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

AG Carinae is one of the prototypes of the class of Luminous Blue Vari ables. Since 1990 the star has continuously brightened in its visual c ontinuum. We report on a multi-instrument and -wavelength observing ca mpaign to monitor the current activity phase of AG Car. Ground-based p hotometry, polarimetry, spectroscopy, and space-ultraviolet spectrosco py and spectropolarimetry have been obtained. From the variability of the polarization at ultraviolet and optical wavelengths we detect sign ificant intrinsic polarization. P(int) greater-than-or-equal-to 0.5% i s a large value for a hot, luminous star, suggesting departures from s pherical 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 a nd returned to 1% in 1992 July. The results are interpreted in terms o f a variable outflow with a density enhancement in the equatorial plan e. A similar model was suggested for the related object R127 in the La rge Magellanic Cloud. This geometry is reminiscent of the large-scale morphology of the gas nebula and dust ''jet'' surrounding AG Car. It i s therefore likely that physical conditions close to the stellar surfa ce are responsible for the geometry of the spatially resolved circumst ellar material around AG Car. The line spectrum in the optical and ult raviolet is dominated by the effects of a massive stellar wind. Two wi nd components are detected: a slow dense wind, where the bulk of recom bination radiation is emitted, and a faster, less dense wind, visible in the absorption components of ultraviolet P Cygni profiles. This win d structure is consistent with the geometry suggested by the spectropo larimetric observations, and it is reminiscent of the wind conditions prevailing in B[e] stars. An analysis of the photospheric and wind par ameters has been performed using an expanding, spherically extended no n-LTE model atmosphere. The temperature of AG Car decreased from 21,00 0 to 14,000 K over approximately 1 yr, with a corresponding increase o f the photospheric radius by a factor of 2. Helium is found to be sign ificantly overabundant, supporting the very evolved state of AG Car. C omparison with evolutionary models leads to an estimate for the zero-a ge main-sequence mass of (50 +/- 10) M.. Therefore the possibility can not 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 correla tion between wind density and stellar radius. This makes models that e xplain the radius increase by opacity effects in the outflow unlikely. The mechanism responsible for the temperature and radius variations i s still unknown but most likely has its origin in subphotospheric regi ons.