Showing papers on "White dwarf published in 1993"
TL;DR: In this paper, absolute magnitudes in the B, V, and I bands were derived for nine well-observed Type Ia supernovae, using host galaxy distances estimated via the surface brightness fluctuations or Tully-Fisher methods.
Abstract: Absolute magnitudes in the B, V, and I bands are derived for nine well-observed Type Ia supernovae, using host galaxy distances estimated via the surface brightness fluctuations or Tully-Fisher methods. These data indicate that there is a significant intrinsic dispersion in the absolute magnitudes at maximum light of Type Ia supernovae, amounting to +/- 0.8 mag in B, +/- 0.6 mag in V, and +/- 0.5 mag in I. Moreover, the absolute magnitudes appear to be tightly correlated with the initial rate of decline of the B light curve, with the slope of the correlation being steepest in B and becoming progressively flatter in the V and I bands. This implies that the intrinsic B - V colors of Type Ia supernovae at maximum light are not identical, with the fastest declining light curves corresponding to the intrinsically reddest events. Certain spectroscopic properties may also be correlated with the initial decline rate. These results are most simply interpreted as evidence for a range of progenitor masses, although variations in the explosion mechanism are also possible. Considerable care must be exercised in employing Type Ia supernovae as cosmological standard candles, particularly at large redshifts where Malmquist bias could be an important effect.
1,991 citations
TL;DR: In this article, it is argued that most of the magnetic energy becomes concentrated in thin flux ropes when the field pressure exceeds the turbulent pressure at the smallest scale of turbulence, and the possibilities for dynamo action during the various (precollapse) stages of convective motion that occur in the evolution of a massive star are examined.
Abstract: Neutron star convection is a transient phenomenon and has an extremely high magnetic Reynolds number In this sense, a neutron star dynamo is the quintessential fast dynamo The convective motions are only mildly turbulent on scales larger than the approximately 100 cm neutrino mean free path, but the turbulence is well developed on smaller scales Several fundamental issues in the theory of fast dynamos are raised in the study of a neutron star dynamo, in particular the possibility of dynamo action in mirror-symmetric turbulence It is argued that in any high magnetic Reynolds number dynamo, most of the magnetic energy becomes concentrated in thin flux ropes when the field pressure exceeds the turbulent pressure at the smallest scale of turbulence In addition, the possibilities for dynamo action during the various (pre-collapse) stages of convective motion that occur in the evolution of a massive star are examined, and the properties of white dwarf and neutron star progenitors are contrasted
917 citations
TL;DR: Close binaries are defined as systems in which at least one of the components has filled or will fill its Roche lobe and attempt to transfer matter to its companion; some of the orbital energy helps drive matter from the common envelope (CE) into interstellar space as mentioned in this paper.
Abstract: The characteristics of many close, evolved binaries can be understood most easily if there exists some agency that can abstract angular momentum or mass, or both, from the precursor system. Close binaries may be defined as systems in which at least one of the components has filled or will fill its Roche lobe and attempt to transfer matter to its companion. If the timescale for mass transfer is considerablyy shorter than the timescale on which the accretor can adjust thermally to the proferred mass, the accreted layer will heat up, expand, and fill the Roche lobe of the accretor. The mass lost by the donor thereafter flows into a "common envelope" (CE) which encomapsses both stars. The frictional interaction between this common envelope and the stellar cores produces drag forces that cause the cores to spiral in toward one another; some of the orbital energy helps drive matter from the CE into interstellar space. Examples of the systems which are experiencing or have experienced this process include some planetary nebulae, cataclysmic variables, and close binary degenerate stars. Similar situations can arise if one of the components can support, of its own accord, a dense wind that flows out of the system; the drag luminosity produced by interaction between the companion and the wind may intensify the wind and contribute tot eh mass loss from the donor. Systems undergoing this "wind-CE" process include novae and close binaries containing a Wolf-Rayet star. Planetary nebulae with close binary central stars are actually ejected CEs, and precursors of many cataclysmic variables were once the central stars of planetary nebulae formed in a CE event. In this review, we (1) describe various initial configurations which will produce a CE, (2) discuss the physics of the CE event, (3) describe attempts to model the event quantiatively, and (4) apply what we have learned to describe, in several real situations, the tarnsformations wrought by evolution through a CE phase.
