Showing papers on "White dwarf published in 1998"

Nucleosynthesis in Classical Novae: CO versus ONe White Dwarfs

Abstract: 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☉) and mixing levels between the accreted envelope and the underlying white dwarf core (25%-75%). The synthesis of each isotope from 1H to 40Ca is discussed, along with its sensitivity to model parameters. Special emphasis is placed on isotopes such as 13C,15N, and 17O, whose synthesis may account for a significant fraction of their Galactic content. Production of the radioactive isotopes 7Be,22Na, and 26Al is also analyzed, since they may provide a direct test of the thermonuclear runaway model through their γ-ray emission. The resulting elemental yields reproduce the spectroscopic abundance determinations of several well-studied classical novae fairly well.

Nucleosynthesis in Classical Novae and Its Contribution to the Interstellar Medium

Abstract: Classical novae, explosions that result from thermonuclear runaways (TNRs) on the surfaces of white dwarfs (WDs) accreting hydrogen-rich matter in close binary systems, are sporadically injecting material processed by explosive hydrogen-burning nucleosynthesis into the interstellar medium (ISM). Although novae probably have processed less than »0.3% of the interstellar matter in the Galaxy, both theoretical and observational evidence suggests that they may be important sources of the nuclides 7 Li, 15 N, and 17 O, as well as the radioactive isotopes 22 Na and 26 Al. The latter nuclides are astrophysically important in that they may have been involved in the production of the 22 Ne (Ne-E) and 26 Mg enrichments identified in meteoritic inclusions, the composition of which is thought to be representative of the chemical and mineral contents of the primitive solar nebula. These inclusions may be partially composed of dust condensed in nova outbursts. We review theoretical expectations for the yields of various isotopes in nova outbursts and conclude that any of the heavy isotope anomalies attributable to novae are most likely produced by the approximately 25%-33% of novae that occur in systems containing massive ( M,) oxygen-neon-magnesium (ONeMg) WDs. We attempt to place quantitative constraints M 1 1.2 ⁄ on the degree to which classical novae participate in the production of chemical anomalies, both in the primitive solar system and on a Galactic scale. Diffuse Galactic g-ray fluxes provide particularly important clues to and constraints on the 22 Na and 26 Al yields from novae. Ultraviolet (UV), optical, and infrared (IR) emission-line spectra of classical novae reveal the abundances of some of the gas-phase elements present in the ejecta; recent results are reviewed. We describe how IR observations of novae reveal dust formation and gas-phase line emission and how they distinguish the temporal development of nova explosions on carbon-oxygen (CO) WDs (CO novae) from those on ONeMg WDs (ONeMg or "neon" novae). Recent studies show that the ejecta in some novae can be strongly cooled by near- and mid-IR forbidden-line radiation from highly ionized ("coronal") atomic states. We compare the abundances deduced from recent UV, optical, and IR observations with theoretical predictions, and we suggest that future studies of IR coronal emission lines may provide additional key information. Novae produce only about 0.1% of the Galactic "stardust" (dust condensed in stellar outflows), but IR observations show that it may be some of the more interesting dust. Novae appear capable of producing astrophysical dust of virtually every known chemical and mineral composition. We summarize recent IR observations of the dust production scenario in novae and argue that neon novae may lead to the formation of dust grains that carry the Ne-E and 26 Mg anomalies.

Type Ia Supernovae: Influence of the Initial Composition on the Nucleosynthesis, Light Curves, and Spectra and Consequences for the Determination of ΩM and Λ

