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At least 19 records

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NASA NTRS1987

Wave energy in white dwarf atmospheres. I - Magnetohydrodynamic energy spectra for homogeneous DB and layered DA stars

The radiative damping of acoustic and MHD waves that propagate through white dwarf photospheric layers is studied, and other damping processes that may be important for the propagation of the MHD waves are calculated. The amount of energy remaining after the damping processes have occurred in different types of waves is estimated. The results show that lower acoustic fluxes should be expected in layered DA and homogeneous DB white dwarfs than had previously been estimated. Acoustic emission manifests itself in an enhancement of the quadrupole term, but this term may become comparable to or even lower than the dipole term for cool white dwarfs. Energy carried by the acoustic waves is significantly dissipated in deep photospheric layers, mainly because of radiative damping. Acoustically heated corona cannot exist around DA and DB white dwarfs in a range T(eff) = 10,000-30,000 K and for log g = 7 and 8. However, relatively hot and massive white dwarfs could be exceptions.

Musielak, Zdzislaw E.↗

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 NTRS1990

X-ray emission from normal stars

The paper addresses the potential for future X-ray missions to determine the fundamental cause of stellar X-ray emissions based on available results and existing analyses. The determinants of stellar X-ray emission are listed, and the relation of stellar X-ray emissions to the 'universal' activity-rotation connection is discussed. The specific rotation-activity connection for evolved stars is mentioned, and the 'decay' of stellar activity at the low-mass end of the main sequence is related to observational data. The data from Einstein and EXOSAT missions that correspond to these issues are found to be sparse, and more observational work is found to be necessary. Also, it is concluded that some issues need to be addressed, such as the X-ray dividing line in evolved stars and the absence of X-ray emission from dA stars. The related observational requirements and instrumental capabilities are given for each significant research focus.

Rosner, Robert↗

NASA NTRS1984

IUE spectroscopy of the degenerate components in the Hyades close binaries V471 Tauri and HZ9

Effective temperatures and gravities have been determined for the degenerate components in the Hyades close binaries V471 Tauri and HZ9 from model atmosphere analysis of IUE ultraviolet spectra. For V471 Tauri, T(e) = 35,000 K and log g is about 8. For HZ9, T(e) = 20,000 K and log g = 8. Both systems contain DA2 white dwarfs. No photospheric metal lines were detected at low IUE resolution in either DA2 star. The 1400 A and 1600 A features are absent in the HZ9 degenerate, despsite being within the temperature range of some other DA stars with the 1400 A absorption feature.

Guinan, E. F.↗

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 NTRS1976

An extrasolar extreme-ultraviolet object. II - The nature of HZ 43

A variety of data are presented concerning the spectrum, distance, temperature, and evolutionary state of the hot white dwarf HZ 43, the first extrasolar object to be detected in the EUV band. The data include spectrophotometry of the star and its red dwarf companion (HZ 43B), a trigonometric parallax for the star, its tangential velocity, and results of soft X-ray and EUV observations. The main conclusions are that: (1) the spectrum of HZ 43A is that of a hot DAwk star, (2) HZ 43B is a dM3.5e star, (3) the distance of the system is about 65 pc, (4) the tangential velocity is not atypical of white dwarfs, and (5) the stellar energy distribution of HZ 43A is well fitted by a black body with an effective temperature of approximately 110,000 K. Evolutionary implications of the existence of an object as hot as HZ 43A are briefly considered, and it is suggested that the progenitors of hot DA stars must include objects hotter than spectral type sdB, with logical possibilities being nuclei of planetary nebulae and sdO stars.

Margon, B.↗

NASA NTRS1986

On the number of comets around other single stars

It is shown how to obtain interesting limits on the number of comets in orbit around other single stars in clouds similar to the one present in our own solar system. The number of comets around white dwarf stars is constrained directly, and this in turn constrains the presence of comets around main-sequence stars. If a white dwarf possesses a cloud of comets similar to the Oort cloud, it will occassionally accrete a comet. The accretion of a comet may add enough heavy elements to the atmosphere of the white dwarf to produce a recognizable spectral signature; it is suggested that the calcium reported in the white dwarf G 74-7 is due to recent comet accretion. This signature will persist until sedimentation has removed the heavy elements from the atmosphere; the mean time between accretion events is comparable to the sedimentation time for cool DA stars, so this signature should be frequently observed if cometary systems are common. The possibility that cometary systems greater than or approximately equal to 3 times more massive than the Oort cloud are present around most white dwarf stars are excluded and how this limit can be improved is indicated. It is shown that a star keeps most of its comets during the vigorous mass loss phase prior to the appearance of the white dwarf star. Thus, the constraints obtained here may be applied to the main-sequence progenitors of the white dwarfs.

