V1405 Cas 极早期光谱表明其前身星为一颗低质量 ONeMg 白矮星
Spectra of V1405 Cas at the very beginning indicate a low-mass ONeMg white dwarf progenitor
AI总结：
通过分析新星 V1405 Cas 极早期光谱的快速演化，发现其前身星为低质量 ONeMg 白矮星，挑战了约 1.1 太阳质量的质量下限理论。
摘要：
尽管 ONeMg 白矮星（WD）的最低可能质量对其形成和演化具有重要意义，但该质量下限尚未明确。我们通过研究 V1405 Cas（仙后座新星 2021）的性质来探讨这一问题，该新星因其极缓慢的光变曲线演化以及近期被认定的氖新星身份而成为一个特例。我们报告了其在初始阶段的快速光谱演化，覆盖了发现后 9.88、23.77、33.94、53.53、71.81 和 81.90 小时的光谱。第一张光谱以高电离物种的谱线为特征，最显著的是 He II 和 N III。这些谱线很快被较低电离的谱线（如 N II、Si II 和 O I）所取代。此外，Al II（6237 Å）在第二个观测时刻开始作为发射线出现。我们进行了发射线强度诊断，表明密度和温度在后期迅速下降。这一行为，连同 Hα、Hβ 和 He I 中观测到的速度下降，表明初始新星动力学可以合理地描述为在膨胀光球之上的膨胀火球。有趣的是，N III 和 Al II 的强度表明丰度大幅增强，指向 ONeMg 白矮星前身星，这与其氖新星分类一致。鉴于其由缓慢光变曲线演化和相对较窄的发射线所推断出的低质量特性，这对恒星演化理论提出了挑战，该理论预测 ONeMg 白矮星质量下限约为 1.1 太阳质量。
英文摘要：
The lowest possible mass of ONeMg white dwarfs (WDs) has not been clarified despite its importance in the formation and evolution of WDs. We tackle this issue by studying the properties of V1405 Cas (Nova Cassiopeiae 2021), which is an outlier given a combination of its very slow light-curve evolution and the recently reported neon-nova identification. We report its rapid spectral evolution in the initial phase, covering 9.88, 23.77, 33.94, 53.53, 71.79, and 81.90 hours after the discovery. The first spectrum is characterized by lines from highly-ionized species, most noticeably He II and N III. These lines are quickly replaced by lower-ionization lines, e.g., N II, Si II, and O I. In addition, Al II (6237 Å) starts emerging as an emission line at the second epoch. We perform emission-line strength diagnostics, showing that the density and temperature quickly decrease toward later epochs. This behavior, together with the decreasing velocity seen in H$α$, H$β$, and He I, indicates that the initial nova dynamics is reasonably well described by an expanding fireball on top of an expanding photosphere. Interestingly, the strengths of the N III and Al II indicate large abundance enhancement, pointing to an ONeMg WD progenitor as is consistent with its neon-nova classification. Given its low-mass nature inferred by the slow light-curve evolution and relatively narrow emission lines, it provides a challenge to the stellar evolution theory that predicts the lower limit of the ONeMg WD mass being $\sim$ 1.1 $M_\odot$.
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