Stars Crushed by Black Holes. III. Mild Compression of Radiative Stars by Supermassive Black Holes

Abstract

A tidal disruption event (TDE) occurs when the gravitational field of a supermassive black hole (SMBH) destroys a star. For TDEs in which the star enters deep within the tidal radius, such that the ratio of the tidal radius to the pericenter distance $β$ satisfies $β\gg 1$, the star is tidally compressed and heated. It was predicted that the maximum density and temperature attained during deep TDEs scale as $\propto β^3$ and $\propto β^2$, respectively, and nuclear detonation triggered by $β\gtrsim 5$, but these predictions have been debated over the last four decades. We perform Newtonian smoothed-particle hydrodynamics (SPH) simulations of deep TDEs between a Sun-like star and a $10^6 M_\odot$ SMBH for $2 \le β\le 10$. We find that neither the maximum density nor temperature follow the $\propto β^3$ and $\propto β^2$ scalings or, for that matter, any power-law dependence, and that the maximum-achieved density and temperature are reduced by $\sim$ an order of magnitude compared to past predictions. We also perform simulations in the Schwarzschild metric, and find that relativistic effects modestly increase the maximum density (by a factor of $\lesssim 1.5$) and induce a time lag relative to the Newtonian simulations, which is induced by time dilation. We also confirm that the time the star spends at high density and temperature is a very small fraction of its dynamical time. We therefore predict that the amount of nuclear burning achieved by radiative stars during deep TDEs is minimal.