526 citations
TL;DR: In this article, an extensive grid of calculations of stellar models from the zero-age horizontal branch (ZAHB) through to a point late in post-HB evolution or a point on the white dwarf cooling track is presented.
Abstract: This series of papers comprises a systematic exploration of the hypothesis that the far-ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the zero-age horizontal branch (ZAHB) through to a point late in post-HB evolution or a point on the white dwarf cooling track. The grid will be used to produce synthesized UV fluxes for the interpretation of existing and future short-wavelength (900-3000 A) observations
454 citations
TL;DR: In this paper, an extensive grid of calculations of stellar models from the Zero Age Horizontal branch through to a point late in post-HB evolution or a point on the white dwarf cooling track is presented.
Abstract: This series of papers comprises a systematic exploration of the hypothesis that the far ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the Zero Age Horizontal Branch through to a point late in post-HB evolution or a point on the white dwarf cooling track. We use the term `Extreme Horizontal Branch' (EHB) to refer to HB sequences of constant mass that do not reach the thermally-pulsing stage on the AGB. These models evolve after core helium exhaustion
350 citations
TL;DR: A review of the current status of both brown dwarf theory and the searches for these elusive substellar objects can be found in this article, where the physics of brown dwarfs and their interiors are reviewed and an analytic model of the lower edge of the hydrogen main sequence is presented.
Abstract: This review summarizes the current status of both brown dwarf theory and the searches for these elusive substellar objects. The conceptual continuity between the brown dwarf and the well-studied M dwarf branches is emphasized throughout. The physics of their atmospheres and interiors is reviewed and an analytic model of both brown dwarf evolution and the lower edge of the hydrogen main sequence is presented. An extensive discussion of brown dwarf searches in the field, in binaries, around white dwarfs, in clusters, in the solar neighborhood, in the galactic halo, and using proper-motion catalogs is provided, as is a tentative list of candidates in the field. The theory near and below the edge of the main sequence, while sophisticated, is only now being successfully challenged by optical and infrared observations. The near future promises a productive explosion in our knowledge of this problematic galactic population.
215 citations
TL;DR: In this article, the origin and evolution of binary radio pulsars in binary X-ray binaries is discussed, and the evolution process of radio pulsar evolution into binary radio stars is discussed.
Abstract: a companion star. Optical obser vations have shown that there are two main classes of such X-ray binaries: the high-mass X-ray binaries, in which the mass donor is an 0 or B star, and the low-mass X-ray binaries, which have donors less massive than the Sun. It has become clear during the past decade that the neutron star in an X-ray binary may switch on as a radio pulsar when the mass transfer stops. Accordingly, two classes of radio pulsars in binaries can be delin eated. Those in which the radio pulsar has another neutron star or a relatively massive (�IM 0) white dwarf as a companion probably have evolved from a high-mass X-ray binary. Those in which the neutron star is accompanied by a low-mass white dwarf (;5 0.5M 0) may have evolved from low-mass X-ray binaries. The origin and evolution of these X-ray binaries and binary radio pulsars are the subject of this review. Origin of high-mass and oflow-mass X-ray binaries and their evolution into binary radio pulsars are discussed in Section 3 and in Section 4. In order not to have to interrupt our discussion of the evolutionary scenarios with the technical explanations of the various processes occurring in them, we start with a separate discussion of these
195 citations
TL;DR: In this article, the evolution of Population II red giants has been studied adopting Reimer's formalism for the efficiency of mass loss, showing that the internal He core is unaffected by mass loss and the luminosity of the red giant clump follows the actual stellar mass.
Abstract: The evolution of Population II red giants has been studied adopting Reimer's formalism for the efficiency of mass loss. Evolutionary computations for a model of 0.8 M ○. , Y=0.23, Z=0.0002 are presented for selected assumptions on the value of the parameter η governing the mass-loss rate. Comparison with canonical models (η=0) indicates that (1) the evolution of the internal He core is unaffected by mass loss, and (2) the luminosity of the red giant clump follows the actual stellar mass. Models with η≥1.0 fail to ignite He, crossing the HR diagram toward the final cooling as He white dwarfs
171 citations
TL;DR: In this article, a model of thin accretion disks around a central white dwarf is presented, which includes a self-consistent description of the boundary layer, and two distinct kinds of solution depending on the mass accretion rate are found.
