Low-mass White Dwarfs Research Articles

Low-mass white dwarfs need friends: five new double-degenerate close binary stars

We have discovered five detached close binary stars out of seven white dwarfs chosen for their low mass (<0.45 Mʘ). The high success rate of our observations supports the notion that evolution within a binary star is needed to obtain white dwarfs with masses below 0.45 Mʘ. We have measured the orbital period and radial velocity amplitude of four of our discoveries. No late-type features are seen in any of our targets at a level which forces the mass of any main-sequence companion to be less than 0.1 Mʘ. This, together with our measured mass functions, implies that the companion stars are in fact white dwarfs. Our observations raise the number of detached, double-degenerate close binaries with known orbital periods from two to six. All of the orbits are circular, a consequence of past interaction. We find periods of 1.1, 3.3 and 4.8 d for 1713+332, 1241–010 and 1317+453. 2331+290 has a very short period, most likely 4h, but with the 1 cycle d–1 alias at 4.8 h also a possibility. Gravitational radiation will cause this star to merge within about 2 ×109 yr. Close double-degenerates go through one or more stages during which the two stars orbit inside a common envelope. The long orbital periods of 1241–010 and 1317+453 suggest that the ejection of the envelope during the common-envelope phase is very efficient. The spectrum of the companion is directly detectable in 1713+332 but in none of the other systems.

The discovery of a short-period double-degenerate binary star

We have found that the white dwarf PG 1101+364 is a double-lined white dwarf/white dwarf binary with an orbital period of 3.47 hours. PG 1101+364 is the shortest period detached double-degenerate yet found and gravitational radiation will cause it to merge in 2.5 X 10**9 years. PG 1101+364 has a mass ratio of 0.87 +/- 0.03. From the relative strengths of the narrow core of Halpha, the lighter and therefore larger star appears to contribute more of the light by an amount that indicates almost equal temperatures for the two stars. We chose to observe PG 1101+364 on the basis of its low spectroscopic mass (0.31 Msun), and the discovery of its binary nature confirms the belief that binary evolution is required to produce such low mass white dwarfs. The radial velocity semi-amplitudes (69.7 +/- 1.7 km/s and 80.3 +/- 1.6 km/s) and the spectroscopic mass show that we see the orbit of PG 1101+364 at a low inclination of ~ 25 deg.

An extended grid of multicycle nova evolution models

view Abstract Citations (321) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Extended Grid of Multicycle Nova Evolution Models Prialnik, Dina ; Kovetz, Attay Abstract According to the nova theory, observed characteristics of novae may be reproduced by varying the values of three basic and independent parameters: the acreting white dwarf's mass MWD, its temperature TWD, and the mass transfer rate dot M. Calculations performed to date have, however, left wide regions of the parameter space unexplored. We carry out a systematic study involving calculations of evolutionary sequences of nova outbursts through several cycles, for 64 parameter combinations spanning the entire parameter space, assuming CO white dwarfs (WDs). An updated stellar evolution code is used, including an extended nuclear reactions network, new opacities (OPAL), diffusion of all elements and the effect of radiation pressure on mass loss. We find that the entire range of observed nova characteristics can be accounted for, including recurrent and symbiotic novae. Recurrent novae may be obtained on relatively low-mass WDs (approximately 1 solar mass). Accretion at rates dot M greater than or equal to 10(wxp -7) solar mass/yr invariably results in an increase of MWD and may, eventually, lead to a type 1a supernova. For accretion rates dot M less than or equal to 10-9 solar mass/yr, MWD decreases under all circumstances. The overall dependence of nova characteristics on the basic parameters is analyzed. Observed correlations between nova properties, as well as the conspicuous lack of correlation between other properties, are verified by the theoretical results. Among all the observed properties of novae there are three that appear to be independent of each other: the time of decline by 3 magnitudes t3, the heavy element abundance of the ejecta Zej, and their helium content Yej. Our calculations yield t3(MWD, TWD, dot M), Yej(MWD, TWD, dot M) at discrete points over the entire parameter space. By matching observed characteristics of a particular nova with calculated counterparts, it is possible to derive the WDs mass and temperature and the (average) accretion rate as well as additional observable properties. We find an excellent match for the measured expansion velocities, but the calculated ejected masses are generally smaller than those estimated from observations. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175741 Bibcode: 1995ApJ...445..789P Keywords: Independent Variables; Mass Transfer; Novae; Stellar Evolution; Stellar Mass; Stellar Models; Stellar Temperature; Abundance; Radiation Pressure; Stellar Interiors; Stellar Mass Accretion; Stellar Mass Ejection; White Dwarf Stars; Astrophysics; ACCRETION; ACCRETION DISKS; STARS: BINARIES: CLOSE; STARS: INTERIORS; STARS: NOVAE; CATACLYSMIC VARIABLES; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (14) Related Materials (2) Part 2: 2005ApJ...623..398Y Part 3: 2010ApJ...725..831S

