Evolution of the number of accreting white dwarfs with shell nuclear burning and the SNe Ia rate

We analyze the time evolution of the number of accreting white dwarfs with surface shell hydrogen burning in semidetached and detached binaries. We consider the case where continuous star formation with a constant rate takes place in a stellar system over 10¹⁰ Gyr and the case of a starburst in which the same mass of stars is formed over 10⁹ Gyr. The evolution of the number of white dwarfs is compared with the evolution of the rate of events that are usually considered as SNe Ia and/or accretion-induced collapses, i.e., the accumulation of a Chandrasekhar mass by white dwarfs or the merger of white dwarf pairs with a total mass greater than or equal to the Chandrasekhar one. In stellar systems with a starburst, the supersoft X-ray sources observed at t = 10¹⁰ yr are most likely not the progenitors of SNe Ia. The same is true for a significant fraction of the sources in systems with a constant star formation rate. In both cases, the merger of white dwarfs is the dominant mechanism of SNe Ia. In symbiotic binaries, accreting CO dwarfs do not accumulate enough mass for an SNe Ia explosion, while ONeMg dwarfs finish their evolution by an accretion-induce collapse with the formation of a neutron star.     

Precollapse evolution of accreting CO white dwarfs

We calculate how accretion affects the evolution of carbon-oxygen white dwarfs up to thermonuclear runaway. The influence of internal temperature (including the possibility of solidification) as well as that of other initial conditions (accretion rate, total mass) are taken into account. We examine in some detail the influence of the adopted pycnonuclear reaction rate and of the lattice impurities on the ignition conditions. The consequences of having a completely ordered C-O crystal instead of a disordered one are also analyzed.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Hydrogen burning on a white dwarf accreting hydrogen in a binary system

We follow the accretion of hydrogen-rich matter onto the surface of a white dwarf in a close binary system. Two phases of accretion are assumed. First—slow accretion from the interstellar clouds, second—fast accretion from the companion.Hydrogen is ignited at the interface between the fast and slowly accreted layers. After a short runaway burning continues in the form of quasistationary deflagration front propagating inwards. The features of this front are discussed. A possibility of mass loss is indicated.     

Relation between the X-ray and optical luminosities in binary systems with accreting nonmagnetic white dwarfs

We investigate the relation between the optical (g-band) and X-ray (0.5–10 keV) luminosities of accreting nonmagnetic white dwarfs. According to the present-day counts of the populations of star systems in our Galaxy, these systems have the highest space density among the close binary systems with white dwarfs. We show that the dependence of the optical luminosity of accreting white dwarfs on their X-ray luminosity forms a fairly narrow one-parameter curve. The typical half-width of this curve does not exceed 0.2–0.3 dex in optical and X-ray luminosities, which is essentially consistent with the amplitude of the aperiodic flux variability for these objects. At X-ray luminosities L _x ～ 10³² erg s^?1 or lower, the optical g-band luminosity of the accretion flow is shown to be related to its X-ray luminosity by a factor ～2–3. At even lower X-ray luminosities (L _x ? 10³⁰ erg s^?1), the contribution from the photosphere of the white dwarf begins to dominate in the optical spectrum of the binary system and its optical brightness does not drop below M _g ～ 13–14. Using the latter fact, we show that in current and planned X-ray sky surveys, the family of accreting nonmagnetic white dwarfs can be completely identified to the distance determined by the sensitivity of an optical sky survey in this region. For the Sloan Digital Sky Survey (SDSS) with a limiting sensitivity m _g ～ 22.5, this distance is ～400–600 pc.     

