New light on the S235A-B star forming region

Abstract

The S235A-B star forming region has been extensively observed in the past from the radio to the near-IR, but what was happening in the immediate surroundings of the water maser, placed in between the two nebulosities, was still unclear because of insufficient resolution especially in the spectral range from the Far-IR to the mm, even though there were sound indications that new young stellar objects (YSOs) are being formed there. We present here new high resolution maps at mm wavelengths in different molecules (HCO+, C34S, H2CS, SO2 and CH3CN), as well as in the 1.2 and 3.3 mm continuum obtained with the Plateau de Bure interferometer, and JCMT observations at 450 micron and 850 micron that unambiguously reveal the presence of new YSOs placed in between the two HII regions S235A and S235B and associated with the water maser. A molecular core and an unresolved source in the mm and in the sub-mm are centred on the maser, with indication of mass infall onto the core. Two molecular bipolar outflows and a jet originate from the same position. Weak evidence is found for a molecular rotating disk perpendicular to the direction of the main bipolar outflow. The derived parameters indicate that one of the YSOs is an intermediate luminosity object (L~1000 Lsun) in a very early evolutionary phase, embedded in a molecular core of ~100 Msun, with a temperature of 30 K. The main source of energy for the YSO could come from gravitational infall, thus making of this YSO a rare example of intermediate luminosity protostar representing a link between the earliest evolutionary phases of massive stars and low mass protostars of class 0-I.