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Adaptive optics observations of young massive stars

The intense stellar radiation of deeply embedded young massive stars leads to the formation of very compact ionized regions. Adaptive optics observations of such objects using ESO's ADONIS system yielded diffraction-limited images at 2.2 and 3.6 micrometers . Supplementary measurements were acquired at mid-IR and radio wavelengths. As a main result, it turned out that the observationally defined class of UCHIIs comprises sources of different nature: ionized dense gas clumps close to massive stars, externally ionized circumstellar disks and photo-evaporating disks around hot stars. In any case, the reservoir provided by the molecule matter prolongs the lifetime of an UCHII over that of a classical HII region which explains the high frequency of galactic UCHIIs. The investigations yielded new insights on the birth of massive stars and the interaction with the ambient matter of the parent molecular cloud which helps to establish a more accurate theory of the formation of massive stars and to describe properly their impact on the stimulation/inhibition of star formation in general.