Revisiting the Rates and Demographics of Tidal Disruption Events: Effects of the Disk Formation Efficiency

Thomas H. T. Wong, Hugo Pfister, Lixin Dai

Published 2021-11-17Version 1

Tidal disruption events (TDEs) are valuable probes of the demographics of supermassive black holes as well as the dynamics and population of stars in the centers of galaxies. In this Letter, we focus on studying how the TDE disk formation and circularization processes can impact the possibility of observing prompt TDE flares, based on recent theoretical developments. First, we investigate how the efficiency of disk formation is determined by the key parameters, namely, the black hole mass $M_{BH}$, the stellar mass $m_\star$, and the stellar orbital penetration parameter $\beta$. Then we apply the loss cone theory to calculate the differential TDE rate as a function of these three parameters. Combining these two results, we find that the rates of TDEs with prompt disk formation are significantly suppressed around lighter black holes, which provides a plausible explanation for why the observed TDE host black hole mass distribution peaks between $10^6$ and $10^7 M_\odot$. Therefore, the consideration of the disk formation efficiency is crucial for recovering the intrinsic black hole demographics from TDEs. Furthermore, we find that the efficiency of the disk formation process also impacts the distributions of both stellar orbital penetration parameter and stellar mass observed in TDEs.