Showing papers on "White dwarf published in 1999"

Nucleosynthesis in Chandrasekhar Mass Models for Type Ia Supernovae and Constraints on Progenitor Systems and Burning-Front Propagation

Abstract: The major uncertainties involved in the Chandrasekhar mass models for Type Ia supernovae (SNe Ia) are related to the companion star of their accreting white dwarf progenitor (which determines the accretion rate and consequently the carbon ignition density) and the flame speed after the carbon ignition. We calculate explosive nucleosynthesis in relatively slow deflagrations with a variety of deflagration speeds and ignition densities to put new constraints on the above key quantities. The abundance of the Fe group, in particular of neutron-rich species like 48Ca,50Ti,54Cr,54,58Fe, and 58Ni, is highly sensitive to the electron captures taking place in the central layers. The yields obtained from such a slow central deflagration, and from a fast deflagration or delayed detonation in the outer layers, are combined and put to comparison with solar isotopic abundances. To avoid excessively large ratios of 54Cr/56Fe and 50Ti/56Fe, the central density of the average white dwarf progenitor at ignition should be as low as 2 ? 109 g cm-3. To avoid the overproduction of 58Ni and 54Fe, either the flame speed should not exceed a few percent of the sound speed in the central low Ye layers or the metallicity of the average progenitors has to be lower than solar. Such low central densities can be realized by a rapid accretion as fast as -->img1.gif 1 ? 10-7 M? yr-1. In order to reproduce the solar abundance of 48Ca, one also needs progenitor systems that undergo ignition at higher densities. Even the smallest laminar flame speeds after the low-density ignitions would not produce sufficient amount of this isotope. We also found that the total amount of 56Ni, the Si-Ca/Fe ratio, and the abundance of some elements like Mn and Cr (originating from incomplete Si burning), depend on the density of the deflagration-detonation transition in delayed detonations. Our nucleosynthesis results favor transition densities slightly below 2.2 ? 107 g cm-3.

A WIDE SYMBIOTIC CHANNEL TO TYPE Ia SUPERNOVAE

Abstract: As a promising channel to Type Ia supernovae (SNe Ia), we have proposed a symbiotic binary system consisting of a white dwarf (WD) and a low-mass red giant (RG) in which strong winds from the accreting WD play a key role increasing the WD mass to the Chandrasekhar mass limit. However, the occurrence frequency of SNe Ia through this channel is still controversial. Here we propose two new evolutionary processes that make the symbiotic channel to SNe Ia much wider. (1) We first show that the WD+RG close binary can form from a wide binary even with an initial separation as large as ai 40,000 R?. Such a binary consists of a low-mass main-sequence (MS) star and an asymptotic giant branch (AGB) star that is undergoing a superwind before becoming a WD. If the superwind at the end of AGB evolution is as fast as or slower than the orbital velocity, the wind outflowing from the system takes away the orbital angular momentum effectively. As a result the wide binary shrinks greatly to become a close binary. Then the AGB star undergoes to form a common envelope (CE) evolution. After the CE evolution, the binary becomes a pair consisting of a carbon-oxygen WD and an MS star. When the MS star evolves to an RG, a WD+RG system is formed. Therefore, the WD+RG binary can form from much wider binaries than our earlier estimate, which was constrained by ai 1500 R?. (2) When the RG fills its inner critical Roche lobe, the WD undergoes rapid mass accretion and blows a strong optically thick wind. Our earlier analysis has shown that the mass transfer is stabilized by this wind only when the mass ratio of the RG to the WD is smaller than 1.15. Our new finding is that the WD wind can strip mass from the RG envelope, which could be efficient enough to stabilize the mass transfer even if the RG-to-WD mass ratio exceeds 1.15. If this mass-stripping effect is strong enough, though its efficiency is subject to uncertainties, the symbiotic channel can produce SNe Ia for a much (10 or more times) wider range of the binary parameters than our earlier estimate predicted. With the above two new effects (1) and (2), the symbiotic channel can account for the inferred rate of SNe Ia in our Galaxy. The immediate progenitor binaries in this symbiotic channel to SNe Ia may be observed as symbiotic stars, luminous supersoft X-ray sources, or recurrent novae, such as T CrB or RS Oph, depending on the wind status.