Abstract: The influence of the initial composition of the exploding white dwarf on the nucleosynthesis, light curves, and spectra of Type Ia supernovae has been studied in order to evaluate the size of evolutionary effects on cosmological timescales, how the effects can be recognized, and how one may be able to correct for them. The calculations are based on a set of delayed detonation models that give a good account of the optical and infrared light curves and of the spectral evolution. The explosions and light curves are calculated using a one-dimensional Lagrangian radiation-hydro code including a nuclear network. Spectra are computed for various epochs using the structure resulting from the light-curve code. Our non-LTE code solves the relativistic radiation transport equations in the comoving frame consistently with the statistical equations and ionization due to γ-radiation for the most important elements (C, O, Ne, Na, Mg, Si, S, Ca, Fe, Co, Ni). About 106 additional lines are included assuming LTE-level populations and an equivalent-two-level approach for the source functions. Changing the initial metallicity Z from Population I to Population II alters the isotopic composition of the outer layers of the ejecta that have undergone explosive O burning. Especially important is the increase of the 54Fe production with metallicity. The influence on the resulting rest-frame visual and blue light curves is found to be small. Detailed analysis of spectral evolution should permit a determination of the progenitor metallicity. Mixing 56Ni into the outer layers during the explosion can produce effects similar to an increased initial metallicity. Mixing can be distinguished from metallicity effects by means of the strong cobalt and nickel lines, by a change of the calcium lines in the optical and IR spectra and, in principle, by γ-ray observations. As the C/O ratio of the white dwarf is decreased, the explosion energy and the 56Ni production are reduced, and the Si-rich layers are more confined in velocity space. A reduction of the C/O ratio by about 60% gives slower rise times by about three days, an increased luminosity at maximum light, a somewhat faster postmaximum decline, and a larger ratio between maximum light and 56Ni tail. A reduction of the C/O ratio has an effect on the colors, light-curve shapes and element distribution similar to a reduction in the deflagration to detonation transition density. However, for the same light-curve shape, the absolute brightness is larger for smaller C/O ratios. An independent determination of the initial C/O ratio and the transition density is possible for local supernovae if detailed analyses of both the spectra and light curves are performed simultaneously. Because the spectra are shifted into different color bands at different redshifts, the effect of metallicity Z on a given observed color is a strong function of redshift. A change of Z by a factor of 3 or of the C/O ratio by 33% alters the peak magnitudes in the optical wavelength range by up to ≈ 0.3 mag for z ≥ 0.2. These variations are comparable to the effect of changes of ΩM and Λ at redshifts of 0.5-1.0. The systematic effects due to changes in composition are expected to remain small up to about z ≈ 0.5 for R-V and up to z ≈ 0.7 for R-I. We discuss how evolution in the progenitor population can be recognized and taken into account. With proper account of evolutionary corrections, supernovae will provide a valuable tool to determine the cosmological parameters of the universe, and they will provide new insight into its chemical evolution.

Low-Metallicity Inhibition of Type Ia Supernovae and Galactic and Cosmic Chemical Evolution

Abstract: We introduce a metallicity dependence of the Type Ia supernova (SN Ia) rate into the Galactic and cosmic chemical evolution models. In our SN Ia progenitor scenario, the accreting white dwarf (WD) blows a strong wind to reach the Chandrasekhar mass limit. If the iron abundance of the progenitors is as low as [Fe/H] -1, then the wind is too weak for SNe Ia to occur. Our model successfully reproduces the observed chemical evolution in the solar neighborhood. We make the following predictions that can test this metallicity effect:(1) SNe Ia are not found in the low iron abundance environments such as dwarf galaxies and the outskirts of spirals. (2) The cosmic SN Ia rate drops at z~1-2 because of the low iron abundance, which can be observed with the Next Generation Space Telescope. At z1-2, SNe Ia can be found only in the environments where the timescale of metal enrichment is sufficiently short as in starburst galaxies and elliptical galaxies. The low-metallicity inhibition of SNe Ia can shed new light on the following issues: (1) The limited metallicity range of the SN Ia progenitors would imply that "evolution effects" are relatively small for the use of high-redshift SNe Ia to determine the cosmological parameters. (2) WDs of halo populations are poor producers of SNe Ia, so that the WD contribution to the halo mass is not constrained from the iron abundance in the halo. (3) The abundance patterns of globular clusters and field stars in the Galactic halo lack of SN Ia signatures in spite of their age difference of several Gyr, which can be explained by the low-metallicity inhibition of SNe Ia. (4) It could also explain why the SN Ia contamination is not seen in the damped Lyα systems over a wide range of redshift.

Interaction of Type Ia Supernovae with Their Surroundings

Abstract: Type Ia supernovae (SNe) are presumed to arise from white dwarf progenitors, which may not appreciably modify their ambient medium. We study the interaction of the resulting supernova remnants with a constant density interstellar medium. Density profiles obtained from detailed explosion models of Type Ia SN explosions can be complex, but an exponential profile gives the best approximate representation of a set of profiles, and we emphasize this case. We describe the time evolution of dynamical quantities (such as radius, velocity, and expansion parameter) as a result of the interaction in terms of dimensionless variables and present the profiles of physical quantities. We compare our results to the power-law and constant ejecta density cases; a characteristic feature of the exponential case is that the shocked ejecta have a relatively constant temperature. The effect of a possible circumstellar wind region close to the supernova is to create a dense, cool shell near the contact discontinuity between the shocked ejecta and the surrounding medium. The complex density structure found in some supernova models persists in the shocked layer, giving rise to density and temperature variations. We apply our results to the two likely historical Type Ia SNe, SN 1006 and Tycho. The observed angular sizes and expansion rates are consistent with a distance of 1.95 ± 0.4 kpc and an ambient H density of 0.05-0.1 cm-3 for SN 1006. For Tycho's SNR, the results are not conclusive but indicate a distance around 2.3 kpc for an ambient density of 0.6-1.1 cm-3. In both cases, the low expansion rate limits the extent of a possible circumstellar wind region. The evidence for temperature variations in the ejecta of Tycho's remnant suggests that the supernova profile was more complex than an exponential profile and contained density inhomogeneities, or that there was early interaction with a circumstellar wind.

