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Greenstein, J. L.

Publications and source records attributed to Greenstein, J. L..

15 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Futher identifications of hydrogen in Grw +708247

Numerous spectra of the magnetic white dwarf Grw +70 8247 were obtained with the double CCD spectrograph at Palomar with signal-to-noise ratio above 100. Five weak absorptions in the blue and four in the red are produced by hydrogen in a strong magnetic field. New, precise, multiparameter, variational energy levels yielding intensity and wavelength, from 100 to 600 MG are presented. A search for those Zeeman components with wavelengths nearly constant with field is carried out. Two such Zeeman components which are relatively sharp and strong Pi transitions, explain the Minkowski band, 4137 A (known since 1938), as the 2s0-4f0 and the 5855 A band as the 2s0-3p0. Both have minimum wavelengths at B of about 300 MG. Their absorptions resemble band heads degraded to the red, as observed. The mixed nature of the 4f0 level results in an intensity ratio (2s0-4f0)/(2s0-3p0) = 0.58 at 280 MG, high for a line forbidden at zero field. Profiles computed for a pole-on dipole, near log Bp = 8.5, fit the two observed lines. Other hydrogen transitions (including the core of Ly-alpha) are consistent with the field strength. Six models with a variety of Bp and inclination are illustrated to show the quality of fit to the observations.

Greenstein, J. L.↗
NASA NTRS1984

The identification of hydrogen in Grw +70 deg 8247

The ultraviolet spectrum of the magnetic degenerate Grw -70 deg 8247 contains a relatively sharp, but resolved absorption feature near 1345 A. This is interpreted as the sigma(-), 1s0 to 2p-1 component of hydrogen Ly-alpha in a very strong magnetic field, near 350 megagauss. A cyclotron absorption may also exist near 2300 A, corresponding to such a field. Review of theoretical predictions of these hydrogen levels shows that a relatively sharp feature near 1335 A would be produced in fields within the range from 280 to 800 megagauss. The predicted wavelength has considerable uncertainty, perhaps + or - 10 A. Diffuse features in the red region of Palomar CCD spectra may arise from two sigma(-) and a pi component, based on even less certain computations for H-alpha. The large field is consistent with polarization data. The original bands discovered by Minkowski are not yet identifiable. But this important, magnetic star contains hydrogen even on the basis of Ly-alpha alone.

Greenstein, J. L.↗
NASA NTRS1983

Iron and magnesium in the white dwarf GD 40 - A test of diffusion theory

An outstanding problem in interpreting the properties of white-dwarf stars is related to the understanding of the chemical abundances in their atmospheres. The hydrogen-rich white-dwarf stars have monoelemental atmospheres, with small quantities of helium and no heavier elements observed in most, possibly all, stars with visible H lines. The helium-rich stars are more complex, and the existence of metallic lines in many of their spectra is now well confirmed. The DB star GD 40 (Gr 384) is the hottest He-rich white-dwarf star to show metal lines, apart from the extremely hot objects, such as HZ 21 and HD 149499B. The present investigation is concerned with IUE spectra which have been obtained of the GD 40. It is found that the near-UV is marked by strong Mg II lines along with broad blends of Fe II lines. An analysis is conducted of the abundances of Fe and Mg at the surface of GD 40 using a limited spectrum-synthesis technique with the measured equivalent widths of the best Fe II features.

Shipman, H. L.↗
NASA NTRS1982

RW Sextantis, a disk with a hot, high-velocity wind

The continuum spectrum of the flickering blue variable RW Sex was observed from 10,000 to 1150 A. The star is a cataclysmic variable currently stabilized at maximum, and the spectrum is dominated by an accretion disk, with flat spectrum in the ultraviolet, except at more than 5000 A, where a blackbody near 7000 K is seen. A distance of 400 pc is derived, if the latter arises from an F type main sequence star. The accretion rate required is near 10 to the -8th solar masses per year. Only weak emission is seen, except for Lyman alpha; strong, broad UV absorption lines are seen with centers displaced up to -3000 km/s, with terminal velocities up to -4500 km/s, the velocity of escape from a white dwarf. The low X-ray flux may arise from absorption within an unusually dense, hot wind from the innermost portions of the disk. The estimated mass loss rate is nearly 10 to the -12th solar masses per year.

Greenstein, J. L.↗
NASA NTRS1982

The interpretation of the spectra of two magnetic degenerates

Ultraviolet and optical fluxes are measured for two magnetic degenerates, Feige 7 (Gr 267) and Grw + 70 deg 8247 (EG 129), from 10,000 to 1150 A. A method is developed for fitting blackbodies and models to this broad range of fluxes. From models, the best effective temperatures average 20% below the blackbody temperatures. Feige 7, a mixed H + He atmosphere, exhibits Ly-alpha absorption and a continuum flux corresponding to between 23,000 and 19,000 K. For a white dwarf, the radius is normal. It is thought that the spectrum may have some weak absorption line features.