Alcock, C.↗

NASA NTRS1980

An absorption line in the ultraviolet spectrum of 40 Eridani B

Two excellent low-resolution spectra show an absorption line of equivalent width 3 A, near 1391 A, in the typical DA (hydrogen atmosphere) white dwarf 40 Eri B. The line is confirmed by a high-resolution spectrum and is the first seen in any DA star. Ultraviolet fluxes and the profile of Lyman-alpha confirm an effective temperature near 17,000 K. If the line is Si IV, it requires a temperature near 40,000 K. Unattractive possibilities are a hot circumstellar absorbing envelope dependent on accretion from companions, or formation at large optical depth in a transparent atmosphere with high Si/H. A suggestion that H2 should be considered leads to the possible interpretation as the (0, 5) transition of the Lyman band, formed at small optical depth. The band should be stronger in cooler DAs.

Greenstein, J. L.↗

NASA NTRS1984

IUE spectrophotometry of the DA4 primary in the short-period white dwarf-red dwarf spectroscopic binary Case 1

Low-resolution ultraviolet International Ultraviolet Explorer spectra of the DA white dwarf Case 1 are presented. The spectra show the presence of the 1400 A feature, already discovered in several other DA stars, and of a shallower trough in the 1550-1700 A range. A model atmosphere analysis of the ultraviolet energy distribution of the Ly-alpha red wing yields T(e) = 13,000 + or - 500 K. Possible interpretations of the 1400 A feature are reviewed. Case 1 is the coolest white dwarf found in a short-period, detached white dwarf-red dwarf binary, and its cooling time is consistent with estimates of the efficiency of angular momentum removal mechanisms in the phases subsequent to common envelope binary evolution.

Sion, E. M.↗

NASA NTRS1990

X-ray studies of coeval star samples. II - The Pleiades cluster as observed with the Einstein Observatory

Coronal X-ray emission of the Pleiades stars is investigated, and maximum likelihood, integral X-ray luminosity functions are computed for Pleiades members in selected color-index ranges. A detailed search is conducted for long-term variability in the X-ray emission of those stars observed more than once. An overall comparison of the survey results with those of previous surveys confirms the ubiquity of X-ray emission in the Pleiades cluster stars and its higher rate of emission with respect to older stars. It is found that the X-ray emission from dA and early dF stars cannot be proven to be dissimilar to that of Hyades and field stars of the same spectral type. The Pleiades cluster members show a real rise of the X-ray luminosity from dA stars to early dF stars. X-ray emission for the young, solarlike Pleiades stars is about two orders of magnitude more intense than for the nearby solarlike stars.

Micela, G.↗

NASA NTRS1985

Emerging monochromatic fluxes and colors of red degenerate stars

The emerging monochromatic fluxes and the B-V, V-I, J-H, and V-K color indices are presented for red degenerate stars with helium atmospheres which were calculated using an equation of state and opacities based on a hot Thomas-Fermi model of the helium gas. The effective temperature range is 4500-2500 K. It is found that although the emerging fluxes resemble blackbody curves, red degenerates emit more radiation than blackbodies in the short wavelength range and less in the long wavelength range. Thus, red degenerates appear bluer than blackbodies of the same temperature. The calculated colors of these models are compared to the colors of some of the coolest known non-DA degenerate stars. In particular it is found that the B-V and V-I colors of the cool white dwarf VB 11, whose temperature had been previously estimated to be higher than 4000 K, suggest a temperature of 3750 K. If this result is correct, then VB 11 is probably the coolest known white dwarf.

Kapranidis, S.↗

NASA NTRS1982

Voyager absolute far-ultraviolet spectrophotometry of hot stars

Voyager observations in the 912-1200 A spectral region are used to indirectly intercompare absolute stellar spectrophotometry from previous experiments. Measurements of hot stars obtained by the Voyager 1 and 2 ultraviolet spectrometers show considerably higher 912-1200 A continuum fluxes than the recent observations of Brune et al. (1979) and Carruthers et al. (1981). The intercomparisons show all observations in basic agreement near 1200 A. The Carruthers et al. flux measurements are preferred down to 1050 A at which point the Voyager and Brune et al. values are respectively 60% higher and 60% lower. Below 1050 A the diasgreement among the observations becomes very large and the fluxes predicted by model atmospheres have been adopted. The pure hydrogen line-blanketed model atmosphere calculations of Wesemael et al. 1980) in comparison with Voyager observations of HZ 43 are used to adjust the Voyager calibration below 1050 A. This adjusted Voyager calibration, which is in good agreement with current model atmosphere fluxes for both early-type stars and DA white dwarfs, will be used for Voyager astronomical observations.

