Engineering topics

Holberg, J. B.

Publications and source records attributed to Holberg, J. B..

At least 37 records · Page 2

NASA NTRS1989

A compact planetary nebula around the hot white dwarf EGB 6/PG 0950 + 139

The remarkable central star (0950 + 139), a very hot DA/DAO white dwarf, of the planetary nebula EGB 6 is described. Follow-up observations relevant to the analyses of both the nebula and the stellar photosphere are presented. Three kinds of scenarios are discussed to account for the existence of this peculiar nebula, but none appears very promising. The first consideration is that the nebula was ejected from the white dwarf as a discret event. This hypothesis is heavily constrained by the nebular size, density, and expansion rate; by the low luminosity and radius of the star; and by the absence of evidence for variation in density-sensitive forbidden lines from 1978 to 1987. No plausible mechanism can cause the observed amount of mass to be lost directly from a white dwarf in a steady or sporadic wind, at outflow velocities orders of magnitude below the escape velocity. Final consideration is given to the possibility that the gas is lost from a close companion star, but there is no evidence that this is a close binary system.

Liebert, James↗

NASA NTRS1989

A new temperature determination for Sirius B from IUE - Implications for the observed soft X-ray fluxes

IUE archive obserbations were used to obtain a new and independent estimate of the effective temperature of Sirius B. The observed Lyman alpha profile of Sirius B was modeled as a function of effective temperature and T(eff) = 24,500 + or - 500 K was found. This temperature is in good agreement with a recent result obtained from analysis of Exosat soft X-ray spectra of Sirius B. In addition to providing confirmation of the soft X-ray result, this temperature implies that Sirius B is significantly cooler and larger in radius than most previous estimates. The radius, R = 0.090 + or - 0.005 solar mass, implied by the lower temperature is well in excess of the radius of fully degenerate star having the mass of Sirius B. This investigation employed a unique set of SWP archive spectra, acquired with Sirius B in the small aperture, which have proved to be free of scattered light contamination from Sirius A.

Kidder, K.↗

NASA NTRS1988

Large time scale variation in hydrogen emission from Jupiter and Saturn

The IUE and Voyager spacecraft observations of Jupiter and Saturn were combined to obtain a consistent measurement of temporal variation of the equatorial subsolar hydrogen emission. The outer planets appear to have rather independent behavior over time scales of the order of 10 yr, particularly in emission from the H Ly alpha line. The time interval from 1978 to the present shows variation of mean equatorial H Ly alpha brightness of 2 at Jupiter and 5 at Saturn. The relative magnitudes of the variations is sufficiently different to suggest that response to input from the Sun is at least nonlinear. The brightness of H2 band emission appears to be relatively more stable than H Ly alpha. There is evidence in IUE observations of a moderate increase in H2 band brightness with increasing time at Jupiter, in opposition to the variation in H Ly alpha.

Shemansky, D. E.↗

NASA NTRS1988

IUE observations of a hot DAO white dwarf: Implications for diffusion theory and photospheric stratification

Observations of the DAO white dwarf PG1210+533, including the first high dispersion spectrum of a hybrid H-He object of this nature were obtained by IUE. In contrast with hot DAs in the 50,000 K temperature range, PG1210+533 shows no narrow interstellar-like metal lines, in spite of an optically observed He/H abundance of 0.1. This lack of metal makes accretion from the ISM an unlikely source for the He in the PG1210+533 photosphere. A significant discovery in the high dispersion spectrum is the existence of a sharp, non-LTE like, core seen in the He II 1640 line. Such features are detected in DO white dwarfs. A small aperture SWP low dispersion observation reveals the Lyman alpha profile of PG1210+533 to be surprisingly weak and narrow. Fits of this profile using pure H models yielded a T(eff) = 56,000 K. Fits of the Balmer H gamma profile however, yield T(eff) = 42,300 K and log g = 8.5 + or - 0.5 for the same models. It is unlikely that homogeneously mixed H-He atmospheres can resolve the inconsistency between the Lyman alpha and H gamma features in this star. Stratified models involving thin H photospheres may be necessary to explain these results.

Holberg, J. B.↗

NASA NTRS1987

The upper atmosphere of Uranus - EUV occultations observed by Voyager 2

EUV (52-170 nm) solar and stellar occultation observations of the Uranian atmosphere from 500 microbar to about 1 pbar, obtained with the UV spectrometer on Voyager 2 during its encounter with Uranus in January 1986, are reported. The data are presented in extensive tables and graphs and characterized in detail. The atmosphere is found to be dominated by H2 (with very small hydrocarbon mixing ratios) out to about 1.25 Uranian radii, where atomic H becomes important. Also noted are a small (relative to Jupiter and Saturn) homopause eddy-diffusion coefficient, little difference between the atmospheres of the day and night hemispheres, high temperatures (800 + or - 100 K) above about 1-10 nbar, and number densities of several hundred H/cu cm at 2 Uranian radii. The implications of the latter finding for ring dynamics and plasma populations are explored.

