[论文解读] On Rapid Disk Accretion and Initial Conditions in Protostellar Evolution

Lee Hartmann, Zhaohuan Zhu+1 more|arXiv (Cornell University)|Jun 16, 2011

Astrophysics and Star Formation Studies参考文献 2被引用 12

一句话总结

本文挑战了低质量原恒星中冷、快速吸积盘的假设，认为高吸积率会导致热的、几何上厚实的吸积盘，从而加热原恒星，使冷吸积模型与观测结果不一致。研究结论指出，初始原恒星半径——由湍流坍缩条件决定——导致年轻恒星的年龄不确定性约为1 Myr，而非10 Myr。

ABSTRACT

Low-mass protostars may accrete most of their material through short-lived episodes of rapid disk accretion; yet until recently evolutionary tracks for these protostars assumed only constant or slowly-varying accretion. Important initial steps toward examining the potential effects of rapid accretion were recently made by Baraffe, Chabrier, & Gallardo, who showed that in the limit of low-temperature ("cold") accretion, protostars may have much smaller radii than found in previous treatments. Such small radii at the end of protostellar accretion would have the effect of making some young stars appear much older - perhaps as much as 10 Myr - than they really are. However, we argue that very rapid disk accretion is unlikely to be cold, because observations of the best-studied pre-main sequence disks with rapid disk accretion outbursts - the FU Ori objects - have spectral energy distributions which imply large, not small, protostellar radii. In addition, theory indicates models that at high accretion rates, protostellar disks become internally hot and geometrically thick, making it much more likely that hot material is added to the star. In addition, the very large luminosity of the accretion disk is likely to irradiate the central star strongly, heating up the outer layers and potentially expanding them. Nevertheless, the Baraffe et al. calculations emphasize the importance of initial protostellar radii for subsequent evolution.

研究动机与目标

• 评估冷吸积模型在低质量原恒星快速吸积盘情景下的有效性。
• 研究快速、冷吸积是否能产生Baraffe等人（2009年）所预测的小型原恒星半径。
• 调和观测到的主序前恒星在赫罗图中的位置与理论演化轨迹之间的关系。
• 评估初始原恒星核心半径在确定恒星表观年龄中的作用。
• 确定快速周期性吸积是否能在年轻恒星中导致较大的年龄不确定性（例如10 Myr）。

提出的方法

• 分析FU Orionis类天体的观测Sed，其显示的大型原恒星半径与冷吸积模型预测不一致。
• 应用理论吸积盘模型，表明高吸积率会导致粘性加热和几何上厚实的吸积盘。
• 利用吸积盘稳定性判据，估算吸积盘变得热厚的半径（对于10⁻⁴ M☉ yr⁻¹的吸积率，R_T ~ 0.23 AU）。
• 通过垂直平均吸积盘方程和消光系数标度，比较吸积盘中心温度和压强分布。
• 评估辐射照射和内部加热对原恒星半径及收缩 timescale 的影响。
• 重新审视原恒星演化中热能添加可忽略的假设，对比冷吸积与热吸积。

实验结果

研究问题

• RQ1 快速吸积盘能否在无显著热能输入的低温状态下发生？
• RQ2 FU Ori类天体的观测Sed支持小半径还是大半径的原恒星？这对年龄估计有何影响？
• RQ3 在何种临界吸积率下，吸积盘的加热和几何增厚效应变得显著？
• RQ4 由湍流流体动力学坍缩决定的初始原恒星核心半径，如何影响后续恒星演化和年龄测定？
• RQ5 初始条件的差异在多大程度上可解释年轻星团中观测到的半径分散？

主要发现

• FU Ori类天体的观测显示原恒星半径较大，与冷吸积模型的预测相矛盾。
• 高吸积率导致粘性加热、几何上厚实的吸积盘，使冷吸积在物理上不可行。
• 对于10⁻⁴ M☉ yr⁻¹的吸积率，吸积盘在R_T ~ 0.23 AU处变得几何上厚实，中心温度超过2×10⁴ K。
• 热而厚实的吸积盘会照射并加热原恒星的外层，可能导致其膨胀并增加半径。
• 快速冷吸积无法解释年轻恒星中观测到的半径变化；年龄不确定性可能仅限于约1 Myr，而非10 Myr。
• 由湍流坍缩决定的初始原恒星核心半径，会显著影响原主序阶段的演化和表观年龄。