1997A&A...327..299P

We present J(1.25µm), H(1.65µm), K(2.2µm), H₂(2.125µm) and 11.2µm infrared images at sub-arcsec resolution of an area centered around the star forming region G 35.20-1.74. In J, H and K a cluster of early type stellar sources with infrared excess clearly stands out with respect to the background distribution and is associated with a diffuse K emission around an UC HII region (which is the brightest source at K). No H₂ emission is detected in narrow-band images at 2.125µm. At 11.2µm six components are detected. The brightest one (MIR3) is extended and coincides with the UC HII region. The source with steepest IR spectrum and the largest infrared excess (MIR1) is associated with an H₂O maser and a near IR source detected only at K. It is separated from the IR cluster and at a distance of 20'' from the UC HII region. The IR emission comes from a local young stellar object (YSO) associated with the maser. The lack of radio continuum emission from MIR1 confirms that H₂O masers can trace the youngest evolutionary stages of massive YSOs, much before the appearance of a radio UC HII region and shows that star formation is not limited to the IR cluster (where most probably it has already come to an end) but is still taking place in other parts of the molecular cloud. Of the other 11.2µm sources, three (MIR2, MIR4 and MIR5) present IR excesses and are similar to MIR1, while MIR6 appears to be a reddened early-type star. The morphology of the entire star forming complex, taking into account also molecular and sub-mm observations, is indicative of different and independent episodes of star formation taking place in the same molecular cloud.