The G9.62+0.19-F hot molecular core - The infrared view on very young massive stars

We present the results of an extensive infrared study of the massive star-forming region G9.62+0.19. The data cover information from broad- and narrow-band filters in the wavelength range from 1 to 19 mum and are obtained with ESO's near- and thermal infrared camera ISAAC at the VLT and with the mid-infrared cameras TIMMI2 (La Silla, ESO) and SpectroCam-10 (Mt. Palomar). The high sensitivity and resolution provided by these facilities revealed intriguing new details of this star-forming region, especially of the embedded hot molecular core (HMC)-component F. We analyse the newly found infrared substructure of four objects in this HMC region. While one of these objects (F2) is probably a foreground field star, the nature of the brightest object in the near- infrared there (F1) remains somewhat enigmatic. Our new astrometry proves that this object is not coincident with the peak of the molecular line emission of the HMC, but displaced by similar to1.7 arcsecs which translates to nearly 10 000 AU on a linear scale. On the basis of the available data we estimate this object to be an additional embedded object with a dense dust shell. Very near the HMC location we find L' band emission which strongly rises in flux towards longer wavelengths. We presume that this emission (F4) arises from the envelope of the HMC which is known to be associated with a molecular outflow roughly aligned along the line of sight. Thus, the clearing effect of this outflow causes strong deviations from spherical symmetry which might allow infrared emission from the HMC to escape through the outflow cavities. This presents the first direct detection of an HMC at a wavelength as short as 3.8 mum. At 11.7 mum and 18.75 mum, the HMC counterpart F4 ultimately proves to be the most luminous IR source within the G9.62+0.19-F region. In addition, within the entire G9.62+0.19 complex our narrow-band data and the K band imaging polarimetry reveal well-defined regions of enhanced Brgamma and H-2 emission as well as a sector where a large contribution comes from scattered light. Combining our results with high-resolution radio data we make predictions about the extinction within this star-forming region which clarifies why some of the associated ultracompact HII regions are not visible in the near- infrared. Our investigations show the complexity of massive star formation in its full grandeur, but they also demonstrate that the related problems can be tackled by observations using the new generation of infrared cameras.