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

We report the discovery of a low-mass pre-main-sequence (PMS) stellar population in the younger subgroup of the Cepheus OB3 association, Cep OB3b, using UBVI CCD photometry and follow-up spectroscopy. The optical survey covers about 1300 square arcmin on the sky and gives a global photometric and astrometric catalogue for more than 7000 objects. The location of a PMS is well defined in a V versus (V-I) colour-magnitude diagram (CMD). Multi-fibre spectroscopic results for optically-selected PMS candidates confirm the T Tauri nature for 10 objects, with equal numbers of classical TTS (CTTS) and weakline TTS (WTTS). There are 6 other objects which we classify as possible PMS stars. The newly discovered TTS stars have masses in the range ∼ 0.9 − 3.0 M⊙ and ages from < 1 to nearly 10 Myr, based on the Siess, Dufour & Forestini (2000) isochrones. Their location close to the O and B stars of the association (especially the O7n star) demonstrates that low-mass star formation is indeed possible in such an apparently 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., λ Ori). We attribute this to the nature of the local environment, speculating that the bulk of molecular material, which shielded lowmass stars from the ionising radiation of their early-type siblings, has only recently been removed.