Formation Rates of Black Hole Accretion Disk Gamma-Ray Bursts

Abstract: While many models have been proposed for GRBs, those currently favored are all based upon the formation of and/or rapid accretion into stellar mass black holes. We present population synthesis calculations of these models using a Monte Carlo approach in which the many uncertain parameters intrinsic to such calculations are varied. We estimate the event rate for each class of model as well as the propagation distance for those having significant delay between formation and burst production, i.e., double neutron star (DNS) mergers and black hole-neutron star (BH/NS) mergers. For reasonable assumptions regarding the many uncertainties in population synthesis, we calculate a daily event rate in the universe for i) merging neutron stars: ~100/day; ii) neutron-star black hole mergers: ~450/day; iii) collapsars: ~10,000/day; iv) helium star black hole mergers: ~1000/day; and v) white dwarf black hole mergers: ~20/day. The range of uncertainty in these numbers however, is very large, typically two to three orders of magnitude. These rates must additionally be multiplied by any relevant beaming factor and sampling fraction (if the entire universal set of models is not being observed). Depending upon the mass of the host galaxy, half of the DNS and BH/NS mergers will happen within 60kpc (for a Milky-Way massed galaxy) to 5Mpc (for a galaxy with negligible mass) from the galactic center. Because of the delay time, neutron star and black hole mergers will happen at a redshift 0.5 to 0.8 times that of the other classes of models. Information is still lacking regarding the hosts of short hard bursts, but we suggest that they are due to DNS and BH/NS mergers and thus will ultimately be determined to lie outside of galaxies and at a closer mean distance than long complex bursts (which we attribute to collapsars).

A New Evolutionary Path to Type Ia Supernovae: A Helium-rich Supersoft X-Ray Source Channel

Abstract: We have found a new evolutionary path to Type Ia supernovae (SNe Ia) that has been overlooked in previous work. In this scenario, a carbon-oxygen white dwarf (C+O WD) is originated not from an asymptotic giant branch star with a C+O core but from a red giant star with a helium core of ~0.8-2.0 M☉. The helium star, which is formed after the first common envelope evolution, evolves to form a C+O WD of ~0.8-1.1 M☉, transferring a part of the helium envelope onto the secondary main-sequence star. This new evolutionary path, together with the optically thick wind from mass-accreting white dwarf, provides a much wider channel to SNe Ia than previous scenarios. A part of the progenitor systems are identified as luminous supersoft X-ray sources or recurrent novae such as U Sco, which are characterized by the accretion of helium-rich matter. The white dwarf accretes hydrogen-rich, helium-enhanced matter from a lobe-filling, slightly evolved companion at a critical rate and blows excess matter into the wind. The white dwarf grows in mass to the Chandrasekhar mass limit and explodes as an SN Ia. A theoretical estimate indicates that this channel contributes a considerable part of the inferred rate of SNe Ia in our Galaxy, i.e., the rate is about 10 times larger than the previous theoretical estimates for white dwarfs with slightly evolved companions.

Simulation of Stellar Objects in SDSS Color Space

Abstract: We present a simulation of the spatial, luminosity and spectral distributions of four types of stellar objects. We simulate (1) Galactic stars, based on a Galactic structure model, a stellar population synthesis model, stellar isochrones, and stellar spectral libraries; (2) white dwarfs, based on model atmospheres, the observed luminosity function, mass distribution, and Galactic distribution of white dwarfs; (3) quasars, based on their observed luminosity function and its evolution, and models of emission and absorption spectra of quasars; and (4) compact emission-line galaxies, based on the observed distribution of their spectral properties and sizes. The results are presented in the color system of the Sloan Digital Sky Survey (SDSS), with realistic photometric error and Galactic extinction. The simulated colors of stars and quasars are compared with observations in the SDSS system and show good agreement. The stellar simulation can be used as a tool to analyze star counts and constrain models of Galactic structure, as well as to identify stars with unusual colors. The simulation can also be used to establish the quasar target selection algorithm for the SDSS.

A New Evolutionary Path to Type Ia Supernovae: Helium-Rich Super-Soft X-Ray Source Channel

Abstract: We have found a new evolutionary path to Type Ia supernovae (SNe Ia) which has been overlooked in previous work. In this scenario, a carbon-oxygen white dwarf (C+O WD) is originated, not from an asymptotic giant branch star with a C+O core, but from a red-giant star with a helium core of $\sim 0.8-2.0 M_\odot$. The helium star, which is formed after the first common envelope evolution, evolves to form a C+O WD of $\sim 0.8-1.1 M_\odot$ with transferring a part of the helium envelope onto the secondary main-sequence star. This new evolutionary path, together with the optically thick wind from mass-accreting white dwarf, provides a much wider channel to SNe Ia than previous scenarios. A part of the progenitor systems are identified as the luminous supersoft X-ray sources or the recurrent novae like U Sco, which are characterized by the accretion of helium-rich matter. The white dwarf accretes hydrogen-rich, helium-enhanced matter from a lobe-filling, slightly evolved companion at a critical rate and blows excess matter in the wind. The white dwarf grows in mass to the Chandrasekhar mass limit and explodes as an SN Ia. A theoretical estimate indicates that this channel contributes a considerable part of the inferred rate of SNe Ia in our Galaxy, i.e., the rate is about ten times larger than the previous theoretical estimates for white dwarfs with slightly evolved companions.

What Can the Accretion-induced Collapse of White Dwarfs Really Explain?

