A Study of the Impact of Triggered Star Formation

This thesis presents a statistical study of the impact of triggered star formation at the boundary of HII regions. HII regions are bubbles of ionised gas created by high energy radiation feedback from a host massive star. HII regions expand into their surrounding molecular gas due to the extreme temperature difference between the hot ionised gas of the HII region and the cool molecular gas of its natal clump. This results in the gas being swept up and compressed by ionising radiation. This compressed gas can then fragment and collapse to form stars. This is known as triggered star formation and is known to occur through mechanisms such as "The collect and collapse method" and "Radiative Driven Implosion". We visually analysed 5410 sources from the ATLASGAL survey, with the help of the GLIMPSE survey, to categorize them into "triggered" and "non-triggered" sources by visually inspecting the source for signs of triggered star formation and using the ATLAS-GAL contours to distinguish between finer details of the sources. We then compared the physical properties of these sources to see if we could find a difference between the sources that we had deemed triggered and those that were not. We also looked for a peak in source counts at the rim of their host bubbles in an attempt to replicate the results from Thompson et al. (2012) and S. Kendrew et al. (2012). Finally we try and set a lower limit of star formation taking place in the inner Galaxy that occurs through a triggering mechanism. We found statistically significant differences between the temperature, luminosity and luminosity to mass ratio when looking between triggered and non-triggered sources and found no significant differences between the radius and mass of the two different source categories. We find a peak in source counts at a distance of 1 bubble radius, this result being in line with the results of Thompson et al. (2012) and S. Kendrew et al. (2012).With this difference in physical properties, it is likely that there is a significant differ-ence in star formation occurring between our triggered and non-triggered sources, likely as a result of their parental HII regions impact on the surrounding molecular gas which points towards triggering.1

Finally with our catalogue providing a reasonable statistical representation of star formation in the inner Galaxy, we set a lower limit of∼12 % for triggered star formation that occurs in the inner Galaxy, once contaminating sources are excluded.