1997A&A...320..594F

We present near infrared broad band (J,H and K) and narrow band (H₂ S(1)1->0 and Brγ) images, and high resolution molecular observations (C³⁴S(2-1), (3-2), (5-4) and ¹³CO(2-1)) around the highly variable H₂O maser located between the S 235 A and B optical nebulosities. These observations are part of an on-going search for the sources of excitation of H₂O masers in regions of star formation or, alternatively, for the earliest evolutionary phases of massive stars. We confirm the presence of a highly obscured stellar cluster between S 235 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 Brγ emission is found mainly in the vicinity of S 235 A, and unresolved Brγ emission is found coincident with S 235 B. Hot molecular hydrogen emission is distributed around the S 235 A nebula, especially 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 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 C³⁴S and ¹³CO. 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 S 235 A and S 235 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 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 proceeds from the outside towards the core of the molecular cloud.