Greenstein, J. L.↗
NASA NTRS1981

Anomalous extinction in the planetary nebula Abell 30

The stellar continuous energy distribution from 8000 to 1250 A in Abell 30 is quite peculiar. The observed flux can be understood if a hot central star (approximately 200,000 K) is immersed in an absorbing dust cloud which has anomalous absorption with its peak near 2470 A. The dust nebula is a strong infrared source, maintained by conversion of the stellar UV flux. The internal reddening is E(B - V) = 0.30 mag and reaches 2.5 mag at 2470 A; it is best matched not by graphite but by laboratory observations of carbon smoke. Intense P Cygni lines show that a strong stellar wind exists. The central star may be an O subdwarf, approaching more nearly a helium-carbon degenerate DO.

Greenstein, J. L.↗
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 NTRS1980

Ultraviolet spectrum synthesis of the helium white dwarf Ross 640

The UV spectrum of the He-rich white dwarf Ross 640 was used to derive Mg, Si, and Fe abundances by a spectrum synthesis technique. The values obtained were: Mg/He = 6.8 x 10 to the -8th; Si/He = 3.2 x 10 to the -8th; Ca/He = 7.6 x 10 to the -10th; and Fe/He = 2.0 x 10 to the -9th. The abundances indicate the operation of diffusion processes within a helium-rich atmosphere, provided some mechanism exists to deposit metals in the observable layers.

Cottrell, P. L.↗
NASA NTRS1979

High time resolution spectrophotometry of nova DQ Herculis /1934/

Thirty-four spectra of DQ Her are analyzed which were obtained with a time resolution greater than 0.01 cycle. It is found that: (1) the 18th mag visual companion of DQ Her has an energy distribution similar to that of a late F star and is not likely to be a member of the system; (2) the time of minimum has no wavelength dependence and agrees better with the elements of Patterson et al. (1978) than with a linear ephemeris; (3) only very weak evidence exists for the detection of a late-type star, which is of spectral type M4 or later, even at mideclipse; (4) the spectrum of the residual light at mideclipse can be represented by optically thin free-free plus bound-free emission at a temperature of 35,000 K, an electron density of (1-10) trillion per cu cm, and a star contributing about 90% of the red flux; and (5) there is a definite asymmetry in the light curve, as well as strong evidence for a localized bright spot on the side of the disk eclipsed last.

Schneider, D. P.↗
NASA NTRS1979

Ultraviolet spectrophotometry of degenerate stars

Observations of one helium- and three hydrogen-atmosphere degenerates made with the International Ultraviolet Explorer are discussed. Fluxes in the UV give temperatures in good accordance with those determined from the ground and from the ANS satellite data. Profiles of the strong L-alpha absorption in two DA's fit predictions for the expected temperatures. Gravity determination is vitiated by their steep temperature dependence. If one accepts that theoretical predictions should be correct, corrections to the absolute IUE calibration derived are an upward shift of 3-5%, with irregular residuals attaining + or - 7%.

Greenstein, J. L.↗
NASA NTRS1979

Masses, radii, temperatures, luminosities and surface compositions of white dwarfs

The masses, radii, temperature, luminosities and compositions of white dwarf stars are discussed, with particular emphasis on those areas in which considerable work remains to be done. It is pointed out that radii for stars of known parallax are available, together with masses and their variance. Consideration is then given to the difficulties involved in the determination of the temperatures of the hottest stars and helium stars by means of Stromgren or multichannel photometry, and it is noted that the frequency of binaries among the hottest degenerates seems too high to be due to chance. Results of IUE observations of the UV spectra of three DA and one DO star are discussed, with attention given to the similarities between UV-derived and ground-based determinations of temperatures and surface gravities. The metal abundances of hydrogen and helium degenerate stars are finally considered, and spectral classes and abundances of the helium atmosphere degenerates are tabulated.

Greenstein, J. L.↗
NASA NTRS1977

Spectrophotometry of the transient X-ray source A0620-00

Optical observations of the transient X-ray source A0620-00 over the wavelength range from about 3200 to 6600 A are analyzed which were made with the 5-m Hale telescope using a multichannel spectrometer, a digital spectrograph with a cooled integrating SIT vidicon, and an image photon-counting system attached to the camera of the coude spectrograph. Infrared observations at 1.65, 2.2, and 3.5 microns are also examined. Individual spectral lines of special interest are discussed, the E(B-V) color excess is estimated to be about 0.40, and a distance of 1200 pc is adopted. Analysis of the continuous spectrum yields several simple binary models. A specific model is proposed in which one of the continuum sources is an optically thick object characterized by a model atmosphere at a temperature of 25,000 to 30,000 K and the second source is either a cool black body at 4000 K or a partially optically thick source of bound-free plus free-free radiation from a hydrogen gas with an electron temperature of at least 100,000 K. Some possible parameters of these sources and of the accretion disk are considered.

Oke, J. B.↗