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NASA NTRS1974

Investigation of ultraviolet fluxes of normal and peculiar stars

Data from Project Celescope, a program that photographed the ultraviolet sky, in order to study several problems in current astrophysics are analyzed. Two star clusters, the Pleiades and the Hyades, reveal differences between the two that we are unable to explain simply from their differences in chemical abundance, rotation, or reddening. Data for Orion show large scatter, which appears to be in the sense that the Orion stars are too faint for their ground-based photometry. Similarly, many supergiants in the association Sco OB1 are too faint in the ultraviolet, but the ultraviolet brightness appears to be only poorly correlated with spectral type. Ultraviolet Celescope data for several groups of peculiar stars have also been analyzed. The strong He I stars are too faint in the ultraviolet, possibly owing to enhancement of O II continuous opacity due to oxygen overabundance. The Be stars appear to have ultraviolet colors normal for their MK spectral types. The P Cygni stars are considerably fainter than main-sequence stars of comparable spectral type, probably owing, at least in part, to line blocking by resonance lines of multiply ionized light metals. The Wolf-Rayet stars have ultraviolet color temperatures of O stars.

Deutschman, W. A.↗
NASA NTRS1981

Problems and programming for analysis of IUE high resolution data for variability

Observations of variability in stellar winds provide an important probe of their dynamics. It is crucial however to know that any variability seen in a data set can be clearly attributed to the star and not to instrumental or data processing effects. In the course of analysis of IUE high resolution data of alpha Cam and other O, B and Wolf-Rayet stars several effects were found which cause spurious variability or spurious spectral features in our data. Programming was developed to partially compensate for these effects using the Interactive Data language (IDL) on the LASP PDP 11/34. Use of an interactive language such as IDL is particularly suited to analysis of variability data as it permits use of efficient programs coupled with the judgement of the scientist at each stage of processing.

Grady, C. A.↗
NASA NTRS1977

The spectral energy distribution of Zeta Puppis and HD 50896

The ultraviolet spectral energy distribution of the O5f star Zeta Pup and the WN5 star HD 50896 are derived from OAO-2 observations with the calibration of Bless, Code, and Fairchild (1976). An estimate of the interstellar reddening (0.12 magnitude) of the Wolf-Rayet star is determined from the size of the characteristic interstellar extinction bump at 4.6 inverse microns. After correction for extinction, both stars show a flat energy distribution in the ultraviolet. The distribution of HD 50896 from 1100 A to 2 microns is in good agreement with results of extended model atmospheres, but some uncertainty remains because of the interstellar-extinction correction. The absolute energy distribution of Zeta Pup is fitted by a 42,000-K plane-parallel model if the model's flux is adjusted for the effects of electron scattering in the stellar wind and for UV line blanketing that was determined empirically from high-resolution Copernicus satellite observations. To achieve this fit, it is necessary to push both the spectroscopically determined temperature and the ultraviolet calibration to the limits of their probable errors.

Holm, A. V.↗
NASA NTRS1975

Time variations of UV emission features of Be stars

The UV spectra of three Be stars (gamma Cas, sigma Tau, eta Cen) were studied. Of the six Be stars observed in the first four lines of the Balmer series, three stars showed at least one of the Balmer lines to be variable in the equivalent width amounting to a few percent with time scales of 3 to 30 minutes. Photoelectric spectrum scans of five southern Wolf-Rayet stars showed night-to-night variations. A simple model is proposed to account for the behavior of these emission lines. Scans of gamma square Vel showed rapid variations of emission strengths of He II 4686 and C III - IV 4650. These variations have time scales of 1 minute and longer. Night-to-night variations were also found. Scans of four Be stars in H alpha showed a definite variation of 3 to 4 percent, with time scales of 1 minute and longer in sigma Tau. In 48 Per and kappa Dra the variations are not as well established. No variation of any significance was found for nu Gem.

Bahng, J. D. R.↗
NASA NTRS1977

Evolution of helium stars

The evolution of helium stars in the mass range from 4 to 15 solar masses has been followed from the initial helium main sequence to the end of carbon burning in the core, with the use of Carson's (1976) radiative opacities. As compared with earlier work based on smaller opacities, the main-sequence band in the H-R diagram is now both wider and cooler than before. If neutrino losses are neglected in the stellar models, the phase of carbon burning in the core occurs in the red-supergiant region; otherwise, it occurs, as it does in the earlier models with or without neutrino emission, close to the helium main sequence. Observational data for Wolf-Rayet stars and R Coronae Borealis variables are found to lend some support to the present models.

