The effect of photospheric heavy elements on the hot DA white dwarf temperature scale

Using the latest non-local thermodynamic equilibrium (non-LTE) synthetic spectra and stellar model calculations, we have evaluated the potential effect of the presence of heavy elements in the photospheres of hot H-rich DA white dwarfs. In particular, we have examined their influence on the effective temperature and surface gravity perceived from analysis of the Balmer line profiles. It is apparent that both the inclusion of non-LTE effects in the models and significant quantities of heavy elements act independently to lower the value of T _eff determined from a particular spectrum. Hence, the true effective temperatures of the heavy element-rich DA white dwarfs, currently estimated to be above 55 000 K, are apparently lower than previously reported from pure-H LTE analyses, by some 4000–7000 K. We do not see any similar influence on measurements of log g . This work concentrates on a group of relatively bright well-studied objects, for which heavy element abundances are known. As a consequence of this, establishment of correct temperatures for all other hot white dwarfs will require a programme of far-UV spectroscopy in order to obtain the essential compositional information. Since only stars with effective temperatures lying notionally in the range from ≈ 55 000 to 70 000 K (52 000–62 000 K when the non-LTE effects and heavy elements are taken into account) have been considered here, important questions remain regarding the magnitude of any similar effects in even hotter white dwarfs and pre-white dwarfs. The resulting implications for the plausibility of the evolutionary link between the main hot DA population and their proposed precursors, the H-rich central stars of planetary nebulae, need to be investigated.     

TheROSAT survey of extreme UV sources: a new window on the universe

This paper discusses the first all-sky suvey of cosmic extreme ultra-violet sources, discovered by theROSAT Wide Field Camera. Details of the instrument and the survey are presented, with comparisons made to previous selected surveys in the X-ray regime. The subsequent optical identification program is described, and the major results summarized. I then discuss the main classes of EUV emitters: active chromosphere stars and hot white dwarfs, and describe the importance of EUV obserevations in understanding the astrophysics of these objects. Many bright, and relatively nearby, sources have been identified with hitherto unrecognized active stars, representing the extremes in chromospheric and coronal activity, be it binary or age related. Many new hot DA white dwarfs have also been indentified, and the most exciting result in this area is the discovery that significant traces of heavier elements (e.g. C, N, O, Si, Fe & Al) exist in their atmospheres, substantially increasing their EUV opacities. The importance of hot white dwarfs as standard candles in probing the local interstellar medium is also discussed. Miscellaneous counterparts (AGN, PNN, O-B stars and CVs) that make up the rest of the sample of EUV sources are also briefly mentioned. I conclude the paper with a discussion of the new magnetic cataclysmic variables discovered at SAAO, which have been the subject of intensive follow-up observations.     

Optical identifications of southern rosat EUV and soft X-ray sources

This paper discusses the first all-sky surveys of cosmic extreme ultra-violet and soft X-ray sources, discovered by ROSAT. Details of the surveys are presented, with comparisons made to previous selected surveys in the X-ray regime. The subsequent optical identification programs are described, and the major results summarized. We then discuss the main classes of EUV emitters: active chromosphere stars and hot white dwarfs, and describe the importance of EUV observations in understanding the astrophysics of these objects. Many bright, and relatively nearby, sources have been identified as hitherto unrecognized active stars, representing the extremes in chromospheric and coronal activity, be it binary or age related. Many new hot DA white dwarfs have also been indentified, and the most exciting result in this area is the discovery that significant traces of heavier elements (e.g. C, N, O, Si, Fe and Al) exist in their atmospheres, substantially increasing their EUV opacities. The importance of hot white dwarfs as standard candles in probing the local interstellar medium is also discussed. Miscellaneous counterparts (AGN, PNN, O-B stars and CVs) that make up the rest of the sample of EUV sources are also briefly mentioned. We finish with a discussion of the on-going ROSAT Galactic Plane Survey (RGPS) identification program.     

