H II and hot dust emission around young massive stars in G9.62+0.19

In this paper we present new near infrared (NIR) observations (J, H, and K broadbands), of the G9.62+0.19 star forming complex. Comparison of our observations with similar resolution centimetric continuum, millimeter continuum and molecular emission show that the mm continuum source F, not detected in the cm wavelength free-free radio continuum and associated with a high density molecular peak, is detected at 2.2mu m, while the ultracompact HII region D, one of the youngest of the HII regions in the complex, is not detected in the near infrared. We propose a simple model that explains why, in the first stages of evolution of a young massive star, the source may be observable at K but not in the cm radio continuum. When the size of the UC HII region is << 10(-3) pc the hot dust present around the YSO strongly emits at K band, but the radio emission will be strongly self-absorbed. At later stages, when the size of the UC HII becomes greater than lower .5exhbox {; \buildrel > \over \sim ;} 10(-3) pc, the dust temperature goes down and the K band dust emission strongly decreases; at the same time the cm radio continuum becomes detectable. As the UCHII expands the extinction drops and the K band emission rises again due to the stellar and ionized gas free-free and free-bound emission. We propose an evolutionary sequence of the different sites of star formation in the complex, based on the radio continuum--infrared morphology and on the association with H_2O masers.