of the theoretical ZZ Ceti instability strip. Our results also indicate that the development of this instability strip is insensitive to the mass of the surface hydrogen layer and to uncertainties in the hydrogen opacities. We demonstrate that existing observations of the DA white dwars are insufficient to determine if such radial pulsations exist in the DA white dwarfs, and we evaluate the prospects for detecting these pulsations in the future

1983-02-15

248

C, O enrichment of the outer layers of a white dwarf  

International Nuclear Information System (INIS)

Evolutionary calculations for a helium white dwarf accreting matter from its companion in a binary system indicate that the outer layers of the dwarf might become enriched with carbon and oxygen. Subsequent accretion onto the dwarf with its enriched envelope could instigate a nova outburst

1980-01-01

249

Meridional circulation in rotating stars. V. Cooling white dwarfs  

International Nuclear Information System (INIS)

In this paper we discuss the large-scale meridonal circulation and concomitant differential rotation in a cooling white dwarf that does not greatly depart from spherical symmetry. It is found that the circulation velocities are utterly negligible in the degenerate interior as well as in the thin nondegenerate envelope, where the presence of a (turbulent) viscous boundary layer allows the velocities to satisfy all of the surface boundary conditions. There are no 1/rho singularities in the meridional flow. The back reaction of the circulatory currents on the overall rotation rate is not very large either, thus implying at most a mild differential rotation when departure from sphericity is small

1983-04-01

250

Diffusion processes in Ap stars and white dwarfs  

International Nuclear Information System (INIS)

Diffusion processes in a fully ionized, multicomponent plasma are investigated in the report. Kinetic phenomena due both to ions and to electrons are considered. The kinetic coefficients are calculated in the two-polynomial approximation of the Chapman--Enskog method. The influence of the magnetic field on transport phenomena is consistently taken into account. The results obtained are compared in detail with the results of other authors. In conclusion, possible astrophysical applications of the theory (surface layers of Ap stars and white dwarfs) are briefly discussed

1982-03-01

251

Magneto-optical effects in magnetic white dwarfs  

International Nuclear Information System (INIS)

The impact on the spectra of magnetic white dwarfs of magneto-optical effects or magnetic birefringence arising from free electrons in the continuum is investigated using model calculations. Linear polarization is the most strongly affected: in the continuum its magnitude is reduced and its sign often reversed, and in the lines its magnitude is greatly reduced. In the outer components of lines, the maximum values of circular polarization are reduced and their positions shifted inwards. Line depths are increased, especially the central component of a Zeeman triplet. (author)

1982-01-01

252

SDSS White Dwarf mass distribution at low effective temperatures  

CERN Document Server

The DA white dwarfs in the Sloan Digital Sky Survey, as analyzed in the papers for Data Releases 1 and 4, show an increase in surface gravity towards lower effective temperatures below 11500 K. We study the various possible explanations of this effect, from a real increase of the masses to uncertainties or deficiencies of the atmospheric models. No definite answer is found but the tentative conclusion is that it is most likely the current description of convection in the framework of the mixing-length approximation, which leads to this effect.

Köster, D; Kleinman, S J; Nitta, A

2008-01-01

253

Properties of Carbon-Oxygen White Dwarf Merger Remnants  

CERN Multimedia

Recent studies have shown that for suitable initial conditions both super- and sub-Chandrasekhar mass carbon-oxygen white dwarf mergers produce explosions similar to observed SNe Ia. The question remains, however, how much fine tuning is necessary to produce these conditions. We performed a large set of SPH merger simulations, sweeping the possible parameter space. We find trends for merger remnant properties, and discuss how our results affect the viability of our recently proposed sub-Chandrasekhar merger channel for SNe Ia.

Zhu, Chenchong; van Kerkwijk, Marten; Wadsley, James

2011-01-01

254

Thermodynamics of nuclear condensates and phase transitions in white dwarfs  

CERN Document Server

We study the thermodynamics of helium at densities relevant for white dwarf physics. We find evidence that, as the temperature is increased, there is first a first order transition between two superconducting phases followed by a second order transition to the normal state. These transitions occur, for realistic densities, at temperatures below the crystallization temperature and the crystalline state is likely to remain as the true ground state of the system. The calculations are performed with a screening but non-dynamical electron background and we comment on the impact of this and other approximations to our result.

Bedaque, Paulo F; Sen, Srimoyee

2012-01-01

255

Thermodynamics of nuclear condensates and phase transitions in white dwarfs  

Science.gov (United States)

We study the thermodynamics of helium at densities relevant for white dwarf physics. We find evidence that, as the temperature is increased, there is first a first-order transition between two superconducting phases followed by a second-order transition to the normal state. These transitions occur, for realistic densities, at temperatures below the crystallization temperature, and the crystalline state is likely to remain as the true ground state of the system. The calculations are performed with a screening but nondynamical electron background, and we comment on the impact of this and other approximations to our result.

Bedaque, Paulo F.; Berkowitz, Evan; Sen, Srimoyee

2014-02-01

256

The ELM Survey. V. Merging Massive White Dwarf Binaries  

CERN Document Server

We present the discovery of 17 low mass white dwarfs (WDs) in short-period P=0.9 Msun companions. If the companions are massive WDs, these four binaries will evolve into stable mass transfer AM CVn systems and possibly explode as underluminous supernovae. If the companions are neutron stars, then these may be milli-second pulsar binaries. These discoveries increase the number of detached, double degenerate binaries in the ELM Survey to 54; 31 of these binaries will merge within a Hubble time.

Brown, Warren R; Prieto, Carlos Allende; Gianninas, A; Kenyon, Scott J

2013-01-01

257

FUSE Observations of the hottest DA white dwarfs  

CERN Multimedia

We report early FUSE observations of the very hot DA white dwarfs PG1342+444 and REJ0558-371. Detection of photospheric absorption lines allows us to estimate the abundances of C, O, Si, P, S and Fe, the first measurements reported for DA stars at such high temperatures. Values of Teff and log g determined for PG1342+444 from the Lyman line series disagree with the results of the standard Balmer line analysis, an issue that requires further investigation.

Barstow, M A; Bannister, N P; Holberg, J B; Hubeny, I

2000-01-01

258

Subluminous B Stars and Progenitors of Helium Core White Dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Subluminous B (sdB) stars can result from stable Roche lobe overflow or common envelope ejection in close binary systems. The companions are either low mass main sequence stars or white dwarfs. We discuss mass determinations for sdB stars from such systems and report the discovery of the bright nearby sdB star, HD 188112, to be a close binary system of unusually low mass (0.23 solar masses). From the mass function it is evident that the companion is a massive compact object ...

Heber, U.

2002-01-01

259

A New Small-Amplitude Variable Hot DQ White Dwarf  

CERN Document Server

We present the discovery of photometric variations in the carbon-dominated atmosphere (hot DQ) white dwarf star SDSS J133710.19-002643.6. We find evidence for two low-amplitude, harmonically-related periodicities at 169 s and 339 s, making it the fastest and smallest amplitude of the four known hot DQ variables and the only variable whose spectrum suggests the presence of hydrogen. The star's fundamental and harmonic amplitudes are roughly equal, and its pulse shape is similar to the other two members of the class with detected harmonics. Like the other variables, it appears relatively stable in frequency and amplitude.

Dunlap, B H; Clemens, J C

2010-01-01

260

Hot DQ Pulsator or Magnetic White Dwarf Binary?  

Science.gov (United States)

We present spectroscopic and photometric observations of a suspected hot carbon atmosphere (hot DQ) white dwarf which has a large-amplitude optical and ultraviolet brightness variation with a period of 84 min. Our observations show that it is not a hot DQ but instead a magnetic DA, and an observed infrared excess and periods of significantly increased luminosity suggest that it might be a polar currently in a low state. We also discuss whether the variability of the hot DQ variable stars might arise from a similar mechanism.

Dunlap, B. H.; Barlow, B. N.; Clemens, J. C.

2013-01-01

 
 
 
 
261

Keck Speckle Imaging of the White Dwarf G29-38 No Brown Dwarf Companion Detected  

CERN Document Server

The white dwarf Giclas 29-38 has attracted much attention due to its largeinfrared excess and the suggestion that excess might be due to a companionbrown dwarf. We observed this object using speckle interferometry at the Kecktelescope, obtaining diffraction-limited resolution (55 milliarcseconds) at Kband, and found it unresolved. Assuming the entire K band excess is due to asingle point-like companion, we place an upper limit on the binary separationof 30 milliarcseconds, or 0.42 AU at the star's distance of 14.1 pc. Thisresult, combined with astroseismological data and other images of G29-38,supports the hypothesis that the source of the near-infrared excess is not acool companion but a dust cloud.

Kuchner, M J; Brown, M E; Kuchner, Marc J.; Koresko, Christopher D.; Brown, Michael E.

1998-01-01

262

HST/STIS spectroscopy of the exposed white dwarf in the short-period dwarf nova EK TrA  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present high resolution Hubble Space Telescope ultraviolet spectroscopy of the dwarf nova EK TrA obtained in deep quiescence. The Space Telescope Imaging Spectrograph data reveal the broad Ly-alpha absorption profile typical of a moderately cool white dwarf, overlayed by numerous broad emission lines of He, C, N, and Si and by a number of narrow absorption lines, mainly of CI and SiII. Assuming a white dwarf mass in the range 0.3-1.4Msun we derive Teff=17500-23400K for th...

Gaensicke, B. T.; Szkody, P.; Sion, E. M.; Hoard, D. W.; Howell, S.; Cheng, F. H.; Hubeny, I.

2001-01-01

263

Three New Eclipsing White-dwarf - M-dwarf Binaries Discovered in a Search for Transiting Planets Around M-dwarfs  

CERN Document Server

We present three new eclipsing white-dwarf / M-dwarf binary systems discovered during a search for transiting planets around M-dwarfs. Unlike most known eclipsing systems of this type, the optical and infrared emission is dominated by the M-dwarf components, and the systems have optical colors and discovery light curves consistent with being Jupiter-radius transiting planets around early M-dwarfs. We detail the PTF/M-dwarf transiting planet survey, part of the Palomar Transient Factory (PTF). We present a Graphics Processing Unit (GPU)-based box-least-squares search for transits that runs approximately 8X faster than similar algorithms implemented on general purpose systems. For the discovered systems, we decompose low-resolution spectra of the systems into white-dwarf and M-dwarf components, and use radial velocity measurements and cooling models to estimate masses and radii for the white dwarfs. The systems are compact, with periods between 0.35 and 0.45 days and semimajor axes of approximately 2 solar radi...

Law, Nicholas M; Street, Rachel; Fulton, Benjamin J; Hillenbrand, Lynne A; Shporer, Avi; Lister, Tim; Baranec, Christoph; Bloom, Joshua S; Bui, Khanh; Burse, Mahesh P; Cenko, S Bradley; Das, H K; Davis, Jack T C; Dekany, Richard G; Filippenko, Alexei V; Kasliwal, Mansi M; Kulkarni, S R; Nugent, Peter; Ofek, Eran O; Poznanski, Dovi; Quimby, Robert M; Ramaprakash, A N; Riddle, Reed; Silverman, Jeffrey M; Sivanandam, Suresh; Tendulkar, Shriharsh

2011-01-01

264

Near-UV absorption in very cool DA white dwarfs  

CERN Document Server

The atmospheres of very cool, hydrogen-rich white dwarfs (Teff <6000 K) are challenging to models because of the increased complexity of the equation of state, chemical equilibrium, and opacity sources in a low-temperature, weakly ionized dense gas. In particular, many models that assume relatively simple models for the broadening of atomic levels and mostly ideal gas physics overestimate the flux in the blue part of their spectra. A solution to this problem that has met with some success is that additional opacity at short wavelengths comes for the extreme broadening of the Lyman alpha line of atomic H by collisions primarily with H2. For the purpose of validating this model more rigorously, we acquired Hubble Space Telescope STIS spectra of 8 very cool white dwarfs (5 DA and 3 DC stars). Combined with their known parallaxes, BVRIJHK and Spitzer IRAC photometry, we analyze their entire spectral energy distribution (from 0.24 to 9.3 micron) with a large grid of model atmospheres and synthetic spectra. We f...

Saumon, D; Kowalski, P M

2014-01-01

265

Calcium and magnesium in the helium white dwarf GD 401  

International Nuclear Information System (INIS)

A calcium abundance (Ca/Heapprox.6--9 x 10-9) has been derived for the DB degenerate object GD 401 using spectrum synthesis techniques and observational data acquired with the SIT spectrograph. From these data we have also derived upper limits for the Mg/He and Fe/He ratios of approx.1 x 10-8 and approx.6 x 10-9, respectively. The errors on these abundances are relatively high (factors of approx.5--10), since we used a cooler model (T/sub e/=8800 K; cf. 9500 K, Greenstein) and abundances of the electron donors (C, N, O, H) which are uncertain because of the lack of observable features. However, two conclusions can be made from this analysis: (1) calcium is deficient relative to the Sun, contrary to the estimates of Liebert et al., and (2) the Mg/Ca ratio limit is much smaller than the solar value, in contrast to the results of the two other well-studied metal-line white dwarfs (Ross 640, van Maanen 2). The second result for GD 401 is at variance with recent diffusion theories for white dwarfs, which generally predict light-to-heavy element ratios larger than the Sun. If more detailed model atmospheres confirm these results, GD 401 may reflect abundance ratios from recently accreted interstellar material which has noncosmic abundances

1980-11-15

266

Search for Metal Pollution in 81 DA White Dwarfs  

CERN Document Server

A total of 81 DA white dwarfs have been observed with the Cosmic Origins Spectrograph on the Hubble Space Telescope in a snapshot program. The targets were selected to be in the $T_{\\rm eff}$ range from 17000 - 25000 K, where optical metal lines become weak and difficult to detect. Because of the strong Si, C, and O resonance lines in the UV, this survey has a sensitivity that is comparable to that of the Keck/VLT searches for CaII K in cooler white dwarfs. These objects also have no convection zone and thus very short diffusion timescales, assuring that accretion is currently ongoing. The spectra have high resolution and in most cases fairly good S/N. About 60% of them show photospheric metal pollution, predominantly of Si, but in some cases additional metals are present. We report the results of a preliminary analysis and discuss the sources of the accreted matter and the possible r\\^ole of radiative levitation.

Koester, Detlev; Girven, Jonathan; Farihi, Jay

2012-01-01

267

White dwarfs as a source of constraints on exotic physics  

CERN Document Server

In this paper we briefly review main ideas underlying the constraints on exotic physics coming from Astrophysics already used by the others. Next we present a new bound coming from the White Dwarf cooling. Such stringent bound is possible due to accurate measurements offered by astro-seismology. Specifically we consider the G117-B15A pulsating white dwarf (ZZ Ceti star) for which the speed of the period increase has been accurately measured for its fundamental oscillation mode. It has been claimed that this mode detected in G117-B15A is perhaps the most stable oscillation ever recorded in the optical band. Then we review our result concerning the bounds on compactification scale in the theory with large extra dimensions according to Arkani-Hamed, Dimopoulos and Dvali (1998). Because an additional channel of energy loss (Kaluza-Klein gravitons) would speed up the cooling rate, one is able to use the aforementioned stability to derive a bound on compactification scale. We find the lower bound on compactificatio...

Malec, B

2001-01-01

268

A spectrophotometric study of some cool white dwarfs  

Science.gov (United States)

Spectrophotometry from the Hale reflector and the IUE of four cool (approximately 7000 K) white dwarfs having large parallaxes is reported. Practically the entire flux is measured, together with the bolometric luminosity, temperature and radius. Stein 2051 is found to be a DC, with an astrometric binary companion of type dM4. The helium-dominated atmosphere of the DC exhibits no metallic lines in the ultraviolet. From the mass-radius relation, the radius of 0.011 earth radius gives a mass of 0.68 solar mass (light-element core) or 0.47 solar mass (iron core); astrometric solutions correspond to either 0.50 or 0.72 solar mass. Ross 627, a DA, exhibits no metals in the ultraviolet. The radius is put between 0.010 and 0.013 earth radius. The star L870-2, a DA, has the best established radius for a white dwarf (unless it itself is an unresolved binary). The radius, which is between 0.016 and 0.020 earth radius, corresponds to a maximum of 0.36 solar mass and minimum of 0.20 solar mass, for a helium core. An 8-hr exposure to investigate the near-ultraviolet spectrum of vMa2 is only partially successful. Within 500 A, the enormously strong line blanketing, probably by Mg II, Mg I and Fe II, reduces the flux by a factor of 100.

Greenstein, J. L.

1983-06-01

269

A spectrophotometric study of some cool white dwarfs  

International Nuclear Information System (INIS)

Spectrophotometry of four cool (approx. 7000 K) white dwarfs with large parallaxes is reported, from the Hale reflector and the IUE. Almost the entire flux is measured, and the bolometric luminosity, temperature and radius. Stein 2051 is a DC, with an astrometric binary companion of type dM4. The helium-dominated atmosphere of the DC shows no metallic lines in the ultraviolet. The radius 0.011 Rsun gives, from the mass-radius relation, a mass of 0.68 Msun (light-element core) or 0.47 Msun (iron core); astrometric solutions correspond either to 0.50 or 0.72 Msun. Ross 627, a DA, shows no metals in the ultraviolet. The radius is between 0.010 and 0.013 Rsun, and its mass is in the range 0.55 to 0.75 Rsun. The star L870-2, a DA, has the best established, larger than average radius for a white dwarf (unless it is itself an unresolved binary). The radius, in the range 0.016 to 0.020 Rsun, corresponds to a maximum 0.36 Msun, and a minimum 0.20 Msun, for a helium core. An 8-hr exposure to study the near-ultraviolet spectrum of vMa2 proved only partially successful. (author)

1983-01-01

270

SGRs and AXPs: Massive Rotating White Dwarfs versus Magnetars  

CERN Multimedia

The recent observations of SGR 0418+5729 offer an authentic Rosetta Stone for deciphering the energy source of Soft Gamma Ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs). The "magnetar" model, appeals to a yet untested new energy source in astrophysical systems: bulk magnetic energy. It leads for SGR 0418+5729 to results in contradiction with observations. It is shown how a consistent model for SGRs and AXPs can be expressed in terms of canonical physics and astrophysics within massive, fast rotating, and highly magnetized white dwarfs. The pioneering work of B. Paczynski (1990) on 1E 2259+586 is extended and further developed to describe the observed properties of all known SGRS and AXPs by assuming spin-down powered massive, fast rotating, and highly magnetized white dwarfs. Within this model, we obtain the theoretical prediction for the lower limit of the first time derivative of the rotational period of SGR 0418+5729, $\\dot{P} \\geq L_X P^3/(4\\pi^2 I)=1.18\\times 10^{-16}$ s/s being $L_X$ and $P$ th...

