Optical and UV Investigation of the Envelope of AG Carinae

Abstract

The galactic LBV object AG Car is unique for assembling several interesting features: (i) a light curve typical of Hubble-Sandage variables, (ii) a largely variable P Cygni-type spectrum, (iii) a strong IR excess, and especially (iv) a small ring nebula. Caputo and Viotti (1970) found that during 1949-59 the AG Car spectrum varied between Al and BO equivalent spectral types. A dramatic evolution occurred in recent years, when the visual magnitude gradually faded from V=6 in 1981 to V=8 in 1985, followed by a rise of the line excitation from Aeq in 1981 (Wolf and Stahl 1982), to Be£ in 1983 (Viotti et al. 1984), and to 0fpe/WN9 in January 1985 (Stahl 1986). This behaviour has represented a unique chance to follow a LBV in different spectral ranges during its major photometric and spectral changes. In the following we present the results of a long term monitoring of the star and of its nebula in the optical and UV. We propose thatAG Car evolved in recent times from a red supergiant phase, and that the present LBV phase is characterized by a critical structure of the stellar atmosphere.