The cooling of liquefying hot white dwarfs

The consequences of gas-liquid phase transitions in the core of hot white dwarf stars are discussed. Expressions for the latent heat and the liquefaction curveT _l =T _l (Q) are obtained. Then amodel for a hot white dwarf is introduced and the corresponding liquefaction sequences are built on the H-R diagram; relations luminosity-central temperature and effective temperature-central temperature are also given for liquefying white dwarfs.Finally the cooling curves are obtained for such stars taking into account the effect of latent heat emission.Our results seem to suggest a possible identification of the central stars of planetary nebulae as hot liquefying white dwarfs.     

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.     

Isentropic stars in general relativity

In an investigation of the evolution of homogeneous, isentropic, stars through stages of diminishing entropy, Rakavy and Shaviv (1968) have recently found that stars of mass less thanM _c (Chandrasekhar's limiting mass for white dwarfs) evolve into white dwarfs, while stars of mass greater thanM _c approach a (singular) state of minimum entropy. An elementary explanation of these results is given and qualitative effects of general relativity are discussed. It is found that stars which are lighter than the Oppenheimer and Volkoff (1939) limit become white dwarfs, while heavier stars must become dynamically unstable at a finite stage in their evolution.     

Relation of Velocity Distribution and Mass for DA White Dwarfstwo

White dwarfs are the evolutionary endpoint of the low-and-medium mass stars. In the studies of white dwarfs, the mass of white dwarf is an important physical parameter. In this paper, we give an analysis about the velocity distribution of DA white dwarfs in the Sloan Digital Sky Survey (SDSS), and hope to find the relation between mass and velocity distribution of white dwarfs. We get the radial velocity and tangential velocity of every DA white dwarf according to their proper motion and spectral shift. Through analyzing the velocity distribution of DA white dwarfs, we find that the small-mass white dwarfs, which are produced from the single-star evolution channel, have a relatively large velocity dispersion.     

A study of optically featureless objects from the EC Survey: searching for new southern BL Lac objects

In an attempt to discover new southern BL Lac objects, 14 optically featureless objects from the Edinburgh–Cape Survey were selected. Optical polarimetry and spectroscopy, radio and IR observations were carried out in order to improve their classifications. The 14 objects were examined according to special criteria that are described. Their UBV and JHK colour–colour distributions, spectrograms, radio observations and visible-region polarimetry were utilized to conclude that the selected objects are not BL Lacs. Most of them are apparently not extragalactic objects. It is suggested that four of the 14 candidates are DC white dwarfs, one is a QSO, three are DA white dwarfs or sdB subdwarfs, one is a cataclysmic variable, one is a DAO/sdO and another is possibly a DA+dM binary (composite system with a 'primary' hot white dwarf and a 'secondary' M-type main-sequence dwarf) or a cataclysmic variable. One object is likely to be a subdwarf, while two remain unclassifiable.     

Interaction of hot plasma with radiation of magnetic white dwarfs

The influence of radiation on the electron velocity distribution in a hot nonrelativistic plasma localized near the surface of magnetic white dwarfs is investigated. The part played by the plasma in the formation of cyclotron features in the optical spectrum of these stars is studied. The region of parameters where the transverse temperature of plasma is defined by the brightness temperature of extraordinary radiation at the gyrofrequency is found. When escaping from the plasma in a homogeneous magnetic field, this component forms a cyclotron line in absorption. The ordinary radiation at the gyrofrequency and both modes at higher cyclotron harmonics are in emission or absorption depending on the magnetic field strength and hot plasma density. Possible interpretation of the observed spectral features of magnetic white dwarfs in terms of the developed theory is discussed.     

A search for hidden white dwarfs in the ROSAT EUV survey — II. Discovery of a distant DA+F6/7V binary system in a direction of low-density neutral hydrogen

