Optically thick winds of very massive stars suppress intermediate-mass black hole formation

Abstract

Intermediate-mass black holes (IMBHs) are the link between stellar-mass and supermassive black holes. Gravitational waves have started unveiling a population of IMBHs in the $\sim 100-300 \, \mathrm{M_{\odot}}$ range. Here, we investigate the formation of IMBHs from non-rotating very massive stars (VMSs, $>100\,{} \mathrm{M_{\odot}}$). We calculate new VMS models that account for the transition from optically thin to optically thick winds, and study how this enhanced mass loss affects IMBH formation and the black hole mass function at intermediate and high metallicity ($Z=10^{-4}-0.02$). We show that optically thick winds suppress the formation of IMBHs from direct VMS collapse at metallicities $Z>0.001$, one order of magnitude lower than predicted by previous models. Our models indicate that the stellar progenitors of GW231123 must have had a metallicity $Z<0.002$, if the primary black hole formed via direct VMS collapse.

Figures (7)

• Figure 1: Masses of the He (circles) and CO cores (diamonds) for the wind model from Sabhahit2023 (full markers) and Chen2015 (void markers). The dark (light) gray shaded areas represent the regime for (pulsational) pair-instability supernovae.
• Figure 2: Final BH mass as a function of $M_{\mathrm{ZAMS}}$ for different metallicities $Z$, for the Sabhahit2023 (upper) and Chen2015 (lower) wind mass-loss prescription. The grey shaded area shows the resulting pair-instability mass gap for the two wind models.
• Figure 3: Contour plot showing the final BH mass as a function of $M_{\mathrm{ZAMS}}$ and $Z$, for the Sabhahit2023 (upper panel) and the Chen2015 (lower panel) models. The white contours highlight the levels at $20 \, {\rm M}_\odot$ and $100 \, {\rm M}_\odot$.
• Figure 4: BH mass distribution from single stars with $M_{\mathrm{ZAMS}} > 50 \,{}{\rm M}_\odot$, for the models adopting the Sabhahit2023 (upper panel) and Chen2015 (lower panel) wind prescriptions and for different metallicities.
• Figure 5: BH mass distribution from binaries with $M_{\mathrm{ZAMS,1}} > 50 {\rm M}_\odot$, for models adopting the Sabhahit2023 (blue, solid line, solid fill) and Chen2015 (orange, dashed line, hatched area) wind prescriptions. The shaded (hatched) areas indicate the BHs in BBHs, while the empty histograms display the single BHs from disrupted binaries, including BHs that form from stellar collisions.