Stellar-mass black holes in the Hyades star cluster?

ABSTRACT

Astrophysical models of binary-black hole mergers in the universe require a significant fraction of stellar-mass black holes (BHs) to receive negligible natal kicks to explain the gravitational wave detections. This implies that BHs should be retained even in open clusters with low escape velocities (≲1 km s−1). We search for signatures of the presence of BHs in the nearest open cluster to the Sun – the Hyades – by comparing density profiles of direct N-body models to data from Gaia. The observations are best reproduced by models with 2–3 BHs at present. Models that never possessed BHs have an half-mass radius $\sim 30~{{\ \rm per\ cent}}$smaller than the observed value, while those where the last BHs were ejected recently (≲150 Myr ago) can still reproduce the density profile. In 50  per cent of the models hosting BHs, we find BHs with stellar companion(s). Their period distribution peaks at ∼103 yr, making them unlikely to be found through velocity variations. We look for potential BH companions through large Gaia astrometric and spectroscopic errors, identifying 56 binary candidates – none of which is consistent with a massive compact companion. Models with 2–3 BHs have an elevated central velocity dispersion, but observations cannot yet discriminate. We conclude that the present-day structure of the Hyades requires a significant fraction of BHs to receive natal kicks smaller than the escape velocity of$\sim 3 \, \mathrm{km \, s^{-1}}$ at the time of BH formation and that the nearest BHs to the Sun are in, or near, Hyades.