A Census of Diverse Environments in Star Forming Regions: Where Do Massive Stars Form?

Massive stars have profound effects on the interstellar medium that lead to chemical and dynamical evolution of the gas. This contributes to galaxy evolution and may also trigger new star formation. The physical conditions of massive star forming environments, and thus the formation mechanism, have been historically less well understood than their lower mass counterparts. This work discusses investigations into massive star formation, primarily in two different phases of the evolution of massive star forming regions.

First, we analyzed the environments of H II regions powered by massive stars for evidence of newly triggered star formation. Triggering may be an important mechanism through which massive star formation propagates through a cloud, contributing to the observed clustering of massive stars. We investigated six H II regions with infrared, bright rimmed bubble or cometary morphology, in search of quantitative evidence for triggered star formation, both "collect and collapse" (CnC) and "radiatively driven implosion" (RDI). We identified and classified 458 Young Stellar Objects (YSOs) in and around the H II regions. YSOs were determined by fitting a collection of radiative transfer model spectral energy distributions (SEDs) to infrared photometry for a large sample of point sources. We determined areas where there exist enhanced populations of relatively unevolved YSOs on the bright rims of these regions, suggesting that star formation has been triggered there. We further investigated the physical properties of the regions by using radio continuum emission as a proxy for ionizing flux powering the H II regions, and 13CO J=1-0 observations to measure masses and gravitational stability of molecular clumps. We used an analytical model of CnC triggered star formation, as well as a simulation of RDI, and compare the observed properties of the molecular gas with those predicted in the triggering scenarios. Notably, those regions in our sample with cometary, or "blister," morphology are more likely to show evidence of triggering.

Second, we focused on Infrared Dark Clouds (IRDCs). IRDCs harbor the earliest phases of massive star formation, and many of the compact cores in IRDCs, traced by millimeter continuum or by molecular emission in high critical density lines, host massive protostars. We used the Robert C. Byrd Green Bank Telescope (GBT) and the Very Large Array (VLA) to map NH3 and CCS in nine IRDCs to reveal the temperature, density, and velocity structures and explore chemical evolution in the dense (>1e22/cm^2) gas. Ammonia is an ideal molecular tracer for these cold, dense environments. The internal structure and kinematics of the IRDCs include velocity gradients, filaments, and possibly colliding sub-clouds that elucidate the formation process of these structures and their protostars. We find a wide variety of substructure including filaments and globules at distinct velocities, sometimes overlapping at sites of ongoing star formation. It appears that these IRDCs are still being assembled from molecular gas clumps even as star formation has already begun, and at least three of the IRDCs in our sample appear consistent with morphology of the "hub-filament structure" discussed in the literature. Furthermore, we find that these clumps are typically near equipartition between gravitational and kinetic energies, so these structures may survive for multiple free-fall times. We also have Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations of dense gas tracers in a large IRDC with diverse physical conditions and star formation content. These data, preliminary analysis, and plans for future work are presented here.