Showing papers on "White dwarf published in 2004"

The effects of binary evolution on the dynamics of core collapse and neutron star kicks

Abstract: We systematically examine how the presence in a binary affects the final core structure of a massive star and its consequences for the subsequent supernova explosion Interactions with a companion star may change the final rate of rotation, the size of the helium core, the strength of carbon burning, and the final iron core mass Stars with initial masses larger than � 11 Mthat experience core collapse will generally have smaller iron cores at the point of explosion if they lost their envelopes through a binary interaction during or soon after core hydrogen burning Stars below � 11 M� , on the other hand, can end up with larger helium and metal cores if they have a close companion, since the second dredge-up phase that reduces the helium core mass dramatically in single stars does not occur once the hydrogen envelope is lost We find that the initially more massive stars in binary systems with masses in the range 8-11 Mare likely to undergo an electron-capture supernova, while single stars in the same mass range would end as ONeMg white dwarfs We suggest that the core collapse in an electron-capture supernova (and possibly in the case of relatively small iron cores) leads to a prompt or fast explosion rather than a very slow, delayed neutrino-driven explosion and that this naturally produces neutron stars with low-velocity kicks This leads to a dichotomous distribution of neutron star kicks, as inferred previously, where neutron stars in relatively close binaries attain low kick velocities We illustrate the consequences of such a dichotomous kick scenario using binary population synthesis simulations and discuss its implications This scenario has also important consequences for the minimum initial mass of a massive star that becomes a neutron star For single stars the critical mass may be as high as 10-12 M� , while for close binaries it may be as low as 6-8 M� T hese critical masses depend on the treatment of convection, the amount of convective overshooting, and the metal- licity of the star, and will generally be lower for larger amounts of convective overshooting and lower metallicity Subject heading

A fossil origin for the magnetic field in A stars and white dwarfs

Abstract: Some main-sequence stars of spectral type A are observed to have a strong (0.03–3 tesla), static, large-scale magnetic field, of a chiefly dipolar shape: they are known as ‘Ap stars’1,2,3,4, such as Alioth, the fifth star in the Big Dipper. Following the discovery of these fields, it was proposed that they are remnants of the star's formation, a ‘fossil’ field5,6. An alternative suggestion is that they could be generated by a dynamo process in the star's convective core7. The dynamo hypothesis, however, has difficulty explaining high field strengths and the observed lack of a correlation with rotation. The weakness of the fossil-field theory has been the absence of field configurations stable enough to survive in a star over its lifetime. Here we report numerical simulations that show that stable magnetic field configurations, with properties agreeing with those observed, can develop through evolution from arbitrary, unstable initial fields. The results are applicable equally to Ap stars, magnetic white dwarfs and some highly magnetized neutron stars known as magnetars. This establishes fossil fields as the natural, unifying explanation for the magnetism of all these stars.

The single-degenerate channel for the progenitors of Type Ia supernovae

Abstract: We have carried out a detailed studyh of one of the most favoured evolutionary channels for the production of Type Ia supernova (SN Ia) progenitors, the single-degenerate channel (CO + MS), where a carbon/oxygen (CO) white dwarf (WD) accretes matter from an unevolved or slightly evolved non-degenerate star until it reaches the Chandrasekhar mass limit. Employing Eggleton's stellar evolution code and adopting the prescription of Hachisu et al. for the accretion efficiency, we performed binary stellar evolution calculations for about 2300 close WD binary systems and mapped out the initial parameters in the orbital period-secondary mass (P-M(2)) plane (for a range of WD masses) which lead to a successful Type Ia supernova. We obtained accurate, analytical fitting formulae to describe this parameter range which can be used for binary population synthesis (BPS) studies. The contours in the P-M(2) plane differ from those obtained by Hachisu et al. for low-mass CO WDs, which are more common than massive CO WDs. We confirm that WDs with a mass as low as 0.67 M. can accrete efficiently and reach the Chandrasekhar limit. We have implemented these results in a BPS study to obtain the birth rates for SNe Ia and the evolution of birth rates with time of SNe Ia for both a constant star formation rate and a single starburst. The birth rates are lower than (but comparable to) those inferred observationally.

The Formation Rate, Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey

Abstract: Spectrophotometric observations at high signal-to-noise ratio were obtained of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey. Fits of observed Balmer lines to synthetic spectra calculated from pure-hydrogen model atmospheres were used to obtain robust values of Teff, log g, masses, radii, and cooling ages. The luminosity function of the sample, weighted by 1/Vmax, was obtained and compared with other determinations. The mass distribution of the white dwarfs is derived, after important corrections for the radii of the white dwarfs in this magnitude-limited survey and for the cooling time scales. The formation rate of DA white dwarfs from the PG is estimated to be 0.6x10^(-12) pc^(-3) yr^(-1). Comparison with predictions from a theoretical study of the white dwarf formation rate for single stars indicates that >80% of the high mass component requires a different origin, presumably mergers of lower mass double degenerate stars. In order to estimate the recent formation rate of all white dwarfs in the local Galactic disk, corrections for incompleteness of the PG, addition of the DB-DO white dwarfs, and allowance for stars hidden by luminous binary companions had to be applied to enhance the rate. An overall formation rate of white dwarfs recently in the local Galactic disk of 1.15+/-0.25x10^(-12) pc^(-3) yr^(-1) is obtained. Two recent studies of samples of nearby Galactic planetary nebulae lead to estimates around twice as high. Difficulties in reconciling these determinations are discussed.

