The initial conditions and early evolution of star-forming clouds

Type Thesis
Names Audra Kathleen Hernandez
Date 2011
Library Catalog
Abstract The formation of star clusters from giant molecular clouds (GMCs) is of fundamental importance for the evolution of the Milky Way and other galaxies and yet there are still many open questions. Why do certain localized regions of GMCs form star clusters, while the rest of the cloud shows little star formation activity? Is this process the result of spontaneous instabilities in the cloud, perhaps regulated by turbulence or magnetic fields, or triggered by external agents? Does the star cluster formation process take few or many dynamical times? What are the processes that control GMC formation and evolution? I have investigated the physical properties of molecular clouds in order to address these questions. First, using 13CO(1-0) molecular line emission data from the Galactic Ring Survey (GRS), I studied the dynamical state of two filamentary infrared dark clouds (IRDCs), which are expected to be representative of the initial conditions for star cluster formation. I compared mass surface densities, Sigma13CO, with those derived from small medium filter (SMF) mid-infrared (MIR) dust absorption, Sigma SMF, finding systematic trends that could be due to CO depletion from the gas phase due to freeze out onto dust grains or decreased excitation temperatures in the highest density regions. Furthermore, one of the filaments was observed at higher angular resolution with the IRAM 30m telescope in multiple molecular species. I analyzed J=2-1 and 1-0 lines of 13CO and C 18O. The latter yielded an excitation temperature and mass surface density map, and by comparison with SigmaSMF, a depletion map of the cloud. With a significance of 10sigma, I confirmed that CO depletion is widespread in the cloud. An estimated several hundred solar masses are being affected, making this one of the most massive clouds in which CO depletion has been observed directly. Additionally, through ellipsoidal and filamentary virial analyses, I found that the filament is in a state consistent with some models of virial equilibrium, although surface pressure terms are still quite important. There is tentative evidence that the regions of the filament with the most star formation activity are more likely to be in virial equilibrium. Secondly, turning to the larger scales of GMC environments around IRDCs, I first studied a small sample of nine relatively nearby IRDCs and their associated GMCs using GRS 13CO emission data. I measured GMC mass, position angle of the rotation axis projected on the plane of the sky, and the virial parameter, alpha. I investigated how these results depended on the scale used to define the GMC and for different methods of defining the GMC boundary. I found masses ˜ 105 M⊙ , a broad range of projected rotation axis position angles and alpha ˜ 10, suggesting quite disturbed kinematics of the molecular gas on these scales. No systematic trends were found between alpha with size scale or with the method used to identify the GMC. Next, I performed a similar study for much larger samples of GMCs. No significant differences were found in the GMC properties of these samples. Both show quite large values of virial parameter, ˜ 1--100, again suggesting that the clouds have quite disturbed kinematics. Both samples show a broad range of projected rotation axis position angles, including about half with rotation in a direction retrograde compared to Galactic rotation. This may have major implications for the formation and evolution of GMCs, perhaps indicating that strong interactions and mergers between clouds occur quite frequently. Finally, I performed a pilot study with data on 16 clumps from the Census of High and Medium-mass Protostars (CHaMP) to understand the chemical evolution between quiescent and active star-forming systems, as measured by their bolometric luminosity to mass ratio. Using ˜ 90 and ˜ 115GHz molecular line data I searched for trends in the abundances of HCO+ and N 2H+, sensitive to carbon and nitrogen chemistry, respectively. The clumps show a large dispersion in relative abundances, especially HCO + and N2H+.
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