DISCOVERY OF THE FIRST B[e] SUPERGIANTS IN M 31

Abstract

B[e] supergiants (B[e]SGs) are transitional objects in the post-main sequence\nevolution of massive stars. The small number of B[e]SGs known so far in the\nGalaxy and the Magellanic Clouds indicates that this evolutionary phase is\nshort. Nevertheless, the strong aspherical mass loss occurring during this\nphase, which leads to the formation of rings or disk-like structures, and the\nsimilarity to possible progenitors of SN1987A emphasize the importance of\nB[e]SGs for the dynamics of the interstellar medium as well as stellar and\ngalactic chemical evolution. The number of objects and their mass loss behavior\nat different metallicities are essential ingredients for accurate predictions\nfrom stellar and galactic evolution calculations. However, B[e]SGs are not\neasily identified, as they share many characteristics with luminous blue\nvariables (LBVs) in their quiescent (hot) phase. We present medium-resolution\nnear-infrared K-band spectra for four stars in M 31, which have been assigned a\nhot LBV (candidate) status. Applying diagnostics that were recently developed\nto distinguish B[e]SGs from hot LBVs, we classify two of the objects as\nbonafide LBVs; one of them currently in outburst. In addition, we firmly\nclassify the two stars 2MASS J00441709+4119273 and 2MASS J00452257+4150346 as\nthe first B[e]SGs in M 31 based on strong CO band emission detected in their\nspectra, and infrared colors typical for this class of stars.\n