Pulsation-driven mass loss from massive stars behind stellar mergers in metal-poor dense clusters

TL;DR

This study investigates how pulsation-driven mass loss affects the formation of massive black holes from stars in dense, metal-poor clusters, suggesting that such stars can still produce supermassive black hole seeds despite mass loss.

Contribution

It provides the first detailed analysis of pulsation-driven mass loss in massive stars with high initial masses and low metallicity, assessing its impact on black hole formation.

Findings

Pulsational instability grows rapidly in massive stars, influencing mass loss.

Mass loss rates increase with stellar mass and metallicity, but remain below critical thresholds.

Massive stars can still form black holes over 200 solar masses despite pulsation-driven winds.

Abstract

The recent discovery of high-redshift (z > 6) supermassive black holes (SMBH) favors the formation of massive seed BHs in protogalaxies. One possible scenario is formation of massive stars ~ 1e3-1e4 Msun via runaway stellar collisions in a dense cluster, leaving behind massive BHs without significant mass loss. We study the pulsational instability of massive stars with the zero-age main-sequence (ZAMS) mass Mzams/Msun = 300-3000 and metallicity Z/Zsun = 0-0.1, and discuss whether or not pulsation-driven mass loss prevents massive BH formation. In the MS phase, the pulsational instability excited by the epsilon-mechanism grows in ~ 1e3 yrs. As the stellar mass and metallicity increase, the mass-loss rate increases to < 1e-3 Msun/yr. In the red super-giant (RSG) phase, the instability is excited by the kappa-mechanism operating in the hydrogen ionization zone and grows more rapidly in ~ 10 yrs. The RSG mass-loss rate is almost independent of metallicity and distributes in the range of ~ 1e-3-1e-2 Msun/yr. Conducting the stellar structure calculations including feedback due to pulsation-driven winds, we find that the stellar models of Mzams/Msun = 300-3000 can leave behind remnant BHs more massive than ~ 200-1200 Msun. We conclude that massive merger products can seed monster SMBHs observed at z > 6.