SUB-ARCSEC RESOLUTION INFRARED IMAGES OF THE STAR-FORMING REGION G-35.20-1.74

We present J(1.25 mu m), H(1.65 mu m), K(2.2 mu m), H-2(2.125 mu m) an d 11.2 mu m infrared images at sub-arcsec resolution of an area center ed around the star forming region G 35.20-1.74. In J, H and K a cluste r of early type stellar sources with infrared excess clearly stands ou t with respect to the background distribution and is associated with a diffuse K emission around an UC HII region (which is the brightest so urce at K). No H-2 emission is detected in narrow-band images at 2.125 mu m. At 11.2 mu m six components are detected. The brightest one (ML RS) is extended and coincides with the UC HII region. The source with steepest IR spectrum and the largest infrared excess (MIR1) is associa ted with an H2O maser and a near IR source detected only at K. It is s eparated from the IR cluster and at a distance of 20 '' from the UC HI I region. The LR 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 sta ges of massive YSOs, much before the appearance of a radio UC Hn regio n and shows that star formation is not limited to the IR cluster (wher e most probably it has already come to an end) but is still taking pla ce in other parts of the molecular cloud. Of the other 11.2 mu m sourc es, three (MIR2, MIR4 and MIR4) present IR excesses and are similar to MIR1, while MIR6 appears to be a reddened early-type star. The morpho logy of the entire star forming complex, taking into account also mole cular and sub-mm observations, is indicative of different and independ ent episodes of star formation taking place in the same molecular clou d.