PhD defense by Paolo Suin: “Impact of high-mass stars’ feedback on the star formation properties”

Date: Friday, September 26, 2025
Time: 14:00 Paris time
Place: Amphitheatre of the Astrophysics Laboratory of Marseille

The defence will be held in English.

Members of the jury:
Peter SCHILKE – University of Cologne (Reporter, President)
Paul CLARK – University of Cardiff (Reporter)
Kate PATTLE – University College London (Examiner)
Doris ARZOUMANIAN – Kyushu University (Invited member)
Patrick HENNEBELLE – Paris Saclay University (Invited member)
Annie ZAVAGNO – Aix-Marseille University (Thesis supervisor)

Abstract:
Understanding how the early radiative feedback of massive stars regulates star formation (SF) within molecular clouds remains one of the most elusive challenges of modern astrophysics. Over the past few decades, interest in pre-supernova feedback has grown: it begins to reshape the natal environment from the birth of the star, rather than only at the end of its life (a few million years later). On the scale of molecular clouds (tens of pc), the ionising radiation of massive stars plays a central role, although its modes of action remain poorly understood. H II regions can reduce local SF by heating and dispersing the gas, or, on the contrary, enhance it by favouring the creation of new SF sites. Observations confirm this two-sided mechanism, but its interpretation remains difficult, due to projection effects, uncertainties on local quantities, or the million-year timescales involved. Numerical simulations, by offering a controlled environment, make it possible to isolate the impact of feedback and to test theoretical predictions against observational signatures.

In this thesis, I carried out high spatial resolution simulations (<0.1 pc) of a massive molecular cloud (10 000 solar masses) in order to study the effects of ionising radiation at different spatial scales (from a few tens to a tenth of a parsec). At the cloud scale, I quantified how radiative feedback modifies the SF law and efficiency. Although it globally reduces SF, H II regions increase its efficiency in the densest areas. By comparing the simulated clouds with observational data, I also showed how instrumental limitations (sensitivity and resolution) can lead to systematic underestimations of these quantities. The analysis also reveals that protostellar jets from low-mass stars are essential to reproduce the observed SF rate.

At smaller scales, I studied how expanding ionised bubbles interact with the filaments that host SF. The initial magnetic field shapes the morphology and dynamics of the cloud. However, after the appearance of H II regions, the bubbles dominate the subsequent evolution: they completely restructure the cloud, regardless of local geometry or magnetic field.

In the final study, I conducted a large set of simulations by varying the position of the ionising source, the intensity of the radiation field, and the initial magnetisation, to determine under which conditions radiative feedback stimulates or reduces SF.

The results of this thesis show that radiative feedback can profoundly transform the evolution of molecular clouds and their ability to form stars. Particular attention was paid to diagnostics directly comparable to observations, paving the way for future studies on the signatures of triggered SF and on the regulation of SF by early radiative feedback.