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Wegner, G.

Publications and source records attributed to Wegner, G..

15 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

The WARPS survey for faint clusters of galaxies

The wide angle Rosat pointed survey (WARPS) of clusters is based on the Rosat position sensitive proportional counter (PSPC) archive of pointed observations. It includes extended X-ray sources and point-like X-ray sources with non-stellar optical counterparts. It was designed to minimize the selection effects while covering a large area of the sky. The purposes of the survey were to measure the low luminosity, high redshift, X-ray luminosity function of clusters and groups and to investigate cluster morphologies and unusual systems.

Jones, L. R.↗
NASA NTRS1990

The CI pressure shift and gravitational redshift of the cool DBQA5 white dwarf LDS678A

A high resolution ultraviolet spectrum of the helium rich degenerate LDS 678A, obtained with the International Ultraviolet Explorer (IUE) satellite is presented. LDS 678A is the coolest metallic line generate (DQ or DZ) yet observed with the IUE scale. These observations provide a detailed line profile of the strong C I 2479 absorption line with equivalent width (W sub 2479 = 2.35 plus or minus 0.06 angstroms) from which theoretical line profile fits yield a C abundance (log C/He = 6.4). The presence of carbon in a helium rich atmosphere lends credence to the notion that LDS 678A is a transitional case between the DB white dwarfs with nearly pure helium atmospheres and the helium rich DQ white dwarfs which exhibit carbon bands. Corrected for an inferred pressure shift for the C I line, a gravitational redshift is deduced from which a most probable mass of 0.55 solar mass is derived.

Sion, E. M.↗
NASA NTRS1990

Model atmosphere analysis of the DZ white dwarf K789 - 37

This paper presents new observations and an analysis using model atmosphere techniques of the recently discovered DZA white dwarf K789 - 37. The effective temperature derived is 9500 + or - 500 K. Abundances are determined for the elements hydrogen, magnesium, calcium, and iron, as well as upper limits for carbon and silicon. The abundances are very low compared to solar values and within the range typical for this spectral class. The implications of the results for the currently accepted explanation of trace elements in cool white dwarfs in terms of the diffusion/accretion scenario are discussed. It is concluded that the accreted matter cannot have had solar composition.

Koester, D.↗
NASA NTRS1987

Ultraviolet and visual spectroscopy of DB white dwarfs

Visual wavelength and ultraviolet spectroscopy and model atmospheres of DB white dwarfs are reported. The results yield no evidence for lines of C or any other element besides H or He in the spectra. Upper limits for carbon are in the range C:He less than 10 to the -5th to the -7th for the ultraviolet data and C:He less than 0.01-0.001 for the visual. The upper limits are consistent with the convective dredging theory for the origin of the atmospheric carbon observed in the cooler DQ members of the helium-rich white dwarf sequence. Additional new DBA stars are reported.

Wegner, G.↗
NASA NTRS1985

An analysis of the bright white dwarf CD - 38 deg 10980

The bright DA white dwarf CD - 38 deg 10980 is comprehensively analyzed using all available data. Detailed comparisons of the overall energy distribution and Lyman-alpha and Balmer profiles are made with theoretical predictions based on a homogeneous grid of model atmospheres. These comparisons yield a consistent, well-defined effective temperature and surface gravity for this star of 24,700 + or - 250 K and log g = 7.95 + or - 0.15, respectively. A new gravitational redshift of 28.4 + or - 4.8 km/s is derived from an IUE high-dispersion spectrum. Conclusions regarding the mass and radius of CD - 38 deg 10980 are also presented.

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 NTRS1984

The discontinuity near 1600 A in the spectra of DA white dwarfs

Ultraviolet spectroscopic observations of two relatively cool DA white dwarfs, L481 - 60 (= WD 1544 - 37) and BPM 1266 ( = WD 2105 - 82), with the International Ultraviolet Explorer (IUE) satellite show a strong drop in their spectral energy distributions below 1600 A. Published model atmospheres and thier visual spectra suggest that these two stars have effective temperatures in the vicinity of 9,000-10,000 K, and it is proposed that the 1600 A feature could be due to the 342(1S) 3s2(1S) photoionization edge of Mg I.

Wegner, G.↗
NASA NTRS1984

The absorption near 1400 A in the spectra of additional DA white dwarfs

Ultraviolet spectroscopic observations of five white dwarfs having published spectral types of DA made with the International Ultraviolet Explorer (IUE) satellite are reported. The three coolest objects: GD279 (0148 + 46), W485A (1327 - 08), and G231 - 40 (2117 + 53) have easily detected absorptions near lambda 1400. Of the two remaining stars, PG0039 + 045 may not be a true DA and GD140 (1134 + 30) is apparently too hot to show any feature near lambda 1400. It is argued that the lambda 1400 feature is produced by the lambda 1420 photoionization edge of Ca II.