Abstract: ACCRETION disks1,2 are found in many astrophysical objects, ranging from newly formed stars and mass-transferring binary systems to quasars and other active galactic nuclei. An important feature of accretion disks is the boundary layer—the interface between the disk and the accreting objects—where up to half the accretion luminosity may be liberated. The lack of a satisfactory description of the flow and thermal structure of this layer has long been a handicap when modelling disk spectra. Here we report numerical solutions of a model of thin accretion disks around a central white dwarf which includes a self-consistent description of the boundary layer. We find two distinct kinds of solution depending on the mass accretion rate Ṁ. At high rates, we find optically thick boundary layers whose radial width and peak temperature decrease with decreasing Ṁ, but when the accretion rate falls below a critical value, the boundary layer becomes optically thin, and the width and temperature increase dramatically. Our results provide an explanation for the hard X-rays observed3 in cataclysmic variables, particularly at low Ṁ. It should be possible to extend our analysis to other accretion-disk systems.
149 citations
TL;DR: In this article, a complete atlas of optical spectra of white dwarf stars is presented, and the variation of the dominant spectra features as a function of effective temperature and abundance is documented.
Abstract: We present a complete atlas of optical spectra of white dwarf stars. Spectra for all major spectral classes and subclasses are illustrated, and the variation of the dominant spectra features as a function of effective temperature and abundance is documented. We present, as well, spectra of several peculiar or rare types of degenerate stars.
118 citations
TL;DR: In this paper, the authors model the evolution of binary systems and explore the possibilities of formation of single and binary planetary nebula nuclei (PNNs) under the assumption that all stars are born in binary systems, and obtain a Galactic rate of PNN formation of 0.87 yr -1.
Abstract: We model the evolution of binary systems and explore the possibilities of formation of single and binary planetary nebula nuclei (PNNs). Under the assumption that all stars are born in binary systems, we obtain a Galactic rate of PNN formation of 0.87 yr -1 . The model suggests that about 20% of all PNNs are single, while 66% are CO white dwarfs accompanied by main-sequence stars. Our results are consistent with the observations of close binary PNNs. However, a direct comparison is difficult due to poor statistics and selection effects which bias the observed sample. We used the obtained results to place constraints on parameters of common envelope evolution
TL;DR: In this article, the authors compare the predictions of their two-phase accretion model with all the observations currently available on the metal-line phenomenon in cool white dwarfs, and find that the gross observed metal abundance patterns are very well accounted for within the framework of the model.
Abstract: We compare the predictions of our two-phase accretion model with all the observations currently available on the metal-line phenomenon in cool white dwarfs. The model is based on a simplified picture of the interstellar medium, but makes specific predictions as to the presence of metals in the photospheres of these stars. We find that the gross observed metal abundance patterns are very well accounted for within the framework of the model. For many individual stars, it is also possible to infer directly the accretion rates necessary to maintain the photospheric metal abundances. These rates agree remarkably well with the estimates of the accretion rates from denser patches of the interstellar medium which we used in our idealized numerical simulations, thus providing an important self-consistency check
TL;DR: In this paper, time-dependent simulations of episodic accretion of metals onto cool white dwarfs are presented, where the surface steady-state abundance of a metal directly scales on the assumed accretion rate.
Abstract: Time-dependent simulations of episodic accretion of metals onto cool white dwarfs are presented. Calculations are made for Mg, Si, Ca, and Fe in H-rich and He-rich white dwarf models having a mass of 0.6 M ○ . in the range 25,000 K≥T eff ≥5000 K. It is first shown that under the assumption of continuous accretion the metal distributions rapidly reach a steady state in the outer layers of the models. The surface steady-state abundance of a metal directly scales on the assumed accretion rate. Using estimates of accretion rates based on a simplified understanding of the interstellar medium, a two-phase accretion model is next developed
TL;DR: In this paper, the authors examined several possible sources (both intrinsic and extrinsic) for the second derivative in the pulsar's rotation rate and found that it is best explained by the presence of a second, weakly bound companion object moving in a wide orbit around the main binary system.
