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Results for “WHITE DWARF STARS”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Instability of g-mode oscillations in white dwarf stars

A white dwarf model with M = 6 solar masses, Te = 12,000 K, and L = 1.2 x 10 to the 31st erg/sec provided by Cox has been tested for linear stability of radial oscillations. The radial mode instability first reported for this model by Cox, et al. (1979) has been confirmed. The growth rates obtained are comparable to the rates found by Cox. A sequence of l = 2 g-modes has also been found to be unstable. The e-folding times range from around 10 to the 11th periods for a 137 second mode (1 radial node) to less than 100 periods for a 629 second mode (17 nodes). It is likely that the latter rate is too high because the eigenfunction has been forced to vanish at the non-zero inner radius of the model, at which the Brunt-Vaisala frequency is barely less than the mode frequency.

Keeley, D. A.↗
NASA NTRS1969

Evolution of white dwarf stars.

Cooling time estimation for white dwarf stars with ion component undergoing condensation, based on plasma thermodynamic properties

Vila, S. C.↗
NASA NTRS1990

White dwarf stars and the age of the Galactic disk

The history of the Galaxy is written in its oldest stars, the white dwarf (WD) stars. Significant limits can be placed on both the Galactic age and star formation history. A wide range of input WD model sequences is used to derive the current limits to the age estimates suggested by fitting to the observed falloff in the WD luminosity function. The results suggest that the star formation rate over the history of the Galaxy has been relatively constant, and that the disk age lies in the range 6-12 billion years, depending upon the assumed structure of WD stars, and in particular on the core composition and surface helium layer mass. Using plausible mixed C/O core input models, the estimates for the disk age range from 8-10.5 Gyr, i.e.,sustantially younger than most age estimates for the halo globular clusters. After speculating on the significance of the results, expected observational and theoretical refinements which will further enhance the reliability of the method are discussed.

Wood, M. A.↗
NASA NTRS1986

An unsuccessful search for brown dwarf companions to white dwarf stars

The results of a survey to detect excess infrared emission from white dwarf stars which would be attributable to a low mass companion are reviewed. Neither a simple comparison of spectroscopically identified white dwarf stars with the IRAS Point Source Catalog nor the coadding of IRAS survey data resulted in a detection of a brown dwarf. The seven nearest stars where the most stringent limits to the presence of a brown dwarf were obtained are listed, and an effort to detect brown dwarfs in the solar neighborhood is discussed.

Shipman, Harry L.↗
NASA NTRS2021

A Jovian analogue orbiting a white dwarf star

Studies have shown that the remnants of destroyed planets and debris-disk planetesimals can survive the volatile evolution of their host stars into white dwarfs, but few intact planetary bodies around white dwarfs have been detected. Simulations predict that planets in Jupiter-like orbits around stars of ≲8 Mꙩ (solar mass) avoid being destroyed by the strong tidal forces of their stellar host, but as yet, there has been no observational confirmation of such a survivor. Here we report the non-detection of a main-sequence lens star in the microlensing event MOA-2010-BLG-477Lb using near-infrared observations from the Keck Observatory. We determine that this system contains a 0.53 ± 0.11 Mꙩ white-dwarf host orbited by a 1.4 ± 0.3 Jupiter-mass planet with a separation on the plane of the sky of 2.8 ± 0.5 astronomical units, which implies a semi-major axis larger than this. This system is evidence that planets around white dwarfs can survive the giant and asymptotic giant phases of their host’s evolution, and supports the prediction that more than half of white dwarfs have Jovian planetary companions. Located at approximately 2.0 kiloparsecs towards the center of our Galaxy, it is likely to represent an analogue to the end stages of the Sun and Jupiter in our own Solar System.

J. W. Blackman↗
NASA NTRS1995

Photospheric, circumstellar, and interstellar features of HE, C, N. O, and Si in the HST spectra of four hot white dwarf stars

