Publications of the Astronomical Society of the Pacific

Hunter, D. A.; Gallagher, J. S.

doi: 10.1086/131711pmid: N/A

A review and intercomparison is conducted of data on various morphologically-chosen groups of irregular galaxies in order to understand the underlying physical mechanisms that differentiate these systems. Particular attention is given to the observational clues to the stellar content, star-formation processes, and star-formation histories of these galaxies, with an emphasis on the uncertainties and the many unanswered questions.

Abbott, D. C.; Garmany, C. D.; Hansen, C. J.; Henrichs, H. F.; Pesnell, W. D.

doi: 10.1086/131712pmid: N/A

Osaki, Y.

doi: 10.1086/131713pmid: N/A

In the context of recent observations involving the use of high-precision spectroscopy, it is now believed that nonradial pulsations (NRP) occur everywhere among massive stars. Certain observations have suggested a possible connection between the NRP phenomenon and the episodic mass loss in Be stars. The excitation mechanisms of NRP in massive stars are discussed, taking into account overstable convection modes, the Kelvin-Helmholtz instability of differentially rotating stars, and the resonant generation of NRP waves by tidal bulge waves. It is pointed out that one of the most important aspects of nonaxisymmetric NRP waves is that they can transport angular momentum from one part of the star to another when they are nonconservative. Attention is given to the angular momentum transport by NRP waves and the Be episode.

Smith, M. A.

doi: 10.1086/131714pmid: N/A

A summary of the derivable properties of temporal line-profile analysis is presented in a table. In the table, a single observation provides unique phase information as well as an estimate of m, the number of mode lines of a surface wave intersecting the star's equator. Observed properties are considered along with nonradial oddities. It appears now that nearly all chemically normal and 'unveiled' stars, and possibly some Wolf-Rayets, exhibit NRP. The oddities among NRP OB stars are found to include mode changing, mode growth/decay, unequal wave amplitudes, unequal wavelengths, and absorption/emission transients.

Penrod, G. D.

doi: 10.1086/131715pmid: N/A

In an observation of about 1500 spectra of a sample of 25 rapidly rotating Bn and Be stars, on 20 different observing runs, it was found that all but two of the program stars show obvious line-profile variations due to nonradial pulsation. The amplitude of the pulsations in the Be star Lambda Eridani is correlated with the occurrence of Be outbursts. The same correlation had been observed by Vogt and Penrod (1983) for the Oe star, Zeta Ophiuchi. Attention is also given to transient blue-shifted absorption components sometimes seen in a number of stars, the suggestion that pulsation is a major contributor to the Be phenomenon, and the He I lambda 6678 line profiles of Lambda Eri on two occasions in October 1983.

Willson, L. A.

doi: 10.1086/131716pmid: N/A

Pulsation produces alterations in the density structure of a stellar atmosphere, and it drives or enhances mass loss. A summary is provided of some very general results which were obtained on the basis of an analysis of models calculated by Bowen (1985) for the atmospheres of radially pulsating cool stars. It is pointed out that four parameters are needed to characterize the structure of the atmosphere of a pulsating star, if simplifying assumptions are made regarding isothermal conditions with respect to shocks and the global temperature distribution. Some possible implications of the nonradial pulsations observed in B and Be stars for the structure of their atmospheres are discussed. Attention is given to the stellar wind, and applications to B and Be stars.

Conti, P.

doi: 10.1086/131717pmid: N/A

Progress made in the determination of mass-loss rates, terminal velocities, and other parameters in the winds of the OB stars and the Wolf-Rayet (W-R) objects is considered. It is thought best to base mass-loss rates on the observations of the free-free emission of electrons in the winds, taking into account, for example, a detection in the radio wavelengths on the Very Large Array. It is found that mass-loss rates in hot stars depend on luminosity to some power. All of the studies, both observational and theoretical, have ignored the issues of variability.Questions regarding variability are examined, and requirements for its observation are discussed.

Barker, P.

doi: 10.1086/131718pmid: N/A

The early-type stars whose spectra show Balmer emission lines are considered, taking into account the division of these stars into two groups, including the OB supergiants and the Be stars. In the case of the Be stars, recent observations at H-alpha, H-beta, and H-gamma with best S/N approximately 500 and resolution approximately 0.1 A have revealed a previously unsuspected 'winebottle', or two-tier, structure to the symmetric emission profile in several Be stars. For many Be stars, the circumstellar material has an apparent two-component structure. Attention is given to H-alpha variations in four typical Be stars, linear polarization, and magnetic fields.

Henrichs, H.

doi: 10.1086/131719pmid: N/A

The frequent presence of blue-shifted, discrete absorption components represents one of the major concerns in the interpretation of mass-loss indicating spectral lines in ultraviolet spectra of O, B, and Be stars. This summary is mainly concerned with the principal properties of discrete absorption components in the ultraviolet resonance lines of early-type stars. Discrete narrow absorption lines in the UV are discussed along with statistical considerations, individual studies, and models. It is concluded that the structure of stellar winds of early-type stars cannot be understood without understanding the origin of the discrete absorption components in the ultraviolet resonance lines.

Castor, J. I.

doi: 10.1086/131720pmid: N/A

Certain modifications of the stellar-wind theory reported by Castor et al. (1975) have improved its agreement with observations of O star winds and answered some questions regarding its approximations. The present status of agreement between radiatively-driven wind theory and observations of hot-star winds is discussed. It is pointed out that the state of the comparison between radiatively-driven wind theory and observation for Wolf-Rayet stars has changed recently with two important discoveries. According to one, multiline scattering can provide mass-loss rates well in excess of the 'single scattering limit'. The second discovery is that at least one W-R star is very hot. The temperature referred to the thermalizing radius of V444 Cygni is about 95,000 K. All instabilities considered so far are drift instabilities, at least in the realistic limit that isothermal oscillations are assumed.