Abstract: The accretion-induced collapse (AIC) of a white dwarf into a neutron star has been invoked to explain gamma-ray bursts, Type Ia supernovae, and a number of problematic neutron star populations and specific binary systems. The ejecta from this collapse has also been claimed as a source of r-process nucleosynthesis. So far, most AIC studies have focused on determining the event rates from binary evolution models and less attention has been directed toward understanding the collapse itself. However, the collapse of a white dwarf into a neutron star is followed by the ejection of rare neutron-rich isotopes. The observed abundance of these chemical elements may set a more reliable limit on the rate at which AICs have taken place over the history of the Galaxy. In this paper, we present a thorough study of the collapse of a massive white dwarf in one- and two-dimensions and determine the amount and composition of the ejected material. We discuss the importance of the input physics (equation of state, neutrino transport, rotation) in determining these quantities. These simulations affirm that AICs are too baryon rich to produce gamma-ray bursts and do not eject enough nickel to explain Type Ia supernovae (with the possible exception of a small subclass of extremely low-luminosity Type Ias). Although nucleosynthesis constraints limit the number of neutron stars formed via AICs to 0.1% of the total Galactic neutron star population, AICs remain a viable scenario for forming systems of neutron stars that are difficult to explain with Type II core-collapse supernovae.

ON THE RELEVANCE OF THE r-MODE INSTABILITY FOR ACCRETING NEUTRON STARS AND WHITE DWARFS

Abstract: We present a case study for the relevance of the r-mode instability for accreting compact stars. Our estimates are based on approximations that facilitate back of the envelope calculations. We discuss two different cases. (1) For recycled millisecond pulsars, we argue that the r-mode instability may be active at rotation periods longer than the Kepler period (which provides the dynamical limit on rotation) as long as the core temperature is larger than about 2 × 105 K. Our estimates suggest that the instability may have played a role in the evolution of the fastest spinning pulsars and that it may be presently active in the recently discovered 2.49 ms X-ray pulsar, SAX J1808.4-3658, as well as the rapidly spinning neutron stars observed in low-mass X-ray binaries (LMXBs). This provides a new explanation for the remarkably similar rotation periods inferred from kilohertz, quasi-periodic oscillations in the LMXBs. The possibility that the rotation of recycled pulsars may be gravitational-radiation-limited is interesting, because the gravitational waves from a neutron star rotating at the instability limit may well be detectable with the new generation of interferometric detectors. (2) We also consider white dwarfs and find that the r-mode instability may possibly be active in short-period white dwarfs. Our order-of-magnitude estimates (for a white dwarf of M=M☉ and R=0.01 R☉ composed of C12) show that the instability could be operating for rotational periods shorter than P≈27-33 s. This number is in interesting agreement with the observed periods (greater than 28 s) of the rapidly spinning DQ Herculis stars. However, we find that the instability grows too slowly to affect the rotation of these stars significantly.

Spectral classification of the cool giants in symbiotic systems

Abstract: We derive the spectral types of the cool gi- ants in about 100 symbiotic systems. Our classication is mainly based on near IR spectra in order to avoid the contamination of the spectrum by the nebula and the hot component in the visual region. The accuracy of our spec- tral types is approximately one spectral subclass, similar to previous near IR classication work, and much better than visual spectral type estimates. Strong, intrinsic spectral type variations (> 2 spectral subtypes) are only seen in systems containing pulsating mira variables. We present a catalogue of spectral types for cool giants in symbiotic systems which also includes determinations taken from the literature. The catalogue gives spectral types for the cool giants in about 170 systems which is nearly the full set of conrmed symbiotics. Based on our classications we discuss the distribution of spectral types of the cool giants in galactic symbiotic bi- naries. We nd that the spectral types cluster strongly be- tween M3 and M6, with a peak at M5. The distribution of systems with a mira variable component peaks even later, at spectral types M6 and M7. This is a strong bias towards late spectral types when compared to red giants in the so- lar neighbourhood. Also the frequency of mira variables is much larger among symbiotic giants. This predominance of very late M-giants in symbiotic systems seems to indi- cate that large mass loss is a key ingredient for triggering symbiotic activity on a white dwarf companion. Further we nd for symbiotic systems a strong corre- lation between the spectral type of the cool giant and the orbital period. In particular we nd a tight relation for the minimum orbital period for symbiotic systems with ? Based on observations obtained with the 1.52 m and 3.6 m telescopes of the European Southern Observatory (ESO), the 1.93 m telescope of the Observatoire de Haute-Provence (OHP), the 2.3 m telescope of the Australian National University (ANU) at Siding Spring, and the William Herschel Telescope (WHT) at La Palma. This research has made use of the AFOEV database, operated at CDS, France. red giants of a given spectral type. This limiting line in the spectral type { orbital period diagram seems to be equivalent with the relation R '1=2, where R is the ra- dius of the red giant and '1 the distance from the center of the giant to the inner Lagrangian point L1. This cor- relation possibly discloses that symbiotic stars are { with probably only one exception in our sample { well detached binary systems.

