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 ≫ 1, the star is tidally compressed and heated. It was predicted that the maximum density and temperature attained during deep TDEs scale as ∝ β 3 and ∝ β 2, respectively, and nuclear detonation is triggered by β ≳ 5, but these predictions have been debated over the last four decades. We perform Newtonian smoothed-particle hydrodynamics simulations of deep TDEs between a Sun-like star and a 106 M ⊙ SMBH for 2 ≤ β ≤ 10. We find that neither the maximum density nor temperature follow the ∝ β 3 and ∝ β 2 scalings or, for that matter, any power-law dependence, and that the maximum-achieved density and temperature are reduced by ∼1 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 ≲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.

On the relative importance of shocks and self-gravity in modifying tidal disruption event debris streams

“…Ho we ver, the importance of shocks on the dynamics of the debris has recently been questioned by Norman, Nixon & Coughlin ( 2021 ), Coughlin &Kundu, Coughlin &, who found that even for deep TDEs, in which the stellar pericentre distance is well within the canonical tidal radius, shocks during the compression of the star were either weak with a Mach number near unity or absent completely. 2 Contrarily, it seems possible that the other physical effect ignored in the frozen-in approximationthe self-gravity of the debris stream -could be responsible for the deviation of the numerically obtained d M /d ǫ curve from the analytic prediction.…”

Ultradeep Cover: An Exotic and Jetted Tidal Disruption Event Candidate Disguised as a Gamma-Ray Burst

“…In a standard TDE, the star approaches the SMBH with a pericenter distance that is close to, but smaller than, the tidal radius at which the star is completely destroyed. When the pericenter distance is much smaller than the tidal radius, the star suffers substantial in-plane distortion and vertical compression near pericenter, and this is the "deep" regime studied by, e.g., Carter & Luminet (1982), Bicknell & Gingold (1983), Carter & Luminet (1983), Laguna et al (1993), Stone et al (2013), Evans et al (2015), Tejeda et al (2017), Steinberg et al (2019), Norman et al (2021), Coughlin & Nixon (2022a), Kundu et al (2022) and . As the pericenter continues to decrease, the star is eventually directly captured by the black hole, particularly where the SMBH is high mass, and no observable emission is produced.…”