Wegner, G.↗
NASA NTRS1984

GD 323 - A white dwarf with a stratified H/He atmosphere?

New optical and ultraviolet (IUE) spectrophotometry of the hot white dwarf GD 323 are presented. The optical spectra exhibit broad, shallow Balmer lines, together with weaker He I lambda-4471, lambda-4921 lines. However, the effective temperature and helium abundance derived from the optical spectrum appear incompatible with parameters derived from the optical colors and the overall energy distribution - thus suggesting that GD 323 does not have a homogeneous, mixed composition atmosphere. The alternative interpretations of substantial reddening, a rapidly rotating subdwarf, an accreting short-period binary, and a detached DA + DB binary system are ruled out. It is suggested instead that GD 323 may have a stratified atmosphere, with an optically thin hydrogen layer overlying a helium envelope at T(e) about 30,000 K; however, this conclusion must be properly tested.

Liebert, J.↗
NASA NTRS1983

The cool DC white dwarf Stein 2051B

Visual scans from the Kitt-Peak 2.1-m telescope and UV spectra from the IUE satellite are combined in a spectroscopic study of the bright DC white dwarf, Stein 2051B. With a cooling age near 2 billion years, and a resulting atmospheric mass of hydrogen of the order of 6 x 10 to the 17th g, the accretion rate over the star's lifetime is less than 2 x 10 to the -25th solar masses/year. This is far below the predictions of standard accretion theories, and requires unreasonably strong suppression of hydrogen accretion in this star if the accretion-diffusion theory is to account for the carbon abundance found. It is suggested that the carbon may have been brought to the surface by convective mixing.

Wegner, G.↗
NASA NTRS1983

Ultraviolet carbon lines in the spectrum of the white dwarf BPM 11668

The southern hemisphere DC white dwarf BPM 11668 has been found to show strong ultraviolet lines of neutral carbon using observations from the IUE satellite. This star seems typical of the growing number of DC white dwarfs found to be of this type and appears to have a carbon abundance near C:He = 0.0001, with an effective temperature of 8500 K.

Wegner, G.↗
NASA NTRS1982

Detection of the 1400 A absorption in the ultraviolet spectrum of the DA white dwarf LB 3303

Low-resolution ultraviolet International Ultraviolet Explorer spectra of the southern white dwarf LB 3303 show the presence of the wavelength 1400 absorption feature reported by Greenstein in the spectrum of 40 Eri B. The equivalent width is 5.7 A, and the measured wavelength is 1394 A. A comparison of the ultraviolet fluxes with model atmospheres confirms that LB 3303 has an effective temperature near 16,000 K, as found earlier from visual wavelength data. There are still problems with the identification of this line. The star is not hot enough to explain the presence of Si IV, and the agreement with the spectrum of the H2 molecule is not convincing.

Wegner, G.↗
NASA NTRS1981

Observations of the strong ultraviolet absorptions in the spectrum of the DC white dwarf G218-8

The spectrum of the 14th magnitude DC white dwarf G218-8 (EG 245) is described. In the visible spectral region the star has been observed with the IIDS scanner attached to the Kitt Peak National Observatory 2.1 m telescope, where it shows a featureless spectrum. However, the ultraviolet observations using the IUE satellite show that strong absorption lines attributable to neutral carbon are present. G218-8 has an effective temperature near 11,500 K and a helium dominated atmosphere with a carbon-to-helium abundance ratio less than or approximately equal to 0.005.

Wegner, G.↗
NASA NTRS1981

Observations of strong ultraviolet absorptions in the spectrum of the DC white dwarf LDS 678 B

Evidence from spectroscopic observations that LDS 678 B has strong ultraviolet lines of neutral carbon in its spectrum is presented. The spectra are found to contain strong lines of C I in the ultraviolet and weak He I lines in the visible. The appearance of the star's spectrum and published photometry are consistent with its having an effective temperature near 10,600 K with a helium-dominated atmosphere and a carbon abundance lower than C:He = 0.001. The star's spectra are compared with those of the white dwarfs L879-14 and L145-141, which are cooler than LDS 678B, have C2 bands in the visible and also show strong evidence of strong ultraviolet C I lines. It is concluded that LDS 678 B has a temperature intermediate between the DB and C2 band white dwarfs and that the ultraviolet C I lines can be a useful means for finding carbon in the atmosphere of the cooler white dwarfs.

Wegner, G.↗