M31-2015-LRN is a likely stellar merger discovered in the Andromeda Galaxy in 2015. We present new optical to mid-infrared photometry and optical spectroscopy for this event. The transient brightened by ∼3 mag as compared to its progenitor. The complex precursor emission, which started ∼2 years before the nova event, may be explained by the binary undergoing Roche-lobe overflow. The dynamical mass loss from the outer Lagrange point L2 creates an optically thick outflow to power the observed brightening of the system. We find two possible periods of 16±0.3 and 28.1±1.4 days at different phases of the precursor lightcurve, possibly related to the geometry of the mass-loss from the binary. Although the progenitor spectral energy distribution shows no evidence of pre-existing warm dust in system, the remnant forms an optically thick dust shell 2−4 months after the outburst peak. The optical depth of the shell increases after 1.5 years, suggesting the existence of shocks that enhance the dust formation process. We propose that the merger remnant is likely an inflated giant obscured by a cooling shell of gas with mass ∼0.2 M⊙ ejected at the onset of the common envelope phase.

中文翻译：

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恒星合并 M31-LRN-2015 尘土飞扬的残余物的祖先、前兆和演化

M31-2015-LRN 很可能是 2015 年在仙女座星系中发现的恒星合并。我们为这次事件提供了新的光学到中红外光度测量和光谱学。与它的前身相比，瞬变光亮了~3 mag。在新星事件前约 2 年开始的复杂前体发射可以用经历罗氏叶溢出的双星来解释。来自外部拉格朗日点 L2 的动态质量损失产生了光学上厚的流出物，为观察到的系统增亮提供动力。我们在前体光曲线的不同阶段发现了两个可能的周期，分别为 16±0.3 天和 28.1±1.4 天，这可能与双星质量损失的几何形状有关。尽管前身光谱能量分布没有显示系统中预先存在暖尘的证据，残余物在爆发峰值后 2-4 个月形成光学厚的尘埃壳。外壳的光学深度在 1.5 年后增加，表明存在增强尘埃形成过程的冲击。我们认为合并残余很可能是一个膨胀的巨星，被一个质量约为 0.2 M⊙ 的冷却气体壳遮蔽，在公共包层相开始时喷出。