Malheiro, Manuel; Ruffini, Remo

2011-01-01

271

Luminosity limits on white dwarfs in a Galactic shroud  

CERN Document Server

We place observational constraints on a recently proposed Galactic population, dubbed the shroud (Gyuk & Gates 1999, Gates & Gyuk 2001). The shroud would be a very thick Galactic disk of low luminosity objects, most likely old white dwarfs, proposed to explain the optical depth seen in microlensing surveys towards the Magellanic clouds. The shroud is a simple alternative to the lenses being distributed in a classical, near-spherical dark halo; the advantage of the shroud is that it would compose only a fraction of a dark halo's total mass. In this paper, we argue that stars of the Galactic shroud would be detectable in the recent proper motion survey of Oppenheimer et al. (2001) if their absolute luminosities were brighter than M_R_59F = 19.4 or approximately M_V = 18.6. We adopt a range of simple models of the shroud's kinematics and morphology, and the colours and luminosities of its white dwarfs; via Monte-Carlo simulations, we predict the numbers expected in the Oppenheimer et al. survey, which wo...

Holopainen, J; Holopainen, Janne; Flynn, Chris

2004-01-01

272

Trigonometric parallaxes of high velocity halo white dwarf candidates  

CERN Multimedia

The status of 38 halo white dwarf candidates identified by Oppenheimer et al. (2001) has been intensively discussed by various authors. In analyses undertaken to date, trigonometric parallaxes are crucial missing data. Distance measurements are mandatory to kinematically segregate halo object from disk objects and hence enable a more reliable estimate of the local density of halo dark matter residing in such objects. We present trigonometric parallax measurements for 15 candidate halo white dwarfs (WDs) selected from the Oppenheimer et al. (2001) list. We observed the stars using the ESO 1.56-m Danish Telescope and ESO 2.2-m telescope from August 2001 to July 2004. Parallaxes with accuracies of 1--2 mas were determined yielding relative errors on distances of $\\sim5$% for 6 objects, $\\sim12$% for 3 objects, and $\\sim20$% for two more objects. Four stars appear to be too distant (probably farther than 100 pc) to have measurable parallaxes in our observations. Distances, absolute magnitudes and revised space ve...

Ducourant, C; Hambly, N C; Oppenheimer, B R; Hawkins, M R S; Rapaport, M; Modolo, J; Lecampion, J F

2007-01-01

273

DA WHITE DWARFS OBSERVED IN THE LAMOST PILOT SURVEY  

Energy Technology Data Exchange (ETDEWEB)

A total of {approx}640, 000 objects from the LAMOST pilot survey have been publicly released. In this work, we present a catalog of DA white dwarfs (DAWDs) from the entire pilot survey. We outline a new algorithm for the selection of white dwarfs (WDs) by fitting Sersic profiles to the Balmer H{beta}, H{gamma}, and H{delta} lines of the spectra, and calculating the equivalent width of the Ca II K line. Two thousand nine hundred sixty-four candidates are selected by constraining the fitting parameters and the equivalent width of the Ca II K line. All the spectra of candidates are visually inspected. We identify 230 DAWDs (59 of which are already included in the Villanova and SDSS WD catalogs), 20 of which are DAWDs with non-degenerate companions. In addition, 128 candidates are classified as DAWDs/subdwarfs, which means the classifications are ambiguous. The result is consistent with the expected DAWD number estimated based on the LEGUE target selection algorithm.

Zhang Yueyang; Deng Licai; Liu Chao; Carrell, Kenneth; Yang Fan; Gao Shuang; Xu Yan; Li Jing; Zhang Haotong; Zhao Yongheng; Luo Ali; Bai Zhongrui; Yuan Hailong [Key Lab for Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 (China); Lepine, Sebastien [Department of Astrophysics, Division of Physical Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY (United States); Newberg, Heidi Jo; Carlin, Jeffrey L. [Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180 (United States); Jin Ge [University of Science and Technology of China, Hefei 230026 (China)

2013-08-01

274

Nucleosynthesis in Classical Novae CO vs. ONe White Dwarfs  

CERN Document Server

Detailed nucleosynthesis in the ejecta of classical novae has been determined for a grid of hydrodynamic nova models. The reported 14 evolutionary sequences, followed from the onset of accretion up to the explosion and ejection stages, span a range of CO and ONe white dwarf masses (0.8-1.35 $M_o$) and mixing levels between the accreted envelope and the underlying white dwarf core (25-75%). The synthesis of each isotope, from ${}^1$H to ${}^{40}$Ca, is discussed along with its sensitivity to model parameters. Special emphasis is given to isotopes such as ${}^{13}$C, ${}^{15}$N, and ${}^{17}$O, which may account for a significant fraction of their Galactic content. Production of the radioactive isotopes ${}^7$Be, ${}^{22}$Na, and ${}^{26}$Al is also analyzed, since they may provide a direct test of the thermonuclear runaway model through their $\\gamma$-ray emission. The resulting elemental yields reproduce fairly well the spectroscopic abundance determinations of several well studied classical novae.

José, J; Jose, Jordi; Hernanz, Margarita

1997-01-01

275

Lyman-alpha wing absorption in cool white dwarf stars  

CERN Multimedia

Kowalski & Saumon (2006) identified the missing absorption mechanism in the observed spectra of cool white dwarf stars as the Ly-alpha red wing formed by the collisions between atomic and molecular hydrogen and successfully explained entire spectra of many cool DA-type white dwarfs. Owing to the important astrophysical implications of this issue, we present here an independent assessment of the process. For this purpose, we compute free-free quasi-molecular absorption in Lyman-alpha due to collisions with H and H2 within the one-perturber, quasi-static approximation. Line cross-sections are obtained using theoretical molecular potentials to describe the interaction between the radiating atom and the perturber. The variation of the electric-dipole transition moment with the interparticle distance is also considered. Six and two allowed electric dipole transitions due to H-H and H-H2 collisions, respectively, are taken into account. The new theoretical Lyman-alpha line profiles are then incorporated in our ...

Rohrmann, R D; Kepler, S O

2010-01-01

276

LIMB-DARKENING COEFFICIENTS FOR ECLIPSING WHITE DWARFS  

Energy Technology Data Exchange (ETDEWEB)

We present extensive calculations of linear and nonlinear limb-darkening coefficients as well as complete intensity profiles appropriate for modeling the light-curves of eclipsing white dwarfs. We compute limb-darkening coefficients in the Johnson-Kron-Cousins UBVRI photometric system as well as the Large Synoptic Survey Telescope (LSST) ugrizy system using the most up to date model atmospheres available. In all, we provide the coefficients for seven different limb-darkening laws. We describe the variations of these coefficients as a function of the atmospheric parameters, including the effects of convection at low effective temperatures. Finally, we discuss the importance of having readily available limb-darkening coefficients in the context of present and future photometric surveys like the LSST, Palomar Transient Factory, and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The LSST, for example, may find {approx}10{sup 5} eclipsing white dwarfs. The limb-darkening calculations presented here will be an essential part of the detailed analysis of all of these systems.

Gianninas, A.; Strickland, B. D.; Kilic, Mukremin [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019 (United States); Bergeron, P., E-mail: alexg@nhn.ou.edu, E-mail: benstrickland@ou.edu, E-mail: kilic@ou.edu, E-mail: bergeron@astro.umontreal.ca [Departement de Physique, Universite de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada)

2013-03-20

277

Radius constraints from high-speed photometry of 20 low-mass white dwarf binaries  

CERN Document Server

We carry out high-speed photometry on 20 of the shortest-period, detached white dwarf binaries known and discover systems with eclipses, ellipsoidal variations (due to tidal deformations of the visible white dwarf), and Doppler beaming. All of the binaries contain low-mass white dwarfs with orbital periods less than 4 hr. Our observations identify the first eight tidally distorted white dwarfs, four of which are reported for the first time here, which we use to put empirical constraints on the mass-radius relationship for extremely low-mass (<0.30 Msun) white dwarfs. We also detect Doppler beaming in several of these binaries, which confirms the high-amplitude radial-velocity variability. All of these systems are strong sources of gravitational radiation, and long-term monitoring of those that display ellipsoidal variations can be used to detect spin-up of the tidal bulge due to orbital decay.

Hermes, J J; Kilic, Mukremin; Gianninas, A; Chote, Paul; Sullivan, D J; Winget, D E; Bell, Keaton J; Falcon, R E; Winget, K I; Mason, Paul A; Harrold, Samuel T; Montgomery, M H

2014-01-01

278

Spitzer Observations of White Dwarfs: the Missing Planetary Debris Around DZ Stars  

CERN Multimedia

We report a Spitzer/IRAC search for infrared excesses around white dwarfs, including 14 newly-observed targets and 16 unpublished archived stars. We find a substantial infrared excess around two warm white dwarfs --- J220934.84+122336.5 and WD 0843+516, the latter apparently being the hottest white dwarf known to display a close-in dust disk. Extending previous studies, we find that the fraction of white dwarfs with dust disks increases as the star's temperature increases; for stars cooler than 10,000 K, even the most heavily polluted ones do not have ~1000 K dust. There is tentative evidence that the dust disk occurrence is correlated with the volatility of the accreted material. In the Appendix, we modify a previous analysis to show that Poynting-Robertson drag might play an important role in transferring materials from a dust disk into a white dwarf's atmosphere.

Xu, S

2011-01-01

279

New Pulsating DB White Dwarf Stars from the Sloan Digital Sky Survey  

CERN Document Server

We are searching for new He atmosphere white dwarf pulsators (DBVs) based on the newly found white dwarf stars from the spectra obtained by the Sloan Digital Sky Survey. DBVs pulsate at hotter temperature ranges than their better known cousins, the H atmosphere white dwarf pulsators (DAVs or ZZ Ceti stars). Since the evolution of white dwarf stars is characterized by cooling, asteroseismological studies of DBVs give us opportunities to study white dwarf structure at a different evolutionary stage than the DAVs. The hottest DBVs are thought to have neutrino luminosities exceeding their photon luminosities (Winget et al. 2004), a quantity measurable through asteroseismology. Therefore, they can also be used to study neutrino physics in the stellar interior. So far we have discovered nine new DBVs, doubling the number of previously known DBVs. Here we report the new pulsators' lightcurves and power spectra.

Nitta, A; Krzesínski, J; Kepler, S O; Metcalfe, T S; Mukadam, Anjum S; Mullally, Fergal; Nather, R E; Sullivan, Denis J; Thompson, Susan E; Winget, D E

2008-01-01

280

Binaries discovered by the SPY project. IV. Five single-lined DA double white dwarfs  

CERN Document Server

We present results from our ongoing follow-up observations of double white dwarf binaries detected in the ESO SN Ia Progenitor SurveY (SPY). We discuss our observing strategy and data analysis and present the orbital solutions of five close double white dwarf binaries: HE0320-1917, HE1511-0448, WD0326-273, WD1013-010 and WD1210+140. Their periods range from 0.44 to 3.22 days. In none of these systems we find any spectral lines originating from the companion. This rules out main sequence companions and indicates that the companion white dwarfs are significantly older and cooler than the bright component. Infrared photometry suggests the presence of a cool, helium-rich white dwarf companion in the binary WD0326-273. We briefly discuss the consequences of our findings for our understanding of the formation and evolution of double white dwarfs.

Nelemans, G; Karl, C; Marsh, T R; Voss, B; Roelofs, G; Izzard, R G; Montgomery, M; Reerink, T; Christlieb, N; Reimers, D

2005-01-01

 
 
 
 
281

A search for kilogauss magnetic fields in white dwarfs and hot subdwarf stars  

CERN Multimedia

We present new results of a survey for weak magnetic fields among DA white dwarfs with inclusion of some brighter hot subdwarf stars. We have detected variable circular polarization in the Halpha line of the hot subdwarf star Feige 34 (SP: sdO). From these data, we estimate that the longitudinal magnetic field of this star varies from -1.1 +/- 3.2 kG to +9.6 +/- 2.6 kG, with a mean of about +5 kG and a period longer than 2 h. In this study, we also confirm the magnetic nature of white dwarf WD1105-048 and present upper limits of kilogauss longitudinal magnetic fields of 5 brightest DA white dwarfs. Our data support recent finding that 25% of white dwarfs have kilogauss magnetic fields. This frequency also confirms results of early estimates obtained using the magnetic field function of white dwarfs.

Valyavin, G; Fabrika, S; Reisenegger, A; Wade, G A; Han, I; Monin, D; Han, Inwoo

2006-01-01

282

A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes  

CERN Document Server

NGC 6791 is a well studied open cluster1 that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (~8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (~6 Gyr) are significantly different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallise and phase separation of the main constituents of the core of a typical white dwarf, 12C and 16O, is expected to occur. This sequence of events is expected to introduce significant delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, solving the age discrepancy for NGC 6791.

García-Berro, Enrique; Althaus, Leandro G; Renedo, Isabel; Lorén-Aguilar, Pablo; Córsico, Alejandro H; Rohrmann, René D; Salaris, Maurizio; Isern, Jordi

2010-01-01

283

The WIRED Survey. IV. New Dust Disks from the McCook & Sion White Dwarf Catalog  

CERN Document Server

We have compiled photometric data from the Wide-field Infrared Survey Explorer All Sky Survey and other archival sources for the more than 2200 objects in the original McCook & Sion Catalog of Spectroscopically Identified White Dwarfs. We applied color-selection criteria to identify 28 targets whose infrared spectral energy distributions depart from the expectation for the white dwarf photosphere alone. Seven of these are previously known white dwarfs with circumstellar dust disks, five are known central stars of planetary nebulae, and six were excluded for being known binaries or having possible contamination of their infrared photometry. We fit white dwarf models to the spectral energy distributions of the remaining ten targets, and find seven new candidates with infrared excess suggesting the presence of a circumstellar dust disk. We compare the model dust disk properties for these new candidates with a comprehensive compilation of previously published parameters for known white dwarfs with dust disks....

Hoard, D W; Wachter, Stefanie; Leisawitz, David T; Cohen, Martin

2013-01-01

284

Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements  

CERN Document Server

A photometric and spectroscopic analysis of 152 cool white dwarf stars is presented. The discovery of 7 new DA white dwarfs, 2 new DQ white dwarfs, 1 new magnetic white dwarf, and 3 weak magnetic white dwarf candidates, is reported, as well as 19 known or suspected double degenerates. The photometric energy distributions, the Halpha line profiles, and the trigonometric parallax measurements are combined and compared to model atmosphere calculations to determine the effective temperature and the radius of each object, and also to constrain the atmospheric composition. New evolutionary sequences with C/O cores with thin and thick hydrogen layers are used to derive masses and ages. We confirm the existence of a range in Teff between 5000 and 6000K where almost all white dwarfs have H-rich atmospheres. There is little evidence for mixed H/He dwarfs, with the exception of 2 He-rich DA stars, and 5 C2H white dwarfs which possibly have mixed H/He/C atmospheres. The DQ sequence terminates near 6500K, below which they...

Bergeron, P; Ruiz, M T; Ruiz, Maria Teresa

2002-01-01

285

The progenitors of magnetic white dwarfs in open clusters  

Science.gov (United States)

Context. White dwarfs are the final stages of stellar evolution for most stars in the galaxy and magnetic white dwarfs (MWDs) represent at least ten percent of the whole sample. According to the fossil-field hypothesis magnetic fields are remnants of the previous stages of evolution. However, population synthesis calculations are unable to reproduce the MWD sample without binary interaction or inclusion of a population of progenitor with unobservable small-scale fields. Aims: One necessary ingredient in population synthesis is the initial-to-final-mass relation (IFMR) which describes the mass-loss processes during the stellar evolution. When white dwarfs are members of open clusters, their evolutionary histories can be assessed through the use of cluster properties. This enables an independent way of determining the mass of their progenitors. The discovery of the magnetic WD 0836+201 in the Praesepe cluster prompted the question whether magnetic fields affect the IFMR. In this work we investigate this suggestion through investigations of all three known MWDs in open clusters. Methods: We assess the cluster membership by correlating the proper-motion of MWDs with the cluster proper-motion and by analyzing the candidates spectroscopically with our magnetic model spectra in order to estimate the effective temperature and radii. Furthermore, we use mass-radius relations and evolutionary models to constrain the histories of the probable cluster members. Results: We identified SDSS J085523.87+164059.0 to be a proper-motion member of Praesepe. We also included the data of the formerly identified cluster members NGC 6819-8, WD 0836+201 and estimated the mass, cooling age and the progenitor masses of the three probable MWD members of open clusters. According to our analysis, the newly identified cluster member SDSS J085523.87+164059.0 is an ultra-massive MWD of mass 1.12 ± 0.11 M?. Conclusions: We increase the sample of MWDs with known progenitor masses to ten, with the rest of the data coming from the common proper motion binaries. Our investigations show that, when effects of the magnetic fields are included in the diagnostics, the estimated properties of these cluster MWDs do not show evidence for deviations from the IFMR. Furthermore we estimate the precision of the magnetic diagnostics which would be necessary to determine quantitatively whether magnetism has any effect on the mass-loss.

Külebi, B.; Kalirai, J.; Jordan, S.; Euchner, F.

2013-06-01

286

The white dwarf luminosity function II. The efect of the measurement errors and other biases  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The disc white dwarf luminosity function is an important tool for studying the solar neighbourhood, since it allows the determination of several Galactic parameters, the most important one being the age of the Galactic disc. However, only the 1/Vmax method has been employed so far for observationally determining the white dwarf luminosity function, whereas for other kind of luminosity functions several other methods have been frequently used. Moreover, the procedures to determine the white ...

Torres Gil, Santiago; Garci?a-berro Montilla, Enrique; Isern Vilaboy, Jordi

2007-01-01

287

THE KEPLER LIGHT CURVE OF THE UNIQUE DA WHITE DWARF BOKS 53856  

International Nuclear Information System (INIS)

The faint (g = 16.9) hot white dwarf BOKS 53856 was observed by the Kepler Mission in short cadence mode during mid-2009. Analysis of these observations reveals a highly stable modulation with a period of 6.1375 hr and a 2.46% half-amplitude. The folded light curve has an unusual shape that is difficult to explain in terms of a binary system containing an unseen companion more luminous than an L0 brown dwarf. Optical spectra of BOKS 53856 show a Teff = 34,000 K, log g = 8.0 DA white dwarf. There are few, if any, known white dwarfs in this temperature range exhibiting photometric variations similar to those we describe. A magnetic spin-modulated white dwarf model can in principle explain the light curve, an interpretation supported by spectral observations of the H? line showing evidence of Zeeman splitting.

2011-08-01

288

THE ACCRETING WHITE DWARF IN SS CYGNI REVEALED  

International Nuclear Information System (INIS)

We have carried out a combined Hubble Space Telescope (HST/GHRS) and Far Ultraviolet Spectroscopic Explorer (FUSE) analysis of the prototype dwarf nova SS Cygni during quiescence. The FUSE and HST spectra were obtained at comparable times after outburst and have matching flux levels where the two spectra overlap. In our synthetic spectral analysis, we have used SS Cygni's accurate HST fine guidance sensor parallax giving d = 166 pc, a newly determined mass for the accreting white dwarf (WD) of Mwd = 0.81 Msun (lower than the previous widely used 1.2 Msun) and the reddening (EB-V) values 0.04 and 0.07 derived from the 2175 A absorption feature in the IUE LWP spectra. From the best-fit model solutions to the combined HST + FUSE spectral energy distribution, we find that the WD is reaching a temperature Teff? 45,000-55,000 K in quiescence, assuming log(g) = 8.3 with a solar composition accreted atmosphere. The exact temperature of the WD depends on the reddening assumed and the inclusion of a quiescent low mass accretion rate accretion disk. Accretion disk models alone fit badly in the FUSE range while, and if we take the distance to be a free parameter, the only accretion disk model that fits well is for a discordant distance of at least several hundred parsecs and an accretion rate (?10-8 Msun yr-1), which is unacceptably high for a dwarf nova in quiescence. We discuss the implications of the WD's temperature on the time-averaged accretion rate and long-term compressional heating models.