Gravitational Radiation Limit on the Spin of Young Neutron Stars

Abstract: A newly discovered instability in rotating neutron stars, driven by gravitational radiation reaction acting on the stars' r-modes, is shown here to set an upper limit on the spin rate of young neutron stars. We calculate the timescales for the growth of linear perturbations due to gravitational radiation reaction, and for dissipation by shear and bulk viscosity, working to second order in a slow-rotation expansion within a Newtonian polytropic stellar model. The results are very temperature-sensitive: in hot neutron stars (T>109 K), the lowest-order r-modes are unstable, while in colder stars they are damped by viscosity. These calculations have a number of interesting astrophysical implications. First, the r-mode instability will spin down a newly born neutron star to a period close to the initial period inferred for the Crab pulsar, probably between 10 and 20 ms. Second, as an initially rapidly rotating star spins down, an energy equivalent to roughly 1% of a solar mass is radiated as gravitational waves, which makes the process an interesting source for detectable gravitational waves. Third, the r-mode instability rules out the scenario in which millisecond pulsars are formed by accretion-induced collapse of a white dwarf; the new star would be hot enough to spin down to much slower rates. Stars with periods less than perhaps 10 ms must have been formed by spin-up through accretion in binary systems, where they remain colder than the Eddington temperature of about 108 K. More accurate calculations will be required to define the limiting spin period more reliably, and we discuss the importance of the major uncertainties in the stellar models, in the initial conditions after collapse, and in the physics of cooling, superfluidity, and the equation of state.

A HUBBLE SPACE TELESCOPE Survey for Resolved Companions of Planetary Nebula Nuclei

Abstract: We report the results of a Hubble Space Telescope "snapshot" survey aimed at finding resolved binary companions of the central stars of Galactic planetary nebulae (PNe) Using the the Wide Field and Planetary Camera and Wide Field Planetary Camera 2, we searched the fields of 113 PNe for stars whose close proximity to the central star suggests a physical association In all, we find 10 binary nuclei that are very likely to be physically associated and another six that are possible binary associations By correcting for interstellar extinction and placing the central stars' companions on the main sequence (or, in one case, on the white dwarf cooling curve), we derive distances to the objects, and thereby significantly increase the number of PNe with reliable distances Comparison of our derived distances with those obtained from various statistical methods shows that all of the latter have systematically overestimated the distances, by factors ranging up to a factor of 2 or more We show that this error is most likely due to the fact that the properties of our PNe with binary nuclei are systematically different from those of PNe used heretofore to calibrate statistical methods Specifically, our PNe tend to have lower surface brightnesses at the same physical radius than the traditional calibration objects This difference may arise from a selection effect: the PNe in our survey are typically nearby, old nebulae, whereas most of the objects that calibrate statistical techniques are low-latitude, high surface brightness, and more distant nebulae As a result, the statistical methods that seem to work well with samples of distant PNe, for example, those in the Galactic bulge or external galaxies, may not be applicable to the more diverse population of local PNe Our distance determinations could be improved with better knowledge of the metallicities of the individual nebulae and central stars, measurements of proper motions and radial velocities for additional candidate companions, and deeper HST images of several of our new binary nuclei

Iron Line Diagnostics of the Postshock Hot Plasma in MagneticCataclysmic Variables Observed with ASCA

Abstract: The ASCA spectra of ~20 magnetic cataclysmic variables are presented. Owing to the high spectral resolution of the solid-state imaging spectrometer, we successfully resolved the iron Kα emission line into the fluorescent (6.4 keV) and plasma (6.7 and 7.0 keV) components. By comparing the ionization temperature, which is obtained from the intensity ratio of the plasma line components of iron, with the continuum temperature, we have obtained the evidence that the postshock plasma has a temperature distribution. Detailed analysis indicates that the observed temperature distribution is consistent with that expected from the postshock plasma model in the bremsstrahlung cooling domain. In the framework of this postshock plasma model, we have constrained the mass of the white dwarf in totally nine intermediate polars and obtained the iron abundances. The obtained masses are generally consistent with the previous X-ray work. We have found that the iron abundance is generally subsolar, and its distribution probably peaks at 0.2-0.6 solar. We have also shown that the reflection from the white dwarf surface makes significant contribution to the observed fluorescent iron Kα emission line.