Asymptotic giant branch stars

Abstract: Preface Chapter 1: Introduction: H.J. Habing and H. Olofsson N.B.: This chapter is not yet completed! 1.1 Bits of history 1.2 The structure of AGB stars 1.3 Observational characteristics of AGB stars 1.4 Distinctive properties of AGB stars Chapter 2: Evolution, Nucleosynthesis and Pulsation: P. Wood and J. Lattanzio 2.1 Basic observational properties 2.2 Pre-AGB evolution 2.3 Stellar evolution on the AGB 2.4 Evolution beyond the AGB: planetary nebula nuclei and white dwarfs 2.5 Nucleosynthesis in AGB stars 2.6 Variability 2.7 Conclusions and outlook Chapter 3: Synthetic AGB Evolution: M. Groenewegen, P. Marigo 3.1 The role of synthetic evolutionary models 3.2 A historical overview 3.3 The main ingredients of a synthetic AGB model 3.4 Stellar yields 3.5 From one star to population synthesis 3.6 Observational constraints 3.7 conclusions and outlook Chapter 4: Atmospheres: B. Gustafsson and S. Hofner 4.1 Introduction 4.2 Observations 4.3 Physics and characteristic conditions 4.4 The microscopic state of matter 4.5 The radiation .eld 4.6 The modelling of AGB star atmospheres 4.7 Dynamics 4.8 Mass loss 4.9 Abundances and other fundamental paramters 4.10 Conclusions and outlook Chapter 5: Molecule and grain formation: T. Miller 5.1 Introduction 5.2 Chemical processes for molecule and dust formation 5.3 Detailed models- carbon-rich envelopes 5.4 Detailed models- oxygen-rich envelopes 5.5 Complications 5.6 Conclusions and outlook Chapter 6: Dynamics and instabilities in dusty winds: Y. Simis and P. Woitke 6.1 Introduction 6.2 Modelling the AGB wind 6.3 Instabilities and structure in the out.ow 6.4 Conclusions and outlook Chapter 7: Circumstellar envelopes: H. Olofsson 7.1 Introduction 7.2 A 'standard' gas AGB-CSE 7.3 Circumstellar line observations 7.4 A 'standard' dustAGB-CSE 7.5 Circumstellar dust observations 7.6 Morphology and kinematics of AGB-CSEs 7.7 Mass-loss rate estimators 7.8 Mass-loss rate 7.9 Conclusions and outlook Chapter 8: AGB stars as tracers of a galactic population: H.J. Habing and P.A. Whitelock 8.1 Introduction 8.2 The Milky Way galaxy and its companions 8.3 M31 and its companions 8.4 The remaining members of the Local Group 8.5 AGB stars in galaxies outside of the Local Group 8.6 Conclusions and outlook Chapter 9: AGB stars in Binaries and their Progeny: A. Jorissen 9.1 The binary-AGB connection 9.2 AGB stars in binary systems 9.3 Impact of binarity on intrinsic properties of AGB stars 9.4 The progeny of AGB stars in binary system 9.5 Conclusions and outlook Chapter 10: Post-AGB stars: C.Waelkens and R.Waters 10.1 Introduction 10.2 Observational de.nition of a post-AGB star 10.3 Observed properties of post-AGB stars: the central star 10.4 Observed properties of post-AGB stars: the envelope 10.5 Binary post-AGB stars 10.6 Confrontation of observations with theory 10.7 Conclusions and outlook Index List of acronyms Some biographical notes about the authors

The binary progenitor of Tycho Brahe's 1572 supernova

Abstract: The brightness of type Ia supernovae, and their homogeneity as a class, makes them powerful tools in cosmology, yet little is known about the progenitor systems of these explosions. They are thought to arise when a white dwarf accretes matter from a companion star, is compressed and undergoes a thermonuclear explosion1,2,3. Unless the companion star is another white dwarf (in which case it should be destroyed by the mass-transfer process itself), it should survive and show distinguishing properties. Tycho's supernova4,5 is one of only two type Ia supernovae observed in our Galaxy, and so provides an opportunity to address observationally the identification of the surviving companion. Here we report a survey of the central region of its remnant, around the position of the explosion, which excludes red giants as the mass donor of the exploding white dwarf. We found a type G0–G2 star, similar to our Sun in surface temperature and luminosity (but lower surface gravity), moving at more than three times the mean velocity of the stars at that distance, which appears to be the surviving companion of the supernova.

A new synthetic model for asymptotic giant branch stars

Abstract: We present a synthetic model for thermally pulsing asymptotic giant branch (TPAGB) evolution constructed by fitting expressions to full evolutionary models in the metallicity range 0.0001 Z 0.02. Our model includes parametrizations of third dredge-up and hot-bottom burning with mass and metallicity. The Large Magellanic Cloud and Small Magellanic Cloud carbon star luminosity functions are used to calibrate third dredge-up. We calculate yields appropriate for galactic chemical evolution models for 1 H, 4 He, 12 C, 13 C, 14 N, 15 N, 16 O and 17 O. The