The formation of double degenerates and related objects

Abstract: I systematically investigate the formation of double degenerates (DDs) via binary interactions. I consider three evolutionary channels for their formation [stable Roche lobe overflow (RLOF) plus common envelope (CE), CE plus CE, exposed core plus CE], and carry out Monte Carlo simulations. I explore the effects of model parameters, such as the tidal-enhancement parameter for stellar wind, the mass transfer efficiency for stable RLOF and the CE ejection efficiency, on my results. I also explore the effects of various assumptions about age, metallicity, mass ratio distribution and wind velocity. My results show that the model is successful in the explanation of the formation of DDs. I explain satisfactorily the distributions of masses, mass ratios, orbital periods and birth rate of the observed DDs. The main conclusions are the following. (i) Stable RLOF plus CE and CE plus CE are the main evolutionary scenarios leading to the formation of DDs. (ii) The Galactic birth rate of DDs is 0.03 yr(-1), and the birth rate of DDs with helium (He) white dwarfs (WDs) as brighter components is 0.017 yr(-1). (iii) The number of detectable DDs in our Galaxy is 3 x 10(6), and DDs with brighter He WDs make up 56 per cent. (iv) The distribution of orbital periods for detectable DDs peaks around 6 h. (v) WD 0957-666 and WD 1101+364 are formed through the stable RLOF plus CE scenario, and WD 0135-052 is possibly a carbon-oxygen (CO) WD pair rather than a helium (He) WD pair. (vi) The Galactic birth rates of close i, respectively. (vii) The mergers of two WD binaries and DD mergers are 0.074 and 0.029 yr(-1), respectively. (vii) The merges of two He WDs and the mergers of He and CO WDs have masses of 0.61 +/- 0.09 and 0.96 +/- 0.13 M., respectively. (viii) Mass transfer during stable RLOF is not conservative. (ix) A tidally enhanced stellar wind exists. I also investigate the formation of Type Ia supernovae (SNe Ia), cataclysmic variables (CVs), subdwarf O-type (sdO) stars and R Coronae Borealis (R CrB) stars. The birth rates of SNe Ia and CVs are successfully explained in the above model. The model also shows that CVs with long orbital periods tend to have CO WDs. The birth rates of the mergers of two He WDs (sdO stars) and the mergers of He and CO WDs (R CrB stars) are 0.006 and 0.018 yr(-1) in the Galaxy, respectively. The birth rates of CVs, DDs, DD mergers and SNe Ia are more sensitive to the recent stellar formation history than to the past one.

The Cool White Dwarf Luminosity Function and the Age of the Galactic Disk

Abstract: We present new optical and infrared data for the cool white dwarfs in the proper motion sample of Liebert, Dahn, & Monet. Stellar properties—surface chemical composition, effective temperature, radius, surface gravity, mass, and luminosity—are determined from these data by using the model atmospheres of Bergeron, Saumon, & Wesemael. The space density contribution is calculated for each star and the luminosity function (LF) for cool white dwarfs is determined. Comparing the LF to the most recent cooling sequences by Wood implies that the age of the local region of the Galactic disk is 8 ± 1.5 Gyr. This result is consistent with the younger ages now being derived for the globular clusters and the universe itself.

An oxygen-rich dust disk surrounding an evolved star in the Red Rectangle

Abstract: The Red Rectangle1 is the prototype of a class of carbon-rich reflection nebulae surrounding low-mass stars in the final stages of evolution. The central star of this nebula has ejected most of its layers (during the red-giant phase), which now form the surrounding cloud, and is rapidly evolving to a white dwarf. This star is also a member of a wide binary system2, which is surrounded by a thick, dusty disk of material3,4. Here we report infrared observations of the Red Rectangle that reveal the presence of oxygen-rich material: prominent emission bands from crystalline silicates, and absorption lines arising from carbon dioxide. The oxygen-rich material is located in the circumbinary disk, in contrast to the previously known carbon-rich dust, which is found mainly in the extended nebula5,6. The properties of the oxygen-rich dust are similar to those of dusty disks surrounding young stars7, which are believed to be the sites of planet formation. Grain processing, and perhaps even planet formation, may therefore also be occurring in the circumbinary disk of this evolved star.