2010-06-20

289

Type Ia Supernovae from Merging White Dwarfs I. Prompt Detonations  

CERN Multimedia

Merging white dwarfs are a possible progenitor of Type Ia supernovae (SNe Ia). While it is not entirely clear if and when an explosion is triggered in such systems, numerical models suggest that a detonation might be initiated before the stars have coalesced to form a single compact object. Here we study such "peri-merger" detonations by means of numerical simulations, modeling the disruption and nucleosynthesis of the stars until the ejecta reach the coasting phase. Synthetic light curves and spectra are generated for comparison with observations. Three models are considered with primary masses 0.96 Msun, 1.06 Msun, and 1.20 Msun. Of these, the 0.96 Msun dwarf merging with an 0.81 Msun companion, with a Ni56 yield of 0.58 Msun, is the most promising candidate for reproducing common SNe Ia. The more massive mergers produce unusually luminous SNe Ia with peak luminosities approaching those attributed to "super-Chandrasekhar" mass SNe Ia. While the synthetic light curves and spectra of some of the models resemb...

Moll, Rainer; Kasen, Daniel; Woosley, Stan

2013-01-01

290

Advection-Dominated Accretion Onto Weakly-Magnetized White Dwarfs  

CERN Document Server

The boundary layers of weakly-magnetized white dwarfs (WDs) accreting at rates <=10^16 g/s are radially extended, hot, optically-thin, and they advect some of their internally-dissipated energy (Narayan & Popham 1993). Motivated by this, I construct here idealized spectral models of an Advection-Dominated Accretion Flow (ADAF) around a WD, for application to quiescent Dwarf Novae (DN). The Bremsstrahlung cooling of the gas in the ADAF, with temperatures ranging from a few keV to a few tens of keV, can account for the X-ray emission properties of quiescent DN. If the energy advected by the flow is thermalized in the WD atmosphere, the resulting emission from the entire stellar surface (blackbody of temperature T_eff ~ 5 eV) outshines the X-ray luminosity substantially. This extreme-UV component provides a flux in the 0.055-0.28 keV band which is sufficient to power the strong HeII lambda4686 emission lines of quiescent DN by photoionization of the disk material. Reprocessing of the ADAF X-ray emission b...

Menou, K

2000-01-01

291

On the Orbits of Companions to White Dwarfs  

CERN Multimedia

The ultimate fates of binary companions to stars (including whether the companion survives and the final orbit of the binary) are of interest in light of an increasing number of recently discovered, low-mass companions to white dwarfs (WDs). In this Letter, we study the evolution of a two-body system wherein the orbit adjusts due to structural changes in the primary, dissipation of orbital energy via tides, and mass loss during the giant phases. For companions ranging from Jupiter's mass to ~0.3 Msun and primaries ranging from 1--3 Msun, we determine the minimum initial semimajor axis required for the companion to avoid engulfment by the primary during post-main-sequence evolution, typically several times the maximum radius on the Asymptotic Giant Branch. We present regions in secondary mass and orbital period space where an engulfed companion might be expected to survive the common envelope phase (CEP), and compare with known M dwarf+WD short-period binaries. Finally, we note that engulfed Earth-like planets...

Nordhaus, J

2012-01-01

292

THE FREQUENCY OF DEBRIS DISKS AT WHITE DWARFS  

Energy Technology Data Exchange (ETDEWEB)

We present near- and mid-infrared photometry and spectroscopy from PAIRITEL, IRTF, and Spitzer of a metallicity-unbiased sample of 117 cool, hydrogen-atmosphere white dwarfs (WDs) from the Palomar-Green survey and find five with excess radiation in the infrared, translating to a 4.3{sup +2.7} {sub -1.2}% frequency of debris disks. This is slightly higher than, but consistent with the results of previous surveys. Using an initial-final mass relation, we apply this result to the progenitor stars of our sample and conclude that 1-7 M {sub Sun} stars have at least a 4.3% chance of hosting planets; an indirect probe of the intermediate-mass regime eluding conventional exoplanetary detection methods. Alternatively, we interpret this result as a limit on accretion timescales as a fraction of WD cooling ages; WDs accrete debris from several generations of disks for {approx}10 Myr. The average total mass accreted by these stars ranges from that of 200 km asteroids to Ceres-sized objects, indicating that WDs accrete moons and dwarf planets as well as solar system asteroid analogs.

Barber, Sara D.; Patterson, Adam J.; Kilic, Mukremin [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019 (United States); Leggett, S. K. [Gemini Observatory, 670 N. A' ohoku Place, Hilo, HI 96720 (United States); Dufour, P. [Departement de Physique, Universite de Montreal, C.P. 6128, Succursale Centre-Ville, Montreal, Quebec H3C 3J7 (Canada); Bloom, J. S.; Starr, D. L., E-mail: barber@nhn.ou.edu [Department of Astronomy, University of California, Berkeley, CA 94720 (United States)

2012-11-20

293

On the possible observational signatures of white dwarf dynamical interactions  

CERN Document Server

We compute the possible observational signatures of white dwarf dynamical interactions in dense stellar environments. Specifically, we compute the emission of gravitational waves, and we compare it with the sensitivity curves of planned space-borne gravitational wave detectors. We also compute the light curves for those interactions in which a detonation occurs, and one of the stars is destroyed, as well as the corresponding neutrino luminosities. We find that for the three possible outcomes of these interactions - which are the formation of an eccentric binary system, a lateral collision in which several mass transfer episodes occur, and a direct one in which just a single mass transfer episode takes place - only those in which an eccentric binary are formed are likely to be detected by the planned gravitational wave mission eLISA, while more sensitive detectors would be able to detect the signals emitted in lateral collisions. On the other hand, the light curves (and the thermal neutrino emission) of these ...

Aznar-Siguán, G; Magnien, M; Lorén-Aguilar, P

2014-01-01

294

Neutron star formation by collapse of white dwarfs  

International Nuclear Information System (INIS)

Mass-accreting carbon-oxygen white dwarfs become thermally and dynamically unstable when they reach high enough central densities. Carbon ignition at the star's center likely propagates subsonically and, in the case of an initially solid core, leads to collapse if the rate of increase of the core's mass is sufficiently fast. Recent results indicate, however, that solidification of the core induces carbon-oxygen separation. The central regions are then made of pure oxygen while carbon is rejected to lower-density layers. Carbon ignition happens only after neutronization of the central (oxygen) regions. Collapse to a neutron star is then independent from the rate of mass increase and the only possible restrictions are set by the behaviour of the outer, accreted layers. X-ray sources, pulsars and Type I supernovae are likely outcomes of this process. (orig.)

1980-01-01

295

Externally-polluted white dwarfs with dust disks  

CERN Multimedia

We report Spitzer Space Telescope photometry of eleven externally-polluted white dwarfs. Of the nine stars for which we have IRAC photometry, we find that GD 40, GD 133 and PG 1015+161 each has an infrared excess that can be understood as arising from a flat, opaque, dusty disk. GD 56 also has an infrared excess characteristic of circumstellar dust, but a flat-disk model cannot reproduce the data unless there are grains as warm as 1700 K and perhaps not even then. Our data support the previous suggestion that the metals in the atmosphere of GD 40 are the result of accretion of a tidally-disrupted asteroid with a chondritic composition.

Jura, M; Zuckerman, B

2007-01-01

296

The pulsating low-mass He-core white dwarfs  

CERN Document Server

Recent years have witnessed the discovery of many low-mass ($\\lesssim 0.45 M_{\\odot}$) white dwarf (WD) stars --- expected to harbor He cores--- in the field of the Milky Way and in several galactic globular and open clusters. Recently, three pulsating objects of this kind have been discovered: SDSS J1840+6423, SDSS J1112+1117, and SDSS J1518+0658. Motivated by these very exciting findings, and in view of the valuable asteroseismological potential of these objects, we present here the main outcomes of a detailed theoretical study on the seismic properties of low-mass He-core WDs based on fully evolutionary models representative of these objects. This study is aimed to provide a theoretical basis from which to interpret present and future observations of variable low-mass WDs.

Córsico, Alejandro H; Romero, Alejandra D

2013-01-01

297

Astrometric determination of white dwarf radial velocities with Gaia?  

CERN Multimedia

Usually, the determination of radial velocities of stars relies on the shift of spectral lines by the Doppler effect. Russel & Atkinson (1931) and Oort (1932) already noted that due to the large proper motion and parallax of the white dwarf (WD) van Maanen 2, a determination of the perspective acceleration of the proper motion would provide a direct astrometric determination of the radial velocity which is independent of the gravitational redshift. If spectroscopic redshift measurements of Halpha and Hbeta NLTE cores exist, a purely astrometric determination would allow disentangling the gravitational redshift from the Doppler shift. The best instrument for measuring the tiny perspective acceleration is the Gaia satellite of the European Space Agency, aiming at absolute astrometric measurements of one billion stars down to 20th magnitude with unprecedented accuracy. At 15th magnitude, the predicted angular accuracy of Gaia is about 20 micro-arcseconds. In this article, we estimate whether it is possible t...

Jordan, Stefan

2012-01-01

298

Thermonuclear explosions of rapidly rotating white dwarfs - II. Detonations  

CERN Document Server

Context: Superluminous type Ia supernovae (SNe Ia) may be explained by super-Chandrasekhar-mass explosions of rapidly rotating white dwarfs (WDs). In a preceding paper, we showed that the deflagration scenario applied to rapidly rotating WDs generates explosions that cannot explain the majority of SNe Ia. Aims: Rotation of the progenitor star allows super-Chandrasekhar-mass WDs to form that have a shallower density stratification. We use simple estimates of the production of intermediate and iron group elements in pure detonations of rapidly rotating WDs to assess their viability in explaining rare SNe Ia. Methods: We numerically construct WDs in hydrostatic equilibrium that rotate according to a variety of rotation laws. The explosion products are estimated by considering the density stratification and by evaluating the result of hydrodynamics simulations. Results: We show that a significant amount of intermediate mass elements is produced for theoretically motivated rotation laws, even for prompt detonation...

Pfannes, J M M; Schmidt, W

2009-01-01

299

Cosmological Physics with Black Holes (and Possibly White Dwarfs)  

CERN Document Server

The notion that microparsec-scale black holes can be used to probe gigaparsec-scale physics may seem counterintuitive, at first. Yet, the gravitational observatory LISA will detect cosmologically-distant coalescing pairs of massive black holes, accurately measure their luminosity distance and help identify an electromagnetic counterpart or a host galaxy. A wide variety of new black hole studies and a gravitational version of Hubble's diagram become possible if host galaxies are successfully identified. Furthermore, if dark energy is a manifestation of large-scale modified gravity, deviations from general relativistic expectations could become apparent in a gravitational signal propagated over cosmological scales, especially when compared to the electromagnetic signal from a same source. Finally, since inspirals of white dwarfs into massive black holes at cosmological distances may permit pre-merger localizations, we suggest that careful monitoring of these events and any associated electromagnetic counterpart...

Menou, Kristen; Kocsis, Bence

2008-01-01

300

THE ELM SURVEY. V. MERGING MASSIVE WHITE DWARF BINARIES  

Energy Technology Data Exchange (ETDEWEB)

We present the discovery of 17 low-mass white dwarfs (WDs) in short-period (P {<=} 1 day) binaries. Our sample includes four objects with remarkable log g {approx_equal} 5 surface gravities and orbital solutions that require them to be double degenerate binaries. All of the lowest surface gravity WDs have metal lines in their spectra implying long gravitational settling times or ongoing accretion. Notably, six of the WDs in our sample have binary merger times <10 Gyr. Four have {approx}>0.9 M{sub Sun} companions. If the companions are massive WDs, these four binaries will evolve into stable mass transfer AM CVn systems and possibly explode as underluminous supernovae. If the companions are neutron stars, then these may be millisecond pulsar binaries. These discoveries increase the number of detached, double degenerate binaries in the ELM Survey to 54; 31 of these binaries will merge within a Hubble time.

Brown, Warren R.; Kenyon, Scott J. [Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138 (United States); Kilic, Mukremin; Gianninas, A. [Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK, 73019 (United States); Allende Prieto, Carlos, E-mail: wbrown@cfa.harvard.edu, E-mail: skenyon@cfa.harvard.edu, E-mail: kilic@ou.edu, E-mail: alexg@nhn.ou.edu, E-mail: callende@iac.es [Instituto de Astrofisica de Canarias, E-38205, La Laguna, Tenerife (Spain)

2013-05-20

 
 
 
 
301

Tidal disruption of white dwarfs by intermediate mass black holes  

Directory of Open Access Journals (Sweden)

Full Text Available Modeling ultra-close encounters between a white dwarf and a spinning, intermediate mass black hole requires a full general relativistic treatment of gravity. This paper summarizes results from such a study. Our results show that the disruption process and prompt accretion of the debris strongly depend on the magnitude and orientation of the black hole spin. On the other hand, the late-time accretion onto the black hole follows the same decay, ? ? ?t?5/3, estimated from Newtonian gravity disruption studies. The spectrum of the fallback material peaks in the soft X-rays and sustains Eddington luminosity for 1–3?yrs after the disruption. The orientation of the black hole spin has also a profound effect on how the outflowing debris obscures the central region. The disruption produces a burst of gravitational radiation with characteristic frequencies of ?3.2?Hz and strain amplitudes of ?10?18 for galactic intermediate mass black holes.

Bode T.

2012-12-01

302

Detecting white dwarf binaries in Mock LISA Data Challenge 3  

International Nuclear Information System (INIS)

We present a strategy for detecting gravitational wave signals from the Galactic white dwarf binaries in the Mock LISA Data Challenge 3 (MLDC3) and estimate their parameters. Our method is based on the matched filtering in the form of the F-statistic. We perform the search on three-dimensional space (sky coordinate and frequency of gravitational wave) below 3 mHz and include the fourth parameter (frequency derivative) at high frequencies. A template bank is used to search for the strongest signal in the data, then we remove it and repeat the search until we do not have signals in the data above a preselected threshold. For the template bank, we construct an optimal grid that realizes the best lattice covering with a constraint such that the nodes of the grid coincide with the Fourier frequencies. This enables the use of the fast Fourier transform algorithm to calculate the F-statistic.

2009-10-21

303

LOW MASS STELLAR AND SUBSTELLAR COMPANIONSHIP AMONG NEARBY WHITE DWARFS  

Directory of Open Access Journals (Sweden)

Full Text Available This work is a systematic, deep search for stellar and substellar objects orbiting nearby white dwarfs (WDs. The scienti c interest spans testing speci c predictions of common envelope evolutionary phase models, as well as providing constraints to planetary system evolution in advanced stages of its parent star (Livio & Soker 1984; Willes & Wu 2005. Additionally, we seek to explore the hypothesis about the origin of metal lines in hydrogen WDs, produced by the accretion of tidal disturbed asteroidal or cometary material. This could be linked to the presence of a undetected substellar object that perturbed the orbits of these asteroids or comets (Debes & Sigurdsson 2002. Here, we show preliminary results of this project.

M. Radiszcz

2009-01-01

304

Calibrating cosmological chronometers: White dwarf masses via astrometry  

International Nuclear Information System (INIS)

In an effort to increase the number of accurate dynamical masses for white dwarfs (WDs), we have begun an initiative using Hubble Space Telescope's Fine Guidance Sensors (FGS) to resolve suspected binary WDs. With the increasing number of WD trigonometric parallaxes becoming available via CTIO's and the USNO's ongoing parallax programs, we have targeted objects that are overluminous at V magnitude and are presumably unresolved multiple systems. A few targets were selected because of spectral anomalies or possible perturbations evident in the residuals of the trigonometric parallax solutions. A total of 16 HST orbits were designated to this program and 12 are completed. Of the eleven WDs observed thus far (one object was observed twice), all but one were unresolved. Analysis of a recent orbit's data indicate a pair was resolved with a separation of 70 mas and a delta V magnitude of ?1.4. Coupled with astrometric data from the USNO parallax program, we have obtained preliminary constraints on component masses.

2009-06-01

305

White Dwarfs and Hot Subdwarfs as Seen from FUSE  

CERN Multimedia

We present a small collection of FUSE spectra representative of the main spectral classes found in white dwarf stars. In addition, we also discuss another family of hot evolved stars, that of the hot subdwarfs. Both families belong to the chemically peculiar stars, and it is thought that a complex interplay of competing processes such as gravitational settling, ordinary diffusion, radiative levitation, weak stellar winds, and accretion is responsible for the rich variety of atmospheric compositions observed in those objects. FUSE is playing a key role in the current quest for establishing a coherent theory of the spectral evolution of these stars as it allows the determination of the patterns of heavy element abundances at a significantly higher level of accuracy than has been possible before on the basis of optical or UV observations. We also briefly present some fascinating FUV light curves of a handful of pulsating subdwarf B stars, thus illustrating the unique potential of FUSE for asteroseismological stu...

Fontaine, G

2004-01-01

306

LISA astronomy of double white dwarf binary systems  

CERN Multimedia

The Laser Interferometer Space Antenna (LISA) will provide the largest observational sample of (interacting) double white dwarf binaries, whose evolution is driven by radiation reaction and other effects, such as tides and mass transfer. We show that, depending on the actual physical parameters of a source, LISA will be able to provide very different quality of information: for some systems LISA can test unambiguously the physical processes driving the binary evolution, for others it can simply detect a binary without allowing us to untangle the source parameters and therefore shed light on the physics at work. We also highlight that simultaneous surveys with GAIA and/or optical telescopes that are and will become available can radically improve the quality of the information that can be obtained.

Stroeer, A; Nelemans, G

2005-01-01

307

The ELM Survey. V. Merging Massive White Dwarf Binaries  

Science.gov (United States)

We present the discovery of 17 low-mass white dwarfs (WDs) in short-period (P double degenerate binaries. All of the lowest surface gravity WDs have metal lines in their spectra implying long gravitational settling times or ongoing accretion. Notably, six of the WDs in our sample have binary merger times systems and possibly explode as underluminous supernovae. If the companions are neutron stars, then these may be millisecond pulsar binaries. These discoveries increase the number of detached, double degenerate binaries in the ELM Survey to 54; 31 of these binaries will merge within a Hubble time. Based on observations obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.

Brown, Warren R.; Kilic, Mukremin; Allende Prieto, Carlos; Gianninas, A.; Kenyon, Scott J.

2013-05-01

308

Dynamical Tides in Compact White Dwarf Binaries: Influence of Rotation  

CERN Document Server

Tidal interactions play an important role in the evolution and ultimate fate of compact white dwarf (WD) binaries. Not only do tides affect the pre-merger state (such as temperature and rotation rate) of the WDs, but they may also determine which systems merge and which undergo stable mass transfer. In this paper, we attempt to quantify the effects of rotation on tidal angular momentum transport in binary stars, with specific calculations applied to WD stellar models. We incorporate the effect of rotation using the traditional approximation, in which the dynamically excited gravity waves within the WDs are transformed into gravito-inertial Hough waves. The Coriolis force has only a minor effect on prograde gravity waves, and previous results predicting the tidal spin-up and heating of inspiraling WDs are not significantly modified. However, rotation strongly alters retrograde gravity waves and inertial waves, with important consequences for the tidal spin-down of accreting WDs. We identify new dynamical tidal...