Holberg, J. B.↗

NASA NTRS1985

Identification of the 1400 and 1600 A features observed in the ultraviolet spectra of DA white dwarfs

The 600 A absorption feature in the spectra of cool DA white dwarf stars has been identified as a resonance broadening of Ly-alpha due to the hydrogen quasi molecule. Likewise, the 1400 A absorption feature in the spectra of cool and moderately warm DA white dwarf stars appears to be due to a Ly-alpha satellite line arising from the hydrogen ion quasi molecule. The strength of both features is gravity sensitive and therefore promises to be an excellent indicator of surface gravity.

Nelan, E. P.↗

NASA NTRS1990

Detection of trace helium in G104 - 27, a 26,000 K DA white dwarf

The detection of the He I 4471 A feature in the moderately hot DA white dwarf G104 - 27 places this star within the rather sparse class of DAB white dwarfs. It does not appear possible, on the basis of the present data, to distinguish between a stratified and a homogeneously mixed atmosphere. While a stratified structure would be natural for a DA white dwarf, G104 - 27 resides near the red edge of the DB gap, in a region where DA stars could transform into DB stars through convective mixing in the underlying He envelope and thereby destroy envelope stratification. Both atmospheres are consistent with the existing soft X-ray flux limit for this star.

Holberg, J. B.↗

NASA NTRS1993

PG 0308 + 096 and PG 1026 + 002 - Two new short period binary stars resulting from common-envelope evolution

Ultraviolet spectroscopy, optical spectroscopy, and spectrophotometry have been used to study the excess UV stars PG 0308 + 096 and PG 1026 + 002. Both objects are short-period binary systems, each containing a DA white dwarf star and a dM star. Orbital periods of approximately 0.284 day for PG 0308 + 096, and aproximately 0.597 day for PG 1026, have been found by spectroscopic analysis of the H-alpha emission line. Ly-alpha and Balmer line profile fitting were used to estimate the mass of white dwarf stars; mass estimates for the dM stars are based on their spectral types. The orbital inclinations are derived from these masses, the periods, and amplitudes of the H-alpha radial velocity curves. The equivalent width of the H-alpha emission line, in each binary system, varies with the orbital phase in such a manner as to imply that it arises, in large part at least, from the hemisphere of the M star that faces the white dwarf star.

Saffer, Rex A.↗

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

A Study of the Ultraviolet Absorptions in the Spectra of DA White Dwarfs and Ultraviolet Spectra of the Star HR6560

Two projects in conjunction with the International Ultraviolet Explorer Satellite are discussed. These projects were to: (1) study the properties of the H2 and H2+ quasi-molecular absorption features at lambda lambda 1600 and 1400 in the ultraviolet spectra of the hydrogen-rich DA white dwarfs and to search for additional spectroscopic features in the spectra of these stars; and (2) use the ultraviolet portion of the spectrum of the peculiar rare earth-rich late F type star, HR6560 (HD159870), to establish whether or not the element abundance anomalies are produced in conjunction with its having a white dwarf binary companion. The data show that HR6560 is probably not associated with any hot subluminous or degenerate star.

Wegner, Gary A.↗

NASA NTRS1992

The constitution of the atmospheric layers and the extreme ultraviolet spectrum of hot hydrogen-rich white dwarfs

An analysis is presented of the atmospheric properties of hot, H-rich, DA white dwarfs that is based on optical, UV, and X-ray observations aimed at predicting detailed spectral properties of these stars in the range 80-800 A. The divergences between observations from a sample of 15 hot DA white dwarfs emitting in the EUV/soft X-ray range and pure H synthetic spectra calculated from a grid of model atmospheres characterized by Teff and g are examined. Seven out of 15 DA stars are found to consistently exhibit pure hydrogen atmospheres, the remaining seven stars showing inconsistency between FUV and EUV/soft X-ray data that can be explained by the presence of trace EUV/soft X-ray absorbers. Synthetic data are computed assuming two other possible chemical structures: photospheric traces of radiatively levitated heavy elements and a stratified hydrogen/helium distribution. Predictions about forthcoming medium-resolution observations of the EUV spectrum of selected hot H-rich white dwarfs are made.

Vennes, Stephane↗