Tidal Disruptions of Main Sequence Stars -- I. Observable Quantities and their Dependence on Stellar and Black Hole Mass

Abstract

This paper introduces a series of papers presenting a quantitative theory for the tidal disruption of main sequence stars by supermassive black holes. Using fully general relativistic hydrodynamics simulations and MESA-model initial conditions, we explore the pericenter-dependence of tidal disruption properties for eight stellar masses ($0.15 \leq M_*/M_\odot \leq 10$) and six black hole masses ($10^5 \leq M_{BH}/M_\odot \leq 5 \times 10^7$). We present here the results most relevant to observations. The effects of internal stellar structure and relativity decouple for both the disruption cross section and the characteristic energy width of the debris. Moreover, the full disruption cross section is almost independent of $M_*$ for $M_*/M_\odot \lesssim 3$. Independent of $M_*$, relativistic effects increase the critical pericenter distance for full disruptions by up to a factor $\sim 3$ relative to the Newtonian prediction. The probability of a direct capture is also independent of $M_*$; at $M_{BH}/M_\odot \simeq 5 \times 10^6$ this probability is equal to that of a complete disruption. The width of the debris energy distribution $ΔE$ can differ from the standard estimate by factors from 0.35 to 2, depending on $M_*$ and $M_{BH}$, implying a corresponding change in the characteristic mass-return timescale. The "frozen-in approximation" is inconsistent with $ΔE$, and mass-loss continues over a long span of time. We provide analytic forms, suitable for use in both event rate estimates and parameter inference, to describe all these trends. For partial disruptions, we find a nearly-universal relation between the star's angular momentum and the fraction of $M_*$ remaining. Within the "empty loss-cone" regime, partial disruptions must precede full disruptions. These partial disruptions can drastically affect the rate and appearance of subsequent total disruptions.