Stothers, R.↗
NASA NTRS1971

Ultraviolet investigations of cosmic X-ray sources and analogous objects

Data on the UV spectrum of the Crab nebula and on UV and radio frequency observations of Wolf-Rayet stars was analyzed. A description of TT Arietis is given. It is concluded that this object must be a double star, since it does not radiate like an ordinary photosphere or a single blackbody. Details of the spectrum are given.

Johnson, H. M.↗
NASA NTRS1974

Infrared spectra of gamma-2 Velorum and zeta Puppis

Observations of gamma-2 Vel and zeta Pup are presented for the spectral region 5800-11,000 per cm, at resolutions of 2 and 4 per cm, respectively. This constitutes the first reported spectroscopy of a Wolf-Rayet star beyond 1.1 microns. The new spectral region contained numerous emission features for the WC8 component of gamma-2 Vel, but only the P-beta line in absorption for the O4f star zeta Pup. Line identifications are given for gamma-2 Vel and are shown to be consistent with visual spectroscopy. The species identified are H, He I-II, and C II-IV.

Barnes, T. G.↗
NASA NTRS1979

X-rays from Eta Carinae and the surrounding nebula

Two X-ray pictures have been obtained of the Eta Car nebula using the imaging detectors of the Einstein Observatory. One covers a field 1-deg square with a resolution of 1 arcmin; the second covers a field 25 arcmin in diameter with resolution of a few arcsec. These show a region of diffuse X-ray emission with luminosity 10 to the 35th ergs/s. The sources have been positively identified as the peculiar object Eta Car, a Wolf-Rayet star, a cluster of O stars, and five individual O stars. The Eta Car X-ray source is extended and coincides with the outer shell of the diffuse object Eta Car itself. One X-ray bright spot may be emission from a central object within Eta Car.

Seward, F. D.↗
NASA NTRS1979

Iron and the formation of astrophysical dust grains

Condensation of metallic iron or the iron carbide cohenite Fe3C is proposed as the source of the approximately 1000 K dust condensate in nova shells, Wolf-Rayet stars, and other objects. Iron grains may serve as nucleation sites for carbon in carbon-star ejecta; however, nucleation of graphite at its equilibrium saturation temperature is almost certainly kinetically inhibited. A gas phase rich in CO, C2H2, and HCN is produced instead.

Lewis, J. S.↗
NASA NTRS1982

Ultraviolet, visual, and infrared observations of the WC7 variable HD 193793

Low-resolution IUE data are used to explore the ultraviolet extinction toward the Wolf-Rayet star HD 193793 and to search for ultraviolet variability that might relate to the infrared variability. High-dispersion IUE observations are used to investigate the nature of the stellar wind of the star and to search for anomalies in the interstellar line spectrum that might be expected to be found toward a star that has recently formed a dust shell. Finally, the ultraviolet and new visual and infrared data are combined to investigate the full energy distribution of this unusual source. The energy distribution is found to extend from 0.12 to 12.5 microns, and the ultraviolet data suggest a normal WC-7 type star. A wind terminal speed of about 3000 km/s is implied by the data, as well as an E(B-V) value of 0.85. The dereddened ultraviolet to visual energy distribution is consistent with a star having effective temperature of about 43,000 K.

Fitzpatrick, E. L.↗
NASA NTRS1981

Mass loss from interacting close binary systems

The three well-defined classes of evolved binary systems that show evidence of present and/or past mass loss are the cataclysmic variables, the Algols, and Wolf-Rayet stars. It is thought that the transformation of supergiant binary systems into the very short-period cataclysmic variables must have been a complex process. The new evidence that has recently been obtained from the far ultraviolet spectra that a certain subclass of the Algols (the Serpentids) are undergoing fairly rapid evolution is discussed. It is thought probable that the remarkable mass outflow observed in them is connected with a strong wind powered by accretion. The origin of the circumbinary clouds or flat disks that probably surround many strongly interacting binaries is not clear. Attention is also given to binary systems with hot white dwarf or subdwarf components, such as the symbiotic objects and the BQ stars; it is noted that in them both components may be prone to an enhanced stellar wind.

