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] /N iii] 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 and are formed in the same zone, so the Ciii]/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 of nitrogen to carbon. We derive a relative abundance ratio of [N/C] > 1.5 for ASASSN-14li, and an even higher one 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 . The chemical abundance of the line-emitting gas provides convincing evidence that the flares originate from stellar tidal disruptions. The coincidence of the weakness of the X-ray emission with the strong broad absorption lines in PTF15af and iPTF16fnl, and the strong X-ray emission without such lines in ASASSN-li14, are analogous to quasars with and without broad absorption lines.
|State||Published - Sep 10 2017|
Bibliographical noteFunding Information:
We acknowledge the financial support by the Fundamental Research Funds for the Central Universities, the Strategic Priority Research Program The Emergence of Cosmological Structures of the Chinese Academy of Sciences (XDB 09000000), NSFC (NSFC-11233002, NSFC-11421303, U1431229), and National Basic Research Program of China (grant No. 2015CB857005). G.J.F. acknowledges support from NSF (1412155), NASA (ATP13-0153), and STScI (HST-AR-13245, GO-12560, HST-GO-12309, GO-13310.002-A, HST-AR-13914, HST-AR-14286.001 and HST-AR-14556).
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- accretion, accretion disks
- black hole physics
- galaxies: abundances
- galaxies: active
- galaxies: nuclei
- line: formation
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science