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

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 8 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 main-sequence 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 AGN, but lacking the outer, lower-ionization BLR gas. We show that the profile of He II 4686 in PS1-10jh is similar to the blueshifted profiles of high-ionization lines in AGNs. We find an He II 4686/Halpha 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 4686/Halpha ratios. A high He II 4686/Halpha ratio can thus be produced with a solar H/He abundance ratio. We demonstrate from photoionization modelling that the estimated He II 4686/Halpha ratio can be produced with a BLR truncated before the He++ Stroemgren length if the density is ~ 10^{11} cm^{-3}. The similarity of the He II 4686 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.