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Thesis



The disc accretion process is an essential component of star and planet formation theories. This process balances the inflow of material against the outward spread of angular momentum, facilitating mass gain from a circumstellar disc which determines the main-sequence position of the central star and its planetary system configuration. Much of the detailed knowledge of the physical processes involved in disc accretion have been gained through observations of nearby (< 1 kpc) pre-main sequence (PMS) stars, in low mass star-forming regions (SFR). Stars formed in such regions are not representative of the massive, dense, and metal-poor environments in which the majority of stars in the Galaxy, and in previous epochs have formed. In particular, ionising radiation from nearby massive stars, and the metallicity (Z) of the local environment are thought to affect PMS disc accretion rates, and disc evolutionary timescales. Therefore, understanding the effect of the surrounding environment on the disc accretion process in PMS stars is necessary to develop a global picture of star formation.

In this thesis, I present new ultraviolet/optical/infrared photometric and spectroscopic observations of pre-main sequence stars that have formed in either metal-poor conditions, or in the vicinity of strong ionising radiation. This includes observations of 235 Classical T Tauri stars in the Lagoon Nebula; 63 Classical T Tauri/Herbig Ae stars in the Carina Nebula open cluster Trumpler 14; 24 intermediate mass T Tauri stars in the low-Z Sh 2-284 SFR; and one Herbig B[e] PMS candidate in the metal-poor 30 Doradus SFR. I measure the accretion rates of these PMS stars using the intensities of the U/Hα band excess measured through either optical spectra or imaging. Where possible, I use archive infrared photometry in the 1.2-8 micron wavelength range to measure the PMS disc evolutionary stage. The influence of the surrounding environment on the accretion rate evolution of pre-main sequence stars in these regions is explored using the spatial, and temporal distributions of accretion rate, mass, age and disc stage of PMS stars. In wide-field photometric data of the Lagoon Nebula, I find that the spatial distributions of PMS stars is a continuum, ranging from dense clustering to relative isolation. Strongly accreting PMS stars are generally clumped together, in close proximity to their natal molecular cloud, whereas weaker, older accretors are relatively spaced apart. Ionising radiation from early-type stars appears to positively affect accretion rates on scales of 2--3 pc, but no evidence for triggered star formation is found. In addition, I find that the accretion rates measured from Hα imaging are well correlated to the accretion rates estimated using U-band photometry. In wide-field photometric data of Trumpler 14, I discover a population of PMS candidates using UV/optical/near-infrared photometry nearly 25 ,Myrs old. I argue that these PMS candidate stars are a foreground PMS population, approximately 5 Myrs old that belong to the Carina Nebula cluster Trumpler 16. Using Hαspectra of 24 intermediate T Tauri stars in Sh 2-284 (where Z = 0.004), I demonstrate that there is no evidence for a systematic change in accretion rates with metallicity, contrary to previous literature results at Z = 0.006--0.002 in the Magellanic clouds. I suggest that previous studies are affected by detection limits and biases. I also present ultraviolet/optical spectra of the Herbig B[e] PMS candidate VFTS 822 located in the 30 Doradus SFR of the Large Magellanic Cloud. I discuss the impact of the discovery of VFTS 822 for star formation studies in the Magellanic Clouds, and external Galaxies. Finally, I suggest that the median accretion rate in a given SFR may correlate with its total mass, although diligent work ruling out the various sources of biases must be thoroughly explored before a relationship can be established.



The whole thesis is available here