Scenarios for the formation of binary and millisecond pulsars – A critical assessment

The evolution of high-and low-mass X-ray binaries (HMXB and LMXB) into different types of binary radio pulsars, the ‘high-mass binary pulsars’(HMBP) and ‘low-mass binary pulsars’ (LMBP) is discussed. The HMXB evolve either into Thorne-Zytkow objects or into short-period binaries consisting of a helium star plus a neutron star (or a black hole), resembling Cygnus X-3. The latter systems evolve (with or without a second common-envelope phase) into close binary pulsars, in which the companion of the pulsar may be a massive white dwarf, a neutron star or a black hole ( some final systems may also consist of two black holes). A considerable fraction of the systems may also be disrupted in the second supernova explosion. We discuss the possible reasons why the observed numbers of double neutron stars and of systems like Cyg X-3 are several orders of magnitude lower than theoretically predicted. It is argued that the observed systems form the tip of an iceberg of much larger populations of unobserved systems, some of which may become observable in the future. As to the LMBP, we consider in some detail the origins of systems with orbital periods in the range 1–20 days. We show that to explain their existence, losses of orbital angular momentum (e.g., by magnetic braking) and in a number of cases: also of mass, have to be taken into account. The masses of the low-mass white dwarf companions in these systems can be predicted accurately. We notice a clear correlation between spin period and orbital period for these systems, as well as a clear correlation between pulsar magnetic field strength and orbital period. These relations strongly suggest that increased amounts of mass accreted by the neutron stars lead to increased decay of their magnetic fields: we suggest a simple way to understand the observed value of the ‘bottom’ field strengths of a few times 108 G. Furthermore, we find that the LMBP-systems in which the pulsar has a strong magnetic field (> 1011 G) have an about two orders of magnitude larger birth rate (i.e., about 4 × 10-4 yr-1 in the Galaxy) than the systems with millisecond pulsars (which have B < 109 G). Using the observational fact that neutron stars receive a velocity kick of ∼450 km/s at birth, we find that some 90% of the potential progenitor systems of the strong-field LMBP must have been disrupted in the Supernovae in which their neutron stars were formed. Hence, the formation rate of the progenitors of the strong-field LMBP is of the same order as the galactic supernova rate (4 × 10-3 yr-1). This implies that a large fraction of all Supernovae take place in binaries with a close low-mass (< 2.3 M⊙) companion.

