Bipolar molecular outflows in massive star formation regions

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 (CO)-C-12(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 how. We find that the molecular outflows have masses between similar to 16 and 72 M(.) and kinetic energies between similar to 10(45) and 10(46) 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 (CO)-C-12 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 L(.) to 2.1 x 10(5) L(.) 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 similar to 10(2) to 10(4) L(.), 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 out flow 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 O stars. Our data are consistent with the ideas that (1) M in a molecular outflow increases continuously with L(bol) of the driving source over a range from similar to 1 L(.) to similar to 10(6) L(.), (2) one can predict the central source luminosity from the measured mass flux in the flow, (3) there is a clear M(flow) versus age (t(d)) relationship for stars of all luminosities, and (4) the larger M for higher luminosity sources implies that these objects produce massive outflows on short timescales relative to low-mass stars.