The secondary stars in cataclysmic variables and low-mass X-ray binaries

Abstract: We critically re-examine the available data on the spectral types, masses and radii of the secondary stars in cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs), using the new catalogue of Ritter & Kolb as a starting point. We find there are 55 reliable spectral type determinations and only 14 reliable mass determinations of CV secondary stars (10 and 5, respectively, in the case of LMXBs). We derive new spectral type–period, mass–radius, mass–period and radius–period relations, and compare them with theoretical predictions. We find that CV secondary stars with orbital periods shorter than 7–8 h are, as a group, indistinguishable from main-sequence stars in detached binaries. We find that it is not valid, however, to estimate the mass from the spectral type of the secondary star in CVs or LMXBs. We find that LMXB secondary stars show some evidence for evolution, with secondary stars which are slightly too large for their mass. We show how the masses and radii of the secondary stars in CVs can be used to test the validity of the disrupted magnetic braking model of CV evolution, but we find that the currently available data are not sufficiently accurate or numerous to allow such an analysis. As well as considering secondary star masses, we also discuss the masses of the white dwarfs in CVs, and find mean values of M1 ¼ 0:69 6 0:13 M ⊙ below the period gap, and M1 ¼ 0:806 0:22 M⊙ above the period gap.

Inward Propagation of Nuclear-burning Shells in Merging C-O and He White Dwarfs

Abstract: We have investigated the consequences of merging double white dwarf systems by calculating evolutionary models of accreting white dwarfs. We have considered two cases: a massive C-O white dwarf of ~1 M☉ accreting C-O mixture and a low-mass white dwarf with an initial mass of 0.4 M☉ accreting matter composed mostly of helium. The accretion rate of the C-O white dwarf is assumed to be 1 × 10-5 M☉ yr-1. After carbon burning is ignited at Mr ~ 1.04 M☉, the flame propagates inward because of heat conduction. By inserting enough grid points to resolve the structure of the flame, we have obtained almost steady burning in most phases of evolution, but we have found a new phenomenon that the strength of flame sometimes oscillates because of a thermal instability in an early phase of the evolution. In calculating evolutionary models, we have occasionally employed a steady-burning approximation, in which the propagation speed of flame is given a priori. We have considered two extreme cases for the interior abundance of the massive white dwarf: XC = 0.5 and XC = 0.2. For XC = 0.5, the flame becomes very weak at Mr ~ 0.4 M☉ and the inward propagation stalls there. But a few thousand years later, the flame is reactivated because of contraction and propagates to the center. For XC = 0.2, the flame propagates smoothly and reaches the center in ~1000 yr. In both cases the C-O mixture has been burned into an O-Ne-Mg mixture without causing an explosive phenomenon. For helium-accreting low-mass white dwarfs, we have considered accretion rates of 1 × 10-7 and 1 × 10-6 M☉ yr-1. After a fraction of M☉ is accreted to the white dwarf, helium is ignited in the outer part and a shell flash occurs. Such a shell flash diminishes when about 10% of helium in the convective shell is burned into carbon and oxygen. The next shell flash occurs at a shell interior to the previously flashed shell. After less than 30 shell flashes, the helium ignition occurs at the center and steady burning begins. Thus, the merging produces a helium star that burns helium at the center. The mechanism of the propagation of the burning shell in this case is compressional heating during interpulse phases, in contrast to the case of the carbon-burning flame where the conduction drives the inward propagation of the flame. We infer that such a low-mass double white dwarf system could be a progenitor of the AM CVn stars.

Testing the White Dwarf Mass-Radius Relation with Hipparcos*

Abstract: We present the Hipparcos parallaxes and resulting radii for 10 white dwarfs in visual binaries or common proper-motion systems and 11 field white dwarfs. For bright stars, Hipparcos parallaxes have uncertainties approaching 1 mas and are thus considerably more accurate than earlier ground-based parallaxes. Overall, our results support the predictions of the white dwarf mass-radius relation and our understanding of stellar degeneracy. Our most important finding for an individual object is the position of 40 Eri B, a well-known puzzle, now consistent with single-star evolution. In addition, we present evidence supporting the existence of a range of atmosphere thicknesses for hydrogen (DA) white dwarfs.

Type Ia Supernovae: An Examination of Potential Progenitors and the Redshift Distribution

Abstract: We examine the possibility that Type Ia supernovae (SNe Ia) are produced by white dwarfs accreting from Roche lobe filling evolved companions, under the assumption that a strong optically thick stellar wind from the accretor is able to stabilize the mass transfer. We show that if a mass-transfer phase on a thermal timescale precedes a nuclear-burning driven phase, then such systems (of which the supersoft X-ray sources are a subgroup) can account for about 10% of the inferred SNe Ia rate. In addition, we examine the cosmic history of the supernova rate and show that the ratio of the rate of SNe Ia to the rate of supernovae produced by massive stars (supernovae of Types II, Ib, Ic) should increase from about z = 1 toward lower redshifts.