Optically thick winds in nova outbursts

view Abstract Citations (243) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optically Thick Winds in Nova Outbursts Kato, Mariko ; Hachisu, Izumi Abstract Using the OPAL opacity, we have calculated a sequence of optically thick wind solutions, which mimics a time-dependent evolution of the decay phase of novae. Strong winds are driven by a large peak in the OPAL opacity and, as a result, theoretical timescale of nova duration is drastically shortened compared with the use of the old opacity. Thus, we are able to resolve a theoretical problem of nova duration: the theoretical timescale of nuclear burning for the initial envelope mass at ignition is too long to reconcile with the observational durations of novae. Good quality light curves are automatically obtained from the combination of the optically thick wind theory and the OPAL opacity. We have compared our theoretical light curves with the observations of a well-studied classical nova, Nova Cygni 1978, and found that our 1.0 solar mass white dwarf model shows an excellent agreement with the observations both for the visual light curves and for the ultraviolet light curves and also for the expansion velocities of the envelope. These results strongly suggest the validity of our steady state approach and indicate that the optically thick wind really occurs on the white dwarf at least in the decay phase of the nova. In other words, optically thick winds, in which the matter is accelerated deep inside the photosphere, are the main acceleration mechanism in the decay phase of novae. Comparison of our theoretical light curves with observational ones enables us to determine the mass of the white dwarf and the distance of the star. The distance to Nova Cygni 1978 is estimated to be 2.9 - 3.1 kpc with the white dwarf mass of 1.0 solar mass. Thus, quantitative studies of light curve fitting will be able to provide/add useful information of binary parameters that have been poorly known. The effects of the drag luminosity in the common envelope phase have also been estimated by a one-dimensional (spherical) model. It is found that the drag luminosity is as small as or smaller than 1% of the photospheric luminosity because the density of the envelope drops sharply near/outside the accelerating region of wind. Only for relatively low-mass white dwarfs, the decline rate of the light curves is much affected by the effects of a companion, for example, the decline rate of a 0.6 solar mass white dwarf and a 0.2 solar mass companion shows a similar rate of 0.7 solar mass without a companion. However, we may conclude that the accuracy of the mass determination of the white dwarf is still within 0.1 solar mass. Publication: The Astrophysical Journal Pub Date: December 1994 DOI: 10.1086/175041 Bibcode: 1994ApJ...437..802K Keywords: Cataclysmic Variables; Novae; Optical Thickness; Stellar Evolution; Stellar Mass Ejection; Stellar Winds; White Dwarf Stars; Astronomical Models; Flux Density; Light Curve; Opacity; Stellar Luminosity; Thermonuclear Reactions; Astrophysics full text sources ADS | data products SIMBAD (4)

Spectroscopic, orbital, and physical properties of the binary Feige 24 and detection of transient He II absorption in the system

view Abstract Citations (36) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectroscopic, Orbital, and Physical Properties of the Binary Feige 24 and Detection of Transient He II Absorption in the System Vennes, S. ; Thorstensen, J. R. Abstract We have obtained new high-dispersion optical spectroscopy at Kitt Peak National Observatory (KPNO) and new International Ultraviolet Explorer (IUE) spectroscopy of the white dwarf+red dwarf binary system Feige 24. The optical range shows a composite DA+dM spectrum, together with H I Balmer and He I emission. The orbital phase dependence of the emission shows that it results from extreme ultraviolet (EUV) light reprocessing in the red dwarf upper atmosphere. The systems close enough and hot enough to show this reprocessing signature only recently emerged from common-envelope evolution. The ultraviolet spectrum exclusively emanates from the white dwarf and shows numerous heavy element absorption lines. We measured accurate radial velocities of the red dwarf component motion, traced by both optical absorption and emission lines, and new radial velocities of the white dwarf, traced by ultraviolet Fe V lines. Combining these measurements, we refined the orbital parameters presented by Vennes et al. (1991), and we confirmed that the white dwarf gravitational redshift is exceptionally small with 9 +/- 2 km/s. From this we deduced that the interior is either pure helium or carbon with a thick hydrogen layer, and we derived, for the combined interior compositions, a white dwarf mass and radius of MWD = 0.44-050 solar mass and RWD = 0.028-0.036 solar radius. We suggest that Feige 24 could be a typical case of close binary evolution leading to the formation of a low-mass helium white dwarf. The mass of the red dwarf and the inclination of the system naturally follow: MdM = 0.26-0.33 solar mass, i greater than or equal to 75 deg. High-dispersion H-alpha line profiles are asymmetrical, strongly enhanced toward the blue, suggesting a moving atmosphere possibly linked to a mass loss rate of 10-10 solar mass/yr. The IUE spectra taken when the system is near inferior conjunction show strong He II 1640 A absorption. The profile is highly variable in width and intensity. Because it is correlated with the passage of the white dwarf at inferior conjunction, the absorption may occur in some foreground plasma emanated by the red dwarf and accumulating near a Lagrangian point or, alternatively, it may originate in an accretion spot on the white dwarf surface coaligned with the major orbital axis. Either way, the He II detection may imply substantial mass loss from the red dwarf with a corollary reclassification of Feige 24 as a mixed He/H DAO white dwarf resulting from accretion of secondary mass-loss material. Feige 24 is the prototype of a class of young, EUV-emitting, binary systems comprising a late main sequence secondary and a hot H-rich white dwarf; the class is characterized by optical and ultraviolet photospheric He II absorption, circumstellar C IV lambda (1550) absorption, and by the presence of EUV-induced, phase-dependent Balmer fluorescence. These young systems present the best opportunity to constrain theory of common-envelope evolution. Publication: The Astronomical Journal Pub Date: November 1994 DOI: 10.1086/117201 Bibcode: 1994AJ....108.1881V Keywords: Absorption Spectra; Binary Stars; Dwarf Stars; Helium; Light (Visible Radiation); Red Dwarf Stars; Stellar Mass Accretion; Stellar Spectra; Ultraviolet Astronomy; Ultraviolet Spectra; Visible Spectrum; Astronomical Spectroscopy; Balmer Series; Emission Spectra; H Alpha Line; Iue; Radial Velocity; Stellar Envelopes; Stellar Mass; Velocity Measurement; Astronomy; STARS: INDIVIDUAL: FEIGE 24; BINARIES: SPECTROSCOPIC full text sources ADS | data products SIMBAD (11) MAST (2) INES (1)

