Star formation in the S235 A-B complex

We confirm the presence of a highly obscured stellar cluster between S235 A and B and, from the colour-colour analysis, we show that the cluster contains many sources with infrared excess, which are believed to be Young Stellar Objects (YSOs) in an early evolutionary stage.

Diffuse Br gamma emission is found mainly in the vicinity of S235 A, and unresolved Br gamma emission is found coincident with S235 B. Hot molecular hydrogen emission is distributed around the S235 A nebula, especially in a belt-like region to the south of S235 A, at the edge of the HII region.

The driving source of the H2O maser does not appear to be either the YSO inside S235 A or S235 B, but is identified with a faint near infrared member of the cluster, with a large (H-K) colour excess, located near the position of the maser. A hot dust envelope around an early type star may be the source of the near IR emission. This identification is supported by the coincidence of the maser and the near IR source with the center of a high density and compact molecular core observed in C{34}S and {13}CO. The lack of radio continuum emission from the area around the maser suggests that the star powering the maser and responsible for the near IR emission must be in a very early evolutionary stage, highly obscured even at K band and surrounded by an envelope with such a high density that any radio continuum emission is strongly self-absorbed. In any case the evolutionary status of such a star is much earlier than those of the exciting stars of S235 A and S235 B. Strong variability of the maser emission and large velocity differences of the maser features with respect to the molecular cloud velocity imply the presence of highly collimated, energetic and short duration jet activity in this YSO. The more evolved members of the cluster S235 A and S235 B lie on the sides of the molecular core, suggesting that star formation in the cluster is not coeval but proceeds from the outside towards the core of the molecular cloud.