Fuller, Jim

2014-01-01

309

THE ELM SURVEY. V. MERGING MASSIVE WHITE DWARF BINARIES  

International Nuclear Information System (INIS)

We present the discovery of 17 low-mass white dwarfs (WDs) in short-period (P ? 1 day) binaries. Our sample includes four objects with remarkable log g ? 5 surface gravities and orbital solutions that require them to be double degenerate binaries. All of the lowest surface gravity WDs have metal lines in their spectra implying long gravitational settling times or ongoing accretion. Notably, six of the WDs in our sample have binary merger times 0.9 M? companions. If the companions are massive WDs, these four binaries will evolve into stable mass transfer AM CVn systems and possibly explode as underluminous supernovae. If the companions are neutron stars, then these may be millisecond pulsar binaries. These discoveries increase the number of detached, double degenerate binaries in the ELM Survey to 54; 31 of these binaries will merge within a Hubble time.

2013-05-20

310

SEISMOLOGY OF A MASSIVE PULSATING HYDROGEN ATMOSPHERE WHITE DWARF  

International Nuclear Information System (INIS)

We report our observations of the new pulsating hydrogen atmosphere white dwarf SDSS J132350.28+010304.22. We discovered periodic photometric variations in frequency and amplitude that are commensurate with nonradial g-mode pulsations in ZZ Ceti stars. This, along with estimates for the star's temperature and gravity, establishes it as a massive ZZ Ceti star. We used time-series photometric observations with the 4.1 m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1 m data, to discover the photometric variability. The light curve of SDSS J132350.28+010304.22 shows at least nine detectable frequencies. We used these frequencies to make an asteroseismic determination of the total mass and effective temperature of the star: M* = 0.88 ± 0.02 M? and Teff = 12, 100 ± 140 K. These values are consistent with those derived from the optical spectra and photometric colors.

2012-10-01

311

The Halo White Dwarf WD 0346+246 Revisited  

CERN Multimedia

The extreme helium-rich atmospheric composition determined for the halo white dwarf WD 0346+246 is reexamined. This solution is shown to be improbable from an astrophysical point of view when accretion of hydrogen and metals from the interstellar medium is taken into account. An alternate solution is proposed where hydrogen and helium are present in the atmospheric regions in equal amounts. The best fit at Teff=3780 K, log g=8.34, and N(He)/N(H)=1.3 is achieved by including in the model calculations a bound-free opacity from the Lyman edge associated with the so-called dissolved atomic levels of the hydrogen atom, or pseudo-continuum opacity.

Bergeron, P

2001-01-01

312

A New Small-amplitude Variable Hot DQ White Dwarf  

Science.gov (United States)

We present the discovery of photometric variations in the carbon-dominated atmosphere (hot DQ) white dwarf star SDSS J133710.19-002643.6. We find evidence for two low-amplitude, harmonically related periodicities at 169 s and 339 s, making it the fastest and smallest amplitude of the four known hot DQ variables and the only variable whose spectrum suggests the presence of hydrogen. The star's fundamental and harmonic amplitudes are roughly equal, and its pulse shape is similar to the other two members of the class with detected harmonics. Like the other variables, it appears relatively stable in frequency and amplitude. Based on observations at the SOAR Telescope, a collaboration between CPNq-Brazil, NOAO, UNC, and MSU.

Dunlap, B. H.; Barlow, B. N.; Clemens, J. C.

2010-09-01

313

On the incidence of weak magnetic fields in DA white dwarfs  

CERN Document Server

Context: About 10% of white dwarfs have magnetic fields with strength in the range between about 10^5 and 3x10^8 G. It is not known whether the remaining white dwarfs are not magnetic, or if they have a magnetic field too weak to be detected with the techniques adopted in the large surveys. Aims. We describe the results of the first survey specifically devised to clarify the detection frequency of kG-level magnetic fields in cool DA white dwarfs. Methods: Using the FORS1 instrument of the ESO VLT, we have obtained Balmer line circular spectropolarimetric measurements of a small sample of cool (DA6 - DA8) white dwarfs. Using FORS and UVES archive data, we have also revised numerous white dwarf field measurements previously published in the literature. Results: We have discovered an apparently constant longitudinal magnetic field of \\sim9.5 kG in the DA6 white dwarf WD2105-820. This star is the first weak-field white dwarf that has been observed sufficiently to roughly determine the characteristics of its field...

Landstreet, J D; Valyavin, G G; Fossati, L; Jordan, S; Monin, D; Wade, G

2012-01-01

314

The Initial-Final Mass Relation among White Dwarfs in Wide Binaries  

CERN Document Server

We present the initial-final mass relation derived from 10 white dwarfs in wide binaries that consist of a main sequence star and a white dwarf. The temperature and gravity of each white dwarf was measured by fitting theoretical model atmospheres to the observed spectrum using a $\\chi^{2}$ fitting algorithm. The cooling time and mass was obtained using theoretical cooling tracks. The total age of each binary was estimated from the chromospheric activity of its main sequence component to an uncertainty of about 0.17 dex in log \\textit{t} The difference between the total age and white dwarf cooling time is taken as the main sequence lifetime of each white dwarf. The initial mass of each white dwarf was then determined using stellar evolution tracks with a corresponding metallicity derived from spectra of their main sequence companions, thus yielding the initial-final mass relation. Most of the initial masses of the white dwarf components are between 1 - 2 M$_{\\odot}$. Our results suggest a correlation between t...

Zhao, Jingkun; Willson, Lee Anne; Wang, Qian; Zhao, Gang

2011-01-01

315

Remarkable spectral variability on the spin period of the accreting white dwarf in V455 And  

CERN Document Server

We present spin-resolved spectroscopy of the accreting white dwarf binary V455 And. With a suggested spin period of only 67s, it has one of the fastest spinning white dwarfs known. To study the spectral variability on the spin period of the white dwarf, we observed V455 And with 2s integration times, which is significantly shorter than the spin rate of the white dwarf. To achieve this cadence, we used the blue arm of the ISIS spectrograph at the 4.2-m William Herschel Telescope, equipped with an electron multiplying CCD (EMCCD). Strong coherent signals were detected in our time series, which lead to a robust determination of the spin period of the white dwarf (Pspin=67.619 +/- 0.002 s). Folding the spectra on the white dwarf spin period uncovered very complex emission line variations in Hgamma, He I 4472 and He II 4686. We attribute the observed spin phase dependence of the emission line shape to the presence of magnetically controlled accretion onto the white dwarf via accretion curtains, consistent with an ...

Bloemen, S; De Smedt, K; Vos, J; Gänsicke, B T; Marsh, T R; Rodriguez-Gil, P

2012-01-01

316

3D kinematics of white dwarfs from the SPY project. II.  

Science.gov (United States)

We present the kinematics of a sample of 398 DA white dwarfs from the SPY project (ESO SN Ia Progenitor surveY) and discuss kinematic criteria for distinguishing of thin-disk, thick-disk, and halo populations. This is the largest homogeneous sample of white dwarfs for which 3D space motions have been determined. Since the percentage of old stars among white dwarfs is higher than among main-sequence stars, they are presumably valuable tools in studies of old populations, such as the halo and the thick disk. Studies of white-dwarf kinematics can help to determine the fraction of the total mass of our Galaxy contained in the form of thick-disk and halo white dwarfs, an issue which is still under discussion. Radial velocities and spectroscopic distances obtained by the SPY project were combined with our measurements of proper motions to derive 3D space motions. Galactic orbits and further kinematic parameters were computed. We calculated individual errors of kinematic parameters by means of a Monte Carlo error propagation code. Our kinematic criteria for assigning population membership were deduced from a sample of F and G stars taken from the literature, for which chemical criteria can be used to distinguish between a thin-disk, a thick-disk and a halo star. Our kinematic population classification scheme is based on the position in the U-V-velocity diagram, the position in the Jz-eccentricity diagram, and the Galactic orbit. We combined this with age information and found seven halo and 23 thick-disk white dwarfs in this brightness limited sample. Another four rather cool white dwarfs probably also belong to the thick disk. Correspondingly 2% of the white dwarfs belong to the halo and 7% to the thick disk. The mass contribution of the thick-disk white dwarfs is found to be substantial, but is insufficient to account for the missing dark matter.

Pauli, E.-M.; Napiwotzki, R.; Heber, U.; Altmann, M.; Odenkirchen, M.

2006-02-01

317

Limits on the neutrino magnetic dipole moment from the luminosity function of hot white dwarfs  

CERN Document Server

Recent determinations of the white dwarf luminosity function (WDLF) from very large surveys have extended our knowledge of the WDLF to very high luminosities. This, together with the availability of new full evolutionary white dwarf models that are reliable at high luminosities, have opened the possibility of testing particle emission in the core of very hot white dwarfs, where neutrino processes are dominant. We use the available WDLFs from the Sloan Digital Sky Survey and the SuperCOSMOS Sky Survey to constrain the value of the neutrino magnetic dipole moment ($\\mu_\

Bertolami, Marcelo Miguel Miller

2014-01-01

318

Theta Hya: Spectroscopic identification of a second B star + white dwarf binary  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We report the identification, in an Extreme Ultraviolet Explorer (EUVE) spectrum, of a hot white dwarf companion to the 3rd magnitude late-B star Theta Hya (HR3665, HD79469). This is the second B star + white dwarf binary to be conclusively identified; Vennes, Berghofer and Christian (1997), and Burleigh and Barstow (1998) had previously reported the spectroscopic discovery of a hot white dwarf companion to the B5V star y Pup (HR2875). Since these two degenerate stars must h...

Burleigh, M. R.; Barstow, M. A.

1998-01-01

319

On the incidence of weak magnetic fields in DA white dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Context: About 10% of white dwarfs have magnetic fields with strength in the range between about 10^5 and 3x10^8 G. It is not known whether the remaining white dwarfs are not magnetic, or if they have a magnetic field too weak to be detected with the techniques adopted in the large surveys. Aims. We describe the results of the first survey specifically devised to clarify the detection frequency of kG-level magnetic fields in cool DA white dwarfs. Methods: Using the FORS1 ins...

Landstreet, J. D.; Bagnulo, S.; Valyavin, G. G.; Fossati, L.; Jordan, S.; Monin, D.; Wade, G.

2012-01-01

320

Thermohaline Instabilities Induced by Heavy Element Accretion onto White Dwarfs: Consequences on the Derived Accretion Rates  

CERN Document Server

Heavy elements are observed in the atmospheres of many DA and DB white dwarfs, and their presence is attributed to the accretion of matter coming from debris disks. Several authors have deduced accretion rates from the observed abundances, taking into account the mixing induced by the convective zones and the gravitational settling. The obtained values are different for DA and DB white dwarfs. Here we show that an important process was forgotten in all these computations: thermohaline mixing, induced by the inverse $\\mu$-gradient built during the accretion process. Taking this mixing into account leads to an increase of the derived accretion rates, specially for DA white dwarfs, and modifies the conclusions.

Deal, M; Vauclair, G

2012-01-01

 
 
 
 
321

Features of the mass transfer in magnetic cataclysmic variables with fast-rotating white dwarfs  

Science.gov (United States)

The flow structure in magnetic cataclysmic variables was investigated taking into account the effects of strong magnetic field and fast rotation of the white dwarf. We modeled the AE Aqr system as a unique object that has the rotation period of the white dwarf is about 1000 times shorter than the orbital period of the binary system. Observations show that in spite of fast rotation of the white dwarf some part of the stream from the inner Lagrange point comes into the Roche lobe region. We analyzed possible mechanisms preventing material to outflow from the system.

Isakova, Polina; Zhilkin, Andrey; Bisikalo, Dmitry

2014-01-01

322

Analysis of High Resolution White Dwarf Spectra from the ESO SNIa Progenitor Survey (SPY)  

Science.gov (United States)

We have analyzed some 830 white dwarf spectra from the SPY survey. The spectra were fitted with theoretical spectra derived from model atmospheres, yielding the so far largest sample of white dwarfs with known atmospheric parameters that were derived from high resolution spectra. Based on these data, the mass distribution of 632 DA white dwarfs is determined and found to show a somewhat lower peak compared to results from other recent surveys. Furthermore, we report on peculiar objects of different type that are found in the sample. 20 objects are found to have effective temperatures and gravities which place them inside the ZZ Ceti instability strip.

Voss, B.; Koester, D.

2005-07-01

323

The Future is Now: the Formation of Single Low Mass White Dwarfs in the Solar Neighborhood  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Low mass helium-core white dwarfs (M < 0.45 Msun) can be produced from interacting binary systems, and traditionally all of them have been attributed to this channel. However, a low mass white dwarf could also result from a single star that experiences severe mass loss on the first ascent giant branch. A large population of low mass He-core white dwarfs has been discovered in the old metal-rich cluster NGC 6791. There is therefore a mechanism in clusters to produce low mass ...

Kilic, Mukremin; Stanek, K. Z.; Pinsonneault, M. H.

2007-01-01

324

HST/STIS spectroscopy of the exposed white dwarf in the short-period dwarf nova EK TrA  

CERN Multimedia

We present high resolution Hubble Space Telescope ultraviolet spectroscopy of the dwarf nova EK TrA obtained in deep quiescence. The Space Telescope Imaging Spectrograph data reveal the broad Ly-alpha absorption profile typical of a moderately cool white dwarf, overlayed by numerous broad emission lines of He, C, N, and Si and by a number of narrow absorption lines, mainly of CI and SiII. Assuming a white dwarf mass in the range 0.3-1.4Msun we derive Teff=17500-23400K for the primary in EK TrA; Teff=18800K for a canonical mass of 0.6Msun. From the narrow photospheric absorption lines, we measure the white dwarf rotational velocity, v*sin i=200+-100kms. Even though the strong contamination of the photospheric white dwarf absorption spectrum by the emission lines prevents a detailed quantitative analysis of the chemical abundances of the atmosphere, the available data suggest slightly sub-solar abundances. The high time resolution of the STIS data allows us to associate the observed ultraviolet flickering with ...

Gänsicke, B T; Sion, E M; Hoard, D W; Howell, S; Cheng, F H; Hubeny, I

2001-01-01

325

Absolute far-ultraviolet spectrophotometry of hot subluminous stars from Voyager  

Energy Technology Data Exchange (ETDEWEB)

Observations, obtained with the Voyager ultraviolet spectrometers, are presented of absolute fluxes for two well-known hot subluminous stars: BD + 28 deg 4211, an sdO, and G191 - B2B, a hot DA white dwarf. Complete absolute energy distributions for these two stars, from the Lyman limit at 912 A to 1 micron, are given. For BD + 28 deg 4211, a single power law closely represents the entire observed energy distribution. For G191 - B2B, a pure hydrogen model atmosphere provides an excellent match to the entire absolute energy distribution. Voyager absolute fluxes are discussed in relation to those reported from various sounding rocket experiments, including a recent rocket observation of BD + 28 deg 4211. 43 refs.

Holberg, J.B.; Ali, B.; Carone, T.E.; Polidan, R.S. (Arizona, University, Tucson (USA) NASA, Goddard Space Flight Center, Greenbelt, MD (USA))

1991-07-01

326

THE HABITABILITY AND DETECTION OF EARTH-LIKE PLANETS ORBITING COOL WHITE DWARFS  

International Nuclear Information System (INIS)

Since there are several ways planets can survive the giant phase of the host star, we examine the habitability and detection of planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, a planet orbiting at 0.01 AU would remain in the continuous habitable zone (CHZ) for ?8 Gyr. We show that photosynthetic processes can be sustained on such planets. The DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, and hence non-magnetic white dwarfs are compatible with the persistence of complex life. Polarization due to a terrestrial planet in the CHZ of a cool white dwarf (CWD) is 102 (104) times larger than it would be in the habitable zone of a typical M-dwarf (Sun-like star). Polarimetry is thus a viable way to detect close-in rocky planets around white dwarfs. Multi-band polarimetry would also allow us to reveal the presence of a planet atmosphere, providing a first characterization. Planets in the CHZ of a 0.6 M? white dwarf will be distorted by Roche geometry, and a Kepler-11d analog would overfill its Roche lobe. With current facilities a super-Earth-sized atmosphereless planet is detectable with polarimetry around the brightest known CWD. Planned future facilities render smaller planets detectable, in particular by increasing the instrumental sensitivity in the blue.

2012-09-20

327

The Low State Temperature Distribution and First Chemical Abundances of White Dwarfs in Polars  

CERN Multimedia

During the low optical brightness states of AM Herculis systems (polars) when accretion has declined to a very low value, the underlying magnetic white dwarf photosphere can be modelled without the complication of thermal bremstrahlung and cyclotron emission from the luminous accretion column. The far ultraviolet spectra can be modelled with high gravity solar composition photospheres. In this way, I present new temperatures and the first chemical abundance estimates for the white dwarfs in three selected polars from the IUE NEWSIPS archive. For the white dwarf in V834 Cen with Teff = 16,000$K, Si/H = 0.1 solar, C/H = 0.5 solar, for BY Cam, Teff = 17,000 K, Si/H = 0.1, C/H = 5 solar and for RX J1313-32, Teff = 22,000 K, Si/H = 0.1 solar, C/H = 0.1 solar. The temperature distribution of 24 white dwarfs in polars with known temperatures above and below the period gap is compared with the distribution of the white dwarf temperatures in dwarf novae during quiescence. In both cases, the magnetic white dwarfs in po...

Sion, E M

2004-01-01

328

High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia Progenitor Survey. III. DA white dwarfs  

CERN Document Server

The ESO Supernova Ia Progenitor Survey (SPY) took high-resolution spectra of more than 1000 white dwarfs and pre-white dwarfs. About two thirds of the stars observed are hydrogen-dominated DA white dwarfs. Here we present a catalog and detailed spectroscopic analysis of the DA stars in the SPY. Atmospheric parameters effective temperature and surface gravity are determined for normal DAs. Double-degenerate binaries, DAs with magnetic fields or dM companions, are classified and discussed. The spectra are compared with theoretical model atmospheres using a chi^2 fitting technique. Our final sample contains 615 DAs, which show only hydrogen features in their spectra, although some are double-degenerate binaries. 187 are new detections or classifications. We also find 10 magnetic DAs (4 new) and 46 DA+dM pairs (10 new).

Köster, D; Napiwotzki, R; Christlieb, N; Homeier, D; Lisker, T; Reimers, D; Heber, U

2009-01-01

329

An upper limit to the secular variation of the gravitational constant from white dwarf stars  

CERN Multimedia

A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound G'/G ~ -1.8 10^{-12} 1/yr. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function.

García-Berro, Enrique; Torres, Santiago; Althaus, Leandro G; Isern, Jordi

2011-01-01

330

One possible solution of peculiar type Ia supernovae explosions caused by a charged white dwarf  

CERN Multimedia

Recently astrophysics observation reveals the existence of some super luminous type Ia supernovae. One natural explanation of such peculiar phenomenon is to require the progenitor of such supernova to be a highly super-Chandrasekhar mass white dwarf. Along this line, in this paper, we propose a possible mechanism to explain this phenomenon based on charged white dwarf. In particular, by choosing suitable new variables and a representative charge distribution, an analytic solution is obtained. The stability issue is also discussed, remarkably, it turns out that the charged white dwarf configuration can be dynamical stable. Moreover, we investigate the general relativistic effects and it is shown that the general relativistic effects can be negligible when the mass of charged white dwarf below about $3M_\\odot$.

