Black Holes at high and low metallicity

Abstract

At the end of their lives the most massive stars collapse into black holes (BHs). The detection of an 85 $M_{\odot}$ BH from GW 190521 appeared to challenge the upper-mass limit imposed by pair-instability (PI). Using systematic MESA calculations with new mass-loss implementations, we show that 100 $M_{\odot}$ stars at metallicities below 0.1 $Z_{\odot}$ can evolve into blue supergiant progenitors with cores small enough to avoid PI, yet with limited envelope loss, yielding remnants within the second mass gap. The key ingredients involve (i) a proper consideration of internal mixing and (ii) physically motivated stellar winds. Our modelling provides a robust pathway that roughly doubles the maximum BH mass permitted by PI theory and establish a physically-consistent framework to explore the upper BH mass limit versus metallicity. For rapid rotation ($\ge$50\% of critical), the upper BH mass comes down to $\simeq$35 $M_{\odot}$, matching the LIGO/Virgo BH mass pile-up.