GEOMETRY AND PHYSICAL CONDITIONS IN THE STELLAR WIND OF AG-CARINAE

AG Carinae is one of the prototypes of the class of Luminous Blue Variables. Since 1990 the star has continuously brightened in its visual continuum. We report on a multi-instrument and -wavelength observing campaign to monitor the current activity phase of AG Car. Ground-based photometry, polarimetry, spectroscopy, and space-ultraviolet spectroscopy and spectropolarimetry have been obtained. From the variability of the polarization at ultraviolet and optical wavelengths we detect significant intrinsic polarization. P(int) greater-than-or-equal-to 0.5% is a large value for a hot, luminous star, suggesting departures from spherical symmetry in the wind of AG Car. The intrinsic polarization is variable on a timescale of 2 months or less. The measured ultraviolet polarization (intrinsic + interstellar) dropped to 0.5% in 1992 May and returned to 1% in 1992 July. The results are interpreted in terms of a variable outflow with a density enhancement in the equatorial plane. A similar model was suggested for the related object R127 in the Large Magellanic Cloud. This geometry is reminiscent of the large-scale morphology of the gas nebula and dust ''jet'' surrounding AG Car. It is therefore likely that physical conditions close to the stellar surface are responsible for the geometry of the spatially resolved circumstellar material around AG Car. The line spectrum in the optical and ultraviolet is dominated by the effects of a massive stellar wind. Two wind components are detected: a slow dense wind, where the bulk of recombination radiation is emitted, and a faster, less dense wind, visible in the absorption components of ultraviolet P Cygni profiles. This wind structure is consistent with the geometry suggested by the spectropolarimetric observations, and it is reminiscent of the wind conditions prevailing in B[e] stars. An analysis of the photospheric and wind parameters has been performed using an expanding, spherically extended non-LTE model atmosphere. The temperature of AG Car decreased from 21,000 to 14,000 K over approximately 1 yr, with a corresponding increase of the photospheric radius by a factor of 2. Helium is found to be significantly overabundant, supporting the very evolved state of AG Car. Comparison with evolutionary models leads to an estimate for the zero-age main-sequence mass of (50 +/- 10) M.. Therefore the possibility cannot entirely be excluded that AG Car is in a post-red supergiant phase. Despite the drastic change of the photospheric conditions, the mass-loss rate did not increase. We find no evidence for a positive correlation between wind density and stellar radius. This makes models that explain the radius increase by opacity effects in the outflow unlikely. The mechanism responsible for the temperature and radius variations is still unknown but most likely has its origin in subphotospheric regions.