Interacting stellar winds in a binary system

Abstract

It is now known that strong stellar winds develop in stars mostly at the red and blue sides of the HR diagram. However, although the mass loss rates observed in 0 and M stars are comparable, the corresponding wind velocities are vastly different. It would thus be of great interest to find a binary system, containing both a cool and a hot star each with its own wind, and observe the resultant interaction. For a long time a Sco (Ml.5 Iab + B2.5 V) was the only known example (Kudritzki and ' Reimers 1978, van der Hucht et al. 1980). The situation in this case is best illustrated by a VLA map made by Gibson (1979) who finds that a shock develops at the surface of interaction of the two winds. In this paper I shall describe another binary system in which two stellar winds are interacting with dramatic effects.HM Sge is a new emission-line star unique in many aspects, brightened by 6 visual magnitudes in 1976 (Dokuchaeva 1976) and has remained in maximum light since (Ciatti et al. 1978). After the brightening, an emission line spectrum emerged, suggesting the presence of a clrcumstellar nebula. Ultraviolet observations of HM Sge show that it has an extremely rich UV emission line spectrum (Bogges et al. 1980). Infrared ob servations by Davidson et al. 1978) found the object to be heavily reddened with most of the energy emitted in the infrared. The 9.7ym silicate feature was also detected, characteristic of a mass-losing M star. The 2.3pm CO absorption feature was