Evolutionary sequences for Nova V1974 Cygni using new nuclear reaction rates and opacities

Abstract: Accepted 1997 November 12. Received 1997 September 26; in original form 1997 January 24 ABSTRA C T The outburst of Nova V1974 Cyg 1992 is arguably the best observed of this century, with realistic estimates now available for the amount of mass ejected, the composition of the ejecta and the total energy budget. These data strongly support the conclusion that this was indeed a ‘neon’ nova that occurred on an oxygen, neon, magnesium white dwarf. In addition, X-ray studies of its outburst imply that the mass of the white dwarf is about 1.25 M(. We, therefore, report on the results of new calculations of thermonuclear runaways on 1.25-M( oxygen, neon, magnesium white dwarfs, using our one-dimensional, fully implicit, hydrodynamic stellar evolution code that includes a large nuclear reaction network. We have updated the nuclear reaction network, with the inclusion of new and improved experimental and theoretical determinations of the nuclear reaction rates. We have also incorporated the OPAL carbon rich tables and have investigated the effects of changes in convective efficiency on the evolution. Our results show that the changes in the reaction rates and opacities that we have introduced produce important changes with respect to our previous studies. For example, relevant to nucleosynthesis considerations, a smaller amount of 26 Al is produced, while the abundances of 31 P and 32 S increase by factors of more than two. This change is attributed to the increased proton-capture reaction rates for some of the intermediate mass nuclei near 26 Al and beyond, such that nuclear fusion to higher mass nuclei is enhanced. The characteristics of our models are then compared to observations of the outburst of V1974 Cyg 1992 and we find that the predicted amount of mass ejected is at least a factor of 10 less than observed. The low values for the amount of ejected mass are a consequence of the fact that the OPAL opacities are larger than those we previously used, which results in less mass being accreted on to the white dwarf. This is a general problem with respect to the comparison of observations and theory for ONeMg novae and we suggest a possible resolution of this discrepancy.

Evolution of 3-9 Mo 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 but show some 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 Mo 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 effectis neglected. We discuss the implications of these results on the variation of SNe Ia, determination of cosmological parameters, luminosity function of white dwarfs, and the galactic chemical evolution.

Old and blue white-dwarf stars as a detectable source of microlensing events

Abstract: The analysis1 of gravitational microlensing events of stars2,3 in the Large Magellanic Cloud places the masses of the lensing objects in the range 0.3–0.8 solar masses, suggesting that they might be old white-dwarf stars. Such objects represent the last stage of stellar evolution: they are the cooling cores of stars that have lost their atmospheres after nuclear fusion has ceased in their centres. If white dwarfs exist in abundance in the halo of our Galaxy, this would have profound implications for our understanding of the early generations of stars in the Universe4,5,6. Previous attempts to constrain theoretically6,7,8 the contribution of white dwarfs to microlensing indicate that they can account for only a small fraction of the events. But these estimates relied on models of white-dwarf cooling that are inadequate for describing the properties of the oldest such objects. Here I present cooling models appropriate for very old white dwarfs. I find, using these models, that the widely held notion that old white dwarfs are red applies only to those with a helium atmosphere; old white dwarfs with hydrogen atmospheres, which could be a considerable fraction of the total population, will appear rather blue, with colours similar to those of the faint blue sources in the Hubble Deep Field. Observational searches for the population of microlensing objects should therefore look for faint blue objects, rather than faint red ones.

Detailed Mid- and Far-Ultraviolet Model Spectra for Accretion Disks in Cataclysmic Binaries

Abstract: We present a large grid of computed far- and mid-ultraviolet spectra (850-2000 A) of the integrated light from steady-state accretion disks in luminous cataclysmic variables. The spectra are tabulated at 0.25 A intervals with an adopted FWHM resolution of 1.0 A, so they are suitable for use with observed spectra from a variety of modern space-borne observatories. Twenty-six different combinations of white dwarf mass M(sub wd) and mass accretion rate dot-m are considered, and spectra are presented for six different disk inclinations i. The disk models are computed self-consistently in the plane-parallel approximation, assuming LTE and vertical hydrostatic equilibrium, by solving simultaneously the radiative transfer, hydrostatic equilibrium, and energy balance equations. Irradiation from external sources is neglected. Local spectra of disk annuli are computed taking into account line transitions from elements 1-28 (H through Ni). Limb darkening as well as Doppler broadening and blending of lines are taken into account in computing the integrated disk spectra. The radiative properties of the models are discussed, including the dependence of ultraviolet fluxes and colors on M(sub wd), dot-m, and i. The appearance of the disk spectra is illustrated, with regard to changes in the same three parameters. Finally, possible future improvements to the present models and spectra are discussed.

