V 439 CYG as a Late Phase of Stellar Evolution

Abstract

V 439 Cyg lies in the nearby (950 pc), heavily reddened open cluster Berkeley 87. This cluster is probably part of the starforming region ON2, where many compact HII regions, strong OH masers, CO and ammonia molecular clouds and high energy sources have been detected (Manchanda et al. 1996, and references therein). The colours of V 439 Cyg cannot easily be reconciled with the cluster CM diagram, as first noticed by Turner and Forbes (1982). These same authors suggested an age of 2 My for the cluster, while stressing that the evolutionary stage of another cluster member (BC Cyg, a RSG with M3.5I spectral type) would not fit in with the cluster HR diagram. In the framework of present-day evolutionary theories for high mass stars (e.g. Maeder 1996), at least one other cluster member (Sand 5, a WO star) also appears too evolved for a 2 My cluster age. V439 Cyg is reported in the historical records alternatively as a ”red” star or as a Be star. In recent years the star shows a BOep spectrum. Its spectral distribution can nevertheless not be fitted with a black body distribution of suitable temperature for any assumed reddening. V 439 Cyg is also identified with the source IRAS 20198+3716, indicating a far-IR excess that is very anomalous for a Be star.