CO J = 2-1 Maps of Bipolar Outflows in Massive Star-forming Regions

Abstract

Using the eight-beam array receiver of the NRAO 12 m telescope, we mapped the CO J=2-1 emission toward 11 high-mass star-forming regions. In the sample, four are previously detected outflows in the CO J=1-0 line, and seven are outflow candidates. A total of six bipolar outflows were identified in the CO J=2-1 line. For the remaining five sources, including one previously detected bipolar outflow, the CO J=2-1 emission shows multiple velocity components. Therefore, high-velocity line wings or bipolar structure cannot be identified. The CO J=2-1 spectra of the four of the nonbipolar outflow sources exhibit broad-line emission due to the blending of weak velocity components. The complex CO spectra underscore the importance of large-scale mapping in identifying outflows. Compared with the outflows detected with the CO J=1-0 line, the CO J=2-1 outflows often have broader line wings and smaller spatial extents, indicating that the high-velocity gas measured with the CO J=2-1 line arises from warm regions closer to the central source. The masses in the outflows range from a few to 60 M solar. The linear momenta in the outflows are as large as a few hundred M solar km s -1. Both parameters are much larger than the typical values in low-mass outflows. The average dynamic timescale of the outflows is 2×10 4 yr. The driving sources of the bipolar outflows are also identified. All bipolar outflows detected have a near-infrared source, except for IRAS 23385+6053, and all are associated with centimeter or millimeter continuum emission, except for IRAS 22506+5549. We investigated the correlation between the outflow parameters and the properties of the driving source. The outflow luminosity and mechanical force correlate with the bolometric luminosity of the star. However, the mechanical force required to drive a CO outflow is more than an order of magnitude higher than the radiation pressure from the star. We reexamined the relation between the mass entrainment rate of the outflows and the bolometric luminosity of the central source with an up-to-date sample. Results show that the mass outflow rate increases with increasing bolometric luminosity, suggesting that the mass outflow rate is related to the luminosity of the central source.