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 (\(\lesssim1~\mathrm{km \, s^{-1}}\)). 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 \(\sim30\%\) smaller than the observed value, while those where the last BHs were ejected recently (\(\lesssim150~\)Myr ago) can still reproduce the density profile. In 50% of the models hosting BHs, we find BHs with stellar companion(s). Their period distribution peaks at \(\sim10^3\) 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 consistent with a massive compact companion. Models with \(2-3\) BHs have an elevated central velocity dispersion, but observations can not 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.