Liu, Helei; Wen, Dehua

2014-01-01

331

Finding habitable earths around white dwarfs with a robotic telescope transit survey  

Science.gov (United States)

I discuss the possibility that white dwarfs might harbor habitable planets at 0.01 AU that arrive there after the red giant phase. These planets would be detectable with ground-based telescopes via deep transits of the star, and a transit survey of the nearest white dwarfs would favor detection of Earth-temperature planets, assuming they could form as close as twice the Roche limit. I show that a robotic survey is required for habitable planet transit detection around white dwarfs due to the large amount of sky coverage, geographical coverage with multiple telescopes, and significant observing time involved. Robotic telescopes such as LCOGT, ATLAS/WIST, and LSST could place interesting limits on the frequency of planets in the 3 Gyr continuously habitable zone of white dwarfs, possibly constraining frequencies as small as 0.1%.

Agol, Eric

2011-03-01

332

A search for radio pulsars around low-mass white dwarfs  

Science.gov (United States)

Low-mass white dwarfs can be produced either in low-mass X-ray binaries by stable mass transfer to a neutron star, or in a common envelope phase with a heavier white dwarf companion. We have searched eight low-mass white dwarf candidates recently identified in the Sloan Digital Sky Survey for radio pulsations from pulsar companions, using the Green Bank Telescope at 340MHz. We have found no pulsations down to flux densities of 0.6-0.8mJykpc-2 and conclude that a given low-mass helium-core white dwarf has a probability of <0.18 +/- 0.05 of being in a binary with a radio pulsar.

van Leeuwen, Joeri; Ferdman, Robert D.; Meyer, Sol; Stairs, Ingrid

2007-02-01

333

A search for radio pulsars around low-mass white dwarfs  

CERN Document Server

Low-mass white dwarfs can either be produced in low-mass X-ray binaries by stable mass transfer to a neutron star, or in a common-envelope phase with a heavier white dwarf companion. We have searched 8 low-mass white dwarf candidates recently identified in the Sloan Digital Sky Survey for radio pulsations from pulsar companions, using the Green Bank Telescope at 340MHz. We have found no pulsations down to flux densities of 0.6-0.8 mJy/kpc^2 and conclude that a given low-mass helium-core white dwarf has a probability of < 0.18+-0.05 of being in a binary with a radio pulsar.

Van Leeuwen, J; Meyer, S; Stairs, I; Leeuwen, Joeri van; Ferdman, Robert D.; Meyer, Sol; Stairs, Ingrid

2006-01-01

334

Constraining white-dwarf kicks in globular clusters : II. Observational Significance  

CERN Document Server

If the winds of an asymptotic-giant-branch stars are sufficiently strong are slightly asymmetric, they can alter the star's trajectory through a globular cluster; therefore, if these winds are asymmetric, one would expect young white dwarfs to be less radially concentrated than either their progenitors or older white dwarfs in globular clusters. This latter effect has recently been observed. Additionally the young white dwarfs should have larger typical velocities than their progenitors. After phase mixing this latter effect is vastly diminished relative to the changes in the spatial distribution of young white dwarfs with kicks, so it is more difficult to detect than the change in the spatial distribution. The most powerful kinematic signature is the change in the eccentricity of the orbits that is revealed through the distribution of the position angles of proper motion.

Heyl, Jeremy S

2007-01-01

335

RX J0648.0--4418: the fastest-spinning white dwarf  

CERN Document Server

RX J0648.0-4418 is a post common-envelope X-ray binary composed of a hot subdwarf and one of the most massive white dwarfs with a dynamical mass measurement (1.28+/-0.05 M_sun). This white dwarf, with a spin period of 13.2 s, rotates more than twice faster than the white dwarf in the cataclysmic variable AE Aqr. The current properties of these two binaries, as well as their future evolution, are quite different, despite both contain a fast-spinning white dwarf. RX J0648.0-4418 could be the progenitor of either a Type Ia supernova or of a non-recycled millisecond pulsars.

Mereghetti, Sandro

2013-01-01

336

One possible solution of peculiar type Ia supernovae explosions caused by a charged white dwarf  

Science.gov (United States)

Recent astrophysics observation reveals the existence of some superluminous type Ia supernovae. One natural explanation of such a peculiar phenomenon is to require the progenitor of such a supernova to be a highly super-Chandrasekhar mass white dwarf. Along this line, in this paper, we propose a possible mechanism to explain this phenomenon based on a charged white dwarf. In particular, by choosing suitable new variables and a representative charge distribution, an analytic solution is obtained. The stability issue is also discussed; remarkably, it turns out that the charged white dwarf configuration can be dynamically stable. In the end, we investigate the general relativistic effects and it is shown that the general relativistic effects can be negligible when the mass of the charged white dwarf is below about 3M?.

Liu, Helei; Zhang, Xiangdong; Wen, Dehua

2014-05-01

337

WhiteDwarf.org - Establishing a permanent endowment for the Whole Earth Telescope  

CERN Document Server

White Dwarf Research Corporation is a 501(c)(3) non-profit organization dedicated to scientific research and public education on topics relevant to white dwarf stars. It was founded in 1999 in Austin, Texas to help fulfill the need for an alternative research center where scarce funding dollars could be used more efficiently, and to provide a direct link between astronomers who study white dwarf stars and the general public. Due to its administrative simplicity, WDRC can facilitate the funding of multi-institutional and international collaborations, provide seamless grant portability, minimize overhead rates, and actively seek non-governmental funding sources. I describe the motivation for, and current status of, one of the long-term goals of WDRC: to establish a permanent endowment for the operation of the Whole Earth Telescope. I pay particular attention to fund-raising efforts through the website at http://WhiteDwarf.org/donate/

Metcalfe, T S

2002-01-01

338

Echoes of a decaying planetary system: the gaseous and dusty disks surrounding three white dwarfs  

CERN Document Server

We have performed a comprehensive ground-based observational program aimed at characterizing the circumstellar material orbiting three single white dwarf stars previously known to possess gaseous disks. Near-infrared imaging unambiguously detects excess infrared emission towards Ton 345 and allows us to refine models for the circumstellar dust around all three white dwarf stars. We find that each white dwarf hosts gaseous and dusty disks that are roughly spatially coincident, a result that is consistent with a scenario in which dusty and gaseous material has its origin in remnant parent bodies of the white dwarfs' planetary systems. We briefly describe a new model for the gas disk heating mechanism in which the gaseous material behaves like a "Z II" region. In this Z II region, gas primarily composed of metals is photoionized by ultraviolet light and cools through optically thick allowed Ca II-line emission.

Melis, C; Albert, L; Klein, B; Zuckerman, B

2010-01-01

339

ECHOES OF A DECAYING PLANETARY SYSTEM: THE GASEOUS AND DUSTY DISKS SURROUNDING THREE WHITE DWARFS  

International Nuclear Information System (INIS)

We have performed a comprehensive ground-based observational program aimed at characterizing the circumstellar material orbiting three single white dwarf stars previously known to possess gaseous disks. Near-infrared imaging unambiguously detects excess infrared emission toward Ton 345 and allows us to refine models for the circumstellar dust around two of the three white dwarf stars. We find that each white dwarf hosts gaseous and dusty disks that are roughly spatially coincident, a result that is consistent with a scenario in which dusty and gaseous material has its origin in remnant parent bodies of the white dwarfs' planetary systems. We briefly describe a new model for the gas disk heating mechanism in which the gaseous material behaves like a 'Z II' region. In this Z II region, gas primarily composed of metals is photoionized by ultraviolet light and cools through optically thick allowed Ca II-line emission.

2010-10-20

340

Arecibo Measurements of Pulsar--White Dwarf Binaries: Evidence for Heavy Neutron Stars  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We summarize constraints on neutron star masses from ongoing timing observations of pulsar-white dwarf binaries at Arecibo. The trend is toward pulsar masses larger than the canonical value of 1.35 solar masses.

Nice, David J.; Splaver, Eric M.; Stairs, Ingrid H.

2004-01-01

 
 
 
 
341

KNOW YOUR NEIGHBORHOOD: A DETAILED MODEL ATMOSPHERE ANALYSIS OF NEARBY WHITE DWARFS  

Energy Technology Data Exchange (ETDEWEB)

We present improved atmospheric parameters of nearby white dwarfs lying within 20 pc of the Sun. The aim of the current study is to obtain the best statistical model of the least-biased sample of the white dwarf population. A homogeneous analysis of the local population is performed combining detailed spectroscopic and photometric analyses based on improved model atmosphere calculations for various spectral types including DA, DB, DC, DQ, and DZ stars. The spectroscopic technique is applied to all stars in our sample for which optical spectra are available. Photometric energy distributions, when available, are also combined to trigonometric parallax measurements to derive effective temperatures, stellar radii, as well as atmospheric compositions. A revised catalog of white dwarfs in the solar neighborhood is presented. We provide, for the first time, a comprehensive analysis of the mass distribution and the chemical distribution of white dwarf stars in a volume-limited sample.

Giammichele, N.; Bergeron, P. [Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation (United States); Dufour, P., E-mail: noemi.giammichele@astro.umontreal.ca, E-mail: pierre.bergeron@astro.umontreal.ca, E-mail: patrick.dufour@astro.umontreal.ca [Departement de Physique, Universite de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada)

2012-04-01

342

The accretion-diffusion scenario for metals in cool white dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We calculated diffusion timescales for Ca, Mg, Fe in hydrogen atmosphere white dwarfs with temperatures between 5000 and 25000 K. With these timescales we determined accretion rates for a sample of 38 DAZ white dwarfs from the recent studies of Zuckerman et al. (2003) and Koester et al. (2005). Assuming that the accretion rates can be calculated with the Bondi-Hoyle formula for hydrodynamic accretion, we obtained estimates for the interstellar matter density around the accre...

Koester, D.; Wilken, D.

2006-01-01

343

Evidence for an external origin of heavy elements in hot DA white dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a series of systematic abundance measurements for 89 hydrogen atmosphere (DA-type) white dwarfs with temperatures spanning 16000-77000K drawn from the FUSE spectral archive. This is the largest study to date of white dwarfs where radiative forces are significant, exceeding our earlier work, based mainly on IUE and HST data, by a factor three. Using heavy element blanketed non-LTE stellar atmosphere calculations, we have addressed the heavy element abundance patter...

Barstow, M. A.; Barstow, J. K.; Casewell, S. L.; Holberg, J. B.; Hubeny, I.

2014-01-01

344

White dwarfs and neutron stars in globular cluster X-ray sources  

International Nuclear Information System (INIS)

It is predicted that globular clusters contain at least as many cataclysmic variables, which contain white dwarfs, as bright X-ray sources, which contain neutron stars. The consequences of the frequent formation of temporarily bound triple systems (resonance scattering) are examined including; the extra enhancement of neutron star capture with respect to white dwarf capture, the considerable probability of actual collisions between stars, and the presence of cataclysmic variables in the outer regions of globular clusters. (U.K.)

1983-02-17

345

Pollution of single white dwarfs by accretion of many small asteroids  

CERN Multimedia

Extrapolating from the solar system's asteroid belt, we propose that externally-contaminated white dwarfs without an infrared excess may be experiencing continuous accretion of gas-phase material that ultimately is derived from the tidal destruction of multiple small asteroids. If this scenario is correct, then observations of metal-polluted white dwarfs may lead to determining the bulk elemental compositions of ensembles of extrasolar minor planets.

Jura, M

2008-01-01

346

Spitzer Observations of the Oldest White Dwarfs in the Solar Neighborhood  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present Spitzer 5-15 micron spectroscopy of one cool white dwarf and 3.6-8 micron photometry of 51 cool white dwarfs with T_eff < 6000 K. The majority of our targets have accurate BVRIJHK photometry and trigonometric parallax measurements available, which enables us to perform a detailed model atmosphere analysis using their optical, near- and mid-infrared photometry with state- of-the-art model atmospheres. We demonstrate that the optical and infrared spectral energy dis...

Kilic, Mukremin; Kowalski, Piotr M.; Reach, William T.; Von Hippel, Ted

2009-01-01

347

On the distance scale of planetary nebulae and white dwarf birth rates  

International Nuclear Information System (INIS)

Arguments are presented which favor an increase of the distance scale of planetary nebulae by 30% compared to the Seaton-Webster scale. The consequences for evolutionary tracks, PN and white dwarf relations, and birth rates are discussed. It is concluded that opposite to Smith jr. (1976) underestimated, and that the proposed change in distance scale of PN brings white dwarf and PN birth rates into almost complete agreement. (orig.)

1977-11-01

348

Supernova 2011fe from an Exploding Carbon-Oxygen White Dwarf Star  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Type Ia supernovae (SNe Ia) have been used empirically as standardized candles to reveal the accelerating universe even though fundamental details, such as the nature of the progenitor system and how the star explodes, remained a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary could be anything from a main sequence star to a red giant, or even another white dwarf. The uncertainty stems from the fact th...

Nugent, Peter E.; Sullivan, Mark; Cenko, S. Bradley; Thomas, Rollin C.; Kasen, Daniel; Howell, D. Andrew; Bersier, David; Bloom, Joshua S.; Kulkarni, S. R.; Kandrashoff, Michael T.; Filippenko, Alexei V.; Silverman, Jeffrey M.; Marcy, Geoffrey W.; Howard, Andrew W.; Isaacson, Howard T.

2011-01-01

349

Accretion of a Terrestrial-Like Minor Planet by a White Dwarf  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present optical and infrared characterization of the polluted DAZ white dwarf GALEX J193156.8+011745. Imaging and spectroscopy from the ultraviolet to the thermal infrared indicate that this white dwarf hosts excess infrared emission consistent with the presence of an orbiting dusty debris disk. In addition to the five elements previously identified, our optical echelle spectroscopy reveals chromium and manganese and enables restrictive upper limits on several other elements. Synthesis of ...

Melis, Carl; Farihi, J.; Dufour, P.; Zuckerman, B.; Burgasser, Adam; Bergeron, P.; Bochanski, J.; Simcoe, Robert A.

2010-01-01

350

White dwarf mergers and the origin of R Coronae Borealis stars  

CERN Document Server

We present a nucleosynthesis study of the merger of a 0.4 solar masses helium white dwarf with a 0.8 solar masses carbon-oxygen white dwarf, coupling the thermodynamic history of Smoothed Particle Hydrodynamics particles with a post-processing code. The resulting chemical abundance pattern, particularly for oxygen and fluorine, is in qualitative agreement with the observed abundances in R Coronae Borealis stars.

Lorén-Aguilar, P; José, J; García-Berro, E; Althaus, L G; Isern, J

2011-01-01

351

The Rotational Velocity of Helium-rich Pre-White Dwarfs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Previous investigations on hydrogen-rich white dwarfs generally yield only very small rotational velocities (v_rot sin i). We have analyzed line profiles in high-resolution optical spectra of eight hydrogen-deficient (pre-) white dwarfs and find deviations from the dominant Stark line broadening in five cases which, interpreted as an effect of stellar rotation, indicate projected rotational velocities of 40 - 70 km/sec. For the three least luminous stars upper limits of v_ro...

Rauch, T.; Koeper, S.; Dreizler, S.; Werner, K.; Heber, U.; Reid, I. N.

2002-01-01

352

The DB gap and a new class of pulsating white dwarfs  

Directory of Open Access Journals (Sweden)

Full Text Available The recent systematic surveys providing enormously massive datasets of white dwarfs show that there is still a deficit of a factor of 2.5 in the DA/non-DA ratio within the temperature range of 30?000?K < Teff < 45?000?K, which has been regarded as the “DB gap” meaning a range with almost no helium atmosphere white dwarfs. Since all white dwarfs have to evolve through this temperature range along almost the identical sequence on the color-magnitude diagram, this implies that most of the helium atmosphere DO stars once evolve into hydrogen atmosphere hot DA stars in the temperature range of the DB gap and then back to helium atmosphere DB stars. Possible scenarios for this chameleon-like disguises of white dwarfs with helium dominant atmospheres are described and a new class of pulsating white dwarfs, named the hot-DAV stars, is predicted from these scenarios. One pulsating DA white dwarf, being consistent with the prediction, has been discovered indeed.

Shibahashi H.

2013-03-01

353

Theta Hya: spectroscopic identification of a second B star+white dwarf binary  

Science.gov (United States)

We report the identification, in an Extreme Ultraviolet Explorer (EUVE) spectrum, of a hot white dwarf companion to the 3rd magnitude late-B star theta Hya (HR3665, HD79469). This is the second B star+white dwarf binary to be conclusively identified; Vennes et al. (1997), and Burleigh & Barstow (1998) had previously reported the spectroscopic discovery of a hot white dwarf companion to the B5V star y Pup (HR2875). Since these two degenerate stars must have evolved from main sequence progenitors more massive than their B star companions, they can be used to place observational lower limits on the maximum mass for white dwarf progenitors, and to investigate the upper end of the initial-final mass relation. Assuming a pure hydrogen composition, we constrain the temperature of the white dwarf companion to theta Hya to lie between 25,000K and 31,000K. We also predict that a third bright B star, 16 Dra (B9.5V), might also be hiding an unresolved hot white dwarf companion.

Burleigh, M. R.; Barstow, M. A.

1999-01-01

354

Theta Hya Spectroscopic identification of a second B star + white dwarf binary  

CERN Document Server

We report the identification, in an Extreme Ultraviolet Explorer (EUVE) spectrum, of a hot white dwarf companion to the 3rd magnitude late-B star Theta Hya (HR3665, HD79469). This is the second B star + white dwarf binary to be conclusively identified; Vennes, Berghofer and Christian (1997), and Burleigh and Barstow (1998) had previously reported the spectroscopic discovery of a hot white dwarf companion to the B5V star y Pup (HR2875). Since these two degenerate stars must have evolved from main sequence progenitors more massive than their B star companions, they can be used to place observational lower limits on the maximum mass for white dwarf progenitors, and to investigate the upper end of the initial-final mass relation. Assuming a pure hydrogen composition, we constrain the temperature of the white dwarf companion to Theta Hya to lie between 25,000K and 31,000K. We also predict that a third bright B star, 16 Dra (B9.5V), might also be hiding an unresolved hot white dwarf companion.

Burleigh, M R

1999-01-01

355

Cosmochronology in Action: Determination of the Mean Core Composition of the White Dwarfs in M 67  

Science.gov (United States)

As the rate of helium thermonuclear burning in stars remains uncertain, the exact proportions of carbon and oxygen in the cores of white dwarfs are still unknown. Given that the cooling time depends on such proportions, the luminosity functions for populations of white dwarfs with various compositions are not be the same. By comparing theoretical luminosity functions computed for populations of white dwarfs with different ratios of carbon and oxygen with the observed luminosity function of M 67 — a star cluster for which we have a precise knowledge of its age — we are able to infer the mean ratio of carbon-to-oxygen in the white dwarfs in that system. To obtain the theoretical luminosity functions, we computed some 100 state-of-the-art evolutionary sequences and combined the results with models of the stellar formation rate, the initial-to-final mass relationship, and the initial mass function. Under the assumption that the M 67 white dwarfs are all DA's with standard “thick” envelopes, we infer that the cores of these white dwarfs are dominated by oxygen.

