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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
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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 NTRS1984

Comments on the ultraviolet spectrum of gamma(2) Velorum

A table of spectral features, including narrow, very broad, and 'photospheric' lines, in the ultraviolet spectrum of the Wolf-Rayet binary gamma(2) Velorum is given together with radial velocities at phase 0.99. The very broad lines display P Cygni profiles. Ten weak absorption features are tentatively assigned to blends of numerous lines of Fe IV, and three other similar features are possibly due to Fe V blends. These iron lines appear to be associated with the Wolf-Rayet component. The narrow features are present with a wide range of ionization. The Si IV, and probably C IV, features are circumstellar, while the other narrow absorptions are either interstellar or possibly a combination of interstellar and circumstellar. A model is proposed for the outer layers of the extended envelope of the Wolf-Rayet star.

Sahade, J.↗
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 NTRS1985

Spectroscopic observations of selected stellar systems

Spectra of very luminous, high temperature stars, the o-type stars with the largest masses among stars, and the Wolf-Rayet (W-R) stars are studied. These stars are characterized by strong stellar winds which produce signiicant mass loss. The nature and evolution of these winds are studied to learn the densities, velocity structue, ionization balance, and composition. The IUE satellite provides the best way to study these phenomena because many of the most important ground state and excited state lines of common highly ionized species are found in the spectral range.

Conti, P. S.↗
NASA NTRS1985

Observations of Nonthermal Radio Emission from Early-type Stars

As a part of a wider survey of radio emission from O, B, and Wolf-Rayet (WR) stars, five new stars whose radio emission is dominated by a nonthermal mechanism of unknown origin were discovered. From statistics of distance-limited samples of stars, it is estimated that the minimum fraction of stars which are nonthermal emitters is 25% for the OB stars and 10% for the WR stars. The characteristics of this new class of nonthermal radio emitter are investigated.

Abbott, D. C.↗
NASA NTRS1983

Observations of two peculiar emission objects in the Large Magellanic Cloud

Ultraviolet and visual wavelength spectra were obtained of two peculiar emission objects, Henize S63 and Sanduleak's star in the Large Magellanic Cloud. Previously not observed in the near- or far-ultraviolet, both objects exhibit strong permitted and semiforbidden line emissions. Estimates based on the absolute continuum flux of the hot companion star in Hen S63 indicate that it rivals the luminosity of the carbon star primary. The emission-line profile structure in both objects does not suggest Wolf-Rayet type emission. Carbon in Sanduleak's star (LMC anonymous) is conspicuously absent, while N V, semiforbidden N IV, and semiforbidden N III dominate the UV emission-line spectrum. Nitrogen is overabundant with respect to carbon and oxygen in both objects. The large overabundance of nitrogen in Sanduleak's star suggests evidence for CNO processes material similar to that seen in Nu Car.

Kafatos, M.↗
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.↗
NASA NTRS1985

Analysis of ultraviolet atmospheric eclipses in the Wolf-Rayet binary CV Serpentis

While no eclipses deeper than 0.04 mag are noted in the present UV spectra, covering one-half of an orbital cycle of CV Ser, in the electron scattering continuum at 2400-3200 A or in fine error sensor observations, marked atmospheric eclipses of up to 0.5 mag depth are observed in individual strong lines and over large ranges of the continuum at shorter wavelengths. The flux above the continuum in the C II 1247 A, Si IV 1400 A, and Si IV 1723 lines showed similar phase dependence with emission weakening, as well as with the emission's going into absorption as phase progresses from superior to inferior conjunction of the WC star (primary eclipse). These observations show effects very similar to the behavior of WN stars in the UV.

Eaton, J. A.↗
NASA NTRS1982

A recent time of minimum for and atmospheric-eclipse in the ultraviolet spectrum of the Wolf-Rayet eclipsing binary V444 Cygni

The 1200-1900 angstrom region and fine error sensor observations in the optical for V444 Cyg were continuously observed. More than half of a primary minimum and almost a complete secondary minimum were observed. It is found that the time of minimum for the secondary eclipse is consistent with that for primary eclipse, and the ultraviolet times of minimum are consistent with the optical ones. The spectrum shows a considerable amount of phase dependence. The general shaps and depths of the light curves for the FES signal and the 1565-1900 angstrom continuum are similar to those for the blue continuum. The FES, however, detected an atmospheric eclipse in line absorption at about the phase the NIV absorption was strongest. It is suggested that there is a source of continuum absorption shortward of 1460 angstrom which exists throughout a large part of the extended atmosphere and which, by implication, must redden considerably the ultraviolet continuua of WN stars. A fairly high degree of ionization for the inner part of the WN star a atmosphere is implied.

Eaton, J. E.↗