Very Massive Stars: a metallicity-dependent upper-mass limit, slow winds, and the self-enrichment of Globular Clusters

TL;DR

This paper investigates how metallicity influences the upper mass limit of stars, emphasizing the role of radiation-driven winds in mass loss, and explores implications for stellar evolution, supernovae, and black hole formation.

Contribution

It provides new predictions for mass-loss rates and wind velocities of very massive stars across different metallicities, highlighting the Z-dependent nature of the stellar upper-mass limit.

Findings

Wind terminal velocities are low (100-500 km/s) across a wide metallicity range.

Mass-loss rates can exceed 0.001 Msun/yr during star formation.

The upper-mass limit is effectively dependent on metallicity due to radiation-driven winds.

Abstract

One of the key questions in Astrophysics concerns the issue of whether there exists an upper-mass limit to stars, and if so, what physical mechanism sets this limit, which might also determine if the upper-mass limit is metallicity (Z) dependent. We argue that mass loss by radiation-driven winds mediated by line opacity is one of the prime candidates setting the upper-mass limit. We present mass-loss predictions (dM/dt_wind) from Monte Carlo radiative transfer models for relatively cool (Teff = 15kK) inflated very massive stars (VMS) with large Eddington Gamma factors in the mass range 100-1000 Msun as a function of metallicity down to 1/100 Z/Zsun. We employ a hydrodynamic version of our Monte Carlo method, allowing us to predict the rate of mass loss (dM/dt_wind) and the terminal wind velocity (vinf) simultaneously. Interestingly, we find wind terminal velocities (vinf) that are low (100-500 km/s) over a wide Z-range, and we propose that the slow winds from VMS are an important source of self-enrichment in globular clusters. We also find mass-loss rates (dM/dt_wind), exceeding the typical mass-accretion rate (dM/dt_accr) of 0.001 Msun/yr during massive-star formation. We express our mass-loss predictions as a function of mass and Z, finding log dM/dt = -9.13 + 2.1 log(M/Msun) + 0.74 log(Z/Zsun) (Msun/yr). Even if stellar winds would not directly halt & reverse mass accretion during star formation, if the most massive stars form by stellar mergers stellar wind mass loss may dominate over the rate at which stellar growth takes place. We therefore argue that the upper-mass limit is effectively Z-dependent due to the nature of radiation-driven winds. This has dramatic consequences for the most luminous supernovae, gamma-ray bursts, and other black hole formation scenarios at different Cosmic epochs.