Gamma-ray light curves and spectra of models for Type IA supernovae

view Abstract Citations (15) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gamma-Ray Light Curves and Spectra of Models for Type IA Supernovae Hoeflich, P. ; Khokhlov, A. ; Mueller, E. Abstract Based on detailed Monte Carlo calculations, we present γ-ray energy deposition functions, γ-ray light curves, and γ-ray spectra for a large set of theoretical models of Type Ia supernovae including "classical" detonation and deflagration, delayed detonation, explosions of low mass white dwarfs, and tamped detonation scenarios. Our computations show that models for Type Ia supernovae can be discriminated and the absolute amount of ^56^Ni synthesized in the event can be determined on the basis of the γ-ray light curves and spectra if γ-ray measurements are combined with observations at other wavelengths, e.g., in the optical band. We discuss at which times γ-ray observations are most suitable and needed from the theoretical point of view. The implication of the upper limit in the γ-ray flux by CGRO experiment for our understanding of SN 1991T is discussed. We find that this limit is consistent with both the optical light curve and the implied distance (12.5 Mpc) i.e., several models can be ruled out by the γ-ray observations. Publication: The Astrophysical Journal Supplement Series Pub Date: June 1994 DOI: 10.1086/192004 Bibcode: 1994ApJS...92..501H Keywords: Gamma Ray Astronomy; Gamma Ray Spectra; Light Curve; Stellar Models; Supernovae; Deflagration; Detonation; Monte Carlo Method; Nickel Isotopes; Astronomy full text sources ADS | data products NED (2) SIMBAD (1)

On the interpretation and implications of nova abundances: an abundance of riches or an overabundance of enrichments

We reexamine the question of the frequency of occurrence of oxygen-neon-magnesium (ONeMg) degenerate dwarfs in classical nova systems, in light of recent observations which have been interpreted as suggesting that 'neon novae' can be associated with relatively low mass white dwarfs. Determinations of heavy-element concentrations in nova ejecta are reviewed, and possible interpretations of their origin are examined. We conclude that, of the 18 classical novae for which detailed abundance analyses are availble, only two (or possibly three) seem unambiguously to demand the presence of an underlying ONeMg white dwarf: V693 CrA 1981, V1370 Aql 1982, and possibly QU Vul 1984. Three other novae which exhibit significant neon enrichments, relative to their total heavy-element concentrations, are RR Pic 1925, V977, Sco 1989, and LMC 1990 No. 1. This result is entirely consistent with present frequency estimates, and our interpretation of the lower levels of enrichment in other systems explains, in a natural way, the existence of relatively low mass white dwarfs in some of the 'neon' novae.

