The structure of the Onsala 1 star forming region

M. S. N. Kumar, M. Tafalla, R. Bachiller

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 arcsec resolution) shows a centrally peaked condensation of 1 arcmin 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 arcsec × 4 arcsec, 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 NH_3 maps of Zheng et al. (cite{zheng85}). 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 arcmin 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 massive star formation begin much later in the evolution of a cluster and/or the phases of UCHII regions last for much longer than 10⁵ yr.

Astronomy and Astrophysics
Volume 426, Page 195
October 2004

DOI: 10.1051/0004-6361:20041234