Simon, A.; Fontaine, G.; Brassard, P.

2013-01-01

356

An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate.  

Science.gov (United States)

There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in a compact binary system. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 10(7) years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of approximately 30-50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate. PMID:20164924

Gilfanov, Marat; Bogdán, Akos

2010-02-18

357

Double white dwarf mergers and elemental surface abundances in extreme helium and R Coronae Borealis stars  

CERN Document Server

The surface abundances of extreme helium (EHe) and R Coronae Borealis (RCB) stars are discussed in terms of the merger of a carbon-oxygen white dwarf with a helium white dwarf. The model is expressed as a linear mixture of the individual layers of both constituent white dwarfs, taking account of the specific evolution of each star. In developing this recipe from previous versions, particular attention has been given to the inter-shell abundances of the asymptotic giant branch star which evolved to become the carbon-oxygen white dwarf. Thus the surface composition of the merged star is estimated as a function of the initial mass and metallicity of its progenitor. The question of whether additional nucleosynthesis occurs during the white dwarf merger has been examined. The high observed abundances of carbon and oxygen must either originate by dredge-up from the core of the carbon-oxygen white dwarf during a cold merger or be generated directly by alpha-burning during a hot merger. The presence of large quantiti...

Jeffery, C Simon; Saio, Hideyuki

2011-01-01

358

Temperature and cooling age of the white dwarf companion of PSR J0218+4232  

CERN Document Server

We report on Keck optical BVRI images and spectroscopy of the companion of the binary millisecond pulsar PSR J0218+4232. A faint bluish (V=24.2, B-V=0.25) counterpart is observed at the pulsar location. Spectra of this counterpart reveal Balmer lines which confirm that the companion is a Helium-core white dwarf. We find that the white dwarf has a temperature of Teff=8060+-150 K. Unfortunately, the spectra are of insufficient quality to put a strong constraint on the surface gravity, although the best fit is for low log g and hence low mass (~0.2 Msun), as expected. We compare predicted white dwarf cooling ages with the characteristic age of the pulsar and find similar values for white dwarf masses of 0.19 to 0.3 Msun. These masses would imply a distance of 2.5 to 4 kpc to the system. The spectroscopic observations also enable us to estimate the mass ratio between the white dwarf and the pulsar. We find q=7.5+-2.4, which is consistent with the current knowledge of white dwarf companions to millisecond pulsars.

Bassa, C G; Kulkarni, S R

2003-01-01

359

HST Spectra of GW Librae A Hot Pulsating White Dwarf in a Cataclysmic Variable  

CERN Multimedia

We have obtained Hubble Space Telescope UV spectra of the white dwarf in GW Lib, the only known non-radially pulsating white dwarf in a cataclysmic variable, and the first known DAZQ variable. The UV light curve reveals large amplitude (10%) pulsationsin the UV with the same periods (646, 376 and 237 s) as those seen at optical wavelengths, but the mean spectrum fits with an average white dwarf temperature (14,700K for a 0.6M_{odot} white dwarf) that is too hot to be in the normal instability strip for ZZ Ceti stars. A better fit is achieved with a dual temperature model (with 63% of the white dwarf surface at a temperature of 13300K and 37% at 17100K), and a higher mass (0.8M_{odot}) white dwarf with 0.1 solar metal abundance. Since the blue edge of the instability strip moves to higher temperature with increasing mass, the lower temperature of this model is within the instability strip. However, the presence of accretion likely causes abundance and atmospheric temperature differences in GW Lib compared to a...

Szkody, P; Howell, S B; Sion, E M

2002-01-01

360

Emission of Z-ray and ultraviolet radiation during accretion on to non-magnetic white dwarfs  

International Nuclear Information System (INIS)

The article has a review character. The theoretical model of radiation from white dwarf is presented. Some mechanisms of emission of X and UV rays are given. Theoretical spectrum of degenerate dwarf is showing. (author)

1980-01-01

 
 
 
 
361

The not-so-extreme white dwarf of the CV GD 552  

International Nuclear Information System (INIS)

GD 552 is a cataclysmic binary which was previously believed to be composed of an M-star and a white dwarf, the latter having an extreme mass of 1.4 solar masses. In a recent paper we showed that this is not compatible with new observational evidence and presented an alternative model in which the white dwarf has a typical mass and the companion is a brown dwarf, making the system a likely member of the elusive group of CVs which have already evolved through minimum orbital period. Here we present additional spectroscopical evidence supporting this conclusion by means of skew mapping.

2009-06-01

362

On possibility of the carbon and oxygen enrichment of white dwarf outer layers  

International Nuclear Information System (INIS)

A possibility of enriching the external layers of a white dwarf with carbon and oxygen is studied. Calculations of evolution of a helium dwarf with the initial mass of 0.4 Msub(Sun), accreting matter with a normal chemical composition from a satellite in a close binary system are presented. It has been shown that during the helium burnout, which proceeds as a flare, carbon and oxygen are produced in the external layers of the white dwarf. A helium flare occurs at the accretion rate of approximately 10-8 Msub(Sun)/yr

1980-01-01

363

SEISMOLOGY OF A MASSIVE PULSATING HYDROGEN ATMOSPHERE WHITE DWARF  

Energy Technology Data Exchange (ETDEWEB)

We report our observations of the new pulsating hydrogen atmosphere white dwarf SDSS J132350.28+010304.22. We discovered periodic photometric variations in frequency and amplitude that are commensurate with nonradial g-mode pulsations in ZZ Ceti stars. This, along with estimates for the star's temperature and gravity, establishes it as a massive ZZ Ceti star. We used time-series photometric observations with the 4.1 m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1 m data, to discover the photometric variability. The light curve of SDSS J132350.28+010304.22 shows at least nine detectable frequencies. We used these frequencies to make an asteroseismic determination of the total mass and effective temperature of the star: M{sub *} = 0.88 {+-} 0.02 M{sub Sun} and T{sub eff} = 12, 100 {+-} 140 K. These values are consistent with those derived from the optical spectra and photometric colors.

Kepler, S. O.; Pelisoli, Ingrid; Pecanha, Viviane; Costa, J. E. S. [Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS (Brazil); Fraga, Luciano [Southern Observatory for Astrophysical Research, Casilla 603, La Serena (Chile); Hermes, J. J.; Winget, D. E.; Castanheira, Barbara [Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712-1083 (United States); Corsico, A. H.; Romero, A. D.; Althaus, Leandro [Facultad de Ciencias Astronomicas y Geofisicas, Universidad Nacional de La Plata (Argentina); Kleinman, S. J.; Nitta, A. [Gemini Observatory, Northern Operations Center, 670 North A' ohoku Place, Hilo, HI 96720 (United States); Koester, D. [Institut fuer Theoretische Physik und Astrophysik, Universitaet Kiel, D-24098 Kiel (Germany); Kuelebi, Baybars [Institut de Ciencies de L' Espai, Universitat Autonoma de Barcelon and Institute for Space Studies of Catalonia, c/Gran Capita 2-4, Edif. Nexus 104, E-08034 Barcelona (Spain); Jordan, Stefan [Astronomisches Rechen-Institut, ZAH, Moenchhofstr. 12-14, D-69120 Heidelberg (Germany); Kanaan, Antonio, E-mail: kepler@if.ufrgs.br [Departamento de Fisica, Universidade Federal de Santa Catarina, Florianopolis, SC (Brazil)

2012-10-01

364

Gemini spectra of 12000 K white dwarf stars  

Science.gov (United States)

We report signal-to-noise ratio (S/N) ~= 100 optical spectra for four DA white dwarf stars acquired with the GMOS spectrograph of the 8-m Gemini north telescope. These stars have 18 ~ 21) optical spectra acquired by the Sloan Digital Sky Survey through model atmosphere fitting are trustworthy. Our spectra range from 3800 to 6000 Å, therefore including H? to H9. The H8 line was only marginally present in the SDSS spectra, but is crucial to determine the gravity. When we compare the values published by Kleinman et al. and Eisenstein et al. with our line-profile technique (LPT) fits, the average differences are: ?Teff ~= 320 K, systematically lower in the SDSS, and ? log g ~= 0.24 dex, systematically larger in the SDSS. The correlation between the gravity and the effective temperature can only be broken at wavelengths bluer than 3800 Å. The uncertainties in Teff are 60 per cent larger, and in log g larger by a factor of 4, than the internal uncertainties of Kleinman et al. and Eisenstein et al. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil) and CONICET (Argentina). E-mail: kepler@if.ufrgs.br

Kepler, S. O.; Castanheira, B. G.; Costa, A. F. M.; Koester, D.

2006-11-01

365

Ultraviolet spectrum synthesis of the helium white dwarf Ross 640  

International Nuclear Information System (INIS)

The ultraviolet spectrum of the helium-rich white dwarf Ross 640 has been used to derive Mg, Si, and Fe abundances by a spectrum synthesis technique. The visible spectrum provides a Ca abundance and another estimate of the Mg abundance. The Values obtained were: Mg/He=6.8 x 10-8: Si/He=3.2 x 10-8; Ca/He=7.6 x 10-10; Fe/He=2.0 x 10-9. These are far below the solar abundances, and the metals have different ratios relative to the Sun. The estimated cooling and diffusion times for Ross 640 predict that no metals should be observed. Our abundances indicate the operation of diffusion processes (Mg/Ca>> cosmic ratio) within a helium-rich atmosphere, provided some mechanism exists to deposit metals in the observable layers. However, no firm conclusion can yet be reached as to whether the metals have somehow been convectively mixed into the atmosphere or accreted from the interstellar medium

1980-06-15

366

Near-UV Absorption in Very Cool DA White Dwarfs  

Science.gov (United States)

The atmospheres of very cool, hydrogen-rich white dwarfs (WDs) (T eff < 6000 K) are challenging to model because of the increased complexity of the equation of state, chemical equilibrium, and opacity sources in a low-temperature, weakly ionized dense gas. In particular, many models that assume relatively simple models for the broadening of atomic levels and mostly ideal gas physics overestimate the flux in the blue part of their spectra. A solution to this problem that has met with some success is that additional opacity at short wavelengths comes for the extreme broadening of the Lyman ? line of atomic H by collisions primarily with H2. For the purpose of validating this model more rigorously, we acquired Hubble Space Telescope STIS spectra of eight very cool WDs (five DA and three DC stars). Combined with their known parallaxes, BVRIJHK, and Spitzer IRAC photometry, we analyze their entire spectral energy distribution (from 0.24 to 9.3 ?m) with a large grid of model atmospheres and synthetic spectra. We find that the red wing of the Lyman ? line reproduces the rapidly decreasing near-UV flux of these very cool stars very well. We determine better constrained values of T eff and gravity as well as upper limits to the helium abundance in their atmospheres.

Saumon, D.; Holberg, J. B.; Kowalski, P. M.

2014-07-01

367

White dwarf axions, PAMELA data, and flipped-SU(5)  

CERN Document Server

Recently, there are two hints arising from physics beyond the standard model. One is a possible energy loss mechanism due to emission of very weakly interacting light particles from white dwarf stars, with a coupling strength ~ 0.7x10^{-13}, and another is the high energy positrons observed by the PAMELA satellite experiment. We construct a supersymmetric flipped-SU(5) model, SU(5)xU(1)_X with appropriate additional symmetries, [U(1)_H]_{gauge}x[U(1)_RxU(1)_\\Gamma]_{global}xZ_2, such that these are explained by a very light electrophilic axion of mass 0.5 meV from the spontaneously broken U(1)_\\Gamma and two component cold dark matters from Z_2 parity. We show that in the flipped-SU(5) there exists a basic mechanism for allowing excess positrons through the charged SU(2) singlet leptons, but not allowing anti-proton excess due to the absence of the SU(2) singlet quarks. We show the discovery potential of the charged SU(2) singlet E at the LHC experiments by observing the electron and positron spectrum. With t...

Bae, Kyu Jung; Kim, Jihn E; Kyae, Bumseok; Viollier, Raoul D

2008-01-01

368

White dwarfs as the maximal soft x-ray scatterers  

International Nuclear Information System (INIS)

In this paper, we explore the effect of density on the structure formation and the electromagnetic wave (EMw) elastic scattering on quantum plasmas, using the generalized quantum hydrodynamic model valid for a wide range of the plasma density and relativistic degeneracy. It is found that the electron quantum diffraction effect caused by the Bohm potential has a fundamental effect on the ion correlations in a degenerate electron fluid and crystallization in quantum plasmas in the solid-density regime and beyond. The ion correlations and structure formation are shown to be fundamentally affected by the plasma density and the relativistic degeneracy parameters. Moreover, distinct behavior is shown to exist between the non-relativistic and relativistic matter density regimes, regarding the normalized EMw elastic scattering cross-sections. It is theoretically discovered that the maximal Thomson scattering coincides with the average density of a typical white dwarf corresponding to the soft X-ray wavelength regime. Current research can be very useful in plasma optical diagnostic methods for a wide range of electron number-density from warm dense matter and inertial confinement fusion to the astrophysical compact objects

2013-09-01

369

Asteroseismology of white dwarf stars. I - Adiabatic results  

Energy Technology Data Exchange (ETDEWEB)

A preliminary investigation of the asteroseismological properties of chemically stratified evolutionary DA and DB white dwarf models is reported. The period and kinetic energy distributions for nonradial g-modes of spherical harmonic indices l = 1-3 are computed in the adiabatic approximation, and the effects of varying the total stellar masss and the surface layer masses on the pulsation properties are studied using an extensive grid of models. Significant resonant mode trapping due to chemical stratification is found. Modes trapped in the outer layers have much lower kinetic energies; these minima also show up as minima in the period spacing between modes of consecutive radial overtone k. Mode trapping occurs at the same or nearly the same value of k for different l-values. Thus, l-values of trapped modes may be identified on the basis of their period ratios. It is shown that observational identification of these period ratios can be used to constrain the mass of the star and its surface layer. 68 refs.

Bradley, P.A.; Winget, D.E. (Texas, University (USA) McDonald Observatory, Austin (USA))

1991-02-01

370

White dwarfs as the maximal soft x-ray scatterers  

Energy Technology Data Exchange (ETDEWEB)

In this paper, we explore the effect of density on the structure formation and the electromagnetic wave (EMw) elastic scattering on quantum plasmas, using the generalized quantum hydrodynamic model valid for a wide range of the plasma density and relativistic degeneracy. It is found that the electron quantum diffraction effect caused by the Bohm potential has a fundamental effect on the ion correlations in a degenerate electron fluid and crystallization in quantum plasmas in the solid-density regime and beyond. The ion correlations and structure formation are shown to be fundamentally affected by the plasma density and the relativistic degeneracy parameters. Moreover, distinct behavior is shown to exist between the non-relativistic and relativistic matter density regimes, regarding the normalized EMw elastic scattering cross-sections. It is theoretically discovered that the maximal Thomson scattering coincides with the average density of a typical white dwarf corresponding to the soft X-ray wavelength regime. Current research can be very useful in plasma optical diagnostic methods for a wide range of electron number-density from warm dense matter and inertial confinement fusion to the astrophysical compact objects.

Akbari-Moghanjoughi, M. [Department of Physics, Faculty of Sciences, Azarbaijan Shahid Madani University, 51745-406 Tabriz (Iran, Islamic Republic of); International Centre for Advanced Studies in Physical Sciences and Institute for Theoretical Physics, Ruhr University Bochum, D-44780 Bochum (Germany)

2013-09-15

371

White dwarfs as the maximal soft x-ray scatterers  

Science.gov (United States)

In this paper, we explore the effect of density on the structure formation and the electromagnetic wave (EMw) elastic scattering on quantum plasmas, using the generalized quantum hydrodynamic model valid for a wide range of the plasma density and relativistic degeneracy. It is found that the electron quantum diffraction effect caused by the Bohm potential has a fundamental effect on the ion correlations in a degenerate electron fluid and crystallization in quantum plasmas in the solid-density regime and beyond. The ion correlations and structure formation are shown to be fundamentally affected by the plasma density and the relativistic degeneracy parameters. Moreover, distinct behavior is shown to exist between the non-relativistic and relativistic matter density regimes, regarding the normalized EMw elastic scattering cross-sections. It is theoretically discovered that the maximal Thomson scattering coincides with the average density of a typical white dwarf corresponding to the soft X-ray wavelength regime. Current research can be very useful in plasma optical diagnostic methods for a wide range of electron number-density from warm dense matter and inertial confinement fusion to the astrophysical compact objects.

Akbari-Moghanjoughi, M.

2013-09-01

372

Pulsations in Hydrogen Burning Low Mass Helium White Dwarfs  

CERN Document Server

Helium core white dwarfs (WDs) with mass M < 0.20 M_sun undergo several Gyrs of stable hydrogen burning as they evolve. We show that in a certain range of WD and hydrogen envelope masses, these WDs may exhibit g-mode pulsations similar to their passively cooling, more massive carbon/oxygen core counterparts, the ZZ Cetis. Our models with stably burning hydrogen envelopes on helium cores yield g-mode periods and period spacings longer than the canonical ZZ Cetis by nearly a factor of two. We show that core composition and structure can be probed using seismology since the g-mode eigenfunctions predominantly reside in the helium core. Though we have not carried out a fully nonadiabatic stability analysis, the scaling of the thermal time in the convective zone with surface gravity highlights several low mass helium WDs that should be observed in search of pulsations: NLTT 11748, SDSS J0822+2753, and the companion to PSR J1012+5307. Seismological studies of these He core WDs may prove especially fruitful, as t...

Steinfadt, Justin D R; Arras, Phil

2010-01-01

373

The structure and fate of white dwarf merger remnants  

CERN Document Server

We present a large parameter study where we investigate the structure of white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD masses and compositions are explored and we also probe the effect of different initial conditions. We investigated the degree of mixing between the WDs, the conditions for detonations as well as the amount of gas ejected. We find that systems with lower mass ratios have more total angular momentum and as a result more mass is flung out in a tidal tail. Nuclear burning can affect the amount of mass ejected. Many WD binaries that contain a helium-rich WD achieve the conditions to trigger a detonation. In contrast, for carbon-oxygen transferring systems only the most massive mergers with a total mass above ~2.1 solar masses detonate. Even systems with lower mass may detonate long after the merger if the remnant remains above the Chandrasekhar mass and carbon is ignited at the centre. Finally, our findings are discussed in the context of several possible observed ...

Dan, M; Brueggen, M; Podsiadlowski, P

2013-01-01

374

The Viscous Evolution of White Dwarf Merger Remnants  

CERN Document Server

The merger of two white dwarfs (WDs) creates a differentially rotating remnant which is unstable to magnetohydrodynamic instabilities. These instabilities can lead to viscous evolution on a time-scale short compared to the thermal evolution of the remnant. We present multi-dimensional hydrodynamic simulations of the evolution of WD merger remnants under the action of an $\\alpha$-viscosity. We initialize our calculations using the output of eight WD merger simulations from Dan et al. (2011), which span a range of mass ratios and total masses. We generically find that the merger remnants evolve towards spherical states on time-scales of hours, even though a significant fraction of the mass is initially rotationally supported. The viscous evolution unbinds only a very small amount of mass $(< 10^{-5} M_\\odot)$. Viscous heating causes some of the systems we study with He WD secondaries to reach conditions of nearly dynamical burning. It is thus possible that the post-merger viscous phase triggers detonation of...