Abstract: MILLISECOND pulsars are usually found in binary systems. This is in keeping with the generally accepted model for the formation of such pulsars14 in which an old neutron star is spun up to high angular velocities by the accretion of matter from a companion star. The millisecond pulsar 1620 – 26 in the globular cluster M4 is no exception: timing measurements1,2 reveal the presence of a 0.3-solar-mass companion star (probably a white dwarf) with an orbital period of 191 days. But subsequent measurements of this pulsar have identified a small but significant deviation from the expected behaviour3,4, suggestive of an unusually large second derivative in the pulsar's rotation rate5. Here we examine several possible sources—both intrinsic and extrinsic—for this derivative, and we find that it is best explained by the presence of a second, weakly bound companion object moving in a wide orbit around the main binary system. The third body in this hierarchical system has an orbital period of ∼100 years and a mass approximately ten times that of Jupiter, and may have been captured during a recent collision with another stellar system6.
01 Jan 1993
TL;DR: In this paper, the luminosity function and population of a white dwarf is discussed. And the theory of white dwarfs structure and evolution is discussed, as well as the X-ray observations of white dwarves.
Abstract: 1. White Dwarf Luminosity Function and Population. 2. Pre-White Dwarf Evolution. 3. Theory of White Dwarf Structure and Evolution 4. EUV and X-Ray Observations of White Dwarfs. 5. Atmospheres and Envelopes I (Theory). 6. Atmospheres and Envelopes II (Observations). 7 White Dwarf in Binaries. 8. Pulsating White Dwarfs.
TL;DR: In this article, the optical detection of the companion star of the nearby binary millisecond pulsar J0437-4715 was reported, and the companion seems to be a very cool, quiescent white dwarf.
Abstract: OPTICAL observations of the companion stars in binary pulsar systems1–3 offer a way to estimate pulsar ages independent of calculations based on timing measurements of the radio pulses, and can therefore provide important constraints for models of pulsar evolution. Here we report the optical detection of the companion star of the nearby binary millisecond pulsar J0437–4715. The companion seems to be a very cool, quiescent white dwarf. The temperature inferred from the observations is less than that obtained from a cooling model of a white dwarf of an age equivalent to the 'spin-down' age of the pulsar. These observations provide direct support for the hypothesis1 that millisecond pulsars are long-lived objects, and that their magnetic fields do not decay.
TL;DR: In this paper, the authors derived the radial velocities of 15 metallic-line white dwarfs from the positions of the H and K lines of Ca II, and compared the present and past spatial distribution of the stars with the distribution of local interstellar matter.
Abstract: From observations of the spectra of 15 cool metallic-line white dwarfs we derive their radial velocities from the positions of the H and K lines of Ca II. A pressure shift correction is included, and the resulting mean gravitational redshift is 35 km/s. By projecting their spatial distribution and space velocity components onto and perpendicular to the galactic plane, and including 13 additional stars not observed by us, we compare the present and past spatial distribution of the white dwarfs with the distribution of local interstellar matter. If the metallic-line white dwarfs are accreting their metals from the interstellar medium, one would expect a spatial correlation between the stars and the local gas. However, we conclude that only very few of the stars could possibly just have accreted or presently be accreting metals from local clouds
TL;DR: In this paper, the authors present model calculations, based on more recent spectra, which point to a mass of the white dwarf of ∼0.7 solar masses, well below the Chandrasekhar mass.
