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High-resolution scans show that the relatively strong (about 90 mA) K lines of Ca II in the early B stars gamma-Peg and iota-Her are almost entirely stellar in origin, although the latter case includes a small interstellar contribution. Such stellar lines can be of great importance in augmenting the interstellar absorption, up through the earliest of the B stars.
Description of a relatively simple stellar-wind-driven accretion model that appears to satisfactorily account for many of the observed features of the pulsing X-ray sources Cen X-3 and Her X-1. A rotating magnetized neutron star orbits a more massive slightly evolved star and accretes mass from the stellar wind emanating from that star. Some of the implications of the model are discussed.
Vertical profiles of microthermal turbulence structure have been obtained from balloon flights to altitudes near 25 km above mean sea level. Comparison of the observed turbulence structure with meteorological data and simultaneously acquired stellar scintillation data has been successful. For this comparison integrals over the observed turbulence structure were computed as required by theory. Turbulence and wind velocity data were also employed to predict successfully stellar irradiance spectra.
The results are reported of a study of the stellar occultation technique for measuring the composition of the atmosphere. The intensity of starlight was monitored during the occultation using the Wisconsin stellar ultraviolet photometers aboard the Orbiting Astronomical Observatory (OAO-A2). A schematic diagram of an occultation is shown where the change in intensity at a given wavelength is illustrated. The vertical projection of the attenuation region is typically 60 km deep for molecular oxygen and 30 km deep for ozone. Intensity profiles obtained during various occultations were analyzed by first determining the tangential columm density of the absorbing gases, and then Abel inverting the column densities to obtain the number density profile. Errors are associated with each step in the inversion scheme and have been considered as an integral part of this study.
In a previous paper, the PPN (parametrized post-Newtonian) formalism was used to analyze relativistic influences on stellar stability in nearly all metric theories of gravity. That analysis omitted all preferred-frame terms. In this paper, possible preferred-frame effects on stellar stability are examined and no new instabilities are found. Although terms linear in the preferred-frame velocity w (time-odd terms, analogous to viscosity and energy generation) change the shapes of the normal modes, their symmetry properties prevent them from changing the characteristic frequencies. Thus, no new vibrational or secular instabilities can occur. Terms quadratic in w do not change either the shapes of the normal modes or the characteristic frequencies for radial pulsations (except for the effects due to the renormalization of the gravitation constant which does not affect stability). Thus, they have no influence on radial stability. Terms quadratic in w do change both the normal modes and the characteristic frequencies of nonradial pulsations; but in the limit of a neutral mode these changes vanish.
The stellar Lyman-alpha line at 1216 A was observed in 29 lightly reddened stars of spectral type B2.5 to B9 by a far-UV spectrophotometer on OAO-2. The equivalent widths obtained range from 15 A at type B2.5 to 65 A at type B8; in the late-B stars, the L-alpha line removes 2 to 3% of the total stellar flux. In this sampling, the strength of the L-alpha line correlates well with measures of the Balmer discontinuity and Balmer line strengths; luminosity classification does not seem to affect the line strength. The observed line widths also agree with the predictions of Mihala's grid of non-LTE model atmospheres. In some cases, the L-alpha line influences the interstellar column densities reported in the interstellar OAO-2 L-alpha survey. Hence, these data toward lightly reddened B2 and B1.5 stars should be regarded as upper limits only.
Stellar surface heavy-element abundances may be substantially higher than interior abundances if there is appreciable accretion of chemically fractionated matter after Hayashi-phase convection has terminated. The plausibility of appreciable accretion is discussed, and some possible observational consequences of enhanced surface heavy-element abundances are investigated. These consequences include a reduced solar neutrino flux, systematic variations in surface abundance along the main sequence and along the giant branch, and relatively low heavy-element abundances among planetary nebulas and white dwarfs and in the interstellar medium. There is marginal observational evidence in support of the hypothesis of enhanced stellar surface heavy-element abundances.
Ultraviolet stellar spectra in the wavelength region from 130 to 500 nm were photographed during the three manned Skylab missions using a 15-cm-aperture objective-prism telescope mounted on the antisolar scientific airlock. Data at wavelengths of 200 nm or shorter were obtained for 1400 stars in 166 fields. Approximately 300 stars show absorption lines of Si IV, C IV, or C II in which the strengths and profiles show striking variations with stellar temperature and luminosity. Numerous weak lines are also observed in supergiant stars, shell stars, peculiar A stars, and Wolf-Rayet stars. Observations of comet Kohoutek show interesting variations in the strength of the OH 3090 A band.
The NASA Balloon-borne Ultraviolet Stellar Spectrometer (BUSS) which is carried to an altitude of 40 km by a 15 million cubic foot balloon for night-time observations of ultraviolet stellar spectra is discussed. The BUSS optical system, comprising an 0.40 m aperture Cassegrain telescope and an Ebert-Fastie spectrometer, points at various selected stars and focuses a portion of their spectra on the photocathode of an image dissector tube. The spectral region between 2,775 Angstroms and 2,825 Angstroms is sampled by the detector at 0.25 Angstroms increments using photon counting techniques. The pointing system for the payload uses a pair of orthogonal magnetometers which sense the earth's magnetic field for an aximuth reference, and a platform potentiometer for an elevation reference. This pointing system places the target star within the 3x1 degree field of view of an outer optical star tracker. The outer star tracker is then used to point the entire instrument to within one arc minute of the target star.