Evolutionary sequences of very hot, low-mass, accreting white dwarfs with application to symbiotic variables and ultrasoft/supersoft low-luminosity x-ray sources

We present the first detailed model results of quasi-static evolutionary sequences of very hot low-mass white dwarfs accreting hydrogen-rich material at rates between 1 x 10(exp -7) and 1 x 10(exp -9) solar mass/yr. Most of the sequences were generated from starting models whose core thermal structures were not thermally relaxed in the thermal pulse cycle-averaged sense of an asymptotic giant branch stellar core. Hence, the evolution at constant accretion rate was not invariably characterized by series of identical shell flashes. Sequences exhibiting stable steady state nuclear burning at the accretion supply rate as well as sequences exhibiting recurrent thermonuclear shell flashes are presented and discussed. In some cases, the white dwarf accretors remain small (less than 10(exp 11) cm) and very hot even during the shell flash episode. They then experience continued but reduced hydrogen shell burning during the longer quiescent intervals while their surface temperatures increase both because of compressional heating and envelope structure readjustment in response to accretion over thousands of years. Both accretion and continued hydrogen burning power these models with luminosities of a few times 10(exp 37) ergs/s. We suggest that the physical properties of these model sequences are of considerable relevance to the observed outburst and quiescent behavior of those symbiotic variables and symbiotic novae containing low-mass white dwarfs. We also suggest that our models are relevant to the observational characteristics of the growing class of low-luminosity, supersoft/ultrasoft X-ray sources in globular clusters, and the Magellanic Clouds.

Origin and Evolution of X-Ray Binaries and Binary Radio Pulsars

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

On symbiotic stars and Type IA supernovae

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

Multiple hydrogen flashes on accreting low-mass white dwarfs - Novae and DAO stars

view Abstract Citations (45) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Multiple Hydrogen Flashes on Accreting Low-Mass White Dwarfs: Novae and DAO Stars Shara, Michael M. ; Prialnik, Dina ; Kovetz, Attay Abstract The evolution of two low-mass white dwarfs, accreting hydrogen-rich matter at a rate of 10 exp -9 solar mass/yr was calculated by means of a hydrodynamic code. A 0.6 solar mass white dwarf, composed of carbon and oxygen, and a 0.4 solar mass helium white dwarf are considered, both of low intrinsic luminosity. They are found to undergo periodic eruptions, which, for the CO white dwarf become regular only after about five cycles. The outbursts are similar in appearance to those of classical novae, although on a largely stretched time scale. The high-luminosity phase lasts for about 200 yr for both stars. The accretion continues for 4 x 10 exp 5 yr and 9 x 10 exp 5 yr, for the 0.6 and 0.4 solar mass WDs, respectively. Both stars lose mass following the outburst. While the more massive white dwarf ejects the entire accreted mass, the smaller, He WD retains some of the accreted material which is added to the core after being burnt into helium. Publication: The Astrophysical Journal Pub Date: March 1993 DOI: 10.1086/172433 Bibcode: 1993ApJ...406..220S Keywords: Cataclysmic Variables; Dwarf Novae; Stellar Evolution; Stellar Mass Accretion; White Dwarf Stars; Binary Stars; Stellar Luminosity; Stellar Mass Ejection; Astrophysics; ACCRETION; ACCRETION DISKS; DIFFUSION; STARS: NOVAE; CATACLYSMIC VARIABLES; STARS: BINARIES: CLOSE; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (5)

The subluminous, spectroscopically peculiar type IA supernova 1991bg in the elliptical galaxy NGC 4374

