STAR-FORMATION IN THE S-235-A S-235-B COMPLEX

We present near infrared broad band (J,H and K) and narrow band (H-2 S (1) 1 --> 0 and Br gamma) images, and high resolution molecular observ ations ((CS)-S-34(2-1), (3-2), (5-4) and (CO)-C-13(2-1)) around the hi ghly variable H2O maser located between the S 235 A and B optical nebu losities. These observations are part of an on-going search for the so urces of excitation of H2O masers in regions of star formation or, alt ernatively, for the earliest evolutionary phases of massive stars. We confirm the presence of a highly obscured stellar cluster between S 23 5 A and B and, from the colour-colour analysis, we show that the clust er contains many sources with infrared excess, which are believed to b e Young Stellar Objects (YSOs) in an early evolutionary stage. Diffuse Br gamma emission is found mainly in the vicinity of S 235 A, and unr esolved Br gamma emission is found coincident with S 235 B. Hot molecu lar hydrogen emission is distributed around the S 235 A nebula, especi ally in a belt-like region to the south of S 235 A, at the edge of the HII region. The driving source of the H2O maser does not appear to be either the YSO inside S 235 A or S 235 B, but is identified with a fa int near infrared member of the cluster, with a large (H-K) colour exc ess, located near the position of the maser. A hot dust envelope aroun d an early type star may be the source of the near IR emission. This i dentification is supported by the coincidence of the maser and the nea r IR source with the center of a high density and compact molecular co re observed in (CS)-S-34 and (CO)-C-13. The lack of radio continuum em ission 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 e arly evolutionary stage, highly obscured even at K band and surrounded by an envelope with such a high density that any radio continuum emis sion is strongly self-absorbed. In any case the evolutionary status of such a star is much earlier than those of the exciting stars of S 235 A and S 235 B. Strong variability of the maser emission and large vel ocity 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 t he cluster S 235 A and S 235 B lie on the sides of the molecular core, suggesting that star formation in the cluster is not coeval but proce eds from the outside towards the core of the molecular cloud.