The impact of stellar feedback on the host star-forming clouds

Stellar feedback from high-mass stars shapes the interstellar medium, and thereby impacts gas that will form future generations of stars. However, due to our inability to track the time evolution of individual molecular clouds, quantifying the exact role of stellar feedback on their star formation history is an observationally challenging task. Therefore, in the first part of this thesis, I perform a detailed study of the impact of feedback on a single high-mass star forming region. For this purpose, I take advantage of the unique properties of the G316.75-00.00 massive-star forming ridge to determine how stellar feedback from O-stars impacts the dynamical stability of massive filaments. The G316.75 ridge is 13.6 pc in length and contains 18,900 solar masses of H₂ gas, half of which is infrared dark and half of which infrared bright. The infrared bright part has already formed four O-type stars over the past 2 Myr, while the infrared dark part remains quiescent. Therefore, by assuming the star forming properties of the infrared dark part represent the earlier evolutionary stage of the infrared bright part, I can quantify how feedback impacts these properties by contrasting the two. I used publicly available Herschel/Hi-GAL and molecular line data to measure the ratio of kinetic to gravitational energy per-unit-length, α-line, across the ridge. By using both dense (i.e. N₂H⁺ and NH₃) and more diffuse (i.e. ¹³CO) gas tracers, I was able to compute α-line over a range of gas volume densities (~1×10²–1×10⁵ cm⁻³). This study shows that despite the presence of four embedded O-stars, the ridge remains gravitationally bound (i.e. α-line > 2) nearly everywhere, except for some small gas pockets near the high-mass stars. In fact, α-line is almost indistinguishable for both parts of the ridge. These results are at odds with most hydrodynamical simulations in which O-star-forming clouds are completely dispersed by stellar feedback within a few cloud free-fall times. From simple theoretical calculations, I show that such feedback inefficiency is expected in the case of high-gas-density filamentary clouds. I conclude that the discrepancy between numerical simulations and the observations presented here originates from different cloud morphologies and average densities at the time when the first O-stars form. In the case of G316.75, I speculate that the ridge could arise from the aftermath of a cloud-cloud collision, and that such filamentary configuration promotes the inefficiency of stellar feedback. This does very little to the dense gas already present, but potentially prevents further gas accretion onto the ridge. These results have important implications regarding, for instance, how stellar feedback is implemented in cosmological and galaxy scale simulations. To place G316.75 in the larger context of the rest of the galaxy, I investigate how expanding HII regions may affect the star formation efficiencies (SFEs) of molecular clouds. For this purpose, I build a catalogue of dense molecular clouds from the Galactic plane using the Herschel Hi-GAL data, determine the number and luminosity of embedded young protostellar objects as a proxy for their SFE, and characterise their association with known HII regions and HII region candidates. Finally, I determine how infrared dark each cloud is and use this as a tracer of their evolutionary stage. I define three categories of clouds: i. clouds that are not associated with any HII regions; ii. clouds that have an embedded HII region; ii. clouds that lie within the boundary layer of an HII region. When comparing boundary layer clouds taken at a similar evolutionary stage with HII and non- HII region related clouds, I find that the boundary layer clouds exhibit SFEs that are more similar to clouds with embedded HII regions than those without. This suggests that boundary layer clouds must have been already present before finding themselves in the proximity of an HII region. Moreover, I find there is little difference in SFEs between boundary layer clouds and clouds with embedded HII regions at all evolutionary stages. The implication, along with the fact that only 20% of clouds are presently being influenced by HII regions, is that star formation triggered by expanding HII regions is not a major contributor to the Milky Way star formation rate as a whole