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arXiv 1708.03548astro-ph.GAastro-ph.HE
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潮破裂事件宽线区碳氮丰度比

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

摘要:

ASASSN-14 li、PTF-15 af和iPTF-16 fnl三个潮汐扰动事件的静止帧紫外光谱显示出较强的氮发射线,但碳发射线较弱或不可检测。在这三个天体中,C III] $λ1908$ /N III] $λ1750$比值的上限比类星体低约两个数量级,表明[N/C]丰度比很高。详细的光电离模拟,我们表明,$C^{2+}$和$N^{2+}$形成在同一个区域,所以C III]/N III]的比例只适度依赖于气体中的物理条件和弱的电离连续体的形状。在给定的金属丰度下,在气体密度和电离参数的宽范围内,线比的变化小于$0.5$ dex。这允许对氮与碳的相对丰度比的稳健估计。我们推导出ASASSN-14 li的[N/C] $>1.5$的相对丰度比,PTF 15 af和iPTF 16 fnl的[N/C]相对丰度比甚至更高。这表明,这些TDE源中的宽线区域是由氮增强的核心材料制成的,其在稍后的时间回落。根据恒星演化模型,破裂恒星的下限应大于0.6 M $_{\odot}$。谱线发射气体的化学丰度为耀斑起源于恒星潮汐破裂提供了有力的证据。PTF 15 af和iPTF 16 fnl中X射线发射的弱性与强宽吸收线的重合,以及ASASSN-li 14中没有这种线的强X射线发射,类似于有和没有宽吸收线的类星体。

英文摘要:

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 $C^{2+}$ 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.

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