Subarcsecond Imaging of the High-mass Star Forming Region Onsala

We report subarcsecond images of the high-mass star forming region Onsala 1 (ON 1) made with the Submillimeter Array (SMA) at 0.85 mm and the Very Large Array at 1.3 cm and 3.6 cm. ON 1 is one of the smallest ultracompact H II regions in the Galaxy and exhibits various star formation signposts. With our VLA and SMA observations, two new cm-wave sources and five sub-mm dust sources, respectively, within a field of ∼5 (corresponding to a linear scale of 0.05 pc) are identified, indicating the multiplicity at the center of the ON 1 region. The dust and gas masses of these sub-mm sources are in the range of 0.8 to 6.4 M⊙. Among the five sub-mm dust sources, SMA2, with a dust and gas mass of 2.6 M⊙, demonstrates several star formation signatures, and hence likely represents an intermediate-mass (or even high-mass) star forming core. Due to the low star formation efficiency of ∼10%, we suggest that star formation in the ON 1 region will continue. For example, SMA4 and SMA5 are not associated with any star formation signatures and likely mark star formation cores at very early evolutionary stages. Subject headings: H II regions — ISM: individual (Onsala 1) — ISM: clouds — stars: formation