Cooling models for old white dwarfs

Abstract: We present new white dwarf cooling models that incorporate an accurate outer boundary condition based on new opacity and detailed radiative transfer calculations. We —nd that helium-atmosphere dwarfs cool considerably faster than has previously been claimed, while old hydrogen-atmosphere dwarfs will deviate signi—cantly from blackbody appearance. We use our new models to derive age limits for the Galactic disk. We —nd that the Liebert, Dahn, & Monet 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. are both consistent with disk ages as large as D11 Gyr. We have also used our models to place constraints on white dwarf dark matter in the Galactic halo. We —nd 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 cooling sequences of white dwarfs in globular clusters. Subject headings: Galaxy: fundamental parametersGalaxy: halosolar neighborhoodstars: evolutionstars: fundamental parameters

Evolution of 3-9 M☉ Stars for Z = 0.001-0.03 and Metallicity Effects on Type Ia Supernovae

Abstract: Recent observations have revealed that Type Ia supernovae (SNe Ia) are not perfect standard candles; they show variations in their absolute magnitudes, light-curve shapes, and spectra. The C/O ratio in the SNe Ia progenitors (C-O white dwarfs) may be related to this variation. In this work, we systematically investigate the effects of stellar mass (M) and metallicity (Z) on the C/O ratio and its distribution in the C-O white dwarfs by calculating stellar evolution from the main sequence through the end of the second dredge-up for M=3-9 M? and Z=0.001-0.03. We find that the total carbon mass fraction just before SN Ia explosion varies in the range 0.36-0.5. We also calculate the metallicity dependence of the main-sequence mass range of the SN Ia progenitor white dwarfs. Our results show that the maximum main-sequence mass to form C-O white dwarfs decreases significantly toward lower metallicity, and the number of SN Ia progenitors may be underestimated if metallicity effect is neglected. We discuss the implications of these results on the variation of SNe Ia, determination of cosmological parameters, luminosity function of white dwarfs, and galactic chemical evolution.

White dwarfs in cataclysmic variables.

Abstract: Prior to the last 15 years, the literature on cataclysmic variables (CVs) contained little on the properties of the underlying white dwarf accreter other than estimates of their masses. They were regarded simply as nondescript potential wells for studies of accretion disk structure and stability. Estimates of their masses and associated core compositions were discussed, but only in the context of thermonuclear runaway theory. With the advent of space spectroscopy, especially HST, IUE, EUVE, and HUT, direct spectroscopic observations of exposed white dwarfs in CVs were carried out during dwarf nova quiescence or low brightness states of nova-like variables and magnetic CVs when accretion rates were very low. This review covers new insights and physical properties of white dwarfs in nonmagnetic and magnetic CVs, including surface temperatures, heating and cooling measurements, rotational velocities, CV white dwarf masses (including Einstein redshift masses), photospheric chemical abundances (including composition relics of ancient novae), accretion belts, the physics of accretion heating, and long-term CV evolution. For an ensemble of 37 CV degenerates with secure temperatures, the average Teff=20,800 K, for nonmagnetic CV degenerates Teff=24,100 K, and for magnetic CV degenerates Teff=16,400 K. The lowest Teff values are associated with magnetic CVs and with systems whose orbital periods are less than 2 hr. Thermal e-folding times of the white dwarf envelope in response to accretion heating are in the range 6-600 days, photospheric chemical abundances range from moderately subsolar to greatly above solar, and rotational velocities lie within the range 50 km s–1 < V sin i < 1200 km s–1. If pre-CV white dwarfs had time to cool to 10-3L⊙ prior to the onset of Roche lobe overflow, then their average lower limit lifetime is 2.5 × 108 yr, and they have been accretion heated to an average luminosity log (L/L⊙) = 1.90 or, on average, by ~ 11,000 K since the end of their pre-CV evolution phase.

Magnetically Driven Warping, Precession and Resonances in Accretion Disks

Abstract: Interaction between an accretion disk and a magnetic star lies at the heart of the physics of a variety of astrophysical systems, including accreting neutron stars, white dwarfs and pre-main-sequence stars (e.g., Frank et al. 1992). The basic picture of disk—magnetosphere interaction was first outlined by Pringle & Rees (1972), following the discovery of accretion-powered X-ray pulsars. These are rotating, highly magnetized(B ti 1012G) neutron stars (NSs) that accrete material from a companion star, either directly from a stellar wind, or in the form of an accretion disk. The strong magnetic field disrupts the accretion flow at the magnetospheric boundary (typically at a few hundreds NS radii), and channels the plasma onto the polar caps of the NS. The magnetosphere boundary is located where the magnetic and plasma stresses balance, rrn_ qµ4/7(GMM2)-1/7, whereMand tt are the mass and magnetic moment of the central star,Mis the mass accretion rate,gis a dimensionless constant of order unity. In low-mass X-ray binaries containing weakly magnetized(B108G) NSs, the magnetosphere lies close to the stellar surface. In this case, complex field topology and general relativity can affect the determination of the magnetosphere boundary (e.g., Lai 1998). Similar magnetosphere—disk interaction also occurs in T Tauri stars (e.g., Hartmann 1998), where the stellar magnetic field(B103G) strongly affects the accretion flow, as well as in certain class (the so-called DQ Her stars or intermediate polars) of magnetic CVs.

