Star formation in extreme environments

Mullins, Aonghus
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The study of star formation bridges many vastly disparate scales. From the small, nearby, quiescent cores of low mass star formation, to the highly turbulent conditions in which the most massive stars form, to the extreme gravitational potential of the inner Galaxy, where clouds pass close to the supermassive black-hole Sgr A∗. This thesis addresses some of the more extreme conditions seen at each of these scales. Low mass cores are far more common than their massive counterparts, and so far more are observed nearby. The molecular line data from these nearby sources is of extremely high spectral resolution. In order for accurate modelling to take place, the molecular physics needs to understood to the finest detail. In this work we have shown that for HCN, an anomaly in the observed relative line-strengths of the hyperfine lines in the J=(1→0) transition emerges naturally when the radiative transfer is carried out over each hyperfine line separately, using a proportional method to calculate scaled collisional rate coefficients for these transitions. A parameter sweep is carried out over a range of parameters typical of low mass star forming regions, and the J=(1→0) line is found to be highly unstable to the optical depth of the cloud, thus making it a poor choice of tracer for dynamical processes. Model fits are also presented for the prototypical low mass core TMC-1, in which these anomalies were first noted in the early 80’s. High mass star formation is less simple than its low mass counterpart. High mass star forming cores accrete enough matter to ignite nuclear fusion before they are finished accreting. The feedback from the protostar is a source of turbulent energy to its surroundings, and the absorption and reemission by dust grains of their photon flux, leads them to shine brightly in the infrared. The ioinising flux from the newly formed massive star leads to the formation of an expanding shock front, surrounding a region of highly ionised hot gas known as a H ii region. These shocks can compress gas in the surrounding interstellar medium, leading to second generation “triggered" star formation. Radiative transfer modelling of the swept up gas surrounding these regions can place limits on the expansion velocity thorough analysis of the shape of spectral line emission. For the bubble H ii region RCW120, an upper limit of 1 km s−1 is found. A model for the object RCW36 is then, through fitting of the HCO+ (1→0) line, shown to be consistent with triggered star formation in the gas surrounding a H ii region. Finally, the clouds near the Galactic centre are subject to some of the most extreme conditions in the Galaxy. The tidal gravitational field can disrupt them, the intense local star formation bombards them with ionising radiation, and the turbulent feedback from the high supernova rate leads to clouds which are far removed from the quiescent cores of low mass star formation. In this work we study one such cloud, G0.253+0.016, “the Brick", which is unusual in its apparent lack of star formation. We model the observed molecular lines for the dense gas tracers HCN, HNC, HCO+ and N2H+, finding a good fit to the observed data for a model in which the Brick is treated as a recent cloud-cloud collision. The observed position of a water maser [an early indicator of massive star formation] consistent with the cloud overlap region of our model strengthens our claim, while widespread SiO [a shock tracer] emission from recent surveys further supports this hypothesis. A comparison is then made to previous modelling work on another galactic centre cloud, Sgr B2, which is rich in ongoing star formation. The Brick is found to be much colder and more chemically depleted than Sgr B2, indicative of widespread freeze-out of molecular ices onto interstellar grains. The turbulent velocities of the two clouds are found to be comparable, which would suggest that similar support against gravitational collapse exists in each. The precise mechanism by which the Brick is currently resisting star-formation remains an open question. How does 105 M⊙ of gas accumulate without any star formation? Our model for cloud-cloud collision modelling alleviates that requirement, by having two smaller turbulently supported clouds. Overall, this thesis shows that over a large range of scales, the radiative transfer modelling of molecular line data can be used to investigate a number of extreme cases, in chemistry, turbulence, and density. From the thin, highly resolved lines of low mass star formation in the Solar neighbourhood, to the broad, velocity shaped lines of high mass star formation, and the Galactic Centre.
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Attribution-NonCommercial-NoDerivs 3.0 Ireland