The discovery of low-mass pre main sequence stars in Cepheus OB3b

Abstract

We report the discovery of low-mass PMS stars in the younger subgroup of the Cepheus OB3 association, Cep OB3b, using {\it UBVI} CCD photometry and spectroscopy. The location of a PMS is well defined in a V versus (V-I) colour-magnitude diagram (CMD). Multi-fibre spectroscopy results of optically-selected PMS candidates confirm the T Tauri nature of 10 objects, with equal numbers of classical and weak-line TTS. There are 6 other objects which we classify as possible PMS stars. The newly discovered TTS have masses in the range 0.9 - 3.0 Msun and ages from <1 to nearly 10 Myr, based on the Siess et al.(2000) isochrones. Their location close to the O and B Their location close to the O and B stars of the association demonstrates that low-mass star formation is possible in this hostile environment dominated by early-type stars and that the latter must have been less effective in eroding the circumstellar discs of their lower mass siblings compared to other OB associations (e.g. lambda Ori). We attribute this to the nature of the local environment, speculating that the bulk of molecular material, which shielded low-mass stars from the ionising radiation of their early-type siblings, has only recently been removed (abridged).