Mass transfer between double white dwarfs

Abstract: Three periodically variable stars have recently been discovered (V407 Vul, P=9.5 min; ES Cet, P=10.3 min; RX J0806.3+1527, P=5.3 min) with properties that suggest that their photometric periods are also their orbital periods, making them the most compact binary stars known. If true, this might indicate that close, detached, double white dwarfs are able to survive the onset of mass transfer caused by gravitational wave radiation and emerge as the semi-detached, hydrogen-deficient stars known as the AM CVn stars. The accreting white dwarfs in such systems are large compared to the orbital separations. This has two effects. First, it makes it likely that the mass-transfer stream can hit the accretor directly. Secondly, it causes a loss of angular momentum from the orbit which can destabilize the mass transfer unless the angular momentum lost to the accretor can be transferred back to the orbit. The effect of the destabilization is to reduce the number of systems which survive mass transfer by as much as one hundredfold. In this paper we analyse this destabilization and the stabilizing effect of a dissipative torque between the accretor and the binary orbit. We obtain analytical criteria for the stability of both disc-fed and direct impact accretion, and we carry out numerical integrations to assess the importance of secondary effects, the chief one being that otherwise stable systems can exceed the Eddington accretion rate. We show that to have any effect upon survival rates, the synchronizing torque must act on a time-scale of the order of 1000 yr or less. If synchronization torques are this strong, then they will play a significant role in the spin rates of white dwarfs in cataclysmic variable stars as well.

Type Ia Supernova Explosion: Gravitationally Confined Detonation

Abstract: We present a new mechanism for Type Ia supernova explosions in massive white dwarfs. The scenario follows from relaxing assumptions of symmetry and involves a detonation born near the stellar surface. The explosion begins with an essentially central ignition of a deflagration that results in the formation of a buoyancy-driven bubble of hot material that reaches the stellar surface at supersonic speeds. The bubble breakout laterally accelerates fuel-rich outer stellar layers. This material, confined by gravity to the white dwarf, races along the stellar surface and is focused at the location opposite to the point of the bubble breakout. These streams of nuclear fuel carry enough mass and energy to trigger a detonation just above the stellar surface that will incinerate the white dwarf and result in an energetic explosion. The stellar expansion following the deflagration redistributes mass in a way that ensures production of intermediate-mass and iron group elements with ejecta having a strongly layered structure and a mild amount of asymmetry following from the early deflagration phase. This asymmetry, combined with the amount of stellar expansion determined by details of the evolution (principally the energetics of deflagration, timing of detonation, and structure of the progenitor), can be expected to create a family of mildly diverse Type Ia supernova explosions.

Nucleosynthesis in multi-dimensional SN Ia explosions

Abstract: We present the results of nucleosynthesis calculations based on multi-dimensional (2D and 3D) hydrodynamical simulations of the thermonuclear burning phase in type Ia supernovae (hereafter SN la). The detailed nucleosynthetic yields of our explosion models are calculated by post-processing the ejecta, using passively advected tracer particles. The nuclear reaction network employed in computing the explosive nucleosynthesis contains 383 nuclear species, ranging from neutrons, protons, and α-particles to 98 Mo. Our models follow the common assumption that SN Ia are the explosions of white dwarfs that have approached the Chandrasekhar mass (M ch ∼ 1.39), and are disrupted by thermonuclear fusion of carbon and oxygen. But in contrast to 1D models which adjust the burning speed to reproduce lighteurves and spectra, the thermonuclear burning model applied in this paper does not contain adjustable parameters. Therefore variations of the explosion energies and nucleosynthesis yields are dependent on changes of the initial conditions only. Here we discuss the nucleosynthetic yields obtained in 2D and 3D models with two different choices of ignition conditions (centrally ignited, in which the spherical initial flame geometry is perturbated with toroidal rings, and bubbles, in which multi-point ignition conditions are simulated), but keeping the initial composition of the white dwarf unchanged. Constraints imposed on the hydrodynamical models from nucleosynthesis as well as from the radial velocity distribution of the elements are discussed in detail. We show that in our simulations unburned C and O varies typically from ∼40% to ∼50% of the total ejected material. Some of the unburned material remains between the flame plumes and is concentrated in low velocity regions at the end of the simulations. This effect is more pronounced in 2D than in 3D and in models with a small number of (large) ignition spots. The main differences between all our models and standard 1D computations are, besides the higher mass fraction of unburned C and O, the C/O ratio (in our case is typically a factor of 2.5 higher than in ID computations), and somewhat lower abundances of certain intermediate mass nuclei such as S, Cl, Ar, K, and Ca, and of 56 Ni. We also demonstrate that the amount of 56 Ni produced in the explosion is a very sensitive function of density and temperature. Because explosive C and O burning may produce the iron-group elements and their isotopes in rather different proportions one can get different 56 Ni-fractions (and thus supernova luminosities) without changing the kinetic energy of the explosion. Finally, we show that we need the high resolution multi-point ignition (bubbles) model to burn most of the material in the center (demonstrating that high resolution coupled with a large number of ignition spots is crucial to get rid of unburned material in a pure deflagration SN Ia model).

A Catalog of Spectroscopically Identified White Dwarf Stars in the First Data Release of the Sloan Digital Sky Survey

Abstract: We present the full spectroscopic white dwarf and hot subdwarf sample from the Sloan Digital Sky Survey (SDSS) first data release, DR1. We find 2551 white dwarf stars of various types, 240 hot subdwarf stars, and an additional 144 objects we have identified as uncertain white dwarf stars. Of the white dwarf stars, 1888 are nonmagnetic DA types and 171 are nonmagnetic DBs. The remaining (492) objects consist of all different types of white dwarf stars: DO, DQ, DC, DH, DZ, hybrid stars such as DAB, etc., and those with nondegenerate companions. We fit the DA and DB spectra with a grid of models to determine the Teff and log g for each object. For all objects, we provide coordinates, proper motions, SDSS photometric magnitudes, and enough information to retrieve the spectrum/image from the SDSS public database. This catalog nearly doubles the known sample of spectroscopically identified white dwarf stars. In the DR1 imaged area of the sky, we increase the known sample of white dwarf stars by a factor of 8.5. We also comment on several particularly interesting objects in this sample.

