The structure of Onsala 1 star forming region

Abstract

We present new high-sensitivity high-resolution mm-wave observations of the Onsala 1 ultra-compact HII region that bring to light the internal structure of this massive star forming cloud. The 1.2 mm continuum map obtained with the IRAM 30-m radiotelescope (~11" resolution) shows a centrally peaked condensation of 1' size (~0.5 pc at the assumed distance of 1.8 kpc) which has been further investigated at higher resolution in the 3 mm continuum and in the emission lines of H13CO+ J=1--0 and SiO J=2--1 with the IRAM Plateau de Bure interferometer. The 3 mm data, with a resolution of ~5" X 4", displays a 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 NH_3 maps of Zheng et al. (1985). 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 unveils the presence of multiple (at least 4) outflows in the region. In particular, there is an important center of outflow activity in the region at about 1' north of the UCHII region. Indeed the different outflows are related to different members of the Onsala 1 cluster. The data presented here support a scenario in which the phases of massive star formation begin much later in the evolution of a cluster and/or UCHII region last for much longer than 10^5 yrs.