Merged binary white dwarf evolution: rapidly accreting carbon–oxygen white dwarfs and the progeny of extreme helium stars

We have examined the evolution of merged low-mass double white dwarfs that become luminous helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a carbon–oxygen (CO) white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. When the mass above the helium-burning shell becomes small enough, the star evolves blueward almost horizontally in the Hertzsprung–Russell diagram. The theoretical models for the merger of a 0.6-M_⊙ CO white dwarf with a 0.3-M_⊙ He white dwarf agree very well with the observed locations of extreme helium stars in the  log  T _eff–log  g   diagram, with their observed rates of blueward evolution, and with luminosities and masses obtained from their pulsations. Together with predicted merger rates for  CO+He  white dwarf pairs, the evolutionary time-scales are roughly consistent with the observed numbers of extreme helium stars. Predicted surface carbon and oxygen abundances can be consistent with the observed values if carbon and oxygen produced in the helium shell during a previous asymptotic giant branch phase are assumed to exist in the helium zone of the initial CO white dwarfs. These results establish the  CO+He  white dwarf merger as the best, if not only, viable model for the creation of extreme helium stars and, by association, the majority of R Coronae Borealis stars.     

Deflagration of white dwarfs as a model for type-I supernovae

Carbon-oxygen white dwarfs may be the progenitors of type-I supernovae. Spherically-symmetric models of such dwarfs have been evolved from an artificial core incineration. The convectively unstable incinerated region was allowed to grow at a velocity prescribed by the mixing-length theory of convection. The mixing length can be varied to give different cases. In all the cases considered the dwarfs exploded and were totally disrupted. The calculations were stopped after the dwarf matter had gone into homologous expansion. The model with the best estimated mixing length incinerated 0.8M . The energy released in burning this amount of carbon-oxygen to⁵⁶Ni provides a disrupted dwarf with velocities suitable for type-I supernovae.Research supported by the Natural Sciences and Engineering Research Council.     

A source of high-velocity white dwarfs

We investigate whether the recently observed population of high-velocity white dwarfs can be derived from a population of binaries residing initially within the thin disc of the Galaxy. In particular, we consider binaries where the primary is sufficiently massive to explode as a Type II supernova. A large fraction of such binaries are broken up when the primary then explodes as a supernova, owing to the combined effects of the mass loss from the primary and the kick received by the neutron star on its formation. For binaries where the primary evolves to fill its Roche lobe, mass transfer from the primary leads to the onset of a common envelope phase during which the secondary and the core of the primary spiral together as the envelope is ejected. Such binaries are the progenitors of X-ray binaries if they are not broken up when the primary explodes. For those systems that are broken up, a large number of the secondaries receive kick velocities ∼100–200 km s⁻¹ and subsequently evolve into white dwarfs. We compute trajectories within the Galactic potential for this population of stars and relate the birth rate of these stars over the entire Galaxy to those seen locally with high velocities relative to the local standard of rest (LSR) . We show that for a reasonable set of assumptions concerning the Galactic supernova rate and the binary population, our model produces a local number density of high-velocity white dwarfs compatible with that inferred from observations. We therefore propose that a population of white dwarfs originating in the thin disc may make a significant contribution to the observed population of high-velocity white dwarfs.     

Estimation of plasma parameters in an accretion column near the surface of accreting white dwarfs from their flux variability

We consider the behavior of matter in the accretion column that emerges under accretion in binary systems near the surface of a white dwarf. The plasma heated in a standing shock wave near the white dwarf surface efficiently radiates in the X-ray energy band. We suggest a method for estimating post-shock plasma parameters, such as the density, temperature, and height of the hot zone, from the power spectrum of its X-ray luminosity variability. The method is based on the fact that the flux variability amplitude for the hot region at various Fourier frequencies depends significantly on its cooling time, which is determined by the parameters of the hot zone in the accretion column. This allows the density and temperature of the hot matter to be estimated. We show that the characteristic cooling time can be efficiently determined from the break frequency in the power spectrum of the X-ray flux variability for accreting white dwarfs. The currently available X-ray instruments do not allow such measurements to be made because of an insufficient collecting area, but this will most likely become possible with new-generation large-area X-ray spectrometers.     

Computation of convective model atmospheres of cool white dwarfs

It is shown that, in constructing model atmospheres of cool white dwarfs with strong convection, the computation of the radiative flux can be omitted in a number of iterations. This does not increase the number of iterations needed to obtain convergent model and, consequently, saves computation time.     

