High-resolution imaging of ultracompact HII regions III. G11.11-0.40 and G341.21-0.21

We present the first near-infrared images of the two massive southern star- forming regions G11.11-0.40 and G341.21-0.21, obtained with adaptive optics at sub-arcsecond resolution. Together with conventional narrow-band near-i nfrared images and polarization maps, mid-infrared data, millimetre and rad io data, we were able to determine the morphological structure and to const rain the stellar content of the regions. In both cases the molecular cloud cores have masses of about 500 M-circle dot and contain clusters of OB star s. This supports the view that massive stars do not form in isolation. In t he case of G11.11-0.40 we could identify the central O star responsible for the ionization of the ultracompact HII region. Our results for G341.21-0.2 1 indicate that this source is a precursor of an ultracompact HII region.