Abstract
Abstract 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.
Highlights
Evidence has been mounting that in the center of most galaxies there is a supermassive black hole
We find that reasonable ranges of the ionization parameter and SED shape have only weak effects on the line ratio
The very small C III] l1908/N III] l1750 line ratio can be naturally explained by the small relative abundance ratio of carbon to nitrogen
Summary
Evidence has been mounting that in the center of most galaxies there is a supermassive black hole. It was proposed that the lines may come from strong outflows launched during the early high-accretion rate stage (Strubbe & Quataert 2009) or from streams of tidal debris (Bogdanović et al 2004; Guillochon et al 2014) Their origin is not fully understood, the emission lines and their time evolution carry important information about the physical, dynamical, and chemical properties of gas streams around the supermassive black hole. The paper is organized as follows: in Section 2, we will briefly summarize the observations of ASASSN-14li, PTF15af, and iPTF16fnl; in Section 3 we will use the numerical photoionization code CLOUDY (Ferland et al 2013) to investigate the ionization structure and the formation of carbon and nitrogen emission lines; in Section 4 we will inspect how the line ratio changes with the gas properties and give constraint of the disrupted stars; and Section 5 is our summary
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