The ROSAT Wide Field Camera (WFC) survey of the extreme ultraviolet (EUV) has provided us with evidence for the existence of a previously unidentified sample of hot white dwarfs in unresolved, detached binary systems. These stars are invisible at optical wavelengths due to the close proximity of their much more luminous companions (spectral type K or earlier). However, for companions of spectral type ∼A5 or later the white dwarfs are easily visible at far-ultraviolet wavelengths, and can be identified in spectra taken by IUE . 16 such systems have been discovered in this way through ROSAT EUVE IUE observations, including four identified by us in Paper I. In the present paper we report the results of our continuing search during the final year of IUE operations. One new system, RE J0500−364 (DA+F6/7V), has been identified. This star appears to lie at a distance of ∼500−1000 pc, making it one of the most distant white dwarfs, if not the most distant, to be detected in the EUV surveys. The very low line-of-sight neutral hydrogen volume density to this object could place a lower limit on the length of the β CMa interstellar tunnel of diffuse gas, which stretches away from the Local Bubble in a similar direction to RE J0500−364. In this paper we also analyse a number of the stars observed where no white dwarf companion was found. Some of these objects show evidence for chromospheric and coronal activity. Finally, we present an analysis of the previously known WD+active F6V binary HD 27483 (Bo¨hm-Vitense 1993), and show that, at T  ≈ 22 000 K, the white dwarf may be contributing significantly to the observed EUV flux. If so, it is one of the coolest such stars to be detected in the EUV surveys.     

The thermomagnetic instability in degenerate cores of white dwarfs

The degenerate cores of sufficiently hot white dwarfs are shown to suffer the thermomagnetic instability which generates magnetic field and excites hydrodynamical motions. The necessary conditions for the instability to occur and the parameters of an unstable zone are determined. It is shown that, in hot white dwarfs, this zone can be very extended. The characteristic time scale of the instability is evaluated and some effects due to this instability are discussed.     

XSS J00564+4548 and IGR J00234+6141: New cataclysmic variables from the RXTE and INTEGRAL all-sky surveys

We present the results of our optical identification of two X-ray sources from the RXTE and INTEGRAL all-sky surveys: XSS J00564+4548 and IGR J00234+6141. Using optical observations with the 1.5-m Russian-Turkish Telescope (RTT150) and publicly accessible X-ray data from the SWIFT Orbital Observatory, we show that these sources are most likely intermediate polars, i.e., binary systems with accreting white dwarfs that possess a moderately strong magnetic field (≲10 MG). We have found periodic optical oscillations with periods of ≈480 and ≈570 s. These periods most likely correspond to the rotation periods of the white dwarfs in these systems. Further optical RTT150 observations of these systems will allow their parameters to be studied in more detail. Published in Russian in Pis’ma v Astronomicheskiĭ Zhurnal, 2006, Vol. 32, No. 9, pp. 655–661. The article was translated by the authors.     

Evolutionary and pulsational properties of white dwarf stars

White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. Since the coolest white dwarfs are very old objects, the present population of white dwarfs contains a wealth of information on the evolution of stars from birth to death, and on the star formation rate throughout the history of our Galaxy. Thus, the study of white dwarfs has potential applications in different fields of astrophysics. In particular, white dwarfs can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, such as our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow these stars to be used as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. Last but not least, since many white dwarf stars undergo pulsational instabilities, the study of their properties constitutes a powerful tool for applications beyond stellar astrophysics. In particular, white dwarfs can be used to constrain fundamental properties of elementary particles such as axions and neutrinos and to study problems related to the variation of fundamental constants. These potential applications of white dwarfs have led to renewed interest in the calculation of very detailed evolutionary and pulsational models for these stars. In this work, we review the essentials of the physics of white dwarf stars. We enumerate the reasons that make these stars excellent chronometers, and we describe why white dwarfs provide tools for a wide variety of applications. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with a unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows one to measure stellar masses with unprecedented precision and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.     

Temperature and surface gravitation of white dwarfs in the fbs survey from the sdss

Empirical formulas for the temperature and surface gravitation of white dwarfs are derived using data on temperature and the acceleration of gravity from the catalog of spectroscopically confirmed white dwarfs WD in the Sloan Digital Sky Survey Release 4 (SDSS DR4). These formulas are used to determine the temperature and acceleration of gravity for five spectroscopically confirmed white dwarfs from the FBS survey that were not included in the WD SDSS DR4 catalog, and also for 82 WD that had not been spectroscopically confirmed. As a result, the temperature and acceleration of gravity have been determined for 87 FBS white dwarfs that were not included in the SDSS WD catalog.     