Abstract: IN the standard model for type Ia supernovae1, a massive white dwarf in a binary system accretes matter from the companion star until it reaches the Chandrasekhar mass (the stability limit for degenerate-electron stars, corresponding to ∼1.4 solar masses), and a runaway thermonuclear explosion ensues. In a popular variant of this model2, the companion star is also a white dwarf. But regardless of the nature of the companion, the invariance of the Chandrasekhar mass implies that all type Ia supernovae will be similar in luminosity3, making them ideal 'standard candles' for determining extragalactic distances, and hence the Hubble constant. In the context of the standard model, the recent type Ia supernova SN1991bg is hard to explain: it was underluminous at all observed epochs, leading to suggestions4,5 that the mass of the progenitor was unusually low. Here we present model calculations, based on more recent spectra, which point to a mass of the white dwarf of ∼0.7 solar masses―well below the Chandrasekhar mass. Moreover, the late spectrum shows evidence of emission from low-velocity hydrogen gas, which might originate in material stripped from an extended, hydrogen-rich companion star. If our interpretation is correct, SN1991bg challenges both the double white-dwarf scenario, and the standard model for type Ia supernovae.
TL;DR: In this article, the authors studied the magnetic field structures of millisecond pulsars formed by the accretion induced collapse of massive white dwarfs or from neutron stars spun-up by accretion from low-mass companions.
Abstract: Millisecond pulsars may be formed by the accretion induced collapse of massive white dwarfs or from neutron stars spun-up by accretion from low-mass companions. Because the solid crust of a neutron star is expected to be moved by strong stresses which build up during spin-up or spin-down, the expected surface magnetic field structures are quite different for millisecond pulsars formed in these two different scenarios. During prolonged spin-up the moving crust compresses all stellar surface magnetic field into a small region around the spin axis
01 Jan 1993
TL;DR: In this paper, it was shown that the mass of the axions should be 0.008 eV in order to account for the properties of G117-B15A, a quantity that is compatible with all the other astrophysical bounds.
Abstract: The cooling rate of the white dwarf star G117-B15A has been recently measured using asteroseismological techniques. It has been found that it is higher than predicted from current theoretical models. As the modifications introduced into the standard models to accomodate such a result are not satisfactory, we propose to interpret this phenomenon in terms of axion cooling. We show in this paper that in order to account for the properties of G117-B15A, the mass of the axions should be 0.008 eV, a quantity that is compatible with all the other astrophysical bounds.
TL;DR: In this paper, the formation of UV emission lines in cataclysmic variables by constructing kinematical models of biconical rotating outflows from disks around white dwarfs is studied.
Abstract: Winds from accretion disks in cataclysmic variable stars are ubiquitous. Observations by IUE reveal P Cygni-shaped profiles of high-ionization lines which are attributed to these winds. We have studied the formation of UV emission lines in cataclysmic variables by constructing kinematical models of biconical rotating outflows from disks around white dwarfs. The photoionization in the wind is calculated taking into account the radiation fields of the disk, the boundary layer, and the white dwarf. The 3D radiative transfer is solved in the Sobolev approximation. Effects on the line shapes of varying basic physical parameters of the wind are shown explicitly. We identify and map the resonant scattering regions in the wind which have strongly biconical character regardless of the assumed velocity and radiation fields. Rotation at the base of the wind introduces a radial shear which decreases the line optical depth and reduces the line core intensity. We find that it is possible to reproduce the observed P Cygni line shapes and make some predictions to be verified in high-resolution observations.
TL;DR: In this article, the authors examined the possibility that symbiotic stars-wide binaries containing a red giant and a white dwarf-produce a significant fraction of Type Ia supernovae (SN Ia).
Abstract: We examine the possibility that symbiotic stars-wide binaries containing a red giant and a white dwarf-produce a significant fraction of Type Ia supernovae (SN Ia). These binaries probably cannot account for SN Ia events if the white dwarf mass must evolve to the Chandrasekhar limit during the expected lifetime of the red giant primary star. However, symbiotic binaries are good candidates for helium detonation supernovae in low-mass white dwarfs. If helium detonations can produce the majority of SN Ia events, then symbiotic stars might account for a large fraction of Type Ia supernovae
TL;DR: In this paper, a parametric survey of evolutionary models of compositionally stratified white dwarfs with helium surface layers is presented, and a comprehensive set of theoretical g-mode pulsation periods for comparison to observations of pulsating DBs is provided.
