年轻吸积星的阵发性爆发：氢电离导致盘热不稳定性的关键作用
Episodic eruptions of young accreting stars: the key role of disc thermal instability due to Hydrogen ionisation
AI总结：
本文提出氢电离导致的盘热不稳定性（TI）可解释年轻吸积星的中间型爆发，并揭示了特定吸积率区间的爆发缺失，指出 TI 是小尺度阵发吸积的必要机制，但大尺度仍需额外物理机制。
摘要：
在年轻吸积星大振幅阵发性变星的经典分类中，FUORs 的亮度比其恒星高出约 100 倍，并且可持续长达数个世纪；EXORs 则较暗，持续数月至一年。此前曾为 FUORs 提出过盘氢电离热不稳定性（TI）模型，但需要不切实际的低盘粘度。在过去十年中，发现了许多中间型天体，例如在亮度和光谱上类似 FUORs，但在持续时间上类似 EXORs。在此我们表明，中间型爆发天体 Gaia20eae、PTF14jg、Gaia19bey 和 Gaia21bty 可以通过具有真实粘度值的 TI 模型自然解释。我们论证了 TI 预测在峰值吸积率介于 $\dot M \sim 10^{-6} M_\odot$/yr 和 $\dot M \sim 10^{-5} M_\odot$/yr 之间的爆发存在缺失（沙漠区），并且在所有已测定 $\dot M$ 爆发的爆发天体样本中观察到了该沙漠区。大多数经典 EXORs（FUORs）在其爆发峰值光变期间似乎处于 S 曲线的冷（热）分支上；因此 TI 可能在这种分类分化中发挥作用。同时，TI 无法解释经典 FUORs 如何能持续长达数个世纪，并且将短周期 FUORs 的发生率高估了至少一个数量级。我们得出结论，TI 是在 R < 0.1 au 处运行的阵发性吸积的必要组成部分，但在更大尺度上必须由额外的物理机制发挥作用。从相关学科了解 TI 的内部运作机制可能使其被用作约束这种额外物理机制本质的工具。
英文摘要：
In the classical grouping of large magnitude episodic variability of young accreting stars, FUORs outshine their stars by a factor of $\sim$ 100, and can last for up to centuries; EXORs are dimmer, and last months to a year. A disc Hydrogen ionisation Thermal Instability (TI) scenario was previously proposed for FUORs but required unrealistically low disc viscosity. In the last decade, many intermediate type objects, e.g., FUOR-like in luminosity and spectra but EXOR-like in duration were found. Here we show that the intermediate type bursters Gaia20eae, PTF14jg, Gaia19bey and Gaia21bty may be naturally explained by the TI scenario with realistic viscosity values. We argue that TI predicts a dearth (desert) of bursts with peak accretion rates between $\dot M \sim 10^{-6} M_\odot$/yr and $\dot M \sim 10^{-5} M_\odot$/yr, and that this desert is seen in the sample of all the bursters with previously determined $\dot M$ burst. Most classic EXORs (FUORs) appear to be on the cold (hot) branch of the S-curve during the peak light of their eruptions; thus TI may play a role in this class differentiation. At the same time, TI is unable to explain how classic FUORs can last for up to centuries, and over-predicts the occurrence rate of short FUORs by at least an order of magnitude. We conclude that TI is a required ingredient of episodic accretion operating at R < 0.1 au, but additional physics must play a role at larger scales. Knowledge of TI inner workings from related disciplines may enable its use as a tool to constrain the nature of this additional physics.
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