Carbon and Nitrogen Abundance Ratio In the Broad Line Region of Tidal Disruption Events

Chenwei Yang, Tinggui Wang, Gary J. Ferland, Liming Dou, Hongyan Zhou, Ning Jiang, Zhenfeng Sheng

Published 2017-08-11Version 1

The rest-frame UV spectra of three recent tidal disruption events (TDEs), ASASSN-14li, PTF15af and iPTF16fnl display strong nitrogen emission lines but weak or undetectable carbon lines. In these three objects, the upper limits of the C III] $\lambda 1908$/N III] $\lambda 1750$ ratio are about two orders of magnitude lower than those of quasars, suggesting a high abundance ratio of [N/C]. With detailed photoionization simulations, we demonstrate that $C^{2+}$ and $N^{2+}$ are formed in the same zone, so the C III]/N III] ratio depends only moderately on the physical conditions in the gas and weakly on the shape of the ionizing continuum. There are smaller than $0.5$ dex variations in the line ratio over wide ranges of gas densities and ionization parameters at a given metallicity. This allows a robust estimate of the relative abundance ratio nitrogen to carbon. We derive a relative abundance ratio of [N/C]$>1.5$ for ASASSN-14li, and even higher for PTF15af and iPTF16fnl. This suggests that the broad line region in those TDE sources is made of nitrogen-enhanced core material that falls back at later times. Based on stellar evolution models, the lower limit of the disrupted star should be larger than 0.6M$_{\sun}$. The chemical abundance of the line emitting gas gives a convincing evidence that the flares origin from stellar tidal disruptions. The coincidence of the weakness of the X-ray emission with the strong broad absorption lines in PTF15af, iPTF16fnl and the strong X-ray emission without such lines in ASASSN-li14 are in analogy to quasars with and without broad absorption lines.