David C. Abbott

A survey of radio emission from Galactic OB stars

Radio continuum observations of 88 O-type and early B-type stars are examined. Results are presented for a subset of these stars which forms a distance-limited sample of all OB stars within 2.5 kpc of the sun. Mass loss rates are derived for HD 15570, HD 166734, HD 151804, HD 152408, Alpha Cam, HD 169454, and Zeta Sco. All of these very luminous OB stars are found to be losing mass at a rate on the order of 10 to the -5 solar masses/yr. Multifrequency observations confirm the free-free interpretation for Zeta Pup and provide evidence for variability in the free-free sources P Cygni and Cyg OB2 No. 12. 63 refs.

Photospheres of hot stars. III - Luminosity effects at spectral type 09.5

Hydrogen and helium line profiles with high signal-to-noise ratios were obtained for four stars of spectral type 09.5 (Alpha Cam, Xi Ori A, Delta Ori A,AE Aur) that form a sequence in luminosity: Ia, Ib, II, V. The basic stellar parameters of these stars are determined by fitting the observed line profiles of weak photospheric absorption lines with profiles from models which include the effect of radiation scattered back onto the photosphere from their stellar winds, an effect referred to as wind blanketing. For these stars, the inclusion of wind blanketing is significant only for the most luminous star, Alpha Cam, for which the effective temperature was shifted about -2000 K relative to an unblanketed model.

Non-LTE model calculations for SN 1987A and the extragalactic distance scale

This paper presents model atmospheres for the first week of SN 1987A, based on the luminosity and density/velocity structure from hydrodynamic models of Woosley (1988). The models account for line blanketing, expansion, sphericity, and departures from LTE in hydrogen and helium and differ from previously published efforts because they represent ab initio calculations, i.e., they contain essentially no free parameters. The formation of the UV spectrum is dominated by the effects of line blanketing. In the absorption troughs, the Balmer line profiles were fit well by these models, but the observed emissions are significantly stronger than predicted, perhaps due to clumping. The generally good agreement between the present synthetic spectra and observations provides independent support for the overall accuracy of the hydrodynamic models of Woosley. The question of the accuracy of the Baade-Wesselink method is addressed in a detailed discussion of its approximations. While the application of the standard method produces a distance within an uncertainty of 20 percent in the case of SN 1987A, systematic errors up to a factor of 2 are possible, particularly if the precursor was a red supergiant. 68 refs.

Atmospheric models for luminous blue variables

The photospheres and stellar winds of luminous blue variables are studied theoretically. Non-LTE calculations of radiation transfer in spherically extended, expanding atmospheres are combined with a hydrodynamics code for radiation-driven winds. Models are calculated which are typical for luminous blue variables in minimum and maximum states. The recombination of iron group elements from double to singly ionized stages, which occurs when the effective temperature falls below 10,000 K, can explain why the mass-loss rates increase when luminous blue variables approach their maximum states; the singly charged ions provide a much larger number of strong lines which can intercept the radiation pressure. However, the mass-loss increase by itself cannot account for the drop of the effective temperature in a self-consistent way. The corresponding increase of the wind opacity causes only a comparativley small change in the radius of continuum formation. It is concluded that the observed variability of the photospheric radius must be induced from deeper, subphotospheric regions. 42 refs.

Photospheres of hot stars. IV, Spectral type O4

The basic stellar parameters of a supergiant (Zeta Pup) and two main-sequence stars, 9 Sgr and HD 46223, at spectral class O4 are determined using line profile analysis. The stellar parameters are determined by comparing high signal-to-noise hydrogen and helium line profiles with those from stellar atmosphere models which include the effect of radiation scattered back onto the photosphere from an overlying stellar wind, an effect referred to as wind blanketing. At spectral class O4, the inclusion of wind-blanketing in the model atmosphere reduces the effective temperature by an average of 10 percent. This shift in effective temperature is also reflected by shifts in several other stellar parameters relative to previous O4 spectral-type calibrations. It is also shown through the analysis of the two O4 V stars that scatter in spectral type calibrations is introduced by assuming that the observed line profile reflects the photospheric stellar parameters. 26 refs.

Thermal Radio Emission from the Winds of Single Stars

Observations of thermal emission at radio wavelengths provides a powerful diagnostic of the rate of mass loss and temperature of the winds of early-type stars. Some winds are also strong sources of nonthermal emission. Case studies of known thermal and nonthermal sources provide empirical criteria for classifying the observed radio radiation. Mass loss rates are derived for 37 OB and Wolf-Rayet stars considered “definite” or “probable” thermal wind sources by these criteria. The rate of mass loss is strongly linked to stellar luminosity in OB stars and probably linked to stellar mass in Wolf-Rayet stars, with no measurable correlation with any other stellar property. A few late-type giants and supergiants also have detectable thermal emission, which arises from extended, accelerating, partially-ionized chromospheres.

VLA observations of A and B stars with kilogauss magnetic fields

The serendipitous discovery that the star 03C3 Ori E [B2 Vp (He Strong)] is a 3.5 mJy radio continuum source at 6 cm has stimulated a radio survey of other early-type stars with strong magnetic fields. No Ap stars have been detected of 8 observed, with typical 3a upper limits of 0.5 mJy at 2 cm. Of 6 Bp stars examined, only HR 1890, also a helium-strong star, was detected. We discuss possible emission mechanisms for the observed radio emission, and conclude that nonthermal emission seems the most plausible, on the basis of the present data.

The Theory of Radiation Driven Stellar Winds and the Wolf-Rayet Phenomenon

Peter Conti has a tradition of always talking about 0-type stars at Wolf-Rayet symposia, and Wolf-Rayet stars at 0 star symposia. Since there is no well-developed theory for the origin of the winds of WR stars, it is my pleasure to join Peter’s tradition, and to talk mainly about the theory of radiation driven winds in OB stars. The advantage of OB stars is that there exists a fairly complete wind theory, which agrees well with the available observations. The question is, can the mass loss observed from Wolf-Rayet stars be explained by a version of this wind theory which is scaled to the conditions found in the envelopes of Wolf-Rayet stars? The topics I consider are: The calculated radiation pressure in OB stars, and its dependence on temperature, density, and chemical composition. A comparison between predicted and observed mass loss rates and terminal velocities for OB stars. The applicability of the standard radiation driven wind models to Wolf-Rayet stars. Speculations on how Wolf-Rayet stars achieve their enormous mass loss rates within the context of the radiation pressure mechanism.