A search for radio pulsars around low-mass white dwarfs

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

Search for and study of photometric variability in magnetic white dwarfs

We report the results of photometric observations of a number of magnetic white dwarfs in order to search for photometric variability in these stars. These V-band observations revealed significant variability in the classical highly magnetized white dwarf GRW+70?8247 with a likely period from several days to several dozen days and a half-amplitude of about 0_. ^m 04. Our observations also revealed the variability of the well-known white dwarf GD229. The half amplitude of its photometric variability is equal to about 0_. ^m 005, and the likely period of this degenerate star lies in the 10–20 day interval. This variability is most likely due to the rotation of the stars considered.We also discuss the peculiarities of the photometric variability in a number of other white dwarfs. We present the updated “magnetic field–rotation period” diagram for the white dwarfs.     

He-shell flashes on the surface of oxygen-neon white dwarfs

Accretion induced collapse(AIC)may be responsible for the formation of some interesting neutron star binaries(e.g.,millisecond pulsars,intermediate-mass binary pulsars,etc).It has been suggested that oxygen-neon white dwarfs(ONe WDs)can increase their mass to the Chandrasekhar limit by multiple He-shell flashes,leading to AIC events.However,the properties of He-shell flashes on the surface of ONe WDs are still not well understood.In this article,we aim to study He-shell flashes on the surface of ONe WDs in a systematic approach.We investigated the long-term evolution of ONe WDs accreting He-rich material with various constant mass-accretion rates by time-dependent calculations with the stellar evolution code Modules for Experiments in Stellar Astrophysics(MESA),in which the initial ONe WD masses range from 1.1 to 1.35 M_⊙.We found that the mass-retention efficiency increases with the ONe WD mass and the mass-accretion rate,whereas both the nova cycle duration and the ignition mass decrease with the ONe WD mass and the mass-accretion rate.We also present the nuclear products in different accretion scenarios.The results presented in this article can be used in the future binary population synthesis studies of AIC events.     

A possible presence of HeH+ in white dwarfs

The equivalent width calculations for the fundamental vibration rotation band lines of HeH⁺ have been carried out for a non-DA white dwarf model with an effective temperature of 12 000 K. BothP andR branch lines with rotational quantum numbersJ=3 to 18 were included in the calculations. A search for these lines in helium rich white dwarfs is suggested.     

A subsample of white dwarfs among the fbs blue stellar objects

Results are reported from studies of a subsample of white dwarfs in the second part of the FBS survey. Of the 217 WD identified in the FBS, most are DA dwarfs, but the subclasses DO, DOB, DB, DAB, DAZ, DZ, and DC are also encountered. Multiwavelength studies are conducted on the sample from the FBS survey: of the 217 white dwarfs, 178 coincide with 2MASS sources, GALEX (ultraviolet) data exist for 155, 23 are ROSAT x-ray sources, and SDSS data with stellar magnitudes in five photometric bands, u, g, r, i, and z, are available for 120. The WD sample from the FBS survey is compared with similar surveys (PG and SDSS). Average B and R magnitudes, as well as an average value of the length of the low dispersion spectra of white dwarfs from the DFBS are also given.     

Resonance model for high-frequency QPOs in white dwarfs, neutron stars and black holes

High-frequency QPOs reflect non-linear, and likely resonant, oscillations in accretion disks. In black holes, and probably in neutron stars, but not in white dwarfs, strong gravity plays a crucial role in their formation.     

Constraints on the gravitational constant from the observations of white dwarfs

Recently some authors have questioned whether Newton's law of gravitation is actually true on scales less than 1 km. The available constraints on the gravitational constant show that is laboratory valueG ₀ may differ from the value at infinityG by 40%. Long (1976) reported experimental evidence for departures from Newton's law. In this note it is shown that the difference betweenG ₀ andG modifies the mass-radius relation of degenerate stars. The observations of white dwarfs are consistent with the theory of stellar evolution only ifG ₀ differs fromG by not more than 10%. This estimate may be improved by a higher accuracy of observations.