恒星并合与化学元素混合：对活动星系核盘内变质恒星演化的启示
Stellar mergers and chemical element mixing: implications for the metamorphic stellar evolution in AGN disks
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中文总结 AI 辅助
本研究通过三维流体动力学模拟，揭示恒星并合的化学混合机制，发现特定恒星并合可使年老变质恒星恢复活力，为AGN恒星演化及其观测特征提供物理框架。
中文摘要 AI 辅助
恒星并合过程中的化学混合会显著影响并合遗迹的后续演化。我们开展了一系列三维恒星并合流体动力学模拟，每个模拟演化约100个恒星动力学时标，直至遗迹达到准流体静力学平衡。这些模拟采用亚格子扩散模型来捕捉化学元素的湍流混合。我们从理想化的多方物态方程（EOS）出发，首先确定主导的混合机制，并研究并合结果如何依赖于质量比、相对速度、碰撞参数和恒星结构。随后，我们将模拟扩展到嵌入活动星系核（AGN）盘的恒星场景，使用基于成分的真实物态方程，以及由恒星演化代码MESA生成的AGN恒星模型。我们发现，由年轻变质恒星与富氢吸积AGN恒星发生的并合，可在热弛豫后通过高效的核心混合使年老的变质恒星显著“恢复活力”。并合遗迹富含氮，氮氧比（N/O）约为1至3，氮碳比（N/C）大于5，与超新星1987A周围星云观测到的丰度相当。在后续恒星演化中，这些遗迹一旦达到吸积-风平衡，可能会收敛到孤立变质AGN恒星的主序上；还可能向AGN盘沉积大量化学富集物质。本工作为将流体动力学恒星并合与AGN恒星的长期演化及其观测、化学特征建立联系提供了物理框架。
英文摘要
Chemical mixing during stellar mergers can significantly influence the subsequent evolution of the merger remnant. We perform a suite of three-dimensional hydrodynamical simulations of stellar mergers, each evolved for $\sim100$ stellar dynamical times until the remnant reaches a quasi-hydrostatic equilibrium. The simulations incorporate subgrid-scale diffusion models to capture the turbulent mixing of chemical elements. Starting with an idealized polytropic equation of state (EOS), we first identify the dominant mixing mechanisms and investigate how the merger outcome depends on the mass ratio, relative velocity, impact parameter, and stellar structure. We then extend our simulations to the context of stars embedded in active galactic nucleus (AGN) disks, using a realistic, composition-dependent EOS and AGN stellar models generated with the stellar evolution code MESA. We find that mergers with both younger metamorphic stars and H-rich accreting AGN stars can substantially rejuvenate old metamorphic stars through efficient core mixing after thermal relaxation. The merger remnants are nitrogen-enriched, with ${\rm N/O}\sim1$--3 and ${\rm N/C}\gtrsim5$, comparable to the abundances observed in the nebula surrounding SN 1987A. During subsequent stellar evolution, the remnants may converge onto the main sequence of isolated metamorphic AGN stars once they reach accretion--wind equilibrium. They may also deposit a significant amount of chemically enriched material into the AGN disk. This work provides a physical framework for connecting hydrodynamical stellar mergers with the long-term evolution of AGN stars and their observational and chemical signatures.