Understanding the Cool DA White Dwarf Pulsator, G29-38

Abstract: The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post-main-sequence evolution, along with their cooling rates, which will allow us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DA variables (DAVs), which have not been explored previously through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning 10 yr, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modeling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the first time, we have made sense out of the complicated power spectra of a large-amplitude DA pulsator. We have shown that its seemingly erratic set of observed frequencies can be understood in terms of a recurring set of normal-mode pulsations and their linear combinations. With this result, we have opened the interior secrets of the DAVs to future asteroseismological modeling, thereby joining the rest of the known white dwarf pulsators.

Photometric separation of stellar properties using sdss filters

Abstract: Using synthetic photometry of Kurucz model spectra, we explore the colors of stars as a function of temperature, metallicity, and surface gravity with Sloan Digital Sky Survey (SDSS) filters, u'g'r'i'z'. The synthetic colors show qualitative agreement with the few published observations in these filters. We find that the locus of synthetic stars is basically two-dimensional for 4500 < T < 8000 K, which precludes simultaneous color separation of the three basic stellar characteristics we consider. Colors including u' contain the most information about normal stellar properties; measurements in this filter are also important for selecting white dwarfs. We identify two different subsets of the locus in which the loci separate by either metallicity or surface gravity. For 0.5 < g' - r' < 0.8 (corresponding roughly to G stars), the locus separates by metallicity; for photometric error of a few percent, we estimate metallicity to within ~0.5 dex in this range. In the range -0.15 < g' - r' < 0.00 (corresponding roughly to A stars), the locus shows separation by surface gravity. In both cases, we show that it is advantageous to use more than two colors when determining stellar properties by color. Strategic observations in SDSS filters are required to resolve the source of a ~5% discrepancy between synthetic colors of Gunn-Stryker stars, Kurucz models, and external determinations of the metallicities and surface gravities. The synthetic star colors can be used to investigate the properties of any normal star and to construct analytic expressions for the photometric prediction of stellar properties in special cases.

Explosion Diagnostics of Type Ia Supernovae from Early Infrared Spectra

Abstract: Models of infrared spectra of Type Ia supernovae around maximum light are presented. The underlying dynamic models are delayed detonation explosions in Chandrasekhar mass carbon/oxygen white dwarfs. In combination with the radiative transport codes employed here, these models provide plausible fits to the optical spectra of "normal" Type Ia supernovae. Two independent radiative transport codes are used, one that assumes LTE and one that computes non-LTE excitations and ionization. The models are compared with the infrared data available in the literature. The independent codes give a reasonable representation of the data and provide physical explanations for their origin independent of the detailed assumptions of the radiative transfer. The infrared gives an especially powerful diagnostic of the dynamic model because it probes different depths at the same epoch within the exploded white dwarf with strongly variable line-blanketing opacity. The velocity of the transition zone between explosive oxygen and carbon burning can be directly determined. The velocity at which the burning to nickel stops can also be probed. These velocities are very sensitive to the explosion physics.

White dwarf masses in magnetic cataclysmic variables: multi-temperature fits to Ginga data

Abstract: One method of obtaining the mass of the white dwarf in magnetic cataclysmic variables (mCVs) is through their hard X-ray spectra. However, previous mass estimates using this method give lower limits because the temperature of the plasma in the post-shock region (where the hard X-rays are emitted) is lower than the temperature of the shock itself. In AM Her systems, the additional cooling of the post-shock plasma by cyclotron emission will further lower the derived mass. Here we present estimates of the masses of the white dwarf in 13 mCVs derived using Ginga data and a model in which X-rays are emitted from a multi-temperature emission region with the appropriate temperature and density profile. We include in the model reflection from the surface of the white dwarf and a partially ionized absorber. We are able to achieve good fits to the data. We compare the derived masses with previous estimates and the masses for larger samples of isolated white dwarfs and those in CVs.

Close Binary White Dwarf Systems: Numerous New Detections and Their Interpretation

Abstract: We describe radial velocity observations of a large sample of apparently single white dwarfs (WDs), obtained in a long-term effort to discover close double-degenerate (DD) pairs, which might comprise viable Type Ia supernova (SN Ia) progenitors. We augment the WD sample with a previously observed sample of apparently single subdwarf B (sdB) stars, which are believed to evolve directly to the WD cooling sequence after the cessation of core helium burning. We have identified 18 new radial velocity variables, including five confirmed sdB + WD short-period pairs. Our observations are in general agreement with the predictions of the theory of binary star evolution. We describe a numerical method to evaluate the detection efficiency of the survey and estimate the number of binary systems not detected because of the effects of varying orbital inclination, orbital phase at the epoch of the first observation, and the actual temporal sampling of each object in the sample. Follow-up observations are in progress to solve for the orbital parameters of the candidate velocity variables.