Three-Dimensional Delayed-Detonation Model of Type Ia Supernova

Abstract: We study a Type Ia supernova explosion using large-scale three-dimensional numerical simulations based on reactive fluid dynamics with a simplified mechanism for nuclear reactions and energy release. The initial deflagration stage of the explosion involves a subsonic turbulent thermonuclear flame propagating in the gravitational field of an expanding white dwarf. The deflagration produces an inhomogeneous mixture of unburned carbon and oxygen with intermediate-mass and iron-group elements in central parts of the star. During the subsequent detonation stage, a supersonic detonation wave propagates through the material unburned by the deflagration. The total energy released in this delayed-detonation process, (1.3-1.6)x10^51 ergs, is consistent with a typical range of kinetic energies obtained from observations. In contrast to the deflagration model that releases only about 0.6x10^51 ergs, the delayed-detonation model does not leave carbon, oxygen, and intermediate-mass elements in central parts of a white dwarf. This removes the key disagreement between three-dimensional simulations and observations, and makes a delayed detonation the mostly likely mechanism for Type Ia supernova explosions.

He 0107-5240, a chemically ancient star. i. a detailed abundance analysis

Abstract: We report on a detailed abundance analysis of HE 0107� 5240, a halo giant with ½Fe=HNLTE ¼� 5:3 This star was discovered in the course of follow-up medium-resolution spectroscopy of extremely metal-poor candidates selected from the digitized Hamburg/ESO objective-prism survey On the basis of high-resolution VLT/UVES spectra, we derive abundances for eight elements (C, N, Na, Mg, Ca, Ti, Fe, and Ni) and upper limits for another 12 elements A plane-parallel LTE model atmosphere has been specifically tailored for the chemical composition of HE 0107� 5240 Scenarios of the origin of the abundance pattern observed in the star are discussed We argue that HE 0107� 5240 is most likely not a post-asymptotic giant branch star and that the extremely low abundances of the iron-peak and other elements are not due to selective dust depletion The abundance pattern of HE 0107� 5240 can be explained by preenrichment from a zero-metallicity Type II supernova (SN II) of 20-25 M� , plus either self-enrichment with C and N or production of these elements in the asymptotic giant branch phase of a formerly more massive companion, which is now a white dwarf However, significant radial velocity variations have not been detected within the 52 days covered by our moderate- and high-resolution spectra Alternatively, the abundance patterncan be explained by enrichment ofthegascloud from which HE 0107� 5240 formedbya 25M� first-generation star exploding as a subluminous SN II, as proposed by Umeda & Nomoto We discuss con- sequences of the existence of HE 0107� 5240 for low-mass star formation in extremely metal-poor environments and for currently ongoing and future searches for the most metal-poor stars in the Galaxy Subject headings: Galaxy: formation — Galaxy: halo — stars: abundances — stars: individual (HE 0107� 5240) — surveys

Is HE 0107–5240 A Primordial Star? The Characteristics of Extremely Metal-Poor Carbon-Rich Stars

Abstract: We discuss the origin of HE 0107-5240, which, with a metallicity of [Fe/H] = -5.3, is the most iron-poor star yet observed. Its discovery has an important bearing on the question of the observability of first-generation stars in our universe. In common with other stars of very small metallicity (-4 [Fe/H] -2.5), HE 0107-5240 shows a peculiar abundance pattern, including large enhancements of C, N, and O, and a more modest enhancement of Na. The observed abundance pattern can be explained by nucleosynthesis and mass transfer in a first-generation binary star, which, after birth, accretes matter from a primordial cloud mixed with the ejectum of a supernova. We elaborate the binary scenario on the basis of our current understanding of the evolution and nucleosynthesis of extremely metal-poor, low-mass model stars and discuss the possibility of discriminating this scenario from others. In our picture, iron-peak elements arise in surface layers of the component stars by accretion of gas from the polluted primordial cloud, pollution occurring after the birth of the binary. To explain the observed C, N, O, and Na enhancements, as well as the 12C/ 13C ratio, we suppose that the currently observed star, once the secondary in a binary, accreted matter from a chemically evolved companion, which is now a white dwarf. To estimate the abundances in the matter transferred in the binary, we rely on the results of computations of model stars constructed with up-to-date input physics. Nucleosynthesis in a helium-flash-driven convective zone into which hydrogen has been injected is followed, allowing us to explain the origin in the primary of the observed O and Na enrichments and to discuss the abundances of s-process elements. From the observed abundances, we conclude that HE 0107-5240 has evolved from a wide binary (of initial separation ~20 AU) with a primary of initial mass in the range 1.2-3 M☉. On the assumption that the system now consists of a white dwarf and a red giant, the present binary separation and period are estimated at 34 AU and a period of 150 yr, respectively. We also conclude that the abundance distribution of heavy s-process elements may hold the key to a satisfactory understanding of the origin of HE 0107-5240. An enhancement of [Pb/Fe] 1-2 should be observed if HE 0107-5240 is a second-generation star, formed from gas already polluted with iron-group elements. If the enhancement of main-line s-process elements is not detected, HE 0107-5240 may be a first-generation secondary in a binary system with a primary of mass less than 2.5 M☉, born from gas of primordial composition, produced in the big bang, and subsequently subjected to surface pollution by accretion of gas from the parent cloud metal-enriched by mixing with the ejectum of a supernova.