We present photometric and spectroscopic observations of SN 1991bg, a very peculiar Type Ia supernova located in the outskirts of the E1 galaxy NGC 4374 (M84) in the Virgo cluster. At maximum brightness SN 1991bg was 1.6 mag subluminous in V and 2.5 mag subluminous in B, compared with normal SNe Ia. The colors were unusually red, but the object was not significantly reddened by dust. The decline from maximum was certainly quite steep; we measure an initial linear V fading of 0.10 mag d^-1^ rather than the typical value of 0.06 mag d^-1^ for SNe Ia, and a late- time decline of 0.034 mag d^-1^ rather than 0.026 mag d^-1^. Furthermore, the knee in the Flight curve of SN 1991bg occurs only ~17 d past B maximum, in contrast to the usual value of 30-40 d. Unlike normal SNe Ia, the I light curve does not exhibit a secondary maximum, and the R light curve shows no sign of a plateau. Despite having prominent Si II absorption near 6150 A, characteristic of SNe Ia, SN 1991bg was spectroscopically peculiar at maximum brightness; e.g. a deep trough was present around 4200 A. Spectral synthesis suggests that much of this feature was produced by Ti II. Indeed, Ti II was present elsewhere in the spectrum whereas Fe II was weak or absent. Lines due to other intermediate-mass elements were also strong near maximum brightness, but the expansion velocity (~10 000 km s^-1^) was slightly lower than average (11 000-13 000 km s^-1^) for SNe Ia. A spectrum obtained 3 weeks past maximum has a rather narrow absorption line attributed to Na I D; the strength of this feature grew with time. In certain other respects the spectrum 3 weeks past maximum more closely resembles spectra of SNe Ib/Ic than of SNe Ia. We identify emission lines of [Ca II] λλ7291, 7324, O I λ7773, the Ca II infrared triplet, and (tentatively) [O I] λ5577, all superimposed on a complex blend of P Cygni scattering lines; apparently the nebular phase began very early in SN 1991bg. Three months past maximum the [Ca II] blend started to dominate the spectrum. The properties of SN 1991bg suggest that this object was produced by the slow deflagration of a white dwarf. The observed expansion velocity, however, is too large unless a sub-Chandrasekhar mass of material was ejected. An unusually low mass for the ejecta expanding at a nearly normal velocity, is consistent with the early transition to the nebular phase as well as with the rapid decline and the early knee in the V light curve. Thus, SN 1991bg may have resulted from the deflagration of only the outer layer of material on top of a C-O core, thereby leaving a compact remnant; alternatively, perhaps the complete explosion of a low-mass (~1.0 M_sun_) white dwarf was responsible. We show that some properties of the peculiar SN 1986G in Cen A were similar to those of SN 1991bg, and it may have been intrinsically dim by several magnitudes. Moreover, SN 1971I in NGC 5055 is known to have resembled SN 1986G; thus, SN 1991bg was probably a close cousin of these two objects. SN 1991bg clearly demonstrates that at least some SNe 1a are mavericks, both photometrically and spectroscopically. These objects must be carefully eliminated from samples of SNe 1a used in determinations of cosmological parameters.

The hot subdwarf in the eclipsing binary HD 185510

High-resolution spectroscopic measurements of radial velocity are employed to characterize the eclipsing binary HD 185510 in terms of masses and evolutionary status. The IUE is used to obtain the radial velocities which indicate a large mass ratio Mp/Ms of 7.45 +/- 0.15, and Teff is given at 25,000 +/- 1000 K based on Ly alpha and UV spectrophotometry. Photometric observations are used to give an orbital inclination of between 90 and 70 deg inclusive, leading to masses of 0.31-0.37 and 2.3-2.8 solar mass for the hot star and the K star, respectively. The surface gravity of HD 185510B is shown to be higher than those values for sdB stars suggesting that the object is a low-mass white dwarf that has not reached its fully degenerate configuration. The object is theorized to be a low-mass helium main-sequence star or a nascent helium degenerate in a post-Algol system.

