Formation of Sub-Chandrasekhar-mass Black Holes and Red Stragglers via Hawking Stars in Ultrafaint Dwarf Galaxies

Abstract Primordial black holes (PBHs) in the asteroid mass window from 10 −16 M ⊙ to 10 −10 M ⊙ are currently 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 and 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 ⊙ . 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 ⊙ within the age of the Universe. This mechanism of producing subsolar-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 ultrafaint 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 for black hole seed masses around 10 −11 M ⊙ and under the assumption that they accrete with high radiative efficiency.