Nucleosynthesis in multi-dimensional SNI a explosions

Abstract: We present the results of nucleosynthesis calculations based on multi-dimensional (2D and 3D) hydrodynamical simulations of the thermonuclear burning phase in type Ia supernovae (hereafter SN Ia). The detailed nucleosynthetic yields of our explosion models are calculated by post-processing the ejecta, using passively advected tracer particles. The nuclear reaction network employed in computing the explosive nucleosynthesis contains 383 nuclear species, ranging from neutrons, protons, and α-particles to 98 Mo. Our models follow the common assumption that SN Ia are the explosions of white dwarfs that have approached the Chandrasekhar mass (Mch ∼ 1.39), and are disrupted by thermonuclear fusion of carbon and oxygen. But in contrast to 1D models which adjust the burning speed to reproduce lightcurves and spectra, the thermonuclear burning model applied in this paper does not contain adjustable parameters. Therefore variations of the explosion energies and nucleosynthesis yields are dependent on changes of the initial conditions only. Here we discuss the nucleosynthetic yields obtained in 2D and 3D models with two different choices of ignition conditions (centrally ignited, in which the spherical initial flame geometry is perturbated with toroidal rings, and bubbles, in which multi-point ignition conditions are simulated), but keeping the initial composition of the white dwarf unchanged. Constraints imposed on the hydrodynamical models from nucleosynthesis as well as from the radial velocity distribution of the elements are discussed in detail. We show that in our simulations unburned C and O varies typically from ∼40% to ∼50% of the total ejected material. Some of the unburned material remains between the flame plumes and is concentrated in low velocity regions at the end of the simulations. This effect is more pronounced in 2D than in 3D and in models with a small number of (large) ignition spots. The main differences between all our models and standard 1D computations are, besides the higher mass fraction of unburned C and O, the C/O ratio (in our case is typically a factor of 2.5 higher than in 1D computations), and somewhat lower abundances of certain intermediate mass nuclei such as S, Cl, Ar, K, and Ca, and of 56 Ni. We also demonstrate that the amount of 56 Ni produced in the explosion is a very sensitive function of density and temperature. Because explosive C and O burning may produce the iron-group elements and their isotopes in rather different proportions one can get different 56 Ni-fractions (and thus supernova luminosities) without changing the kinetic energy of the explosion. Finally, we show that we need the high resolution multi-point ignition (bubbles) model to burn most of the material in the center (demonstrating that high resolution coupled with a large number of ignition spots is crucial to get rid of unburned material in a pure deflagration SN Ia model).

The White Dwarf cooling sequence in the Globular Cluster Messier 4

Abstract: We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with a limiting magnitude of V ~ 30 and I ~ 28. The white dwarf luminosity function rises sharply for I > 25.5, consistent with the behavior expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 mag fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age difference between the Galactic disk and the halo globular cluster M4. Using the same standard white dwarf models to fit each luminosity function yields ages of 7.3 ± 1.5 Gyr for the disk and 12.7 ± 0.7 Gyr for M4 (2 σ statistical errors).

Carbon Ignition in Type Ia Supernovae: An Analytic Model

Abstract: The observable properties of a Type Ia supernova are sensitive to how the nuclear runaway ignites in a Chandrasekhar-mass white dwarf: at a single point at its center, off-center, or at multiple points and times. We present a simple analytic model for the runaway guided by a combination of stellar mixing-length theory and analogy to Rayleigh-Benard convection. The convective flow just prior to runaway is likely to have a strong dipolar component, although this dipole may be unstable at the very high Rayleigh number (1025) appropriate to the white dwarf core. A likely outcome is multipoint ignition with an exponentially increasing number of ignition points during the few tenths of a second that it takes the runaway to develop. The first sparks ignite approximately 150-200 km off-center, followed by ignition at smaller radii. Rotation may be important to break the dipole asymmetry of the ignition and give a healthy explosion.

Disk and Halo Wide Binaries from the Revised Luyten Catalog: Probes of Star Formation and MACHO Dark Matter

Abstract: We present a catalog of 1147 candidate common proper motion binaries selected from the revised New Luyten Two-Tenths Catalog (NLTT). Among these, we identify 999 genuine physical pairs using the measured proper-motion difference and the relative positions of each binary's components on a reduced proper motion (RPM) diagram. The RPM positions also serve to classify them as either disk main-sequence pairs (801), halo subdwarf (116) pairs, or pairs containing at least one white dwarf (82). The disk and halo samples are complete to separations of Δθ = 500'' and Δθ = 900'', which correspond to ~0.1 and ~1 pc, respectively. At wide separations, both distributions are well described by single power laws dN/dΔθ (Δθ)-α, with α = 1.67 ± 0.07 for the disk and α = 1.55 ± 0.10 for the halo. The fact that these distributions have similar slopes (and similar normalizations as well) argues for similarity of the star formation conditions of these two populations. The fact that the halo binaries obey a single power law out to ~1 pc permits strong constraints on halo dark matter candidates. At somewhat closer separations (10'' Δθ 25''), the disk distribution shows a pronounced flattening, which is detected at very high statistical significance and is not due to any obvious systematic effect. We also present a list of 11 previously unknown halo stars with parallaxes that are recognized here as companions of Hipparcos stars.

