Carbon and Nitrogen Abundance Ratio In the Broad Line Region of Tidal
  Disruption Events
  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$_{\odot}$. 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.
