non-DA White Dwarfs Research Articles

Molecules in white dwarfs

Molecular dissociation equilibrium calculations were done for the model atmospheres of DA and non-DA white dwarfs. Our calculations show that He 2 + and HeH+ appear as most abundant molecules in the atmospheres of non-DA white dwarfs while H2 and H 2 + are most abundant molecules in DA white dwarfs. It is suggested that these molecules should be searched for in the atmospheres of white dwarfs.

On the number-mass distribution of degenerate dwarfs produced by interacting binaries and evidence for mergers of low-mass helium dwarfs

view Abstract Citations (99) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Number-Mass Distribution of Degenerate Dwarfs Produced by the Interacting Binaries and Evidence for Mergers of Low-Mass Helium Dwarfs Iben, Icko, Jr. ; Tutukov, Alexander V. Abstract We construct a theoretical distribution function giving birthrate versus mass for degenerate dwarfs produced in close binaries as well as one giving the birthrate versus mass for dwarfs produced by single stars and wide binaries. The overall birthrate distribution function has two major peaks one which has a sharp maximum at 0.55 Msun, near the minimum white dwarf mass which a single star or component of a wide binary can produce, and another which has a maximum at∼ 0.2 Msun. This second peak is made up primarily of helium degenerate dwarfs of mean mass 0.25 Msun produced in low-mass close binaries. Our birthrate distribution function can be made consistent with number-mass distributions defined by observed white dwarfs if we assume that the majority of low-mass helium dwarfs merge on a time scale short compared with the age of our Galaxy. This assumption is consistent with rough estimates of the degree of orbital shrinkage caused by a magnetic stellar wind and/or by common envelope action during the formation of each degenerate dwarf and with the rate of orbital shrinkage due to gravitational wave radiation after the formation of each degenerate pair. Using parameterized algorithms, we estimate that the minimum mass of a merged product is ∼0.3 Msun and that the maximum mass depends on the initial mass ratio, being typically 0.65 Msun. Some of the hydrogen caught between two merging dwarfs may mix outward by turbulent mixing, and gravitational diffusion may lead to a nearly pure hydrogen surface layer. The lightest merged products may therefore appear initially as DA white dwarfs, 40 Eri B being a possible example. The most massive merged products may ignite helium and become sdB stars. Since these stars require a long wait between the formation of the degenerate pair precursors and merging, the scale height defined by sdB products will be significantly larger than that defined by young disk stars, consistent with observations. The thin layer of surface hydrogen may be abstracted by a modest radiative wind, and all merged products may eventually become non-DA white dwarfs. The total number of non-DA white dwarfs formed in this way relative to the total number of white dwarfs may be ∼10/100. Publication: The Astrophysical Journal Pub Date: December 1986 DOI: 10.1086/164813 Bibcode: 1986ApJ...311..753I Keywords: Binary Stars; Dwarf Stars; Mass Distribution; Star Distribution; Stellar Evolution; Stellar Mass; Helium; Stellar Luminosity; Stellar Models; Astrophysics; LUMINOSITY FUNCTION; STARS: BINARIES; STARS: EVOLUTION; STARS: WHITE DWARFS full text sources ADS |

On the formation and evolution of a helium degenerate dwarf in a close binary

view Abstract Citations (144) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Formation and Evolution of a Helium Degenerate Dwarf in a Close Binary Iben, Icko, Jr. ; Tutukov, Alexander V. Abstract The authors explore the possible evolution of one component in a close binary system when it is assumed that this component fills its Roche lobe after it has become a giant with a degenerate helium core of mass ≡0.3 M_sun;. The model experiences two hydrogen shell flashes which begin when the envelope is compact and which force the model to again fill its Roche lobe. The second flash is due to the diffusion of hydrogen inward to regions of high temperature. After all nuclear burning has ceased and all explicitly calculated mass-loss episodes are completed, the total mass of hydrogen remaining near the surface of the model is about 1.4×10-4M_sun;. A modest radiative wind could remove the surface layer of hydrogen, transforming the model into a non-DA white dwarf. The radius of the model remains significantly larger than that of a zero-temperature model until well over 5×109yr of cooling have elapsed. Using the cooling curve, the authors construct a number-magnitude distribution function for helium dwarfs. Publication: The Astrophysical Journal Pub Date: December 1986 DOI: 10.1086/164812 Bibcode: 1986ApJ...311..742I Keywords: Binary Stars; Helium; Star Formation; Stellar Evolution; Stellar Models; White Dwarf Stars; Giant Stars; Roche Limit; Stellar Envelopes; Stellar Interiors; Astrophysics; DIFFUSION; STARS: BINARIES; STARS: EVOLUTION; STARS: INTERIORS; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (1)

NEWLY OBSERVED DEGENERATE STARS AND RELATED OBJECTS FROM THE PRAESEPE, CASE AND KISO SURVEYS.