Detached white dwarf main-sequence star binaries

Abstract: We initiated a comprehensive state of the art binary population synthesis study of white dwarf main-sequence star (WDMS) binaries to serve as a foundation for subsequent studies on pre-cataclysmic variables, double white dwarfs, and white dwarf + B-star binaries. We considered seven distinct formation channels subdivided into three main groups according to the evolutionary process that gives rise to the formation of the white dwarf or its helium-star progenitor: dynamically stable Roche- lobe overflow (Algol-type evolution), dynamically unstable Roche-lobe overflow (common-envelope evolution), or stellar winds (single star evolution). For each formation channel, we examine the sensitivity of the population to changes in the amount of mass lost from the system during dynamically stable Roche-lobe overflow, the common-envelope ejection efficiency, and the initial mass ratio or initial secondary mass distribution. In the case of a flat initial mass ratio distribution, the local space density of WDMS binaries is of the order of ∼10 −3 pc −3 . This number decreases to ∼10 −4 pc −3 when the initial mass ratio distribution is approximately proportional to the inverse of the initial mass ratio. More than 75% of the WDMS binary population originates from wide systems in which both components essentially evolve as if they were single stars. The remaining part of the population is dominated by systems in which the white dwarf is formed in a common-envelope phase when the primary ascends the first giant branch or the asymptotic giant branch. When dynamically stable mass transfer proceeds highly conservative and the common-envelope ejection process is very efficient, the birthrate of WDMS binaries forming through a common-envelope phase is about 10 times larger than the birthrate of WDMS binaries forming through a stable Roche-lobe overflow phase. The ratio of the number of helium white dwarf systems to the number of carbon/oxygen or oxygen/neon/magnesium white dwarf systems derived from large samples of observed WDMS binaries by, e.g., future planet-search missions such as SuperWASP, COROT, and Kepler may furthermore constrain the common-envelope ejection efficiency.

Theoretical Modeling of the Thermal State of Accreting White Dwarfs Undergoing Classical Nova Cycles

Abstract: White dwarfs (WDs) experience a thermal renaissance when they receive mass from a stellar companion in a binary. For accretion rates of less than 10-8 M☉ yr-1, the freshly accumulated hydrogen/helium envelope ignites in a thermally unstable manner that results in a classical nova (CN) outburst and ejection of material. We have undertaken a theoretical study of the impact of the accumulating envelope on the thermal state of the underlying WD. This has allowed us to find the equilibrium WD core temperatures (Tc), the CN ignition masses (Mign), and the thermal luminosities for WDs accreting at rates of 10-11 to 10-8 M☉ yr-1. These accretion rates are most appropriate for WDs in cataclysmic variables (CVs) of Porb 7 hr, many of which accrete sporadically as dwarf novae. We have included 3He in the accreted material at levels appropriate for CVs and find that it significantly modifies the CN ignition mass. We compare our results with several others from the CN literature and find that the inclusion of 3He leads to lower values of Mign for 10-10 M☉ yr-1 and that for values below this the particular author's assumption concerning Tc, which we calculate consistently, is a determining factor. Initial comparisons of our CN ignition masses with measured ejected masses find reasonable agreement and point to ejection of material comparable to that accreted.

Thermal Timescale Mass Transfer and the Evolution of White Dwarf Binaries

Abstract: The evolution of binaries consisting of evolved main-sequence stars (1 < Md/M☉ < 3.5) with white dwarf companions (0.7 < Mwd/M☉ < 1.2) is investigated through the thermal mass-transfer phase. Taking into account the stabilizing effect of a strong, optically thick wind from the accreting white dwarf surface, we have explored the formation of several evolutionary groups of systems for progenitors with initial orbital periods of 1 and 2 days. The numerical results show that CO white dwarfs can accrete sufficient mass to evolve to a Type Ia supernova, and ONeMg white dwarfs can be built up to undergo accretion-induced collapse for donors more massive than about 2 M☉. For donors less massive than ~2 M☉, the system can evolve to form an He and CO or ONeMg white dwarf pair. In addition, sufficient helium can be accumulated (~0.1 M☉) in systems characterized by 1.6 Md/M☉ 1.9 and 0.8 Mwd/M☉ 1 such that sub-Chandrasekhar-mass models for Type Ia supernovae, involving off-center helium ignition, are possible for progenitor systems evolving via the Case A mass-transfer phase. For systems characterized by mass ratios 3, the system likely merges as a result of the occurrence of a delayed dynamical mass-transfer instability. We develop a semianalytical model to delineate these phases that can be easily incorporated in population synthesis studies of these systems.

Presupernova evolution of accreting white dwarfs with rotation

Abstract: We discuss the effects of rotation on the evolution of accreting carbon-oxygen white dwarfs, with the emphasis on possible consequences in Type Ia supernova (SN Ia) progenitors. Starting with a slowly rotating white dwarf, we consider the accretion of matter and angular momentum from a quasi-Keplerian accretion disk. Numerical simulations with initial white dwarf masses of 0.8, 0.9 and 1.0 Mand accretion of carbon-oxygen rich matter at rates of 3 ... 10 × 10 −7 M� /yr are performed. The models are evolved either up to a ratio of rotational to potential energy of T/W = 0.18 - as angular momentum loss through gravitational wave radiation will become important for T/W < 0.18 - or to central carbon ignition. The role of the various rotationally induced hydrodynamic instabilities for the transport of angular momentum inside the white dwarf is investigated. We find that the dynamical shear instability is the most important one in the highly degenerate core, while Eddington-Sweet circulations, Goldreich-Schubert-Fricke instability and secular shear instability are most relevant in the non- degenerate envelope. Our results imply that accreting white dwarfs rotate differentially throughout, with a shear rate close to the threshold value for the onset of the dynamical shear instability. As the latter depends on the temperature of the white dwarf, the thermal evolution of the white dwarf core is found to be relevant for the angular momentum redistribution. As found previously, significant rotation is shown to lead to carbon ignition masses well above 1.4 M� . Our models suggest a wide range of white dwarf explosion masses, which could be responsible for some aspects of the diversity observed in SNe Ia. We analyze the potential role of the bar-mode and the r-mode instability in rapidly rotating white dwarfs, which may impose angular momentum loss by gravitational wave radiation. We discuss the consequences of the resulting spin-down for the fate of the white dwarf, and the possibility to detect the emitted gravitational waves at frequencies of 0.1 ... 1.0 Hz in nearby galaxies with LISA. Possible implications of fast and differentially rotating white dwarf cores for the flame propagation in exploding white dwarfs are also briefly discussed.