The results from the Celescope Catalog of Ultraviolet Stellar Observations (Davis, Deutschman, and Haramundanis 1973) are studied, emphasizing statistical groups and subgroups of stars. In spite of the rather large observational errors, the great number of Celescope observations make it possible to do that kind of study with reasonable accuracy. Some stars have ultraviolet colors that differ significantly from those of the group to which they have been assigned on the basis of ground-based observations. An attempt is made to describe the general properties of the ultraviolet colors of the stars as observed by the Celescope experiment on board NASA's Orbiting Astronomical Observatory (OAO-2). Besides the direct applicability of these results to the general study of stellar atmospheres and of interstellar extinction, they are also considered as a frame of reference for further studies of individual stars and groups of stars.
A sensitive soft X-ray survey of about one-tenth of the sky, carried out as part of the Skylab program and flown on 1973 July 28, revealed no evidence for soft X-ray emission from stellar sources. We present upper limits to the soft X-ray luminosity of about 50 nearby stars. With nominal assumed values for stellar space densities, we show it unlikely that emission from giants, main sequence stars, white dwarfs, or stars in known binary systems can account for the soft X-ray diffuse background.
The formation of the Balmer-series member H-epsilon in the near-red wing of the Ca II H line is discussed for two cases: the sun (H-epsilon absorption profile) and Arcturus (H-epsilon emission profile). It is shown that although the H-epsilon source functions in both stars are dominated by the Balmer-continuum radiation field through photoionizations, the line-formation problems in the two stars are quantitatively different, owing to a substantial difference in the relative importance of the stellar chromosphere temperature inversion as compared with the stellar photosphere.
An important difference between the radio and stellar occultation techniques for probing planetary atmospheres is that a crucial equation relating refraction angle with ray impact parameter is solved in opposite directions in the two techniques. The solution is tolerant of small geometrical errors for stellar occultations but, unfortunately, not for radio occultations.
Evolutionary sequences for stellar models with 10, 15, 30, and 60 solar masses, as well as four different initial chemical compositions, are calculated to the end of core helium burning using the Schwarzschild criterion for convection. The results are analyzed in terms of the modifications of interior structure and surface parameters induced by semiconvective mixing as a result of adopting the Schwarzschild criterion. It is found that the main differences from results based on the Ledoux criterion are the great extent of the convectively unstable layers in the intermediate zone and the eventual development of a fully convective zone at the base of the semiconvective one. It is shown that semiconvection develops outside the convective core just after the ZAMS stage for masses greater than 12 solar masses and just before the stage of central hydrogen exhaustion for masses greater than 6 solar masses. The present models are found to be insufficiently hot in comparison with the bulk of observed stable blue supergiants and to predict far too many red supergiants fro the range above 20 solar masses. It is concluded that something is fundamentally wrong with the models, the most likely suspects being the stellar opacities adopted and the neglect of mass loss.
Effects of atmospheric turbulence on stellar-occultation inversion procedures are investigated using a heuristic scattering model that is believed to reproduce the essential features of turbulence. A quantitative estimate is made of the size of the error in deducing the mean refractivity profile of a planetary atmosphere, taking into account constant as well as exponential scattering. It is shown that ordinary turbulence has no important effect on the average intensity profile in a stellar occultation but could have an important instantaneous effect. A critical examination of possible manifestations of turbulent scattering during occultations of Beta Sco by Jupiter indicates that all observed phenomena during these events can be understood in terms of scintillations produced by turbulence.
Spectra between 1000 and 1450 A for 60 O- and B-type stars observed by Copernicus at 0.2-A resolution are presented in three forms: tables containing the numerical data, plots showing renormalized spectra, and synthetic photographic spectra. The data have been corrected for all instrument effects of importance for the photometric accuracy except fluctuations in continuum level caused by small variations in spacecraft guidance. Spectrometer sensitivity curves are provided for use in converting to absolute fluxes. It is expected that this catalog will be of use for research on many aspects of stellar UV spectra, including spectral classification, line identification, abundance determinations, spectrum synthesis, model atmosphere calculations, flux distributions, bolometric corrections, stellar winds, and mass loss.
It is noted that any reasonable assumption for the rotation rates of the interstellar clouds which must collapse to produce stars will result in stars which rotate much faster than suggested by observations if angular momentum is conserved throughout the collapse. The contribution of thermomagnetic torque, which can exist when a rarefied gas of polyatomic molecules is subjected to a magnetic field and a nonuniform temperature gradient, to the angular momentum of a collapsing protostellar object is examined in order to determine whether the magnitude of this torque is sufficient to slow down contracting protostellar clouds. Calculations are performed for two times during protostellar collapse when conditions may be favorable for thermomagnetic torque to play a role in angular-momentum considerations. The results indicate that this torque is apparently of no importance in determining or modifying stellar rotation during the early stages of stellar evolution.
A method is developed for estimating the nonradiative heating of stellar chromospheres by measuring the net radiative losses in strong Fraunhofer line cores. This method is applied to observations of the Mg II resonance lines in a sample of 32 stars including the sun. At most a small dependence of chromospheric nonradiative heating on stellar surface gravity is found, which is contrary to the large effect predicted by recent calculations based on acoustic-heating theories.