B. Bohannan

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.

Quantitative Near-Infrared Spectroscopy of Of and WNL Stars

From new high signal-to-noise ratio (S/N) 1-2.2 μm spectroscopy of nine extreme early-type stars—including O Iaf, O Iafpe and WN9 types—we determine stellar parameters from detailed atmospheric analysis and evaluate results from near-IR analogues of well-known spectral diagnostics in the optical. We conclude that accurate stellar parameters can be measured from near-IR spectroscopy alone, an analysis technique important to studies of luminous stars in the Galactic center and other galaxies. Derived stellar parameters—mass-loss rates, luminosities, surface abundances, temperatures—show good agreement between optical and near-IR analyses, provided that IR data are of sufficient spectral resolution (R>2000) and S/N (S/N>30). Wind velocities derived from He I 1.0830 μm are consistent with those from ultraviolet P Cygni profiles. Temperatures 200-1300 K systematically lower are determined from the near-IR diagnostics, a difference not significant in determining the stellar properties of these objects; which set of spectral lines provides the more accurate physical parameters—optical or IR—cannot at present be ascertained. The strength of He I 2.0581 μm is very sensitive to the extreme ultraviolet energy distribution where line blanketing by heavy elements plays an important role; this line should not on its own be considered a reliable temperature diagnostic. The three peculiar, extreme emission-line stars—the O Iafpe stars HD 152386, HD 152408, and HDE 313846—are more similar in both morphological and physical characteristics to WNL-type Wolf-Rayet stars than to normal O Iaf supergiants and should be classified as W-R. Their classification should be WN9ha, in which they remain a unique subgroup.

Failure of continuum methods for determining the effective temperature of hot stars

It is demonstrated here that, for hot stars, methods based on the integrated continuum flux are completely unreliable discriminators of the effective temperature. Absorption line profiles provide much more accurate values of these parameters. It is not necessary to invoke nonradiative energy and momentum effects to explain the spectroscopic appearance of O-type stars of very different spectral type; rather, the observed spectra can be well modeled and fully interpreted by normal interaction of gas and radiation in stellar atmospheres of differing effective temperature and gravity.

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.

Evidence for processed material in the atmospheres of Large Magellanic Cloud B supergiants

We present a high-dispersion study of the optical spectra of 11 LMC B-type supergiants. These stars had been shown previously to have a wide range in nitrogen absorption line strengths, despite having very similar temperatures and luminosities. The current study shows that the relatively rare nitrogen-weak stars also have weaker photospheric helium lines and stronger photospheric oxygen lines than the more spectroscopically typical supergiants. This pattern suggests that the vast majority of LMC B-type supergiants have had their surfaces contaminated by material from their original hydrogen-burning cores - with a resultant enhancement of surface nitrogen and helium and a depletion in oxygen - while the spectroscopically nitrogen-weak stars are more likely to have retained their main-sequence surface abundances.

Stellar properties of Galactic Centre He I sources

The Galactic Centre (GC) region has in recent years attracted a lot of attention from the hot star community following the discovery of numerous He I emission line sources (Krabbe et ale 1991). Recent studies (e.g., Najarro et ale 1994) revealed stellar properties reminiscent of cool Wolf-Rayet stars, providing clues to the origin of the Lyman and He+ ionizing fluxes, and the luminosity in the central parsec of our galaxy. They allow the recent star formation history to be unveiled, and assess the effect of high metallicity on massive star evolution.