The Solar Neighborhood. X. New Nearby Stars in the Southern Sky and Accurate Photometric Distance Estimates for Red Dwarfs

Abstract: Photometric (VJRCIC) and spectroscopic (6000–9500 A) observations of high–proper-motion stars discovered during the first phase of the SuperCOSMOS RECONS (SCR) search are used to estimate accurate distances to eight new nearby red dwarfs, including probable 10 pc sample members SCR 1845-6357 (M8.5 V at 4.6 pc), the binary SCR 0630-7643AB (M6.0 V J at 7.0 pc), and SCR 1138-7721 (M5.0 V at 9.4 pc). Distance estimates are determined using a suite of new photometric color-M relations defined using a robust set of nearby stars with accurate VRIJHKs photometry and trigonometric parallaxes. These relations are used with optical and infrared photometry to estimate distances on a uniform system (generally good to 15%) for two additional samples of red nearby star candidates: several recently discovered members of the solar neighborhood and known faint stars with proper motions in excess of 10 yr-1 south of decl. = -575. Of those without accurate trigonometric parallax measurements, there are five stars in the first sample and three in the second that are likely to be within 10 pc. The two nearest are SO 0253+1652 (M7.0 V at 3.7 pc) and DENIS 1048-3956 (M8.5 V at 4.5 pc). When combined with SCR 1845-6357, these three stars together represent the largest increase in the 5 pc sample in several decades. Red spectra are presented for the red dwarfs, and types are given on the RECONS standard spectral system. Red spectra are also given for two new nearby white dwarfs for which we estimate distances from the photometry of less than 20 pc: WD 0141-675 (LHS 145; 9.3 pc) and SCR 2012-5956 (17.4 pc). WD 0141-675 brings the total number of systems nearer than 10 pc discussed in this paper to 12.

The spin periods and magnetic moments of white dwarfs in magnetic cataclysmic variables

Abstract: We have used a model of magnetic accretion to investigate the rotational equilibria of magnetic cataclysmic variables (mCVs). The results of our numerical simulations demonstrate that there is a range of parameter space in the Pspin=Porb versus � 1 plane at which rotational equilibrium occurs. This has allowed us to calculate the theoretical histogram describing the distribution of mCVs as a function of Pspin=Porb. We show that this agrees with the observed distribution, assuming that the number of systems as a function of white dwarf magnetic moment is distributed approximately according to N � 1 ðÞ d� 1 / � � 1 1 d� 1 . The rotational equilibria also allow us to infer approximate values for the magnetic moments of all known intermediate polars. We predict that intermediate polars with � 1 k5 ; 10 33 Gc m 3 and Porb > 3 hr will evolve into polars, while those with � 1 P5 ; 10 33 Gc m 3 and Porb > 3 hr will either evolve into low field strength polars that are (presumably) unobservable, and possibly EUV emitters, or, if their fields are buried by high accretion rates, evolve into conventional polars, once their magnetic fields resurface when the mass accretion rate reduces. We speculate that EX Hya‐like systems may have low magnetic field strength secondaries and so avoid synchronization. Finally, we note that the equilibria we have investigated correspond to a variety of different types of accretion flow, including disklike accretion at small Pspin=Porb values, streamlike accretion at intermediate Pspin=Porb values, and accretion fed from a ring at the outer edge of the white dwarf Roche lobe at higher Pspin=Porb values.

Mass accumulation efficiency in helium shell flashes for various white dwarf masses

Abstract: We have calculated the mass accumulation efficiency during helium shell flashes on white dwarfs (WDs) of mass 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, and 1.35 M☉ for the helium accretion rates log He (M☉ yr-1) = -7.4 to -5.8. This efficiency is a crucial factor for binary evolutions of Type Ia supernovae. For less massive WDs (<0.8 M☉) no wind mass loss occurs, and all the accreted mass accumulates on the WD if the Roche lobe size is large enough. The efficiency takes the minimum values in between 1.1 and 1.2 M☉ WD for a given mass accretion rate, as well as increases in both less and more massive WDs. The mass accumulation efficiency is larger than 0.5 for log He ≥ -6.72 in all the WD masses.

SN 2003du: Signatures of the circumstellar environment in a normal type Ia supernova?