This paper reports on the observations of four hot white dwarf stars with the spectrographs on the Hubble Space Telescope (HST). The higher resolving power and higher signal/noise, in comparison with IUE, reveals a very rich phenomomenology, including photospheric features from heavy elements, circumstellar features, and the first direct detection of accretion onto the white dwarf component of a binary system. Specific results include the following: Our observations of the ultrahot degenerate H1504+65 confirm that it has a photosphere which is depleted in both H and He, and reveals features of C IV and O VI. The spectrum fits previously published models extremely well. The intermediate-temperature DO star PG 1034+001 has an ultraviolet spectrum showing complex profiles of the well-known resonance doublets of C IV, N v, and Si IV. The O V 1371 line shows a clear separation into a photospheric and a circumstellar component, and it is likely that the same two components can explain the other lines as well. The cooler DA star GD 394 has an extensive system of heavy-element features, but their radial velocity is such that it is highly unlikely that they are formed in the stellar photosphere. Time-resolved spectra of the accreting white dwarf in the V 471 Tau binary system are briefly presented here; they do show the presence of C IV, Si IV, and He II. However, the C IV and He II lines are in emission, rather than in aborption as had been expected.

Shipman, Harry L.↗
NASA NTRS1995

Hubble Space Telescope observations of cool white dwarf stars: Detection of new species of heavy elements

Observations of cool white dwarf stars with the Hubble Space Telescope (HST) has uncovered a number of spectral features from previouslly unobserved species. In this paper we present the data on four cool white dwarfs. We present identifications, equivalent width measurements, and brief summaries of the significance of our findings. The four stars observed are GD 40 (DBZ3, G 74-7 (DAZ), L 745-46A (DZ), and LDS 749B (DBA). Many additional species of heavey elements were detected in GD 40 and G 74-7. In L 745-46A, while the detections are limited to Fe 1, Fe II, and Mg II, the quality of the Mg II h and K line profiles should permit a test of the line broadening theories, which are so crucial to abundance determinations. The clear detection of Mg II h and k in LDS 749 B should, once an abundance determination is made, provide a clear test of the hypothesis that the DBA stars are the result of accretion from the interstellar medium. This star contains no other clear features other than a tantalizing hint of C II 1335 with a P Cygni profile, and some expected He 1 lines.

Shipman, Harry↗
NASA NTRS1989

Origin of the DA and non-DA white dwarf stars

Various proposals for the bifurcation of the white dwarf cooling sequence are reviewed. 'Primordial' theories, in which the basic bifurcation of the white dwarf sequence is rooted in events predating the white dwarf stage of stellar evolution, are discussed, along with the competing 'mixing' theories in which processes occurring during the white dwarf stage are responsible for the existence of DA or non-DA stars. A new proposal is suggested, representing a two-channel scenario. In the DA channel, some process reduces the hydrogen layer mass to the value of less than 10 to the -7th. The non-DA channel is similar to that in the primordial scenario. These considerations suggest that some mechanism operates in both channels to reduce the thickness of the outermost layer of the white dwarf. It is also noted that accretion from the interstellar medium has little to do with whether a particular white dwarf becomes a DA or a non-DA star.

Shipman, Harry L.↗
NASA NTRS1988

A low-temperature companion to a white dwarf star

An infrared object located about 120 AU from the white dwarf GD165 has been discovered. With the exception of the possible brown dwarf companion to Giclas 29-38 reported last year, the companion to GD165 is the coolest (2100 K) dwarf star ever reported and, according to some theoretical models, it should be a substellar brown dwarf with a mass between 0.06 and 0.08 solar mass. These results, together with newly discovered low-mass stellar companions to white dwarfs, change the investigation of very low-mass stars from the study of a few chance objects to that of a statistical distribution. In particular, it appears that very low-mass stars and perhaps even brown dwarfs could be quite common in the Galaxy.

Becklin, E. E.↗
NASA NTRS1986

An X-ray survey of hot white dwarf stars - Evidence for a m(He)/n(H) versus Teff correlation

Observations of 13 white dwarf and subdwarf stars using the Einstein Observatory High Resolution Image are reported. Included are stars of classes DA, DB, DAV, sDO, and sDB, with optically determined effective temperatures in the range 10,000-60,000 K. X-ray emission was detected from two of the 13: the very hot (55,000 K) DA1 star WD 2309 + 105 (= EG 233), with a count rate one-fifth that of HZ 43, and the relatively cool (26,000 K) DA3 star WD 1052 - 273 (=GD 125). The effective temperatures determined from ultraviolet and optical observations were used to place limits on the He content of the white dwarf photospheres, presuming that trace photospheric He is the missing opacity source which quenches the thermal X-rays in these stars. When presently obtained results were combined with those available from the literature evidence was found for a correlation between Teff and n(He)/n(H), in which HZ 43 is a conspicuous exception to the general trend. Both this correlation and the exceptional behavior of HZ 43 are qualitatively accounted for by a radiative acceleration model, in which the rate of upward movement of the He is a function of temperature and surface gravity

