The production of strong, broad He II emission after the tidal disruption of a main-sequence star by a supermassive black hole

Abstract

The tidal disruption event (TDE) PS1-10jh lacked strong Balmer lines but showed strong, broad He II emission both before maximum light and for at least eight months thereafter. Gezari et al. interpreted this as evidence for the disruption of a rare hydrogen-deficient star. However, Guillochon et al. have argued instead that the disrupted star was a normal mainsequence star and that the strength of the He II emission compared with the Balmer lines is a result the emission being similar to the broad-line region (BLR) of an active galactic nucleus (AGN), but lacking the outer, lower ionization BLR gas. We show that the profile of He II γ4686 in PS1-10jh is similar to the blueshifted profiles of high-ionization lines in AGNs. We find an He II γ4686/Ha ratio for PS1-10jh of ̃3.7.We show that both the high-velocity gas of the inner BLR of normal AGNs and the spectra of Type II-P supernovae right after shock break out also produce very high He II γ4686/Ha ratios. A high He II γ4686/Ha ratio can thus be produced with a solar H/He abundance ratio. We demonstrate from photoionization modelling that the estimated He II γ4686/Ha ratio can be produced with a BLR truncated before the He++ Str̈omgren length if the density is ̃1011 cm-3. The similarity of the He II γ4686 emission in PS1-10jh to the emission from the inner BLRs of AGNs supports the idea that the emission after a TDE event is similar to that of normal AGNs. © 2013 The Authors.