Very massive stars and Nitrogen-emitting galaxies

TL;DR

This paper introduces a new framework for modeling very massive stars (VMSs) with physics-based wind mechanisms, explaining their role in nitrogen enrichment of high-redshift galaxies and proposing their inclusion in population synthesis models.

Contribution

The authors develop a novel VMS evolution model that accounts for physics-driven wind physics, challenging previous recipes and explaining nitrogen enrichment in galaxies.

Findings

VMSs can produce significant nitrogen in star-forming galaxies.

New models show VMSs evolve vertically in the HR diagram due to wind effects.

VMSs are the most plausible source of nitrogen enrichment compared to other hypotheses.

Abstract

Recent studies of high-redshift galaxies using JWST, such as GN-z11 revealed highly elevated levels of nitrogen (N). This phenomenon extends to gravitationally-lensed galaxies like the Sunburst Arc at z = 2.37, as well as to globular clusters (GCs). We propose that this originates from the presence of very massive stars (VMSs) with masses ranging from 100 to 1000\,\Msun. The He {\sc ii} observed in the Sunburst Arc could also stem from the disproportionately large contribution of VMSs. We build an entirely new Framework for massive star evolution which is no longer set by Dutch or other mass-loss "recipes" but which take the physics of $\Gamma$ or $L/M$-dependent winds into account. We discuss the mass-loss kink and the transition mass-loss rate between optically thin and thick winds, before we study the evaporative mass-loss history of VMSs. Our novel evolution models exhibit vertical evolution in the HR-diagram from the zero-age main sequence due to a self-regulatory effect driven by their wind-dominated nature, and we discuss what wind physics sets the stellar upper-mass limit. Our estimate for the Sunburst Arc in Vink (2023) suggests that the significant amounts of N found in star-forming galaxies likely arise from VMSs. We evaluate the strengths and weaknesses of previous hypotheses, including fast rotating massive stars and supermassive stars (SMSs), and we conclude that only our VMS model satisfies the relevant criteria. Finally, we advocate for the inclusion of VMSs in population synthesis and chemical evolution models, emphasizing the need for a self-consistent wind approach, which currently does not exist. Even minor inaccuracies in mass-loss rates dramatically impact the stellar evolution of VMS, as well as their ionizing and chemical feedback.