A Comparative Study of the Mass Distribution of Extreme-Ultraviolet selected White Dwarfs

Abstract: We present new determinations of effective temperature, surface gravity, and masses for a sample of 46 hot DA white dwarfs selected from the Extreme Ultraviolet Explorer (EUVE) and ROSAT Wide Field Camera bright source lists in the course of a near-infrared survey for low-mass companions. Our analysis, based on hydrogen non-LTE model atmospheres, provides a map of LTE correction vectors, which allow a thorough comparison with previous LTE studies. We find that previous studies underestimate both the systematic errors and the observational scatter in the determination of white dwarf parameters obtained via fits to model atmospheres. The structure of very hot or low-mass white dwarfs depends sensitively on their history. To compute white dwarf masses, we thus use theoretical mass-radius relations that take into account the complete evolution from the main sequence. We find a peak mass of our white dwarf sample of 0.59 M☉, in agreement with the results of previous analyses. However, we do not confirm a trend of peak mass with temperature reported in two previous analyses. Analogous to other EUV-selected samples, we note a lack of low-mass white dwarfs and a large fraction of massive white dwarfs. Only one white dwarf is likely to have a helium core. While the lack of helium white dwarfs in our sample can be easily understood from their high cooling rate, and therefore low detection probability in our temperature range, this is not enough to explain the large fraction of massive white dwarfs. This feature very likely results from a decreased relative sample volume for low-mass white dwarfs caused by interstellar absorption in EUV-selected samples.

Merging White Dwarf/Black Hole Binaries and Gamma-Ray Bursts

Abstract: The merger of compact binaries, especially black holes and neutron stars, is frequently invoked to explain gamma-ray bursts (GRBs). In this paper, we present three-dimensional hydrodynamical simulations of the relatively neglected mergers of white dwarfs and black holes. During the merger, the white dwarf is tidally disrupted and sheared into an accretion disk. Nuclear reactions are followed, and the energy release is negligible. Peak accretion rates are ~0.05 M☉ s-1 (less for lower mass white dwarfs) and last for approximately a minute. Many of the disk parameters can be explained by a simple analytic model that we derive and compare to our simulations. This model can be used to predict accretion rates for white dwarf and black hole (or neutron star) masses that are not simulated here. Although the mergers studied here create disks with larger radii and longer accretion times than those from the merger of double neutron stars, a larger fraction of the white dwarf's mass becomes part of the disk. Thus the merger of a white dwarf and a black hole could produce a long-duration GRB. The event rate of these mergers may be as high as 10-6 yr-1 per galaxy.

Cooling Models for Old White Dwarfs

Abstract: 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 \sim 11 Gyr. We have also used our models to place constraints on white dwarf 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 cooling sequences of white dwarfs in Globular clusters.

Spectroscopic Studies of DB White Dwarfs: The Instability Strip of the Pulsating DB (V777 Herculis) Stars

Abstract: We have secured optical spectra for the eight currently known variable DB, or V777 Her, stars. With the help of a new generation of synthetic spectra, spectroscopic effective temperatures are derived for these objects, as well as for 15 other DB or DBA stars above 20,000 K. We find that the location of the boundaries of the instability strip is sensitive to the atmospheric hydrogen abundance assumed for DB stars: the strip covers the range 22,400-27,800 K if atmospheres of pure helium are used and the range 21,800-24,700 K if undetectable traces of hydrogen are allowed for in the DB models. These determinations provide independent constraints for current seismological analyses of the V777 Her stars. More sensitive searches for weak hydrogen features in hot DB stars should help decide between the two temperature scales.

A survey for cool white dwarfs and the age of the Galactic disc

Abstract: We describe a new multi-colour proper motion survey for cool white dwarfs (CWDs). The observational database consists of ~300 digitally scanned Schmidt plates in ESO/SERC field 287. The entire survey procedure, from the raw Schmidt plate data to final white dwarf luminosity function (WDLF) is described, with special emphasis on completeness concerns. We obtain a sample of 58 WDs, for which we have follow up CCD photometry and spectroscopy of a representative sub--sample. Effective temperatures and luminosities of our sample objects are determined by comparing photometry with the model atmosphere predictions of Bergeron, Saumon and Wesemael. Space densities are calculated using an adaptation of Schmidts (1/Vmax) method, from which a WDLF is constructed. Comparison of our observational LF with the models of both Wood and Garcia-Berro et al. indicate an age for the local Galactic Disc of 10{-1}_{+3}Gyr. Importantly, we find no evidence of incompleteness in our survey sample. Proper motion number counts imply the survey is complete, and the WD sample passes the (V/V_{max}) completeness test.