Superhumps in Cataclysmic Binaries. XV. EG Cancri, King of the Echo Outbursts

Abstract: We report photometry and spectroscopy of the dwarf nova EG Cancri in its 1996/1997 episode of eruptions. The main eruption was clearly a superoutburst featuring common superhumps with a period of 0.06036(2) days, establishing the star as a new member of the SU UMa class of dwarf novae. At the end of the main eruption, the superhumps were replaced by another wave of slightly longer period (0.06045 days), which endured for at least another 90 days. The properties of the latter wave suggest identification as a "late superhump." The recurrence time for superoutbursts is long, probably in the range 7-20 yr. After the 15 day superoutburst, the star displayed six remarkable surges in light, with an average interval of 7 days; these "echo outbursts" strongly resemble ordinary dwarf nova eruptions. This suggests that a high accretion rate persisted in the disk for ~40 days after the main superoutburst. By a year after outburst, the star faded to V = 18.7, with most of the continuum light from a white dwarf of T ~ 15,000 K. We estimate a distance of 320 pc and a Mv = +11.5 for accretion light. The extreme faintness, the rarity of eruptions, the enormous duration of superhumps, and the quiescent spectrum together establish membership in the WZ Sge subclass, the most sluggish of dwarf novae. Spectroscopy at quiescence and a transient wave in early outburst establish an orbital period of 0.05997(9) days. This suggests a secondary star with M ~ 0.02 M⊙, yet R ~ 0.08 R⊙. A binary can reach such a state after the secondary is forced to lose mass on a timescale shorter than its Kelvin-Helmholtz timescale.

Pure hydrogen atmosphere 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 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 bands, and we confirm that they become bluer in most color indices as they cool below Teff ~ 3500K.

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 (GRB's). 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 Msun/s (less for lower mass white dwarfs) lasting for approximately a minute. Many of the disk parameters can be explained by a simple analytic model which 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 which are not simulated in this paper. 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 merging star'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 1/Myr per galaxy.

The X-Ray Spectral Evolution of Classical Nova V1974 Cygni 1992: A Reanalysis of the ROSAT Data

Abstract: We present a spectral analysis of the archival X-ray data of classical Nova V1974 Cygni 1992 (Nova Cygni 1992) obtained by the ROSAT Position Sensitive Proportional Counter (PSPC). The X-ray spectrum is fitted with a two-component model. The first component is a white dwarf atmosphere emission model developed for the remnants of classical novae near the Eddington luminosity. The model is used to fit the soft X-ray data in the ~0.1-1.0 keV range, where the bulk of emission is below 0.7 keV. The second component is a Raymond-Smith model of thermal plasma applied to the hard X-ray emission above ~1.0 keV. The postoutburst X-ray spectrum of the remnant white dwarf is examined in the context of evolution on the Hertzsprung-Russell diagram using an O-Ne- and a C-O-enhanced atmosphere emission model. A constant bolometric luminosity evolution is detected with increasing effective temperature and decreasing photospheric radius using the O-Ne-enhanced model. The unabsorbed soft X-ray flux for the constant bolometric luminosity phase is found to be in the range (1.7-2.2) × 10-7 ergs s-1 cm-2. A peak effective temperature of 51 eV (5.9 ×105 K) is detected 511 days after outburst. We also present the spectral development of the hard X-ray component. It is found to evolve independently of the soft one. The maximum of the hard X-ray emission is reached at ~150 days after outburst with an unabsorbed flux of ~2.0 × 10-11 ergs s-1 cm-2 corresponding to a luminosity of (0.8-2.0) × 1034 ergs s-1 at a 2-3 kpc source distance. The time evolution of the hard X-ray flux and the plasma temperatures decreasing from 10 keV to 1 keV suggest emission from shock-heated gas as the origin of the hard X-ray component.

