2004A&A...426..195K

We present new high-sensitivity high-resolution mm-wave observations of the Onsala1 ultra-compact HII region that bring to light the internal structure of this massive star forming cloud. The 1.2mm continuum map obtained with the IRAM 30-m radiotelescope (∼11" resolution) shows a centrally peaked condensation of 1' size (∼0.5pc at the assumed distance of 1.8kpc) which has been further investigated at higher resolution in the 3mm continuum and in the emission lines of H¹³CO⁺ J= 1-0 and SiO J= 2-1 with the IRAM Plateau de Bure interferometer. The 3mm data, with a resolution of ∼5"x4", display an unresolved continuum source at the peak of the extended 1.2 mm emission and closely associated with the ultra-compact HII region. The H¹³CO⁺map traces the central condensation in good agreement with previous NH₃ maps of Zheng et al. (1985ApJ...293..522Z). However, the velocity field of this central condensation, which was previously thought to arise in a rapidly rotating structure, is better explained in terms of the dense and compact component of a bipolar outflow. This interpretation is confirmed by SiO and CO observations of the full region. In fact, our new SiO data unveil the presence of multiple (at least 4) outflows in the region. In particular, there is an important center of outflow activity in the region about 1' north of the UCHII region. Indeed the different outflows are related to different members of the Onsala1 cluster. The data presented here support a scenario in which massive star formation begin much later in the evolution of a cluster and/or the phases of UCHII regions last for much longer than 10⁵yr.