Abstract: We present the results of a parametric survey of evolutionary models of compositionally stratified white dwarfs with helium surface layers (DB white dwarfs). Because white dwarfs are the most common final end state of stellar evolution, determining their internal structure will offer us many clues about stellar evolution and the physics of matter under extreme conditions. As a first step towards determining the internal structure of DB white dwarf stars, we provide a comprehensive set of theoretical g-mode pulsation periods for comparison to observations of pulsating DB white dwarfs
TL;DR: The detection by the Rosat satellite of GQ Muscae as a very soft blackbody-like source is reported in this article, and it is concluded that the white dwarf is burning hydrogen-rich material near its surface.
Abstract: The detection by the Rosat satellite of GQ Muscae as a very soft blackbodylike source is reported. If the observed X-ray flux is being radiated at the Eddington luminosity from a one solar mass white dwarf, its effective temperature must be about 350,000 K. It is concluded that the white dwarf is burning hydrogen-rich material near its surface. GQ Mus is, however, the only one of 26 recent novae detected in the all-sky Rosat study; this suggests that either most novae eject all their accreted material during outburst or GQ Mus is now burning recently accreted material. GQ Mus appears identical to the supersoft X-ray sources CAL83, CAL87, and RX J0527.8-6954, lending support to the suggestion that these sources are white dwarfs accreting and burning material from a companion.
TL;DR: In this paper, the authors present an analysis of the EXOSAT medium energy (ME) archival data on dwarf novae, showing little correlation between the count rate and the hardness ratio, contrary to an earlier report.
Abstract: We present our analysis of the EXOSAT medium energy (ME) archival data on dwarf novae. Many dwarf novae were observed with EXOSAT; unfortunately, a significant fraction resulted in nondetection or marginal detection with the ME instrument. Our analysis of the brightest dwarf nova, SS Cyg, shows little correlation between the count rate and the hardness ratio, contrary to an earlier report. This is likely to be the result of a slight error in background subtraction performed. The apparent lack of correlation favors a compact boundary layer between the accretion disk and the white dwarf as the hard X-ray emitting region, rather than an extended corona
TL;DR: In this paper, a detailed study of a sample of 28 hot DA white dwarfs detected in the ROSAT EUV and soft X-ray all-sky-survey is presented.
Abstract: We have made a detailed study of a sample of 28 hot DA white dwarfs detected in the ROSAT EUV and soft X-ray all-sky-survey.
TL;DR: In this paper, a 13.38 hour period in the helium absorption-line profiles of the AM Canum Venaticorum was found, and the lines display blue and red troughs which alternate in depth on this period.
Abstract: We have found a 13.38 hr period in the helium absorption-line profiles of the cataclysmic variable AM Canum Venaticorum. The lines display blue and red troughs which alternate in depth on this period. Comparison of the data with theoretical line profiles of elliptical accretion disks suggests that this is the signature of an elliptical disk, with the line of apsides advancing slowly around the white dwarf. This interpretation of the 13.38 hr spectroscopic period as apsidal precession of the disk supports the idea that the familiar 1051 s photometric signal is the orbital frequency's precessional sideband, which is also believed to be present in the superhumps of dwarf novae
01 Jan 1993
TL;DR: In this article, a detailed study of a sample of 28 hot DA white dwarfs detected in the ROSAT EUV and soft X-ray all-sky-survey is presented.
Abstract: We have made a detailed study of a sample of 28 hot DA white dwarfs detected in the ROSAT EUV and soft X-ray all-sky-survey.
TL;DR: In this paper, the evolution of a 10.5 M ○, Z=Z ○, and Y=0.27 star model has been traced through the two major episodes of mass transfer in order to make a direct comparison with the previous computations of Iben & Tutukov, which we reproduce quite well.
Abstract: The evolution of primary components of initially close binary systems has been followed assuming specific initial conditions such as to have the formation of an O-Ne white dwarf at the end of a nonconservative evolution of the system parameters. More specifically, the evolution of a 10.5 M ○. , Z=Z ○. , and Y=0.27 star model has been traced through the two major episodes of mass transfer in order to make a direct comparison with the previous computations of Iben & Tutukov, which we reproduce quite well. In this case, mass transfer finishes at the beginning of the C-burning phase. Analogous experiments have been performed with both a 10 M ○. and a 10.5 M ○. (Z=2×Z ○. and Y=0.32) star models. In these cases, the second mass-transfer episode continues also during the carbon-burning phase