The radial velocity of the white dwarf Giclas 29-38

view Abstract Citations (15) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Radial Velocity of the White Dwarf Giclas 29-38 Barnbaum, C. ; Zuckerman, B. Abstract The white dwarf Giclas 29-38 displays mysterious excess infrared emission at wavelengths between 2 and 10 microns. The radial velocity of G29-38 was measured 15 times during the latter half of 1990 and 5 times during the latter half of 1991. The present data, in combination with those of Graham et al. (1990), which were obtained during the latter half of 1989, imply that the radial velocity of G29-38 is probably varying with a period of about 11.2 months and total amplitude of about 10 km/s. If confirmed, the variation is likely due to either a high-mass brown dwarf or low-mass white dwarf companion. Publication: The Astrophysical Journal Pub Date: September 1992 DOI: 10.1086/186510 Bibcode: 1992ApJ...396L..31B Keywords: Brown Dwarf Stars; Infrared Radiation; Radial Velocity; Stellar Rotation; White Dwarf Stars; Near Infrared Radiation; Stellar Magnitude; Stellar Spectrophotometry; Astrophysics; STARS: BINARIES: CLOSE; STARS: LOW-MASS; BROWN DWARFS; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (4)

OUTBURSTS BY LOW-MASS WHITE DWARFS IN SYMBIOTIC VARIABLES

ABSTRACT We have investigated the nature of P Cygni-like features in the high resolution IUE spectra of the symbiotic variables BF Cyg and EG And by looking for orbital phase-dependent variations in the structure and velocity of the C IV (1550 A) resonance doublet in 10 archival IUE high-dispersion spectra of these systems and by comparing their profiles with P Cygni-like features seen in IUE high-dispersion spectra of other symbiotic stars. For BF Cyg, we find maximal P Cygni absorption equivalent widths and terminal velocities of 900 mA and -240 Km s-1 respectively, full emission velocity widths at the 10% intensity level of 300-400 km s-1, depending on phase, Phi, variations in the width of the P Cygni absorption, shortward velocity shifts of the C IV emission line in all but two orbital phases, relative to the C IV velocity at Phi = 0.01, and absorption sub-structure within the C IV P Cygni lines which coincide in velocity from image to image. In EG And the C IV P Cygni absorption equivalent width is, on average, 300 mA with a terminal velocity of -217 km s-1 (which includes the systemic radial velocity contribution of -93 km s-1). The presence in one image, of a strong absorption feature at He II (1640), having a precise velocity coincidence with the C IV P Cygni absorption component, suggests that the absorbing material responsible for both line features is associated with outflow and P Cygni self-absorption from the vicinity of the hot component. In both BF Cyg and EG And, the structure and behavior of the P Cygni profiles may rule out their origin in the red giant wind or in an expanding circumbinary shell. Our results support the hot component wind interpretation of Mikolajewska et al. (1989). Motivated by the low-mass white dwarf shown to be present in BF Cyg (Mikolajewska et al. 1989), we have carried out quasi-static evolutionary model calculations with accretion onto a 0.55 solar mass white dwarf accreting at the rate of 2 X 10-8 solar mass yr-1. The sequence of 3800 quasi-static models, the first to be reported for a low-mass hot white dwarf (Mwd < 0.8 solar mass) accreting at a high rate, yields an unexpected outburst behavior in response to accretion at that rate, when compared to evolutionary sequences with accretion onto higher mass white dwarfs. A series of three major thermonuclear shell flashes and one lower energy shell flash is revealed with the timescale during outburst and the inter-outburst timescale substantially shorter than expected for a 0.55 solar mass white dwarf. Implications for the observations of symbiotic systems thought to contain low-mass white dwarfs, are discussed.