Pure Hydrogen Model Atmospheres for Very Cool White Dwarfs

Abstract: 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 H-R diagrams of several globular clusters. Because of the unavailability of white dwarf atmospheres for Teff < 4000 K, the cooling timescales 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 Letter, we present the first detailed model atmospheres and spectra of very cool hydrogen white dwarfs for Teff < 4000 K. We include the latest description of the opacities of hydrogen, and, significantly, we introduce a nonideal equation of state in the atmosphere calculation. We find that due to strong absorption from H2 in the infrared, very old white dwarfs are brightest in the V, R, and I bands, and we confirm that they become bluer in most color indices as they cool below Teff ≈ 3500 K.

Effects of gravity on the structure of post-shock accretion flows in magnetic cataclysmic variables

Abstract: We calculate the temperature and density structure of the hot post-shock plasma in magnetically confined accretion flows, including the gravitational potential. This avoids the inconsistency of previous calculations which assume that the height of the shock is negligible. We assume a stratified accretion column with 1D flow along the symmetry axis. We find that the calculations predict a lower shock temperature than previous calculations, with a flatter temperature profile with height. We revise previous determinations of the masses of the white dwarf primary stars, and find that for higher mass white dwarfs there is a general reduction in derived masses when the gravitational potential is included. This is because the spectrum from such flows is harder than that of previous prescriptions at intermediate energies.

The origin of the diversity of type Ia supernovae and the environmental effects

Abstract: Observations suggest that the properties of Type Ia supernovae (SNe Ia) may depend on environmental characteristics, such as the morphology, metallicity, and age of the host galaxies. The influence of these environmental properties on the resulting SNe Ia is studied in this Letter. First, it is shown that the carbon mass fraction X(C) in the C + O white dwarf SN Ia progenitors tends to be smaller for a lower metallicity environment and an older binary system. It is then suggested that the variation of X(C) causes the diversity in the brightness of SNe Ia: a smaller X(C) leads to a dimmer SN Ia. Further studies of the propagation of the turbulent flame are necessary to confirm this relation. Our model for the SN Ia progenitors then predicts that when the progenitors belong to an older population or to a low-metallicity environment, the number of bright SNe Ia is reduced, so that the variation in brightness among the SNe Ia is also smaller. Thus, our model can explain why the mean SN Ia brightness and its dispersion depend on the morphology of the host galaxies and on the distance of the SN from the center of the galaxy. It is further predicted that at higher redshift (z 1), both the mean brightness of SNe Ia and its variation should be smaller in spiral galaxies than in elliptical galaxies. These variations are within the range observed in nearby SNe Ia. Insofar as the variation in X(C) is the most important cause for the diversity among SNe Ia, the light-curve shape method that is currently used to determine the absolute magnitude of SNe Ia can also be applied to high-redshift SNe Ia.

Measuring the Remnant Mass Function of the Galactic Bulge

Abstract: I show that by observing microlensing events both astrometrically and photometrically, the Space Interferometry Mission (SIM) can measure the mass function of stellar remnants in the Galactic bulge including white dwarfs, neutron stars, and black holes. Neutron stars and black holes can be identified individually, while white dwarfs are detected statistically from the sharp peak in their mass function near M~ 0.6Msun. This peak is expected to be more than twice as high as the `background' of main-sequence microlenses. I estimate that of order 20% of the ~400 bulge microlensing events detected to date are due to remnants, but show that these are completely unrecognizable from their time scale distribution (the only observable that `normal' microlensing observations produce). To resolve the white-dwarf peak, the SIM mass measurements must be accurate to ~5%, substantially better than is required to measure the mass function of the more smoothly distributed main sequence. Nevertheless, SIM could measure the masses of about 20 bulge remnants in 500 hours of observing time.

The Origin of Diversity of Type Ia Supernovae and Environmental Effects

Abstract: Observations suggest that the properties of Type Ia supernovae (SNe Ia) may depend on environmental characteristics, such as morphology, metallicity, and age of host galaxies. The influence of these environmental properties on the resulting SNe Ia is studied in this paper. First it is shown that the carbon mass fraction X(C) in the C+O white dwarf SN Ia progenitors tends to be smaller for lower metallicity and older the binary system age. It is then suggested that the variation of X(C) causes the diversity in the brightness of SNe Ia: a smaller X(C) leads to a dimmer SN Ia. Further studies of the propagation of the turbulent flame are necessary to confirm this relation. Our model for the SN Ia progenitors then predicts that when the progenitors belong to an older population or to a low metallicity environment, the number of bright SNe Ia is reduced, so that the variation in brightness among the SNe Ia is also smaller. Thus our model can explain why the mean SN Ia brightness and its dispersion depend on the morphology of the host galaxies and on the distance of the SN from the center of the galaxy. It is further predicted that at higher redshift (z >~ 1) both the the mean brightness of SNe Ia and its variation should be smaller in spiral galaxies than in elliptical galaxies. These variations are within the range observed in nearby SNe Ia. In so far as the variation in X(C) is the most important cause for the diversity among SNe Ia, the light curve shape method currently used to determine the absolute magnitude of SNe Ia can be applied also to high redshift SNe Ia.

