Formation of Sub-Chandrasekhar Mass Black Holes and Red Stragglers via Hawking Stars in Ultra-Faint Dwarf Galaxies

TL;DR

This paper models how primordial black holes can be captured by stars in ultra-faint dwarf galaxies, leading to the formation of sub-Chandrasekhar mass black holes and red straggler stars, with implications for dark matter and stellar evolution.

Contribution

It introduces stellar evolution models for Hawking stars with 0.5-1.4 M$_$ and explores accretion schemes, showing how asteroid-mass PBHs can produce sub-solar mass black holes within the universe's age.

Findings

Wide range of PBHs can accrete stars within a Hubble time.

Lower metallicity stars accrete faster, favoring early universe formation.

Number of red stragglers matches observations in ultra-faint dwarf galaxies.

Abstract

Presently, primordial black holes (PBHs) in the asteroid-mass window from $10^{-16}$ M$_\odot$ to $10^{-10}$ M$_\odot$ are a popular dark matter candidate. If they exist, some stars would capture them upon formation, and they would slowly accrete the star over gigayears. Such Hawking stars -- stars with a central PBH -- provide a novel channel for the formation of both sub-Chandrasekhar mass black holes as well as red straggler stars. Here we report on stellar evolution models that extend our previous work to Hawking stars with masses between 0.5 and 1.4 M$_\odot$. We explore three accretion schemes, and find that a wide range of PBHs in the asteroid-mass window can robustly accrete stars as small as 1 M$_\odot$ within the age of the Universe. This mechanism of producing sub-solar mass black holes is highly dependent on the assumed accretion physics and stellar metallicity. Lower-metallicity stars are generally accreted more rapidly, suggesting that it may be more likely for sub-Chandrasekhar mass Hawking stars formed in the early universe, such as those in ultra-faint dwarf (UFD) galaxies, to transmute their star into a sub-Chandrasekhar mass black hole within a Hubble time. We present a stellar population synthesis of a Draco II-like UFD galaxy containing Hawking stars and show that the number of red stragglers they produce can qualitatively match the observed population \mb{for black hole seed masses around 10$^{-11}$ M$_\odot$ and under the assumption that they accrete with high radiative efficiency}.