Internal Physical and Chemical Characteristics of Starless Cores on the Brink of Gravitational Collapse

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dc.contributor.authorChitsazzadeh, Shadi
dc.contributor.supervisorDi Francesco, James
dc.contributor.supervisorEllison, Sara L.
dc.date.accessioned2014-08-25T22:51:58Z
dc.date.available2015-08-09T11:22:06Z
dc.date.copyright2014en_US
dc.date.issued2014-08-25
dc.degree.departmentDepartment of Physics and Astronomy
dc.degree.levelDoctor of Philosophy Ph.D.en_US
dc.description.abstractUsing various molecular line and continuum emission criteria, we have selected a sample of six isolated, dense concentrations of molecular gas, i.e., “cores”, which are either starless (L694-2, L429, L1517B, and L1689-SMM16) or contain a protostellar Very Low Luminosity Object (VeLLO) and are currently experiencing gravitational collapse (L1014 and L1521F). Studying the molecular emission from dense gas tracers toward this sample of cores will help us gain a more detailed image of the internal physical conditions of dense cores and their evolution. We observed the cores in our sample in NH3 (1,1) and (2,2) emission using the Green Bank Telescope (GBT) and in N2H+ (1−0) emission using the Nobeyama Radio Observatory (NRO). L429 shows the most complicated structure among the cores in our sample. Also, the maxima of molecular line integrated intensities and dust continuum emission toward L429 show a significant offset. The rest of the cores in our sample are roughly round and the morphologies of line integrated intensities follow that of the corresponding continuum emission closely. Cores in our sample have gas kinetic temperatures ∼ 9 − 10 K and therefore show comparable thermal velocity dispersions. L429 and L1517B are, respectively, the most turbulent and most quiescent cores in our sample. Finally, L1521F is the most centrally concentrated core of our sample. L1689-SMM16 is the least previously studied core in our sample and had not yet been probed in molecular emission. Jeans and virial analyses made using updated measurements of core mass and size confirm that L1689-SMM16 is prestellar, i.e., gravitationally bound. It also has accumulated more mass compared to its corresponding Jeans mass in the absence of magnetic fields and therefore is a “super-Jeans” core. The high levels of X(NH3)/X(N2H+) and deuterium fractionation reinforce the idea that the core has not yet formed a protostar. Comparing the physical parameters of the core with those of a Bonnor-Ebert sphere reveals the advanced evolutionary stage of L1689-SMM16 and shows that it might be unstable to collapse. We do not detect any evidence of infall motions toward the core, however. Instead, red asymmetry in the line profiles of HCN (1−0) and HNC (1−0) indicates expansion of the outer layers of the core at a speed of ∼ 0.2 − 0.3 km s−1. For a gravitationally bound core, expansion in the outer layers might indicate that L1689-SMM16 is experiencing oscillations. Radiative transfer modelling of NH3 emission toward L694-2 and L1521F at low and high spatial resolutions show that the less evolved core, L694-2, is best described by relatively constant radial profiles of temperature and fractional NH3 abundance. On the other hand, L1521F, which contains a protostellar VeLLO, is best described by a radial abundance profile that is enhanced toward the core centre and a radial temperature profile that decreases toward the core centre. Comparison of our results with previous studies on L1544, a well-studied starless core, imply that as dense cores evolve and progress toward the moment of collapse, they become more centrally concentrated. As a result, the gas temperatures at their centres decrease, leading to increase in levels of CO depletion factor and increase in NH3 fractional abundance toward the centre.en_US
dc.description.scholarlevelGraduateen_US
dc.identifier.bibliographicCitationChitsazzadeh et al. 2014, ApJ, 790, 129en_US
dc.identifier.urihttp://hdl.handle.net/1828/5600
dc.language.isoenen_US
dc.rights.tempAvailable to the World Wide Weben_US
dc.subjectastronomyen_US
dc.subjectastrophysicsen_US
dc.subjectstar formationen_US
dc.subjectkinematicsen_US
dc.subjectdynamicsen_US
dc.subjectmoleculesen_US
dc.subjectstructureen_US
dc.subjectradio linesen_US
dc.subjectinterstellar mediumen_US
dc.titleInternal Physical and Chemical Characteristics of Starless Cores on the Brink of Gravitational Collapseen_US
dc.typeThesisen_US

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