Grids of white dwarf evolutionary models with masses from M=0.1 to 1.2 m⊙

Abstract: We present detailed evolutionary calculations for carbon--oxygen- and helium-core white dwarf models with masses ranging from M= 0.1 to 1.2 M⊙ and for metallicities Z = 0.001 and 0. The sequences cover a wide range of hydrogen envelopes as well. We have taken finite-temperature effects fully into account by means of a detailed white dwarf evolutionary code, in which updated radiative opacities and equations of state for hydrogen and helium plasmas are considered. The energy transport by convection is treated within the formalism of the full-spectrum turbulence theory, as given by the self-consistent model of Canuto, Goldman & Mazzitelli. Convective mixing, crystallization, hydrogen burning and neutrino energy losses are taken into account as well. The set of models presented here is very detailed and should be valuable, particularly for the interpretation of observational data on low-mass white dwarfs recently discovered in numerous binary configurations, and also for the general problem of determining the theoretical luminosity function for white dwarfs. In this context, we compare our cooling sequences with the observed white dwarf luminosity function recently improved by Leggett, Ruiz & Bergeron and we obtain an age for the Galactic disc of ≈ 8 Gyr. Finally, we apply the results of this paper to derive stellar masses of a sample of low-mass white dwarfs.

Limb Darkening of a K Giant in the Galactic Bulge: PLANET Photometry of MACHO 97-BLG-28

Abstract: We present the PLANET photometric data set10 for the binary-lens microlensing event MACHO 97- BLG-28, consisting of 696 I- and V -band measurements, and analyze it to determine the radial surface brightness pro—le of the Galactic bulge source star. The microlensed source, demonstrated to be a K giant by our independent spectroscopy, crossed an isolated cusp of the central caustic of the lensing binary, generating a sharp peak in the light curve that was well-resolved by dense (3¨30 minute) and continuous monitoring from PLANET sites in Chile, South Africa, and Australia. This is the —rst time that such a cusp crossing has been observed. Analysis of the PLANET data set has produced a measure- ment of the square-root limb-darkening coefficients of the source star in the I and V bands; the resulting stellar pro—les are in excellent agreement with those predicted by stellar atmospheric models for K giants. The limb-darkening coefficients presented here are the —rst derived from microlensing. They are also among the —rst found for normal giants by any technique and the —rst for any star as distant as the Galactic bulge. Modeling of our light curve for MACHO 97-BLG-28 indicates that the lensing binary has a mass ratio q \ 0.23 and an (instantaneous) separation in units of the angular Einstein ring radius of d \ 0.69. For a lens in the Galactic bulge, this corresponds to a typical stellar binary with a projected separation between 1 and 2 AU. If the lens lies closer (i.e., in the Galactic disk), the separation is smaller, and one or both of the lens objects is in the brown dwarf regime. Assuming that the source is a bulge K2 giant at 8 kpc, the relative lens-source proper motion is k \ 19.4 ^ 2.6 km s~1 kpc~1, consistent with a disk or bulge lens. If the nonlensed blended light is due to a single star, it is likely to be a young white dwarf in the bulge, consistent with the blended light coming from the lens itself. Subject headings: binaries: visualgravitational lensingstars: fundamental parameters ¨ stars: late-type

Effects of Gravity on the Structure of Postshock Accretion Flows in Magnetic Cataclysmic Variables

Abstract: We have calculated the temperature and density structure of the hot postshock plasma in magnetically confined accretion flows, including the gravitational potential. This avoids the inconsistency of previous calculations which assume that the height of the shock is negligible. We assume a stratified accretion column with 1-d flow along the symmetry axis. We find that the calculations predict a lower shock temperature than previous calculations, with a flatter temperature profile with height. We have revised previous determinations of the masses of the white dwarf primary stars and find that for higher mass white dwarfs there is a general reduction in derived masses when the gravitational potential is included. This is because the spectrum from such flows is harder than that of previous prescriptions at intermediate energies.

The fraction of double degenerates among DA white dwarfs

Abstract: We present the results of a radial velocity survey designed to measure the fraction of double degenerates among DA white dwarfs. The narrow core of the Hα line was observed twice or more for 46 white dwarfs yielding radial velocities accurate to a few km s−1. This makes our survey the most sensitive to the detection of double degenerates undertaken to date. We found no new double degenerates in our sample, though Hα emission from distant companions is seen in two systems. Two stars known to be double degenerates prior to our observations are included in the analysis. We find a 95 per cent probability that the fraction of double degenerates among DA white dwarfs lies in the range [0.017, 0.19].