The Interplay of Star Formation and Radiation Feedback in the Interstellar Medium
Creators: name: Harimohan Menon, Shyam
Publisher: name: The Australian National University
Year: 2023

description: The radiation emitted by massive stars, and the energy and momentum it deposits via interaction with gas and dust in the interstellar medium (ISM), plays a crucial role in star formation . Observational efforts, enabled by recent instrumental facilities, have highlighted the pivotal role of radiative feedback in regulating the star formation efficiency (SFE) in giant molecular clouds (GMCs) and in controlling their dynamical evolution. Significant theoretical progress on these fronts have also been made in recent years spurred by advances in computing capability and numerical techniques to model the coupled evolution of gas and radiation -- i.e. radiation hydrodynamics (RHD). However, the nature of star formation and feedback in GMCs forming in regions of high ISM pressure, where so-called super star clusters (SSCs) are born, are largely unconstrained. This is primarily because existing RHD methods struggle at the high optical depths achieved in these GMCs. On more local scales, the dynamical effects of radiation feedback on the turbulent ISM play a direct role on the formation and evolution of observed structures in HII regions such as pillars and globules; yet, this complex interaction is not well understood. In this thesis, I approach these unexplored directions with numerical RHD simulations and subsequent comparison with observations. I develop VETTAM, a state-of-the-art RHD scheme using the Variable Eddington Tensor closure, which for the first time enables accurate and feasible modelling of radiation feedback even in regions of high optical depth. VETTAM has been implemented to model all the important pathways of radiative feedback from massive stars and has been coupled to the widely-used FLASH magnetohydrodynamics code. I use VETTAM to quantify the competition between gravitational collapse and radiation feedback in SSC-forming GMCs, finding that radiation feedback is unable to limit the SFE and disrupt GMCs -- unlike the case of typical Milky Way-like clouds explored in previous works. On achieving a high SFE, radiation pressure on dust launches parsec-scale outflows with velocities ~ 15 - 50 km/s -- a potential explanation for recently observed molecular outflows emanating from star clusters in starburst galaxies. I also quantify the relative importance of feedback mechanisms in this regime and their momentum contribution to the ISM on larger scales, and demonstrate the importance of using appropriate temperature-dependent IR dust opacities. These findings lay the foundation for understanding the nature of star formation and the dynamical state of the ISM in high-pressure environments. On local scales -- using idealised RHD simulations of feedback interacting with a turbulent ISM -- I find that photoionisation can drive turbulence in the neutral gas around HII regions. Moreover, I find this turbulence is dominated by a larger fraction of compressive modes than is typically encountered in the ISM, driven by the overpressurised ionised gas surrounding the pillars. I test this hypothesis on high-resolution observations of pillars in the Carina Nebula, finding that the density and velocity structures in these pillars are consistent with a scenario of compressive turbulence. This could potentially lead to elevated star formation rates in the pillars -- implying a possible pathway for triggered star formation. Several aspects of my work can be tested with the recently launched James Webb Space Telescope (JWST), which can identify and characterize feedback in massive star clusters in a wide range of galactic environments. Moreover, my results will act to inform physically-motivated sub-grid models for pre-supernova feedback in cosmological/galaxy-scale simulations that are unable to resolve the competition between feedback and gravitational collapse occurring within GMCs. These efforts would deepen our understanding of the multiscale, feedback-regulated picture of star formation and galaxy evolution.