Schwab, Josiah; Quataert, Eliot; Dan, Marius; Rosswog, Stephan

2012-01-01

375

Thermonuclear explosions of rapidly rotating white dwarfs. II. Detonations  

Science.gov (United States)

Context. Superluminous type Ia supernovae (SNe Ia) may be explained by super-Chandrasekhar-mass explosions of rapidly rotating white dwarfs (WDs). In a preceding paper, we showed that the deflagration scenario applied to rapidly rotating WDs generates explosions that cannot explain the majority of SNe Ia. Aims: Rotation of the progenitor star allows super-Chandrasekhar mass WDs to form that have a shallower density stratification. We use simple estimates of the production of intermediate and iron group elements in pure detonations of rapidly rotating WDs to assess their viability in explaining rare SNe Ia. Methods: We numerically construct WDs in hydrostatic equilibrium that rotate according to a variety of rotation laws. The explosion products are estimated by considering the density stratification and by evaluating the result of hydrodynamics simulations. Results: We show that a significant amount of intermediate mass elements is produced for theoretically motivated rotation laws, even for prompt detonations of WDs. Conclusions: Rapidly rotating WDs that detonate may provide an explanation of rare superluminous SNe Ia in terms of both burning species and explosion kinematics.

Pfannes, J. M. M.; Niemeyer, J. C.; Schmidt, W.

2010-01-01

376

The viscous evolution of white dwarf merger remnants  

Science.gov (United States)

The merger of two white dwarfs (WDs) creates a differentially rotating remnant which is unstable to magnetohydrodynamic instabilities. These instabilities can lead to viscous evolution on a time-scale short compared to the thermal evolution of the remnant. We present multidimensional hydrodynamic simulations of the evolution of WD merger remnants under the action of an ?-viscosity. We initialize our calculations using the output of eight WD merger simulations from Dan et al., which span a range of mass ratios and total masses. We generically find that the merger remnants evolve towards spherical states on time-scales of hours, even though a significant fraction of the mass is initially rotationally supported. The viscous evolution unbinds only a very small amount of mass (?10-5 M). Viscous heating causes some of the systems we study with He WD secondaries to reach conditions of nearly-dynamical burning. It is thus possible that the post-merger viscous phase triggers detonation of the He envelope in some WD mergers, potentially producing a Type Ia supernova via a double-detonation scenario. Our calculations provide the proper initial conditions for studying the long-term thermal evolution of WD merger remnants. This is important for understanding WD mergers as progenitors of Type Ia supernovae, neutron stars, R Coronae Borealis stars and other phenomena.

Schwab, Josiah; Shen, Ken J.; Quataert, Eliot; Dan, Marius; Rosswog, Stephan

2012-11-01

377

MEASURING THE ROTATIONAL PERIODS OF ISOLATED MAGNETIC WHITE DWARFS  

Energy Technology Data Exchange (ETDEWEB)

We present time-series photometry of 30 isolated magnetic white dwarfs, surveyed with the Jacobus Kapteyn Telescope between 2002 August and 2003 May. We find that 9 were untestable due to varying comparison stars, but of the remaining 21, 5 (24%) are variable with reliably derived periods, while a further 9 (43%) are seen to vary during our study, but we were unable to derive the period. We interpret the variability to be the result of rotation of the objects. We find no correlation between rotation period and mass, temperature, magnetic field, or age. We have found variability in 9 targets with low magnetic field strengths and temperatures low enough for partially convective atmospheres, which we highlight as candidates for polarimetry to search for starspots. Most interestingly, we have found variability in one target, PG1658+441, which has a fully radiative atmosphere in which conventional starspots cannot form, but a magnetic field strength that is too low to cause magnetic dichroism. The source of variability in this target remains a mystery.

Brinkworth, Carolyn S. [Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125 (United States); Burleigh, Matthew R.; Lawrie, Katherine [Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH (United Kingdom); Marsh, Thomas R. [Department of Physics and Astronomy, University of Warwick, Coventry CV4 7AL (United Kingdom); Knigge, Christian [School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ (United Kingdom)

2013-08-10

378

The Nature of Millisecond Pulsars with Helium White Dwarf Companions  

CERN Document Server

We examine the growing data set of binary millisecond pulsars that are thought to have a helium white dwarf companion. These systems are believed to form when a low- to intermediate-mass companion to a neutron star fills its Roche lobe between central hydrogen exhaustion and core helium ignition. We confirm that our own stellar models reproduce a well-defined period-companion mass relation irrespective of the details of the mass transfer process. With magnetic braking this relation extends to periods of less than 1d for a 1Msun giant donor. With this and the measured binary mass functions we calculate the orbital inclination of each system for a given pulsar mass. We expect these inclinations to be randomly oriented in space. If the masses of the pulsars were typically 1.35Msun then there would appear to be a distinct dearth of high-inclination systems. However if the pulsar masses are more typically 1.55 to 1.65Msun then the distribution of inclinations is indeed indistinguishable from random. If it were as ...

Smedley, Sarah L; Ferrario, Lilia; Wickramasinghe, Dayal T

2013-01-01

379

Hot White Dwarf Donors in Ultracompact X-Ray Binaries  

CERN Document Server

The discovery of two accreting millisecond X-ray pulsars in binaries with 43 minute orbital periods allows for a new probe of the donor's structure. For XTE J1751-305, only a hot white dwarf (WD) can fill the Roche Lobe. A cold He WD is a possible solution for XTE J0929-314, though I will show that evolutionary arguments make a hot WD more likely. In addition to being larger than the T=0 models, these finite entropy, low-mass (<0.03 solar masses) WDs have a minimum mass for a fixed core temperature. If they remain hot as they lose mass and expand, they can ``evaporate'' to leave an isolated millisecond radio pulsar. They also adiabatically expand upon mass loss at a rate faster than the growth of the Roche radius if the angular momentum deposited in the disk is not returned to the donor. If the timescale of the resulting runaway mass transfer is shorter than the viscous timescale in the outer disk, then the mass transfer instability of Ruderman and Shaham for He WDs would be realized. However, my estimates...

Bildsten, L

2002-01-01

380

The thermonuclear explosion of Chandrasekhar mass white dwarfs  

CERN Document Server

The flame born in the deep interior of a white dwarf that becomes a Type Ia supernova is subject to several instabilities. We briefly review these instabilities and the corresponding flame acceleration. We discuss the conditions necessary for each of the currently proposed explosion mechanisms and the attendant uncertainties. A grid of critical masses for detonation in the range 10^7 - 2 \\times 10^9 g cm^{-3} is calculated and its sensitivity to composition explored. Prompt detonations are physically improbable and appear unlikely on observational grounds. Simple deflagrations require some means of boosting the flame speed beyond what currently exists in the literature. ``Active turbulent combustion'' and multi-point ignition are presented as two plausible ways of doing this. A deflagration that moves at the ``Sharp-Wheeler'' speed, 0.1 g_{\\rm eff} t, is calculated in one dimension and shows that a healthy explosion is possible in a simple deflagration if the front moves with the speed of the fastest floating...

Niemeyer, J C

1996-01-01

 
 
 
 
381

Electrical conductivity of plasmas of DB white dwarf atmospheres  

CERN Document Server

The static electrical conductivity of non-ideal, dense, partially ionized helium plasma was calculated over a wide range of plasma parameters: temperatures $1\\cdot 10^{4}\\textrm{K} \\lesssim T \\lesssim 1\\cdot 10^{5}\\textrm{K}$ and mass density $1 \\times 10^{-6} \\textrm{g}/\\textrm{cm}^{3} \\lesssim \\rho \\lesssim 2 \\textrm{g}/\\textrm{cm}^{3}$. Calculations of electrical conductivity of plasma for the considered range of plasma parameters are of interest for DB white dwarf atmospheres with effective temperatures $1\\cdot 10^{4}\\textrm{K} \\lesssim T_{eff} \\lesssim 3\\cdot 10^{4}\\textrm{K}$. Electrical conductivity of plasma was calculated by using the modified random phase approximation and semiclassical method, adapted for the case of dense, partially ionized plasma. The results were compared with the unique existing experimental data, including the results related to the region of dense plasmas. In spite of low accuracy of the experimental data, the existing agreement with them indicates that results obtained in th...

Sreckovic, V A; Mihajlov, A A; Dimitrijevic, M S; 10.1111/j.1365-2966.2010.16702.x

2012-01-01

382

Pulsation properties of DB white dwarfs: A preliminary analysis  

International Nuclear Information System (INIS)

We report preliminary results of a numerical investigation of the nonradial g-mode pulsation properties of evolutionary DB white dwarf models. We have solved the fully nonadiabatic equations for modes corresponding to spherical harmonic index l = 1 through 3. For each of the sequences of models we have examined (M/sub asterisk/ = 0.6 M/sub sun/; and helium layer masses of 10"-"6 M/sub asterisk/ and 10"-"4 M/sub asterisk/), we find a nonradial g-mode instability strip about 3000 K wide. For models with standard ML1 convection, this strip lies in the effective temperature range 19,000 K> or approx. =T/sub e/> or approx. =16,000 K. The boundaries of the instability strip are extremely sensitive to the assumed efficiency of convection, however, and for sequences with more efficient (ML3) convection, we find the instability strip to be in the range 29,000 K> or approx. =T/sub e/> or approx. = 26,000 K. Extrapolation of our calculations to 0.4 M/sub sun/ and 0.9 M/sub sun/ indicates that that the instability strip boundaries are insensitive to uncertainties in the total stellar mass. The most unstable modes have e-folding times of the order of days

1983-05-01

383

Calibrating cosmological chronometers: White dwarf masses via astrometry  

Energy Technology Data Exchange (ETDEWEB)

In an effort to increase the number of accurate dynamical masses for white dwarfs (WDs), we have begun an initiative using Hubble Space Telescope's Fine Guidance Sensors (FGS) to resolve suspected binary WDs. With the increasing number of WD trigonometric parallaxes becoming available via CTIO's and the USNO's ongoing parallax programs, we have targeted objects that are overluminous at V magnitude and are presumably unresolved multiple systems. A few targets were selected because of spectral anomalies or possible perturbations evident in the residuals of the trigonometric parallax solutions. A total of 16 HST orbits were designated to this program and 12 are completed. Of the eleven WDs observed thus far (one object was observed twice), all but one were unresolved. Analysis of a recent orbit's data indicate a pair was resolved with a separation of 70 mas and a delta V magnitude of approx1.4. Coupled with astrometric data from the USNO parallax program, we have obtained preliminary constraints on component masses.

Subasavage, John P; Henry, Todd J; Jao, W-C [Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30302-4106 (United States); Nelan, Edmund P [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Harris, Hugh C; Dahn, Conard C, E-mail: subasavage@chara.gsu.ed, E-mail: thenry@chara.gsu.ed, E-mail: jao@chara.gsu.ed, E-mail: nelan@stsci.ed, E-mail: hch@nofs.navy.mi, E-mail: dahn@nofs.navy.mi [US Naval Observatory, Flagstaff Station, PO Box 1149, Flagstaff, AZ 86002-1149 (United States)

2009-06-01

384

COSMOLOGICAL FAST RADIO BURSTS FROM BINARY WHITE DWARF MERGERS  

International Nuclear Information System (INIS)

Recently, Thornton et al. reported the detection of four fast radio bursts (FRBs). The dispersion measures indicate that the sources of these FRBs are at cosmological distance. Given the large full sky event rate ?104 sky–1 day–1, the FRBs are a promising target for multi-messenger astronomy. Here we propose double degenerate, binary white-dwarf (WD) mergers as the source of FRBs, which are produced by coherent emission from the polar region of a rapidly rotating, magnetized massive WD formed after the merger. The basic characteristics of the FRBs, such as the energetics, emission duration and event rate, can be consistently explained in this scenario. As a result, we predict that some FRBs can accompany type Ia supernovae (SNe Ia) or X-ray debris disks. Simultaneous detection could test our scenario and probe the progenitors of SNe Ia, and moreover would provide a novel constraint on the cosmological parameters. We strongly encourage future SN and X-ray surveys that follow up FRBs

2013-10-20

385

Mass Transfer and Tidal Dynamics in White Dwarf Binary Systems  

Science.gov (United States)

Compact white dwarf (WD) binary systems (with orbital periods ranging from minutes to hours) can produce a variety of interesting astrophysical objects (e.g., type Ia supernovae, AM CVn systems, R Cor Bor stars, sdB stars) upon the onset of mass transfer. These systems are driven toward Roche lobe overflow by the emission of gravitational radiation, but it is not known whether the mass transfer will be stable (forming an Am CVn system) or become unstable (resulting in a merger). We analyze how the combined effects of mass transfer and tidal torques affect the evolution of these systems by creating numerical models with the MESA stellar evolution program. Using new calculations of the tidal torque in rotating WDs, we predict the outcome of mass transfer in these systems as a function of the masses of the WD components. We find that the stability of mass transfer depends primarily on the peak mass transfer rate near the period minimum, which is highly dependent on the WD masses and on the strength of the tidal torques. Except for low WD accretor masses, the tidal torques are insufficient to significantly increase the stability of mass transfer. We find that mass transfer is generally unstable for WD donor masses greater than about 0.25 solar masses, and that the 12 minute system SDSS J0615 will end its inspiral in a WD merger, likely producing an R Cor Bor star.

Gerber, Jeffrey; Fuller, J.

2014-01-01

386

The Shortest Period Detached Binary White Dwarf System  

CERN Document Server

We identify SDSS J010657.39-100003.3 (hereafter J0106-1000) as the shortest period detached binary white dwarf (WD) system currently known. We targeted J0106-1000 as part of our radial velocity program to search for companions around known extremely low-mass (ELM, ~ 0.2 Msol) WDs using the 6.5m MMT. We detect peak-to-peak radial velocity variations of 740 km/s with an orbital period of 39.1 min. The mass function and optical photometry rule out a main-sequence star companion. Follow-up high-speed photometric observations obtained at the McDonald 2.1m telescope reveal ellipsoidal variations from the distorted primary but no eclipses. This is the first example of a tidally distorted WD. Modeling the lightcurve, we constrain the inclination angle of the system to be 67 +- 13 deg. J0106-1000 contains a pair of WDs (0.17 Msol primary + 0.43 Msol invisible secondary) at a separation of 0.32 Rsol. The two WDs will merge in 37 Myr and most likely form a core He-burning single subdwarf star. J0106-1000 is the shortest...

Kilic, Mukremin; Kenyon, S J; Prieto, Carlos Allende; Andrews, J; Kleinman, S J; Winget, K I; Winget, D E; Hermes, J J

2011-01-01

387

Gemini spectra of 12000K white dwarf stars  

CERN Multimedia

We report signal-to-noise ratio SNR ~ 100 optical spectra for four DA white dwarf stars acquired with the GMOS spectrograph of the 8m Gemini north telescope. These stars have 18 ~ 21) optical spectra acquired by the Sloan Digital Sky Survey through model atmosphere fitting are trustworthy. Our spectra range from 3800A to 6000A, therefore including H beta to H9. The H8 line was only marginally present in the SDSS spectra, but is crucial to determine the gravity. When we compare the values published by Kleinman et al. (2004) and Eisenstein et al. (2006) with our line-profile (LPT) fits, the average differences are: Delta Teff ~ 320 K, systematically lower in SDSS, and Delta log g ~ 0.24 dex, systematically larger in SDSS. The correlation between gravity and effective temperature can only be broken at wavelengths bluer than 3800 A. The uncertainties in Teff are 60% larger, and in log g larger by a factor of 4, than the Kleinman et al. (2004) and Eisenstein et al. (2006) internal uncertainties.

Kepler, S O; Costa, A F M; Köster, D

2006-01-01

388

A Search for Pulsations in Helium White Dwarfs  

CERN Multimedia

The recent plethora of sky surveys, especially the Sloan Digital Sky Survey, have discovered many low-mass (M < 0.45 Msun) white dwarfs that should have cores made of nearly pure helium. These WDs come in two varieties; those with masses 0.2 < M < 0.45 Msun and H envelopes so thin that they rapidly cool, and those with M < 0.2 Msun (often called extremely low mass, ELM, WDs) that have thick enough H envelopes to sustain 10^9 years of H burning. In both cases, these WDs evolve through the ZZ Ceti instability strip, Teff ~= 9,000-12,000 K, where g-mode pulsations always occur in Carbon/Oxygen WDs. This expectation, plus theoretical work on the contrasts between C/O and He core WDs, motivated our search for pulsations in 13 well characterized helium WDs. We report here on our failure to find any pulsators amongst our sample. Though we have varying amplitude limits, it appears likely that the theoretical expectations regarding the onset of pulsations in these objects requires closer consideration. In ...

Steinfadt, Justin D R; Kaplan, David L; Fulton, Benjamin J; Howell, Steve B; Marsh, T R; Ofek, Eran O; Shporer, Avi

2011-01-01

389

A Spectroscopic Analysis of White Dwarfs in the Kiso Survey  

CERN Document Server

We present a spectroscopic analysis of white dwarfs found in the Kiso survey. Spectroscopic observations at high signal-to-noise ratio have been obtained for all DA and DB stars in the Kiso Schmidt ultraviolet excess survey (KUV stars). These observations led to the reclassification of several KUV objects, including the discovery of three unresolved DA+DB double degenerate binaries. The atmospheric parameters (Teff and log g) are obtained from detailed model atmosphere fits to optical spectroscopic data. The mass distribution of our sample is characterized by a mean value of 0.606 Msun and a dispersion of 0.135 Msun for DA stars, and 0.758 Msun and a dispersion of 0.192 Msun for DB stars. Absolute visual magnitudes obtained from our spectroscopic fits allow us to derive an improved luminosity function for the DA and DB stars identified in the Kiso survey. Our luminosity function is found to be significantly different from earlier estimates based on empirical photometric calibrations of Mv for the same sample....

Limoges, M -M

2010-01-01

390

Seismology of the Accreting White Dwarf in GW Lib  

CERN Multimedia

We present a first analysis of the g-mode oscillation spectrum for the white dwarf (WD) primary of GW Lib, a faint cataclysmic variable (CV). Stable periodicities have been observed from this WD for a number of years, but their interpretation as stellar pulsations has been hampered by a lack of theoretical models appropriate to an accreting WD. Using the results of Townsley and Bildsten, we construct accreting models for the observed effective temperature and approximate mass of the WD in GW Lib. We compute g-mode frequencies for a range of accreted layer masses, Macc, and long term accretion rates, . If we assume that the observed oscillations are from l=1 g-modes, then the observed periods are matched when M ~= 1.02 Msun, Macc ~= 0.31 x 10^-4 Msun and ~= 7.3 x 10^-11 Msun/yr. Much more sensitive observations are needed to discover more modes, after which we will be able to more accurately measure these parameters and constrain or measure the WD's rotation rate.

