Bipolar Molecular Outflows in Massive Star Formation Regions

Abstract

Bipolar molecular outflows are a basic component of the star formation process. This is true for stars of all masses, although it has not yet been well established how outflows associated with massive stars differ from those associated with low-mass stars. We present results from a project to identify bipolar outflows from massive young stellar objects (YSOs) and determine how they compare with low-mass YSO systems. Ten massive star formation regions with high-velocity 12CO(J = 1-0) line wings were mapped with the Kitt Peak 12 m telescope using the On-the-Fly (OTF) mapping technique. Five of the regions have bipolar outflows. We determine accurate mass estimates of the molecular gas in the red- and blueshifted lobes by taking into account variations in the optical depth as a function of velocity in the flow. We find that the molecular outflows have masses between ˜16 and 72 Msun and kinetic energies between ˜1045 and 1046 ergs. The outflows have significantly more mass and kinetic energy than those from low-mass YSOs. Of the remaining five regions, two have a clumpy distribution in 12CO with multiple velocity components within the cloud complex, and three sources did not have sufficient signal-to- noise ratio (S/N) to map the high-velocity line wings. We combine our data with 18 additional outflow sources with stellar luminosities that range from 0.6 Lsun to 2.1 x 105 Lsun to predict the luminosity of the star responsible for the outflow. We find that the stellar luminosities of the sources in our sample range from ˜102 to 104 Lsun, which correspond to mid- to early-B type stars; the precursors of Herbig Be stars. One source, G173.58, has an IRAS source on the flow axis with the appropriate luminosity to drive the observed outflow. For the remaining four sources, there is no detectable ultracompact (UC) H II region or isolated IRAS source with the appropriate luminosity to produce the observed molecular outflow. The outflows mapped in this work begin to fill in a region of outflow parameter space in which relatively few sources have been studied, and they help to bridge the gap between low-luminosity outflow sources and the few isolated outflows from massive 0 stars. Our data are consistent with the ideas that (1) Mdot in a molecular outflow increases continuously with Lbol of the driving source over a range from ˜1 Lsun to ˜106 Lsun, (2) one can predict the central source luminosity from the measured mass flux in the flow (3) there is a clear Mflow versus age (td) relationship for stars of all luminosities, and (4) the larger Mdot for higher luminosity sources implies that these objects produce massive outflows on short timescales relative to low-mass stars.