The Carbon and Nitrogen Abundance Ratio in the Broad Line Region of Tidal Disruption Events
- CAS Key Laboratory for Researches in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, Anhui 230026 (China)
- Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506 (United States)
- Center for Astrophysics, Guangzhou University, Guangzhou 510006 (China)
- Polar Research Institute of China, 451 Jinqiao Road, Shanghai 200136 (China)
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] λ1908/N iii] λ1750 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{sup 2+} and N{sup 2+} 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 0.6 M{sub ⊙}. 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.
- OSTI ID:
- 22875816
- Journal Information:
- Astrophysical Journal, Vol. 846, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
Similar Records
MUSE Reveals a Recent Merger in the Post-Starburst Host Galaxy of the TDE ASASSN-14li
Muse reveals a recent merger in the post-starburst host galaxy of the tde ASASSN-14li