Herbert, Floyd↗

NASA NTRS1987

Further observations of the peculiar hot helium-rich degenerate KPD 0005 + 5106

KPD 0005 + 5106 observations including high- and low-resolution IUE and Voyager UV spectra, high-resolution spectra at optical wavelengths, and a low-resolution spectrum at red wavelengths are presented. The only detected transitions from a heavier element assignable to the stellar photosphere is the N V 1240 A doublet; hydrogen and some probable CNO ions are also detected in the emission. It is suggested that the detected nitrogen is most likely due to selective radiative acceleration processes.

Downes, Ronald A.↗

NASA NTRS1987

Stellar occultation probes of the Uranian rings at 0.1 and 2.2 microns - A comparison of Voyager UVS and earth-based results

A comparison between Voyager 2 UV spectrometer data for stellar occultations of the Uranian ring system obtained at 0.11 microns and 2.2-micron earth-based occultation data reveals the anticipated factor-of-two reduction in observed optical depths relative to those observed from earth. This is due to Voyager's proximity to the rings, which allows light diffracted out of the direct beam by ring particles to be replaced by light diffracted into the direct beam light from other particles, and further permits the placing of firm lower limits on typical particle sizes of 0.3 and 1 cm for the epsilon and delta rings, respectively. As a function of true anomaly, the epsilon ring profile is noted to remain very similar in shape and essentially constant in equivalent depth, even very near periapse.

Holberg, J. B.↗

NASA NTRS1987

Hydrogen emission from Jupiter: Hydrogen emission from sunlit atmosphere of Saturn

Successful IUE observations of the equatorial sunlit atmosphere of Jupiter and Saturn have been obtained. Spectra containing atomic and molecular hydrogen and solar reflection continuum emissions have been analyzed, with the purpose of determining the long term temporal behavior of the electroglow process. Quantitative estimates have been established for the first time using a model analysis of the short wavelength region of the spectrum. Both systems show varying degrees of long term variability in hydrogen emission rate, but the time scale is too short to determine whether there is a dependence on solar cycle activity. As part of the emission modeling program, a preliminary point source spreading function for the IUE SWP instrument has been established, suggesting a wavelength dependence in spectral line width different from previous analyses. Further IUE observations are planned for both Jupiter and Saturn.

Shemansky, D. E.↗

NASA NTRS1987

Multiwavelength monitoring of the dwarf nova VW Hydri. IV - Voyager observations

Results from Voyager far-ultraviolet (500-1200 A) observations of the dwarf nova VW Hyi are presented as part of a coordinated, multiwavelength program. Data from one normal outburst and one superoutburst are discussed in detail. Far-ultraviolet (1050 A) light curves are produced showing a significant delay (0.5 day) in the rise to maximum at 1050 A with respect to optical wavelength, followed by a simultaneous decline. The superoutburst data show a distinct double-peaked light curve with the first rise and decline closely resembling that of a normal outburst. These data suggest that the rise to supermaximum in the far-ultraviolet is also delayed with respect to optical wavelengths. The spectral distribution of VW Hyi shows the steeply falling spectrum shortward of 1200 A, characteristic of dwarf novae in outburst and absorption features at 985 (N III, C III and Ly gamma) and 1030 (Ly beta and O VI). No flux shortward of 912 A was detected in VW Hyi.

Polidan, R. S.↗

NASA NTRS1987

Observations of hot DA white dwarfs

Observations of the hottest white dwarfs offer the opportunity to study the earliest stages of degenerate evolution. For those white dwarfs having pure or nearly pure hydrogen photosphere it is now possible with the current generation of high gravity model atmospheres to achieve substantial refinements in the estimation of atmospheric parameters, such as effective temperature, gravity and trace elements abundance. Such improvements can in turm lead to a clearer picture of DA cooling have a significant impact on the interpretation of a growing number of soft X-ray observations of hot DAs, and contribute to the understanding of He and trace metal abundance patterns in hot DAs.

Holberg, J. B.↗

NASA NTRS1986

Far-ultraviolet background observations at high galactic latitude. II - Diffuse emission

A single spectrum, of extremely long integration time, is used to place stringent new limits on the diffuse far-UV background in the 500-1200 A region. This spectrum, obtained in the vicinity of the North Galactic Pole with the Voyager 2 UV spectrometer, can be explained solely on the basis of resonant scattering from interplanetary H I and He I. Limits on any additional sources of sky background radiation corresponding to 100-200 photons/sq cm per sec/sr per A for continuum emission and 6000 photons/sq cm per sec/sr for line emission are demonstrated. Comparisons of these upper limits with existing measurements and various potential source mechanisms are discussed. The first detection of resonantly scattered interplanetary H I Ly-gamma (973 A) emission is reported.

Holberg, J. B.↗

NASA NTRS1986

H 1504 + 65 - An extraordinarily hot compact star devoid of hydrogen and helium

The optical identification of the bright soft X-ray source H 1504 + 65, originally discovered by the HEAO I A-2 low energy detectors, is reported. The optical counterpart exhibits a steep, virtually featureless continuum from optical wavelengths through the ultraviolet. The spectrum is remarkably featureless except for a broad O VI adsorption feature at 3494.7 A and C IV absorption at 3934 A and emission at 4658 A. The soft X-ray data, which are not consistent with any published models for degenerate stars having predominantly H or He compositions, suggest that H 1504 + 65 is a metal-rich, near-degenerate object of very high temperature. Qualitative considerations indicate that it is definitely hotter than PG 1159 - 035 and similar objects. A temperature of roughly 160,000 K is suggested.

