猩红研究——II. 中等光度红暂现源样本的光谱性质
A study in scarlet -- II. Spectroscopic properties of a sample of Intermediate Luminosity Red Transients
AI总结：
本文通过对四个中等光度红暂现源的光学与近红外光谱监测，揭示了其Balmer线和Ca线主导特征、双峰线光度演化及晚期宽发射特征，表明快速抛射物与缓慢星周介质相互作用，并发现约6170 Å和7000 Å处前所未有的宽发射线可能源于铁峰元素窄线混合。
摘要：
我们研究了中等光度红暂现源（ILRTs）的光谱特征，这是一类峰值光度介于经典新星和标准超新星之间的难以捉摸的天体。我们展示了对四个ILRTs，即NGC 300 2008OT-1、AT 2019abn、AT 2019ahd和AT 2019udc的广泛光学和近红外（NIR）光谱监测。首先我们关注低分辨率光谱中观测到的最显著光谱特征的演化，然后更详细地讨论利用配备UVES的甚大望远镜（Very Large Telescope）为NGC 300 2008OT-1采集的高分辨率光谱。最后我们通过与合成光谱和观测光谱的比较，分析NGC 300 2008OT-1和AT 2019ahd的晚期光谱。Balmer线和Ca线主导光学光谱，揭示了这些天体周围存在缓慢运动的星周介质（CSM）。Hα、Hβ和Ca II近红外三重线的线光度随时间呈现双峰演化，可能表明快速抛射物与缓慢CSM之间存在相互作用。NGC 300 2008OT-1的高分辨率光谱揭示了一个复杂的星周环境，该暂现源被缓慢的（约30 km s^-1）前身星风所包围。在晚期阶段，NGC 300 2008OT-1和AT 2019ahd的光学光谱在约6170 Å和约7000 Å处显示出宽（约2500 km s^-1）发射特征，这对ILRTs而言是前所未有的。我们发现这些谱线最可能来源于若干窄线的混合，可能来自铁峰元素。
英文摘要：
We investigate the spectroscopic characteristics of Intermediate Luminosity Red Transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. We present the extensive optical and near-infrared (NIR) spectroscopic monitoring of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd and AT 2019udc. First we focus on the evolution of the most prominent spectral features observed in the low resolution spectra, then we discuss more in detail the high resolution spectrum collected for NGC 300 2008OT-1 with the Very Large Telescope equipped with UVES. Finally we analyse late time spectra of NGC 300 2008OT-1 and AT 2019ahd through comparisons with both synthetic and observed spectra. Balmer and Ca lines dominate the optical spectra, revealing the presence of slowly moving circumstellar medium (CSM) around the objects. The line luminosity of H$α$, H$β$ and Ca II NIR triplet presents a double peaked evolution with time, possibly indicative of interaction between fast ejecta and the slow CSM. The high resolution spectrum of NGC 300 2008OT-1 reveals a complex circumstellar environment, with the transient being surrounded by a slow ($\sim$30 km s$^{-1}$) progenitor wind. At late epochs, optical spectra of NGC 300 2008OT-1 and AT 2019ahd show broad ($\sim$2500 km s$^{-1}$) emission features at $\sim$6170 A and $\sim$7000 A which are unprecedented for ILRTs. We find that these lines originate most likely from the blending of several narrow lines, possibly of iron-peak elements.
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