Using GALEX-SDSS-PanSTARRS-HST-Gaia to understand post-AGB evolution

Hot WDs in binary systems with a less evolved star are particularly invaluable astrophysical probes, the unevolved companion enabling better derivation of distance and age than is usually possible for post-AGB objects, and therefore also of their radius and luminosity. But hot white dwarfs (WD) are elusive at all wavelengths except the UV (Bianchi et al. 2011a). From our GALEX UV source catalogs (Bianchi et al. 2011a,b, 2014, 2017) matched to SDSS, we identified thousands of candidate hot WDs including WDs in binary systems consisting of a hot WD and a companion of spectral type from A to M. The identification and preliminary characterization of the stellar parameters is based on the analysis of the photometric SED from far-UV to z-band.We have observed subsamples of the UV-selected WDs with the Hubble Space Telescope (HST) to better characterize their stellar parameters. We obtained (1) UV spectroscopy with STIS and analyzed the UV spectra together with optical SDSS spectra, and (2) multi-band imaging with WFC3 (\(0.04^{\prime\prime}/\mbox{pixel}\)) to measure angular separation and individual SEDs of the pair’s components in binary systems. In our HST/WFC3 sample of 59 hot-WD binaries with late-type companions, we found that at least a dozen have possibly evolved without exchanging mass. The UV STIS spectroscopy led to the revision of previous results based on optical spectra only, because of the often undetectable or unquantifiable contribution of the hot component to the optical fluxes.     

Mass distribution of DA white dwarfs

Several weak points in a recent paper by Guseinovet al. (1983) are discussed that cast doubt on their conclusions regarding mean masses and the mass distribution of DA white dwarfs. A selection effect in a magnitude limited sample (Shipman, 1979) is studied in detail and found to be of minor importance for DA.     

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

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

Hot vibrating white dwarf models of pulsating X-ray sources

A number of white dwarf models have been calculated which correspond to various radial and nonradial modes of vibration with eigenfrequencies in agreement with the observed pulsation frequencies of the X-ray sources Hercules X-1 and Centaurus X-3. Most of the white dwarf models have hot interiors, but for calculational purposes these were simplified so that the bulk of the interior was isothermal, and the surface layers were designed to produce an energy generation rate of 10³⁷ erg s^?1 and to transport this energy continuously to the surface by radiative transfer. Cold white dwarfs have a fairly large spread of masses corresponding to the different overtone modes with the given eigenfrequencies, but in the hot models this spread of masses is greatly reduced, for both radial and nonradial modes. It is concluded that if the pulsating X-ray sources are hot white dwarfs, the mass of Cen X-3 probably lies in the range 0.7–1.2M _⊙, and the mass of Her X-1 probably lies in the range 1.1–1.25M _⊙ (in accord with observation).     

Past history of star formation in the solar neighbourhood from the observed white dwarf luminosity distribution

It is known that intermediate and low-mass stars evolve finally to white dwarfs of mass characteristically centred around 0.6M . The observed luminosity distribution and the theoretical cooling curves of such white dwarfs are used in this work to estimate the rate of formation of these and, hence, of their progenitors (although not uniquely) in the solar neighbourhood as a function of time. It is found that the star formation rate has remained fairly constant over the past 10–12 billion years, and that the observed number density of the local white dwarfs match quite well with the one expected from the mass functions of the local stars.     

White dwarf models of supernovae and cataclysmic variables

If the accreting white dwarf increases its mass to the Chandrasekhar mass, it will either explode as a Type I supernova or collapse to form a neutron star. In fact, there is a good agreement between the exploding white dwarf model for Type I supernovae and observations. We describe various types of evolution of accreting white dwarfs as a function of binary parameters (i.e, composition, mass, and age of the white dwarf, its companion star, and mass accretion rate), and discuss the conditions for the precursors of exploding or collapsing white dwarfs, and their relevance to cataclysmic variables. Particular attention is given tohelium star cataclysmics which might be the precursors of some Type I supernovae or ultrashort period X-ray binaries. Finally we present new evolutionary calculations using the updated nuclear reaction rates for the formation of O+Ne+Mg white dwarfs, and discuss the composition structure and their relevance to the model forneon novae.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.