Petre, R.↗
NASA NTRS1986

EX-111 Thermal Emission from Hot White Dwarfs: The Suggested He Abundance-Temperature Correlation. EX-112: The Unique Emission Line White Dwarf Star GD 356

Progress in the EXOSAT data analysis program is reported. EXOSAT observations for four white dwarfs (WD1031-115, WD0004+330, WD1615-154, and WD0109-264) were obtained. Counting rates were unexpectedly low, indicating that these objects have a substantial amount of x-ray absorbing matter in their photosheres. In addition, soft x-ray pulsations characterized by a 9.25 minute cycle were discovered in the DA white dwarf V471 Tauri. A residual x-ray flux from the K dwarf companion can be seen during the white dwarf eclipse at orbital phase 0.0. Pronounced dips in the soft x-ray light curve occur at orbital phases 0.15, 0.18, and 0.85. The dips may be correlated with the triangular Lagrangian points of the binary orbit. Smaller dips at phases near the eclipse may be associated with cool loops in the K star corona. Data for the white dwarf H1504+65 was also analyzed. This object is particularly unusual in that its photoshere is devoid of hydrogen and helium. Finally, existing data on the white dwarf Sirius B were analyzed to see what constraints from other data can be placed on the properties of this star. Interrelationships between radius, rotational velocity, and effective temperature were derived.

Shipman, H. L.↗
NASA NTRS1982

White dwarf stars with chemically stratified atmospheres

Recent observations and theory suggest that some white dwarfs may have chemically stratified atmospheres - thin layers of hydrogen lying above helium-rich envelopes. Models of such atmospheres show that a discontinuous temperature inversion can occur at the boundary between the layers. Model spectra for layered atmospheres at 30,000 K and 50,000 K tend to have smaller decrements at 912 A, 504 A, and 228 A than uniform atmospheres would have. On the basis of their continuous extreme ultraviolet spectra, it is possible to distinguish observationally between uniform and layered atmospheres for hot white dwarfs.

Muchmore, D.↗
NASA NTRS1997

Asteroseismology of White Dwarf Stars

The primary purpose of this investigation has been to study various aspects of multimode pulsations in variable white dwarfs. In particular, nonlinear interactions among pulsation modes in white dwarfs (and, to some extent, in other variable stars), analysis of recent observations where such interactions are important, and preliminary work on the effects of crystallization in cool white dwarfs are reported.

Hansen, Carl J.↗
NASA NTRS1984

The effective temperature of the white-dwarf star and ZZ Ceti candidate Wolf 485A

Previous multichannel observations of W485A (WD 1327-08) have placed it in the instability strip, the effective temperature range 11,000-13,000 K. In the instability strip, most of the stars (the ZZ Ceti stars) are variable, but W485A has not been detected to be variable. In this paper, high-resolution spectra of W485A and improved hydrogen-line broadening routines are used in the ATLAS model-atmospheres program to find the temperature of W485A; the estimate of effective temperature most consistent with the other data on the star is 14,600 K, outside the instability strip.

Digel, S. W.↗
NASA NTRS1989

The high-resolution spectrum of the pulsating, pre-white dwarf star PG 1159-035 (GW VIR)

High-resolution and low-resolution UV spectra and a high-resolution optical spectrum were obtained for PG 1159-035, revealing apparent photospheric absorption features with defined cores from N V 1240 A, N IV 1270 A, O V 1371 A, and C IV 1550 A. The photospheric velocity derived using all of these lines except for C IV is about +35 km/s. Equivalent-width measurements determined for all of the features may provide a tighter constraint on the photospheric temperature in a detailed model atmosphere analysis treating the CNO ions.

Liebert, James↗
NASA NTRS1995

Do some x-ray stars have white dwarf companions

Some Be stars which are intermittent X-ray sources may have white dwarf companions rather than neutron stars. It is not possible to prove or rule out the existence of Be + WD systems using X-ray or optical data. However, the presence of a white dwarf could be established by the detection of its EUV continuum shortward of the Be star's continuum turnover at 100 A. Either the detection or the nondetection of Be + WD systems would have implications for models of Be star variability, models of Be binary system formation and evolution, and models of wind-fed accretion.

Mccollum, Bruce↗