Townsley, D M; Bildsten, L

2004-01-01

391

Measuring White Dwarf Accretion Rates via their Effective Temperatures  

CERN Multimedia

Our previous theoretical study of the impact of an accreting envelope on the thermal state of an underlying white dwarf (WD) has yielded equilibrium core temperatures, classical nova ignition masses and thermal luminosities for WDs accreting at time averaged rates of = 10^-11 - 10^-8 Msun/yr. These 's are appropriate to WDs in cataclysmic variables (CVs) of P_orb predicted for CV's as they transit the period gap. For DN below the period gap, we show that if is that given by gravitational radiation losses alone, then the WD masses are > 0.8 Msun. An alternative conclusion is that the masses are closer to 0.6 Msun and is 3-4 times larger than that expected from gravitational radiation losses. In either case, it is very plausible that a subset of CVs with P_orb < 2 hours will have T_eff's low enough for them to become non-radial pulsators, as discovered by van Zyl and collaborators in GW Lib.

Townsley, D M; Townsley, Dean M.; Bildsten, Lars

2003-01-01

392

On the white dwarf cooling sequence with extremely large telescopes  

CERN Multimedia

We present new diagnostics of white dwarf (WD) cooling sequences and luminosity functions (LFs) in the near-infrared (NIR) bands that will exploit the sensitivity and resolution of future extremely large telescopes. The collision-induced absorption (CIA) of molecular hydrogen causes a clearly defined blue turn-off along the WD (WDBTO) cooling sequences and a bright secondary maximum in the WD LFs. These features are independent of age over a broad age range and are minimally affected by metal abundance. This means that the NIR magnitudes of the WDBTO are very promising distance indicators. The interplay between the cooling time of progressively more massive WDs and the onset of CIA causes a red turn-off along the WD (WDRTO) cooling sequences and a well defined faint peak in the WD LFs. These features are very sensitive to the cluster age, and indeed the K-band magnitude of the faint peak increases by 0.2--0.25 mag/Gyr for ages between 10 and 14 Gyr. On the other hand, the faint peak in the optical WD LF incre...

Bono, G; Gilmozzi, R

2012-01-01

393

A DOUBLE WHITE-DWARF COOLING SEQUENCE IN {omega} CENTAURI  

Energy Technology Data Exchange (ETDEWEB)

We have applied our empirical-point-spread-function-based photometric techniques on a large number of calibration-related WFC3/UVIS UV-B exposures of the core of {omega} Cen, and found a well-defined split in the bright part of the white-dwarf cooling sequence (WDCS). The redder sequence is more populated by a factor of {approx}2. We can explain the separation of the two sequences and their number ratio in terms of the He-normal and He-rich subpopulations that had been previously identified along the cluster main sequence. The blue WDCS is populated by the evolved stars of the He-normal component ({approx}0.55 M{sub Sun} CO-core DA objects), while the red WDCS hosts the end products of the He-rich population ({approx}0.46 M{sub Sun} objects, and {approx}10% CO-core and {approx}90% He-core WDs). The He-core WDs correspond to He-rich stars that missed the central He ignition, and we estimate their fraction by analyzing the population ratios along the cluster horizontal branch.

Bellini, A.; Anderson, J. [Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218 (United States); Salaris, M. [Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD (United Kingdom); Cassisi, S. [Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Collurania, via Mentore Maggini, I-64100 Teramo (Italy); Bedin, L. R. [Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, v.co dell' Osservatorio 5, I-35122, Padova (Italy); Piotto, G. [Dipartimento di Fisica e Astronomia ' Galileo Galilei' , Universita di Padova, v.co dell' Osservatorio 3, I-35122, Padova (Italy); Bergeron, P., E-mail: bellini@stsci.edu [Departement de Physique, Universite de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada)

2013-06-01

394

A MULTI-SURVEY APPROACH TO WHITE DWARF DISCOVERY  

Energy Technology Data Exchange (ETDEWEB)

By selecting astrometric and photometric data from the Sloan Digital Sky Survey (SDSS), the Lepine and Shara Proper Motion North Catalog (LSPM-North), the Two Micron All Sky Survey (2MASS), and the USNO-B1.0 catalog, we use a succession of methods to isolate white dwarf (WD) candidates for follow-up spectroscopy. Our methods include reduced proper motion diagram cuts, color cuts, and atmospheric model adherence. We present spectroscopy of 26 WDs obtained from the CTIO 4 m and APO 3.5 m telescopes. Additionally, we confirm 28 WDs with spectra available in the SDSS DR7 database but unpublished elsewhere, presenting a total of 54 WDs. We label one of these as a recovered WD while the remaining 53 are new discoveries. We determine physical parameters and estimate distances based on atmospheric model analyses. Three new WDs are modeled to lie within 25 pc. Two additional WDs are confirmed to be metal-polluted (DAZ). Follow-up time series photometry confirms another object to be a pulsating ZZ Ceti WD.

Sayres, Conor; Davenport, James R. A.; AlSayyad, Yusra; Tofflemire, Benjamin M. [Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 (United States); Subasavage, John P. [US Naval Observatory, 10391 West Naval Observatory Road, Flagstaff, AZ 86001-8521 (United States); Bergeron, P.; Dufour, P., E-mail: csayres@u.washington.edu [Departement de Physique, Universite de Montreal, C.P.6128, Succursale Centre-Ville, Montreal, QC H3C 3J7 (Canada)

2012-04-15

395

The White Dwarf Population in NGC 1039 (M34) and the White Dwarf Initial-Final Mass Relation  

Science.gov (United States)

We present the first detailed photometric and spectroscopic study of the white dwarfs (WDs) in the field of the ~225 Myr old (log ?cl = 8.35) open cluster NGC 1039 (M34) as part of the ongoing Lick-Arizona White Dwarf Survey. Using wide-field UBV imaging, we photometrically select 44 WD candidates in this field. We spectroscopically identify 19 of these objects as WDs; 17 are hydrogen-atmosphere DA WDs, one is a helium-atmosphere DB WD, and one is a cool DC WD that exhibits no detectable absorption lines. We find an effective temperature (T eff) and surface gravity (log g) for each DA WD by fitting Balmer-line profiles from model atmospheres to the observed spectra. WD evolutionary models are then invoked to derive masses and cooling times for each DA WD. Of the 17 DAs, five are at the approximate distance modulus of the cluster. Another WD with a distance modulus 0.45 mag brighter than that of the cluster could be a double-degenerate binary cluster member, but is more likely to be a field WD. We place the five single cluster member WDs in the empirical initial-final mass relation and find that three of them lie very close to the previously derived linear relation; two have WD masses significantly below the relation. These outliers may have experienced some sort of enhanced mass loss or binary evolution; however, it is quite possible that these WDs are simply interlopers from the field WD population. Eight of the 17 DA WDs show significant Ca II K absorption; comparison of the absorption strength with the WD distances suggests that the absorption is interstellar, though this cannot be confirmed with the current data. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Rubin, Kate H. R.; Williams, Kurtis A.; Bolte, M.; Koester, Detlev

2008-06-01

396

A Hubble Space Telescope study of the underlying white dwarf in the dwarf nova VW Hydri during quiescence  

Science.gov (United States)

We have analyzed a far-ultraviolet spectrum of the dwarf nova VW Hyi obtained during quiescence with the Hubble Space Telescope Faint Object Spectrograph (HST FOS). The observation occurred 10 days after the return to optical quiescence from a superoutburst of VW Hyi. The spectrum reveals a very strong Stark-broadened Ly-alpha absorption with narrow geocoronal emission, and a very rich metallic absorption-line spectrum dominated by strong resonance absorption features of singly and doubly ionized silicon and carbon, the first solid identification of metallic absorption features arising in the accreted atmosphere of the white dwarf. We confirm the reported low-resolution IUE detection of the underlying white dwarf photosphere by Mateo & Szkody. A synthetic spectral analysis with hot, high-gravity LTE model atmospheres yields a best-fit model with the following parameters: T(sub eff) = 22,000 +/- 1000 K, log g = 8.0 +/- 0.3, with chemical abundances for oxygen of 0.3 times solar, for nitrogen of 5 times solar, and for all other heavy elements of 0.15 times solar. Based upon our absorption-line measurements of the observations at different orbital phases, we find no conclusive evidence of equivalent width variations with orbital phase. In the absence of any significant reduction of the white dwarf's core mass by past nova explosions, its lower limit cooling age is approximately 50 million years.

Sion, Edward M.; Szkody, Paula; Cheng, Fu-Hua; Huang, Min

1995-01-01

397

The eclipsing post-common envelope binary CSS21055: a white dwarf with a probable brown-dwarf companion  

CERN Document Server

We report photometric observations of the eclipsing close binary CSS21055 (SDSS J141126+200911) that strongly suggest that the companion to the carbon-oxygen white dwarf is a brown dwarf with a mass between 0.030 and 0.074 Msun. The measured orbital period is 121.73min and the totality of the eclipse lasts 125s. If confirmed, CSS21055 would be the first detached eclipsing WD+BD binary. Spectroscopy in the eclipse could provide information about the companion's evolutionary state and atmospheric structure.

Beuermann, K; Hessman, F V; Backhaus, U; Boesch, A; Husser, T -O; Nortmann, L; Schmelev, A; Springer, R

2013-01-01

398

Post-common-envelope binaries from SDSS - V. Four eclipsing white dwarf main-sequence binaries  

Science.gov (United States)

We identify SDSS011009.09+132616.1, SDSS030308.35+005444.1, SDSS143547.87+ 373338.5 and SDSS154846.00+405728.8 as four eclipsing white dwarf plus main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS), and report on follow-up observations of these systems. SDSS0110+1326, SDSS1435+3733 and SDSS1548+4057 contain DA white dwarfs, while SDSS0303+0054 contains a cool DC white dwarf. Orbital periods and ephemerides have been established from multiseason photometry. SDSS1435+3733, with Porb = 3h has the shortest orbital period of all known eclipsing WDMS binaries. As for the other systems, SDSS0110+1326 has Porb = 8h, SDSS0303+0054 has Porb = 3.2h and SDSS1548+4057 has Porb = 4.4h. Time-resolved spectroscopic observations have been obtained and the H? and CaII??8498.02, 8542.09, 8662.14 triplet emission lines, as well as the NaI??8183.27, 8194.81 absorption doublet were used to measure the radial velocities of the secondary stars in all four systems. A spectral decomposition/fitting technique was then employed to isolate the contribution of each of the components to the total spectrum, and to determine the white dwarf effective temperatures and surface gravities, as well as the spectral types of the companion stars. We used a light-curve modelling code for close binary systems to fit the eclipse profiles and the ellipsoidal modulation/reflection effect in the light curves, to further constrain the masses and radii of the components in all systems. All three DA white dwarfs have masses of MWD ~ 0.4-0.6Msolar, in line with the expectations from close binary evolution. The DC white dwarf in SDSS0303+0054 has a mass of MWD >~ 0.85Msolar, making it unusually massive for a post-common-envelope system. The companion stars in all four systems are M dwarfs of spectral type M4 and later. Our new additions raise the number of known eclipsing WDMS binaries to 14, and we find that the average white dwarf mass in this sample is MWD> =0.57 +/- 0.16Msolar, only slightly lower than the average mass of single white dwarfs. The majority of all eclipsing WDMS binaries contain low-mass (<0.6Msolar) secondary stars, and will eventually provide valuable observational input for the calibration of the mass-radius relations of low-mass main-sequence stars and of white dwarfs.

Pyrzas, S.; Gänsicke, B. T.; Marsh, T. R.; Aungwerojwit, A.; Rebassa-Mansergas, A.; Rodríguez-Gil, P.; Southworth, J.; Schreiber, M. R.; Nebot Gomez-Moran, A.; Koester, D.

2009-04-01

399

A PARAMETER-SPACE STUDY OF CARBON-OXYGEN WHITE DWARF MERGERS  

Energy Technology Data Exchange (ETDEWEB)

The merger of two carbon-oxygen white dwarfs can lead to either a spectacular transient, stable nuclear burning or a massive, rapidly rotating white dwarf. Simulations of mergers have shown that the outcome strongly depends on whether the white dwarfs are similar or dissimilar in mass. In the similar-mass case, both white dwarfs merge fully and the remnant is hot throughout, while in the dissimilar case, the more massive, denser white dwarf remains cold and essentially intact, with the disrupted lower mass one wrapped around it in a hot envelope and disk. In order to determine what constitutes ''similar in mass'' and more generally how the properties of the merger remnant depend on the input masses, we simulated unsynchronized carbon-oxygen white dwarf mergers for a large range of masses using smoothed-particle hydrodynamics. We find that the structure of the merger remnant varies smoothly as a function of the ratio of the central densities of the two white dwarfs. A density ratio of 0.6 approximately separates similar and dissimilar-mass mergers. Confirming previous work, we find that the temperatures of most merger remnants are not high enough to immediately ignite carbon fusion. During subsequent viscous evolution, however, the interior will likely be compressed and heated as the disk accretes and the remnant spins down. We find from simple estimates that this evolution can lead to ignition for many remnants. For similar-mass mergers, this would likely occur under sufficiently degenerate conditions that a thermonuclear runaway would ensue.

Zhu Chenchong; Van Kerkwijk, Marten H. [Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario, M5S 3H4 (Canada); Chang, Philip [Department of Physics, University of Wisconsin-Milwaukee, 1900 E. Kenwood Blvd., Milwaukee, Wisconsin 53211 (United States); Wadsley, James, E-mail: cczhu@astro.utoronto.ca [Department of Physics and Astronomy, ABB-241, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4M1 (Canada)

2013-04-20

400

A PARAMETER-SPACE STUDY OF CARBON-OXYGEN WHITE DWARF MERGERS  

International Nuclear Information System (INIS)

The merger of two carbon-oxygen white dwarfs can lead to either a spectacular transient, stable nuclear burning or a massive, rapidly rotating white dwarf. Simulations of mergers have shown that the outcome strongly depends on whether the white dwarfs are similar or dissimilar in mass. In the similar-mass case, both white dwarfs merge fully and the remnant is hot throughout, while in the dissimilar case, the more massive, denser white dwarf remains cold and essentially intact, with the disrupted lower mass one wrapped around it in a hot envelope and disk. In order to determine what constitutes ''similar in mass'' and more generally how the properties of the merger remnant depend on the input masses, we simulated unsynchronized carbon-oxygen white dwarf mergers for a large range of masses using smoothed-particle hydrodynamics. We find that the structure of the merger remnant varies smoothly as a function of the ratio of the central densities of the two white dwarfs. A density ratio of 0.6 approximately separates similar and dissimilar-mass mergers. Confirming previous work, we find that the temperatures of most merger remnants are not high enough to immediately ignite carbon fusion. During subsequent viscous evolution, however, the interior will likely be compressed and heated as the disk accretes and the remnant spins down. We find from simple estimates that this evolution can lead to ignition for many remnants. For similar-mass mergers, this would likely occur under sufficiently degenerate conditions that a thermonuclear runaway would ensue.

2013-04-20

 
 
 
 
401

TOWARD A SPECTROSCOPIC CENSUS OF WHITE DWARFS WITHIN 40 pc OF THE SUN  

Energy Technology Data Exchange (ETDEWEB)

We present the preliminary results of a survey aimed at significantly increasing the range and completeness of the local census of spectroscopically confirmed white dwarfs. The current census of nearby white dwarfs is reasonably complete only to about 20 pc of the Sun, a volume that includes around 130 white dwarfs, a sample too small for detailed statistical analyses. This census is largely based on follow-up investigations of stars with very large proper motions. We describe here the basis of a method that will lead to a catalog of white dwarfs within 40 pc of the Sun and north of the celestial equator, thus increasing by a factor of eight the extent of the northern sky census. White dwarf candidates are identified from the SUPERBLINK proper motion database, allowing us to investigate stars down to a proper motion limit {mu} > 40 mas yr{sup -1}, while minimizing the kinematic bias for nearby objects. The selection criteria and distance estimates are based on a combination of color-magnitude and reduced proper motion diagrams. Our follow-up spectroscopic observation campaign has so far uncovered 193 new white dwarfs, among which we identify 127 DA (including 9 DA+dM and 4 magnetic), 1 DB, 56 DC, 3 DQ, and 6 DZ stars. We perform a spectroscopic analysis on a subsample of 84 DAs, and provide their atmospheric parameters. In particular, we identify 11 new white dwarfs with spectroscopic distances within 25 pc of the Sun, including five candidates to the D < 20 pc subset.

Limoges, M.-M.; Bergeron, P. [Departement de Physique, Universite de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, Quebec H3C 3J7 (Canada); Lepine, S., E-mail: limoges@astro.umontreal.ca, E-mail: bergeron@astro.umontreal.ca, E-mail: lepine@amnh.org [Department of Astrophysics, Division of Physical Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY 1002 (United States)

2013-05-15

402

White dwarfs in cataclysmic variables - Low state IUE observations of V794 Aquilae, MR Serpentis, and AN Ursae Majoris  

Science.gov (United States)

IUE low-dispersion observations of one novalike system (V794 Aql) and two AM Her systems (MR Ser and AN UMa) during low or intermediate-low accretion-rate states are reported. The decreased UV emission during these times was used to place limits on the white dwarf temperatures. When these results are added to the existing data on white dwarf temperatures, it appears that the white dwarfs in systems with normal high accretion rates and only occasional low-accretion-rate states remain hotter than the white dwarfs in systems with low average accretion rates.

Szkody, Paula; Mateo, Mario; Downes, Ronald

1988-01-01

403

White dwarfs in cataclysmic variables - low state IUE observations of V794 Aquilae, MR Serpentis, and AN Ursae Majoris  

International Nuclear Information System (INIS)

IUE low-dispersion observations of one novalike system (V794 Aql) and two AM Her systems (MR Ser and AN UMa) during low or intermediate-low accretion-rate states are reported. The decreased UV emission during these times was used to place limits on the white dwarf temperatures. When these results are added to the existing data on white dwarf temperatures, it appears that the white dwarfs in systems with normal high accretion rates and only occasional low-accretion-rate states remain hotter than the white dwarfs in systems with low average accretion rates. 28 references

1988-01-01

404

White dwarfs in cataclysmic variables - low state IUE observations of V794 Aquilae, MR Serpentis, and AN Ursae Majoris  

Energy Technology Data Exchange (ETDEWEB)

IUE low-dispersion observations of one novalike system (V794 Aql) and two AM Her systems (MR Ser and AN UMa) during low or intermediate-low accretion-rate states are reported. The decreased UV emission during these times was used to place limits on the white dwarf temperatures. When these results are added to the existing data on white dwarf temperatures, it appears that the white dwarfs in systems with normal high accretion rates and only occasional low-accretion-rate states remain hotter than the white dwarfs in systems with low average accretion rates. 28 references.

Szkody, P.; Mateo, M.; Downes, R.

1988-03-01

405

On the Number of Comets Around White Dwarf Stars: Orbit Survival During the Late Stages of Stellar Evolution  

International Nuclear Information System (INIS)

The accretion of comets onto DA white dwarfs can produce observable metal absorption lines. We show here that comet systems around the progenitor main-sequence star are vulnerable to being lost during asymptotic giant branch mass loss, if the mass loss is sufficiently asymmetric to impart modest linear momentum to the white dwarf. This may have bearing on the frequency of observation of heavy elements in white dwarf stars and on inferences regarding the frequency of comet systems, if the imparted linear velocities of white dwarfs can be estimated. copyright copyright 1998. The American Astronomical Society

1998-07-01

406

The nature of millisecond pulsars with helium white dwarf companions  

Science.gov (United States)

We examine the growing data set of binary millisecond pulsars that are thought to