[论文解读] Forbidden hugs in pandemic times - II. The luminous red nova variety: AT 2020hat and AT 2020kog

A. Pastorello, G. Valerin+33 more|arXiv (Cornell University)|Mar 12, 2021

Gamma-ray bursts and supernovae参考文献 87被引用 6

一句话总结

本研究对两颗明亮红新星 AT 2020hat 和 AT 2020kog 进行了多波段监测，揭示了其在数月内缓慢上升的光度，随后发生剧烈爆发。主要发现包括：光谱演化显著——从 IIn 型特征逐渐转变为类似 K/M 型恒星的特征；并通过哈勃空间望远镜档案影像对前身星进行了强有力的约束，确认其为中等 K 型恒星，与大质量核心塌缩超新星前身星模型不一致。

ABSTRACT

We present the results of our monitoring campaigns of the luminous red novae (LRNe) AT 2020hat in NGC 5068 and AT 2020kog in NGC 6106. The two objects were imaged (and detected) before their discovery by routine survey operations. They show a general trend of slow luminosity rise, lasting at least a few months. The subsequent major LRN outbursts were extensively followed in photometry and spectroscopy. The light curves present an initial short-duration peak, followed by a redder plateau phase. AT 2020kog is a moderately luminous event peaking at ∼7 × 1040 erg s-1, while AT 2020hat is almost one order of magnitude fainter than AT 2020kog, although it is still more luminous than V838 Mon. In analogy with other LRNe, the spectra of AT 2020kog change significantly with time. They resemble those of type IIn supernovae at early phases, then they become similar to those of K-type stars during the plateau, and to M-type stars at very late phases. In contrast, AT 2020hat already shows a redder continuum at early epochs, and its spectrum shows the late appearance of molecular bands. A moderate-resolution spectrum of AT 2020hat taken at +37 d after maximum shows a forest of narrow P Cygni lines of metals with velocities of 180 km s-1, along with an Hα emission with a full-width at half-maximum velocity of 250 km s-1. For AT 2020hat, a robust constraint on its quiescent progenitor is provided by archival images of the Hubble Space Telescope. The progenitor is clearly detected as a mid-K type star, with an absolute magnitude of MF606W = -3.33 ± 0.09 mag and a colour of F606W - F814W = 1.14 ± 0.05 mag, which are inconsistent with the expectations from a massive star that could later produce a core-collapse supernova. Although quite peculiar, the two objects nicely match the progenitor versus light curve absolute magnitude correlations discussed in the literature.

研究动机与目标

• 表征 AT 2020hat 和 AT 2020kog 在爆发阶段的光变曲线与光谱演化特征。
• 利用档案哈勃空间望远镜影像，确定这些事件宁静前身星的性质。
• 检验观测到的光度与前身星特性是否与明亮红新星中前身星星等与爆发光度之间既定的相关性一致。
• 探究 LRN 光变曲线中缓慢上升与平台期的物理机制。

提出的方法

• 在爆发后多个观测历元对 AT 2020hat 和 AT 2020kog 进行了测光与光谱监测。
• 分析哈勃空间望远镜 F606W 与 F814W 波段的档案影像，以识别并表征宁静前身星。
• 测量前身星的绝对星等与色指数（F606W - F814W），以分类其光谱型，并评估其与大质量恒星模型的一致性。
• 追踪从早期 IIn 型特征到晚期 K/M 型恒星特征的光谱演化，包括分子带的出现。
• 通过峰值总光度估计量化爆发光度，AT 2020kog 的峰值光度约为 ∼7 × 10⁴⁰ erg s⁻¹。
• 利用 P Cygni 深线的径向速度测量（如 AT 2020hat 中金属线的 180 km s⁻¹）推断喷流动力学。

实验结果

研究问题

• RQ1 明亮红新星 AT 2020hat 和 AT 2020kog 的光变曲线与光谱演化特征是什么？
• RQ2 AT 2020hat 的宁静前身星性质如何通过哈勃档案影像被约束？
• RQ3 这些 LRNe 的观测光度与前身星特性与文献中既定的相关性相比如何？
• RQ4 为何 AT 2020hat 表现出比 AT 2020kog 更红的连续谱和早期分子带特征？
• RQ5 通过 P Cygni 深线追踪的喷流动力学特性揭示了爆发机制的哪些信息？

主要发现

• AT 2020hat 和 AT 2020kog 均表现出持续至少数月的缓慢光度上升，随后发生剧烈爆发，初期出现短暂峰值，随后进入更红的平台期。
• AT 2020kog 的峰值光度达到 ∼7 × 10⁴⁰ erg s⁻¹，而 AT 2020hat 的光度低约一个数量级，但仍比 V838 Mon 更明亮。
• AT 2020hat 的前身星被识别为中等 K 型恒星，其绝对星等 MF606W = -3.33 ± 0.09 mag，色指数 F606W - F814W = 1.14 ± 0.05 mag。
• 根据其光度与色指数，AT 2020hat 的前身星与预期产生核心塌缩超新星的大质量恒星不一致。
• AT 2020kog 的光谱在早期表现为 IIn 型特征，平台期演变为类似 K 型恒星的特征，晚期则呈现类似 M 型恒星的特征。
• AT 2020hat 在 +37 天时获得的一张中等分辨率光谱显示，金属线处出现大量窄 P Cygni 线，速度为 180 km s⁻¹，同时 Hα 发射线的 FWHM 速度为 250 km s⁻¹。