Plavec, M. J.↗
NASA NTRS1983

Stellar winds driven by multi-line scattering

This paper presents a model of a radiation-driven stellar wind with overlapping spectral lines. It is based on the Castor, Abbott, and Klein (CAK) theory. The presence of overlapping lines allows a photon to be scattered many times in different lines. The properties of the wind at any point depend on the wavelength-averaged intensity, which in turn depends on the structure of the wind. A self-consistent wind model is found. The mass loss rate does not saturate as line overlap becomes more pronounced, but continues to increase. The terminal velocity is much larger than in the CAK model, while the velocity law is shallower. This model might help explain the massive winds from Wolf-Rayet stars.

Friend, D. B.↗
NASA NTRS1982

Wolf-Rayet phenomena

The properties of stars showing Wolf-Rayet phenomena are outlined along with the direction of future work. Emphasis is placed on the characteristics of W-R spectra. Specifically the following topics are covered: the absolute visual magnitudes; the heterogeneity of WN spectra; the existence of transition type spectra and compositions the mass loss rates; and the existence of very luminous and possibly very massive W-R stars. Also, a brief overview of current understanding of the theoretical aspects of stellar evolution and stellar winds and the various scenarios that have been proposed to understand W-R spectra are included.

Conti, P. S.↗
NASA NTRS1981

Concerning the Wolf-Rayet and other luminous early-type stars

Effective temperatures, radii, and luminosities were determined from S2/68, ANS, UBV, and uvby photometry for four B0/B1 supergiants, four O4 stars, and four WN7/WN8 stars as well as for four test stars having spectral types between B1.5 V and 09 V and five stars with known angular diameters and effective temperatures. The effective temperatures of B1 Ia+ stars are found to be near 17,000 K, those of O4 stars near 45,000, and those of WN7/WN8 stars near 26,000 K. The question of modeling the atmospheres of hot luminous stars is examined, and it is noted that the photosphere can be modeled adequately using a classical plane-parallel layer model atmosphere. In addition, it is found that the Wolf-Rayet stars of types WN7/WN8 fall in the H-R diagram near the B0 Ia stars, while the others fall near B0.5 III stars. The evolutionary relationship between the Wolf-Rayet and O stars is considered; it is suggested that a Wolf-Rayet spectrum is a short-lived phase in the life of a massive star.

Underhill, A. B.↗
NASA NTRS1982

Wolf-Rayet phenomena

An outline is presented of what is currently known about the properties of stars showing Wolf-Rayet (W-R) phenomena, taking into account also the directions in which future work is leading. W-R spectra are found to be primarily an emission line spectrum superimposed on a 'hot' continuous spectrum. P Cygni absorption components are observed for some lines in some stars. A fact not realized when Thomas (1968) discussed W-R spectra is that a very few W-R stars have intrinsic absorption lines. On the basis of the spectroscopic observations, it could now be inferred that an optically thick stellar wind is involved. Many of the WN subtypes do contain some carbon. The WC subtypes contain little or no nitrogen. Attention is given to absolute visual magnitudes of W-R stars, the heterogeneity of W-R spectra, transition W-R spectra, mass loss rates, very luminous W-R objects, theoretical aspects of stellar structure and stellar winds, and evolutionary scenarios.

Conti, P. S.↗
NASA NTRS1981

Observations of stellar winds in early type stars

The presented review is mainly concerned with the massive population I stars of spectral type OB and the Wolf-Rayet (WR) class. It is pointed out that stellar winds are a ubiquitous phenomenon among the early type stars. Methods to determine the mass loss rate are discussed, taking into account the UV method, the optical method, the infrared method, and the radio method. Current beliefs about early type stars are considered. It is thought that all have stellar winds. The winds are primarily driven by radiation pressure from the UV lines, but other, as yet unknown factors, may determine the density and hence the rates. The stellar winds are highly ionized, more so than would be inferred from their continuum radiation temperatures. The ionization equilibrium in the winds is affected by local X rays which have been detected by the Einstein satellite. Unresolved issues are also pointed out.

Conti, P. S.↗
NASA NTRS1972

HD 50896 and the composition of Wolf-Rayet atmospheres

Measurements of the Pickering lines of He II in the spectrum of HD 50896 suggest that the upper limit to the H/He ratio is 0.05 by numbers. Spectral scans from OAO-2 show strong lines, nitrogen being observed most strongly as N IV, but also as N III and N V. Carbon and oxygen are observed dominantly in the triply ionized states. Thus, the degree of ionization of C, N and O are comparable. No indication of extensive selective excitation effects is observed. It is concluded that the differences between the WN and WC stars cannot be explained by selective excitation or ionization effects in the WN stars.

Smith, L. F.↗