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A COMPARISON OF STAR FORMATION WITHIN THE GALACTIC CENTRE AND GALACTIC DISC

Barnes, AT (2018) A COMPARISON OF STAR FORMATION WITHIN THE GALACTIC CENTRE AND GALACTIC DISC. Doctoral thesis, Liverpool John Moores University.

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Abstract

Stars are of fundamental importance to the entire field of astronomy. The conversion of elements and the distribution of energy throughout the lifetime of stars drives the evolution of the Universe. Despite this, we do not have a unified understanding of the formation process for all stars. This thesis attempts to move forward this understanding, by focussing on the question: How do the initial conditions of star-forming regions vary across environments, and do these influence the process of star formation? To investigate the initial conditions of star formation, regions on the verge of forming stars have to be first identified and analysed. These regions have to be untouched by the disruptive effects of stellar feedback, such that the natal conditions of the gas – e.g. kinematics and chemistry – are not destroyed. Quiescent regions that are expected to form low-mass stars have been well studied over the past few decades, and the general process of low-mass star formation is well understood. Only relatively recently, however, has a group of objects being identified as being potential hosts of these initial stages of high-mass star formation: Infrared Dark Clouds (IRDCs). The study of these objects is difficult, due to both their rarity and complexity. An end-to-end understanding of high-mass star formation is, therefore, much less developed compared to their lower mass counterparts. This thesis presents the study of a sample of IRDCs within the Disc and Centre of the Milky Way; two very different environments. Several key aspects of the star formation process within IRDCs from these environments are investigated. Firstly, a chemical signpost – the deuterium fraction of N2H+ – is used to identify the regions of dense and cold gas on the verge of forming high-mass stars within a quiescent Disc IRDC, which can be used to study the initial conditions for star formation. Omitting potential beam dilution effects, chemical modelling suggests that the cloud could have reached a global chemical equilibrium, and, if so, would also be dynamically old (survived for several free-fall times). This timescale, with estimates of the embedded stellar mass, is used to determine star formation rates and efficiencies. Secondly, the kinematic structures within two apparently similar Disc IRDCs are identified using dense gas tracers – C18O and N2H+. The properties of these structures appear to be very similar, hinting at a similar formation scenario for both clouds, or, potentially, that these may be inherent to the larger Disc IRDC population. The dynamics of these filaments also show that they may be merging, which would suggest a compressive mode of turbulence driving. These structures are then linked to the larger kinematic structures – identified using a lower density tracing molecule, 13CO – and found to show good coherence with the brightest, most extended structures. These are then placed in the context of the previously identified Galactic scale structures, and in doing so show that IRDCs could be the densest parts of the much larger arm or inter-arm filamentary structures. Thirdly, the level of star formation within the Galactic Centre is investigated on both global (∼ 100 pc) and local (∼ 1 pc) scales. On a global scale, the star formation rate has been determined from all the available observational star formation diagnostics – i.e. direct counting of young stellar objects and integrated light measurements – and found to be in agreement with previous studies; i.e. around one-to-two orders of magnitude lower than predicted by the star formation models. On individual cloud scales, the star formation efficiency per free-fall time is in better agreement with the model predictions. However, uncertainties on the properties of these regions, such as the mode of turbulence driving, limit the further verification or falsification of the star formation theories. Lastly, the investigation of the local scale star formation within the Galactic Centre highlighted a particular part of the parameter space as the most promising to further test the star formation theories. In light of this, high-spatial resolution ALMA observations have been taken of two Galactic Centre clouds within this regime. Early results show that they have a complex structure, similar to that seen within Disc IRDCs, containing both filamentary and core-like features. Investigation of the brightest, most compact core region shows that it contains a very rich chemistry, and, of particular interest, is the rigorous detection of the pre-biotic molecule formamide (NH2CHO). When placing the results of this thesis in the bigger context of star formation theory, they appear to show interesting implications for the initially posed question – what is the influence of environment on the process of star formation? It is found here that despite the very different cloud scale properties of these regions, the star formation efficiency per free-fall time is surprisingly similar. To investigate this, the properties of the individual sites of high-mass star formation, the high-mass star-forming cores, are compared. Interestingly, despite the different environmental conditions, several key properties of the cores, such as their size and mass distribution, are also found to be very similar. The similarity of high-mass core properties and star formation rate per free fall time implies that once a region has produced high-mass cores, the evolution of these cores towards star formation must be similar. The difference in the global/environmental properties of the gas must then be setting the total star formation rate within these regions, by limiting the number of cores that can form. In particular, the mode of turbulence driving may play a major role in governing the fraction of gas that can be converted into stars per free-fall time within these two environments.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: star formation; molecular clouds; Milkyway; Galactic Centre
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Date Deposited: 10 May 2018 10:47
Last Modified: 10 May 2018 10:47
DOI or Identification number: 10.24377/LJMU.t.00008633
Supervisors: Longmore, S
URI: http://researchonline.ljmu.ac.uk/id/eprint/8633

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