Star formation in the S 235 A

We present near infrared broad band (J,H and K) and narrow band (H 2 S(1) 1 ! 0 and Br) images, and high resolution molecular observations (C 34 S(2-1), (3-2), (5-4) and 13 CO(2-1)) around the highly variable H 2 O maser located between the S235 A and B optical nebulosi-ties. These observations are part of an ongoing search for the sources of excitation of H 2 O masers in regions of star formation or, alternatively, for the earlieast evolutionary phases of massive stars. We connrm 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. Diiuse Br emission is found mainly in the vicinity of S235 A, and unresolved Br emission is found coinci-dent 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 H 2 O maser does not appear to be either the YSO inside S235 A or S235 B, but is iden-tiied 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 identii-cation 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 Send ooprint requests to: M. Felli diierences of the maser features with respect to the molecular cloud velocity imply the presence of highly collimated, energetic and short duration …