The maximum mass of a black hole which can tidally disrupt a star: measuring black hole spins with tidal disruption events

The tidal acceleration experienced by an object at the event horizon of a black hole decreases as one over the square of the black hole's mass. As such there is a maximum mass at which a black hole can tidally disrupt an object outside of its event horizon and potentially produce observable emission. This maximum mass is known as the ``Hills mass'', and in full general relativity is a function of both the black hole's spin $a_\bullet$ and the inclination angle of the incoming object's orbit with respect to the black hole's spin axis $ψ$. In this paper we demonstrate that the Hills mass can be represented by a simple analytical function of $a_\bullet$ and $ψ$, the first general solution of this problem. This general solution is found by utilising the symmetries of a class of critical Kerr metric orbits known as the innermost bound spherical orbits. Interestingly, at fixed black hole spin the maximum Hills mass can lie at incoming orbital inclinations outside of the black hole's equatorial plane $ψ\neq π/2$. When compared to previous results in the literature this effect can lead to an increase in the maximum Hills mass (at fixed spin) by as much as a factor of $\sqrt{11/5} \simeq 1.48$ for a maximally rotating black hole. We then demonstrate how Bayesian inference, coupled with an estimate of the mass of a black hole in a tidal disruption event, can be used to place conservative constraints on that black hole's spin. We provide a publicly available code tidalspin which computes these spin distributions.