Nousek, J. A.↗

NASA NTRS1986

Stellar observations with the Voyager EUV objective grating spectrograph

During the periods of interplanetary cruise the Voyager ultraviolet spectrometers are used to provide unique and otherwise unobtainable observations in the extreme ultraviolet (EUV, 500 to 1200) and the far ultraviolet (FUV, 912 to 1220 A). These observations include the spectra of hot stellar sources as well as emission from the interplanetary medium. Recent results of note include: (1)Extensive spectrophotometric coverage of a superoutburst of the dwarf nova VW Hydri, which showed a clear 1/2 day delay in the outburst at 1000 A relative to that observed in the optical and a curious dip in the FUV light curve near maximum light. The Voyager observations were part of a comprehensive and highly successful campaign involving EXOSAT, IUE and ground based observations of this dwarf nova; (2)A comprehensive study of Be star spectra and variability. These results show the critical importance of FUV observations in the study of the effects of stellar rotation in hot stars; (3)The detection of a strong O VI absorption feature in the spectrum of the PG 1159-like object H1504+65. This detection along with the optical identification of weak O IV lines was a key to the interpretation of this object; which is of extremely high (>150,000K) temperature and appears to be a unique example of a stellar atmosphere devoid of H and He; (4)An analysis of an extremely long duration spectrum of the EUV and FUV sky background, which establishes important new upper limits on both continuum and line emission. This result also provide the first detection of interplanetary Lyman gamma.

Holberg, J. B.↗

NASA NTRS1986

DA white dwarf effective temperatures determined from IUE Lyman-alpha profiles

The Lyman-alpha profiles of 12 DA white dwarfs have been obtained with the International Ultraviolet Explorer Satellite. Analysis of these profiles provides an improved, uniform, and relatively bias-free measure of effective temperature for these stars over the range 20,000-60,000 K. Simultaneous estimates of surface gravity yield a mean gravity of log g = 7.96 for the entire sample, with the hottest stars tending to have the lowest gravities. A significant exception to this trend occurs in the case of the gravitation of HZ 43. An important by-product of this work has been the determination of a correction to IUE fluxes over the 1150-1350 A range.

Holberg, J. B.↗

NASA NTRS1986

Ultraviolet spectrometer observations of Uranus

The Voyager 2 UV spectrometer was used to scan the Uranus atmosphere at wavelengths from 500-1700 A with a field of view of 0.1 x 0.86 deg. The temperature and composition of the upper atmosphere were determined through occultations of light from gamma Pegasi, nu Geminorum and the sun. The data indicated a substantial gas density (100 million H atoms/cu cm) at about 28,000 km from the Uranus center, suggesting that gas drag plays a significant role in ring evolution. The distributions of CH4 and C2H2 in the lower atmosphere were also estimated. An electroglow emission was detected on the sunlit side, and attributed to emissions from atomic and molecular hydrogen excited by low energy electrons. An auroral glow was also observed, and exhibited evidence of an energy input equal to that of the electroglow. Finally, estimates of the C2H2 mixing ratio and the vertical column abundance of H2 are calculated.

Broadfoot, A. L.↗

NASA NTRS1986

The detection of photospheric X-ray pulsations from PG 1159-035 with EXOSAT

The detection of soft X-ray pulsations from the hot, helium-rich, degenerate object, PG 1159-035 is reported. These observations, obtained with the Low Energy experiment on EXOSAT, show the presence of large-amplitude soft X-ray (44-150 A) pulsations analogous to several of the low-amplitude, nonradial, g-mode pulsations which are observed in the optical. These soft X-ray pulsations, with periods of 516 s, 524 s, and 539 s, arise from the photosphere of PG 1159-035 and constitute the first observations of stellar atmospheric pulsational phenomena in the X-ray band.

Barstow, M. A.↗

NASA NTRS1985

Studies of hot B subdwarfs. II - Energy distributions of three bright sdB/sdOB stars in the 950-5500 A range

Voyager ultraviolet spectrometer observations of the subdwarf B or OB stars HD 205805, UV 1758+36 and Feige 66 are presented. All three objects display the H I Layman series in absorption. These observations are combined with low dispersion IUE spectrophotometry and with Stromgren photometry to construct virtually complete energy distributions, which extend over the range 950-5500 angstroms. Effective temperatures based on model atmosphere calculations for high gravity, hygrogen rich stars are determined. Our analyses yield T sub e 28,200 + or - 1300 K for HD 205805, T sub e 31, 800 + or - 1100 K for UV 1758+36, and T sub e 35,700 + or 1500 K for Feige 66. The importance of far ultraviolet observations below L sub alpha in reducing the uncertainties associated with the interstellar extinction and the degradation of the IUE sensitivity is emphasized.

Wesemael, F.↗

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.↗