Abstract: We present observations of the Type Ia supernova 2003du obtained with the Hobby-Eberly Telescope and report the detection of a high-velocity component in the Ca II infrared triplet near 8000 A, similar to features previously observed in SN 2000cx and SN 2001el. This feature exhibits a large expansion velocity (≈18,000 km s-1), which is nearly constant between -7 and +2 days relative to maximum light and disappears shortly thereafter. Other than this feature, the spectral evolution and light curve of SN 2003du resemble those of a normal SN Ia. We consider a possible origin for this high-velocity Ca II line in the context of a self-consistent spherical delayed-detonation model for the supernova. We find that the Ca II feature can be caused by a dense shell formed when circumstellar material of solar abundance is overrun by the rapidly expanding outermost layers of the SN ejecta. Model calculations show that the optical and infrared spectra are remarkably unaffected by the circumstellar interaction and the resulting shell. In particular, no hydrogen lines are detectable in either absorption or emission after the phase of dynamic interaction. The only qualitatively different features in the model spectra are the strong, high-velocity feature in the Ca II IR triplet around 8000 A and a somewhat weaker O I feature near 7,300 A. The Doppler shift and time evolution of these features provides an estimate for the amount of accumulated matter (decreasing Doppler shift with increasing shell mass) and also an indication of the mixing within the dense shell. For high shell masses (≈5 × 10-2 M☉), the high-velocity component of the Ca II line merges with the photospheric line forming a broad feature. A cutoff of the blue wings of strong, unblended lines (particularly the Si II feature at about 6,000 A) may also be observable for larger shell masses. The model SN Ia light curves are little effected except at very early times when the shell is partially optically thick because of Thomson scattering, resulting in larger (B-V) colors by up to 0.3 mag. We apply these diagnostic tools to SN 2003du and infer that about 2 × 10-2 M☉ of solar abundance material may have accumulated in a shell prior to the observations. Furthermore, in this interpretation, the early light-curve data imply that the circumstellar material was originally very close to the progenitor system, perhaps from an accretion disk, Roche lobe, or common envelope. Because of the observed confinement of Ca II in velocity space and the lack of ongoing interaction inferred from the light curve, the matter cannot be placed in the outer layers of the exploding white dwarf star or related to a recent period of high mass loss in the progenitor system prior to the explosion. We note that the signatures of circumstellar interaction could be rather common in Type Ia supernovae and may have eluded discovery because optical spectra often do not extend significantly beyond 7500 A.

Deflagrations and detonations in thermonuclear supernovae

Abstract: We study a type Ia supernova explosion using three-dimensional numerical simulations based on reactive fluid dynamics. We consider a delayed-detonation model that assumes a deflagration-to-detonation transition. In contrast with the pure deflagration model, the delayed-detonation model releases enough energy to account for a healthy explosion, and does not leave carbon, oxygen, and intermediate-mass elements in central parts of a white dwarf. This removes the key disagreement between simulations and observations, and makes a delayed detonation the mostly likely mechanism for type Ia supernovae.

Off-Center Carbon Ignition in Rapidly Rotating, Accreting Carbon-Oxygen White Dwarfs

Abstract: We study the effect of stellar rotation on the carbon ignition in a carbon-oxygen white dwarf accreting CO-rich matter. Including the effect of the centrifugal force of rotation, we have calculated evolutionary models up to the carbon ignition for various accretion rates. The rotational velocity at the stellar surface is set to the Keplerian velocity. The angular velocity in the stellar interior is determined by taking into account the transport of angular momentum due to turbulent viscosity. We have found that an off-center carbon ignition occurs even when the effect of stellar rotation is included if the accretion rate is sufficiently high; the critical accretion rate for the off-center ignition is hardly changed by the effect of rotation. Rotation, however, delays the ignition, i.e., the mass coordinate of the ignition layer and the mass of the white dwarf at the ignition are larger than those for the corresponding nonrotating model. The result supports our previous conclusion that a double-white dwarf merger would not be a progenitor of a Type Ia supernova (SN Ia).

Hubble Space Telescope Observations of the White Dwarf Cooling Sequence of M4

Abstract: We investigate in detail the white dwarf cooling sequence of the globular cluster Messier 4. In particular, we study the influence of various systematic uncertainties, both observational and theoretical, on the determination of the cluster age from the white dwarf cooling sequence. These include uncertainties in the distance to the cluster and the extinction along the line of sight, as well as the white dwarf mass, envelope, and core compositions and the white dwarf-main-sequence mass relation. We find that fitting to the full two-dimensional color-magnitude diagram offers a more robust method for age determination than the traditional method of fitting the one-dimensional white dwarf luminosity function. After taking into account the various uncertainties, we find a best-fit age of 12.1 Gyr, with a 95% lower limit of 10.3 Gyr. We also perform fits using two other sets of cooling models from the literature. The models of Chabrier et al. yield an encouragingly similar result, although the models of Salaris et al. do not provide as good a fit. Our results support our previous determination of a delay between the formation of the Galactic halo and the onset of star formation in the Galactic disk.

On the purity of the zz ceti instability strip: discovery of more pulsating da white dwarfs on the basis of optical spectroscopy

Abstract: We report the discovery of two new ZZ Ceti pulsators, LP 133-144 and HE 1258+0123, selected on the basis of model atmosphere fits to optical spectroscopic data. The atmospheric parameters for LP 133-144 (Teff = 11,800 ± 200 K and log g = 7.87 ± 0.05) and for HE 1258+0123 (Teff = 11,410 ± 200 K and log g = 8.04 ± 0.05) place them within the empirical boundaries of the ZZ Ceti instability strip. This brings the number of known ZZ Ceti stars to a total of 36, a quarter of which have now been discovered using the spectroscopic approach for estimating their atmospheric parameters. This method has had a 100% success rate so far in predicting the variability of candidate ZZ Ceti stars. We have also analyzed additional spectra of known nonvariable white dwarfs in the vicinity of the ZZ Ceti instability strip. Our study further strengthens the idea that ZZ Ceti stars occupy a pure region in the log g-Teff plane, a region where no nonvariable stars are found. This result supports the thesis that ZZ Ceti pulsators represent a phase through which all DA stars must evolve.