Hydrogen-accreting Carbon-Oxygen White Dwarfs

Abstract: Matter of solar system composition has been added to the surfaces of two initially cool carbon-oxygen (CO) white dwarfs of masses 0.5 M☉ and 0.8 M☉ at rates in the range 10-8 to 10-6 M☉ yr-1. Four different regimes are encountered. (1) At the highest accretion rates, models become red giants after the accretion of only a very small amount of mass. As the accretion rate is decreased, models are encountered that (2) burn hydrogen at the same rate at which it is accreted, (3) experience a series of nondynamical hydrogen shell flashes followed eventually by a powerful helium shell flash, and, finally, (4) experience nova-like hydrogen shell flashes. Although all of the regimes have been explored, special attention has been given to models that experience recurrent mild hydrogen-burning pulses or burn hydrogen at a stationary rate. For lower accretion rates, the helium flash is so powerful that the convective layer forced by helium burning penetrates deeply into the hydrogen-rich envelope; this penetration may lead to the ejection of external layers even if the helium flash would not of itself have become dynamical. For higher accretion rates, even when convection does not penetrate into hydrogen-rich layers, the helium layer expands, and much, if not most, of the accreted matter is lost during the event because of the interaction of the expanded envelope with the companion star. Analysis of the results suggests that it is unlikely that, in the real world, a hydrogen-accreting CO white dwarf with a typical initial mass will attain the Chandrasekhar mass. Dynamical helium-burning flashes are probable.

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

Abstract: We present detailed evolutionary calculations for carbon - oxygen - and helium - core white dwarf (WD) models with masses ranging from M= 0.1 to M= 1.2 solar masses and for metallicities Z= 0.001 and Z= 0. The sequences cover a wide range of hydrogen envelopes as well. We employed a detailed WD evolutionary code. In particular, the energy transport by convectcion is treated within the formalism of the full spectrum turbulence theory. The set of models presented here is very detailed and should be valuable for the interpretation of the observational data on low - mass WDs recently discovered in numerous binary configurations and also for the general problem of determining the theoretical luminosity function for WDs. In this context, we compare our cooling sequences with the observed WD luminosity function recently improved by Leggett, Ruiz and Bergeron (1998) and we obtain an age for the Galactic disc of approximately 8 Gyr. Finally, we applied the results of this paper to derive stellar masses of a sample of low - mass white dwarfs.

The White Dwarf Cooling Age of M67

Abstract: A deep imaging survey covering the entire 23' diameter of the old open cluster M67 to V=25 has been carried out using the mosaic imager (UHCam) on the Canada-France-Hawaii Telescope. The cluster color-magnitude diagram (CMD) can be traced from stars on its giant branch at MV=+1 down through main-sequence stars at least as faint as MV=13.5. Stars this low in luminosity have masses below 0.15 M☉. A modest white dwarf (WD) cooling sequence is also observed commencing slightly fainter than MV=10 and, after correction for background galaxy and stellar field contamination, terminating near MV=14.6. The observed WDs follow quite closely a theoretical cooling sequence for 0.7 M☉ pure carbon core WDs with hydrogen-rich atmospheres (DA WDs). The cooling time to an MV of 14.6 for such WDs is 4.3 Gyr, which we take as the WD cooling age of the cluster. A fit of a set of isochrones to the cluster CMD indicates a turnoff age of 4.0 Gyr. The excellent agreement between these results suggests that ages derived from white dwarf cooling should be considered as reliable as those from other dating techniques. The WDs currently contribute about 9% of the total cluster mass, but the number seen appears to be somewhat low when compared with the number of giants observed in the cluster.

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

Abstract: We present the PLANET photometric dataset 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 profile of the Galactic bulge source star. The microlensed source, demonstrated to be a K giant by our independent spectroscopy, crossed the central isolated cusp 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. Our modeling of these data has produced stellar profiles for the source star in the I and V bands that are in excellent agreement with those predicted by stellar atmospheric models for K giants. The limb-darkening coefficients presented here are the first derived from microlensing, among the first for normal giants by any technique, and the first for any star as distant as the Galactic bulge. Modeling 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, 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 mu = 19.4 +/- 2.6 km/s /kpc, consistent with a disk or bulge lens. If the non-lensed 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.

Close Binary White Dwarf Systems: Numerous New Detections and Their Interpretation

Abstract: We describe radial velocity observations of a large sample of apparently single white dwarfs (WDs), obtained in a long-term effort to discover close, double-degenerate (DD) pairs which might comprise viable Type Ia Supernova (SN Ia) progenitors. We augment the WD sample with a previously observed sample of apparently single subdwarf B (sdB) stars, which are believed to evolve directly to the WD cooling sequence after the cessation of core helium burning. We have identified 18 new radial velocity variables, including five confirmed sdB+WD short-period pairs. Our observations are in general agreement with the predictions of the theory of binary star evolution. We describe a numerical method to evaluate the detection efficiency of the survey and estimate the number of binary systems not detected due to the effects of varying orbital inclination, orbital phase at the epoch of the first observation, and the actual temporal sampling of each object in the sample. Follow-up observations are in progress to solve for the orbital parameters of the candidate velocity variables.