Outbursts by Low-Mass White Dwarfs in Symbiotic Variables

We have investigated the nature of P Cygni-like features in the high resolution IUE spectra of the symbiotic variables BF Cyg and EG And by looking for orbital phase-dependent variations in the structure and velocity of the C IV (1550 Å) resonance doublet in 10 archival IUE high-dispersion spectra of these systems and by comparing their profiles with P Cygni-like features seen in IUE high-dispersion spectra of other symbiotic stars. For BF Cyg, we find maximal P Cygni absorption equivalent widths and terminal velocities of ~ 900 mÅ and ~ −240 km s−1 respectively, full emission velocity widths at the 10% intensity level of 300–400 km s−1, depending on phase, Φ, variations in the width of the P Cygni absorption, shortward velocity shifts of the C IV emission line in all but two orbital phases, relative to the C IV velocity at Φ = 0.01, and absorption sub-structure within the C IV P Cygni lines which coincide in velocity from image to image. In EG And the C IV P Cygni absorption equivalent width is, on average, 300 mÅ with a terminal velocity of −217 km s−1 (which includes the systemic radial velocity contribution of -93 km s−1). The presence in one image, of a strong absorption feature at He II (1640), having a precise velocity coincidence with the C IV P Cygni absorption component, suggests that the absorbing material responsible for both line features is associated with outflow and P Cygni self-absorption from the vicinity of the hot component. In both BF Cyg and EG And, the structure and behavior of the P Cygni profiles may rule out their origin in the red giant wind or in an expanding circumbinary shell. Our results support the hot component wind interpretation of Mikolajewska et al. (1989). Motivated by the low-mass white dwarf shown to be present in BF Cyg (Mikolajewska et al. 1989), we have carried out quasi-static evolutionary model calculations with accretion onto a 0.55 M⊙ white dwarf accreting at the rate 2 × 10−8 M⊙ yr−1. The sequence of 3800 quasi-static models, the first to be reported for a low-mass hot white dwarf (Mwd &lt; 0.8 M⊙) accreting at a high rate, yields an unexpected outburst behavior in response to accretion at that rate, when compared to evolutionary sequences with accretion onto higher mass white dwarfs. A series of three major thermonuclear shell flashes and one lower energy shell flash is revealed with the timescale during outburst and the inter-outburst timescale substantially shorter than expected for a 0.55 M⊙ white dwarf. Preliminary results on other sequences are also reported. Implications for the observations of symbiotic systems thought to contain low-mass white dwarfs, are discussed.

Low-mass red giants and masses of NPNs

The evolutionary track of low-mass red giant stars (0.7–0.9M⊙) is computed with the aim to demonstrate the conditions under which low-mass white dwarfs (WDs) can form through the evolution of single stars. Also, the influence of the mixing length to the scale height ratio on the radius of the star is calculated and the coupling between the mixing-length and the mass-loss rate parameters is investigated. Our conclusions are that the uncertainties in mass-loss and mixing-length to scale-height ratio leave enough parameter space to allow the formation of low-mass WD via single star evolution. We also conclude that the gap between proto-WD stars without any nebula and stars with well-defined nebulae is bridged by stars which have a dilute gas cloud around them which cannot be observed as a nebula.

On the mass of 40 ERI B

New observations of the gravitational redshift of 40 Eri B are combined with the recent improvement in the parallax determination to derive a mass of 0.53 + or - 0.04 solar masses for the white dwarf. This is much closer to the typical value for white dwarfs, and is consistent with mass-radius relations, but inconsistent with the astrometric mass determination (0.43 + 0.02 solar masses). While the error of the astrometric mass determination might be larger than usually assumed, a discrepancy remains, which cannot be explained. If the high mass could be confirmed, it would remove 40 Eri B as a case in favor of the merging hypothesis for the explanation of the existence of low-mass white dwarfs.

Equation of state in metals and cold stars - Evaluation of statistical models

The Thomas-Fermi-Dirac (TFD) statistical model for electronic structures if refined by Weizsacker gradient and correlation energy corrections. The resulting FTD-λWc model is then used to calculate the nonrelativistic, cold matter equation of state (EOS) in the Wigner-Seitz spherical cell approximation. The correction terms are important mainly at low densities near matter equilibrium (i.e., zero pressure). Inclusion of the gradient term removes many of the unphysical features of the TFD model. Results are summarized for several elements of astrophysical and theoretical importance. The mass-radius relation of a low-mass white dwarf model is calculated using the TDF-λWc EOS.