ABSTRACT The authors report the discovery of 15 white dwarfs from among blue candidates in the Low Dispersion Northern Sky Case Survey and the Kiso Schmidt camera survey. The observations were made at the 2.3-m Steward telescope, the KPNO 2.1-m telescope, and the Multiple Mirror Telescope. Spectroscopy of Praesepe white-dwarf candidates revealed a DA star, and a possible DC star, which if confirmed would be only the second known non-DA white dwarf in an open cluster. One Praesepe white-dwarf candidate is a QSO with z = 2.1.

An analysis of nonradial pulsations of the central star of the planetary nebula K1-16

view Abstract Citations (50) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An analysis of nonradial pulsations of the central star of the planetary nebula K1-16. Starrfield, S. ; Cox, A. N. ; Kidman, R. B. ; Pensnell, W. D. Abstract A linear, nonradial, nonadiabatic pulsation stability analysis of stellar models with effective temperatures 150,000, 155,000, 160,000, and 165,000 K, and luminosities characteristic of stars evolving from AGB to the white dwarf cooling sequence, has been performed. The purpose is to find unstable pulsation modes with periods as long as those observed in the central star of the planetary nebula, K1-16. These models have masses of 0.6 solar masses and homogeneous compositions of half-carbon and half-oxygen by mass. All these stellar models exhibit unstable nonradial modes with periods ranging from 1000 to 4000 s. The longest periods were found in the model with T(e) = 155,000 K, L = 2000 solar luminosities. The pulsation driving mechanism is the cyclical ionization of carbon and oxygen at temperatures of 2-5 million K. This analysis demonstrates that K1-16 is probably a member of the PG 1159-035 class of sdO variable stars. This star is, therefore, an evolutionary link from the central stars of the OVI planetary nebulae to the hottest non-DA white dwarfs as proposed by Sion, Liebert, and Starrfield (1985). It must also be evolving very rapidly. Publication: The Astrophysical Journal Pub Date: June 1985 DOI: 10.1086/184484 Bibcode: 1985ApJ...293L..23S Keywords: Planetary Nebulae; Stellar Evolution; Stellar Oscillations; Variable Stars; Asymptotic Giant Branch Stars; Carbon; Hot Stars; Oxygen; Stellar Models; Astrophysics full text sources ADS | data products SIMBAD (1)

Implications of the absolute magnitude distribution functions of DA and non-DA white dwarfs

view Abstract Citations (67) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Implications of the absolute magnitude distribution functions of DA and non-DA white dwarfs. Sion, E. M. Abstract The number distribution of absolute magnitudes in the sample of proper-motion-dominated white dwarfs of Liebert et al. (1983) and in the Palomar-Green sample is determined, and the distributions of DA and non-DA spectral types are analyzed on the basis of color-luminosity calibrations. The DA/non-DA ratios for the cool and hot ends of the distribution are found to be 2:1 and 6:1, respectively. Two mechanisms which can help explain the distribution functions are examined: a DA-to-DB transition at Te above 15,000 K via the diffusion-induced burning of surface H (as suggested by Michaud et al., 1984) and convective mixing of surface H in cooling DA stars at Te = 10,000-12,000 K. Publication: The Astrophysical Journal Pub Date: July 1984 DOI: 10.1086/162240 Bibcode: 1984ApJ...282..612S Keywords: Distribution Functions; Stellar Color; Stellar Evolution; Stellar Magnitude; Stellar Spectrophotometry; White Dwarf Stars; Balmer Series; Carbon; Helium; Iue; Astrophysics full text sources ADS | data products SIMBAD (1)

On the evolution of those nuclei of planetary nebulae that experience a final helium shell flash

view Abstract Citations (500) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the evolution of those nuclei of planetary nebulae that experiencea final helium shell flash. Iben, I., Jr. ; Kaler, J. B. ; Truran, J. W. ; Renzini, A. Abstract The authors suggest that some of the central stars of planetary nebulae experience a final thermal pulse after having achieved a white dwarf configuration and begun their descent along a cooling white dwarf sequence of nearly constant radius. Theoretical calculations show that there are two possible paths for the evolution of a PN nucleus past the final thermal pulse. In the first case, the pulse occurs well up on the white dwarf cooling track while the star is surrounded by a visible planetary nebula. The subsequent giant phase is of too short a duration to be observable, and the star next appears on the high-luminosity portion of the quiescent helium-burning track, where it may be identified with the nuclei of planetaries like Abell 30 and Abell 78. In the second case, the pulse occurs farther down on the track, when the surrounding nebulae would no longer be visible on existing surveys. The star remains longer in its giant state, where it is identified with the R CrB stars. Eventually, both of these fade to become non-DA white dwarfs. Publication: The Astrophysical Journal Pub Date: January 1983 DOI: 10.1086/160631 Bibcode: 1983ApJ...264..605I Keywords: Planetary Nebulae; Stellar Envelopes; Stellar Evolution; White Dwarf Stars; Abundance; Helium; Hertzsprung-Russell Diagram; Hydrogen; Stellar Luminosity; Stellar Winds; Thermonuclear Reactions; Astrophysics full text sources ADS | data products SIMBAD (2)

The peculiar structure of very cool non-DA white dwarf atmospheres

Model atmospheres of very cool (T/sub eff/<4000 K) non-DA degenerate stars have a rather peculiar structure provided the influence of trace elements on the equation of state and the opacity can be neglected. Below an almost completely transparent outermost layer (showing only some Rayleigh scattering), the actual photosphere always appears just in those layers in which pressure ionization sets in and the resulting free electrons permit a buildup of opacity. The total density variation in such a photosphere is much smaller than in other stellar atmospheres.

On the surface compositions of magnetic white dwarfs

Magnetic white dwarfs have mixed spectra (of hydrogen and helium) much more frequently than nonmagnetic white dwarfs. Many authors have suggested that this is caused by the slowing down of gravitational settling by the magnetic field. This is shown not to be the case. The magnetic field can effectively stop diffusion only at very small optical depths (tau< or approx. =10/sup -3/), not in the usual line forming region. Instead it is suggested that the magnetic field stops the selective wind of hydrogen postulated by Michaud and Fontaine to explain the near complete absence of hydrogen from nonmagnetic, non-DA white dwarfs.

Photometry of white dwarfs on the UVBY system

view Abstract Citations (57) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photometry of white dwarfs on the uvby system. Wegner, G. Abstract This paper reports intermediate band photometry using the Stromgren uvby system of 112 faint stars, mostly southern hemisphere white dwarfs. The internal errors and comparison with published measurements suggest that these data are on the same photometric system as Graham (1972) and have comparable errors. For the DA stars, the distribution of masses is determined using delta log g found from 66 objects. The mass distribution appears slightly asymmetrical toward higher masses, but for most stars, can be fitted by a Gaussian distribution of width + or - 0.24 in delta log g, corresponding to + or - 0.15 solar mass. Probably the observational errors are too large to determine this width accurately. Some aspects of the non-DA white dwarfs are also discussed. Of note is the possible separation of the carbon band white dwarfs into subclasses. Publication: The Astronomical Journal Pub Date: September 1979 DOI: 10.1086/112555 Bibcode: 1979AJ.....84.1384W Keywords: Stellar Spectrophotometry; Ubv Spectra; Visual Photometry; White Dwarf Stars; Error Analysis; Hot Stars; Mass Distribution; Optical Filters; Stellar Temperature; Tables (Data); Astronomy; Stroemgren Photometry:White Dwarfs; White Dwarfs:Mass Distribution full text sources ADS | data products SIMBAD (102) GCPD (1)

ON THE EVOLUTIONARY STATUS OF THE CARBON-BAND WHITE DWARFS.

ABSTRACT The kinematics and statistics of the updated sample of C2 (4670-A) stars are examined. A blanketing correction for weak- to moderate-band-strength C2 stars is estimated based upon C2 parallax stars. Space motions are calculated for the entire C2 sample. A production rate for C2 stars as a group is derived and compared with estimated production rates for carbon-rich, hydrogen-poor stars at higher luminosity. It is concluded that C2 white dwarfs cannot be linked exclusively with carbon-rich hydrogen-deficient giants as progenitors. The C2 stars represent 7% of all white dwarfs in their observed absolute-magnitude interval. Thus, the present number ratio of non-DA white dwarfs to DA white dwarfs implies that one out of six DB stars should be hotter versions of the 4670-A stars.

LP182-44: A FAINT, HIGH-VELOCITY DEGENERATE STAR WITH PROBABLE CARBON FEATURES.

ABSTRACT UBVR observations and image-tube spectra of the faint blue star LP182-44 are analyzed. It is shown that this star is a high-space-velocity non-DA white dwarf with probable weak carbon features in its spectrum. An absolute visual magnitude of about 13.0 and a tangential velocity of approximately 190 km/s are estimated. Properties of the group of high-velocity C2 degenerates are discussed.

Mass differences and convective mixing in white dwarfs

Very different amounts of convective mixing are predicted for DA and for non-DA white dwarfs if we accept the recently determined mass differences between these two classes of stars. Using convection zone models calculated by Fontaine (1973) we show that this effect contributes strongly to the atmospheric abundance differences provided DA and non-DA stars have originally comparable total hydrogen abundances.

18. Model Atmospheres for Hydrogen-Deficient White Dwarfs

The determination of atmospheric parameters for non-DA white dwarfs is investigated with the computed helium-rich model atmospheres by Bues (1970). Only poor predictions are possible from UBV colors alone for DB and DC stars. From uvby colors a determination of effective temperature is possible within 1000 K. Profiles of lines in different parts of the spectrum are necessary for better results.A deficiency of metal abundances for the cooler non-DA stars is obtained.