[论文解读] X-Shooter spectroscopy of young stellar objects in Lupus: Accretion properties of class II and transitional objects

J. M. Alcalá, C. F. Manara+12 more|arXiv (Cornell University)|Dec 21, 2016

Astrophysics and Star Formation Studies参考文献 93被引用 126

一句话总结

本研究使用 VLT/X-Shooter 对 Lupus 的几乎完整样本的 class II 和过渡型 YSO，在同质推导出吸积与恒星性质，揭示一个紧密的 L_acc–L_star 关系，斜率约为 1.6，并呈现 Mdot_acc–M_star 行为的双峰性。

ABSTRACT

We present the results of a study of the stellar and accretion properties of the (almost) complete sample of class II and transitional YSOs in the Lupus I, II, III and IV clouds, based on spectroscopic data acquired with the VLT/X-Shooter spectrograph. Our study combines the dataset from our previous work with new observations of 55 additional objects. We have investigated 92 YSO candidates in total, 11 of which have been definitely identified with giant stars unrelated to Lupus. The stellar and accretion properties of the 81 bona fide YSOs, which represent more than 90% of the whole class~II and transition disc YSO population in the aforementioned Lupus clouds, have been homogeneously and self-consistently derived, allowing for an unbiased study of accretion and its relationship with stellar parameters. The accretion luminosity, Lacc, increases with the stellar luminosity, Lstar, with an overall slope of ~1.6, similar but with a smaller scatter than in previous studies. There is a significant lack of strong accretors below Lstar~0.1Lsun, where Lacc is always lower than 0.01Lstar. We argue that the Lacc-Lstar slope is not due to observational biases, but is a true property of the Lupus YSOs. The logMacc-logMstar correlation shows a statistically significant evidence of a break, with a steeper relation for Mstar<0.2Msun and a flatter slope for higher masses. The bimodality of the Macc-Mstar relation is confirmed with four different evolutionary models used to derive the stellar mass. The bimodal behaviour of the observed relationship supports the importance of modelling self-gravity in the early evolution of the more massive discs, but other processes, such as photo evaporation and planet formation during the YSO's lifetime, may also lead to disc dispersal on different timescales depending on the stellar mass. We also refined the empirical Lacc vs. Lline relationships.

研究动机与目标

• 表征 Lupus class II 与过渡 YSO 群体的恒星与吸积性质，采用自洽的方法。
• 通过整合新的与存档的 X-Shooter 数据，对 Lupus I–IV 云的接近完整普查。
• 通过连续体遮蔽推导 L_acc，并用线强度-光度关系进行验证，以确保吸积估计的稳健性。
• 研究吸积性质与恒星参数（L_star、M_star）的关系，并与盘质量研究进行比较。

提出的方法

• 为大量 Lupus YSOs 获取广波段（330–2500 nm）的 X-Shooter 光谱。
• 将 UV/光学连续体超出量建模为以光球模板为底的氢类板状发射，以自洽地推导 L_acc。
• 通过用模板和光球模型拟合光谱来确定光谱型、消光和恒星参数。
• 从 slab-model 拟合和多条发射线光度推导 L_acc，以交叉验证一致性。
• 使用多种演化模型来分析 L_acc–L_star 与 Mdot_acc–M_star 的关系，以推断恒星-盘早期演化的影响。

实验结果

研究问题

• RQ1 在同质分析下，Lupus class II 与过渡型 YSO 的吸积光度和质量吸积率是多少？
• RQ2 L_acc 如何随 L_star 增长，在 Lupus 中存在多少散射？
• RQ3 Mdot_acc–M_star 关系是否存在断点，且是否取决于恒星质量区间？
• RQ4 在 Lupus 中，过渡盘是否在吸积性质上与完整盘不同？
• RQ5 所推导的吸积性质与自重力、光致蒸发和行星形成等盘演化理论有何关联？

主要发现

• L_acc 相对于 L_star 增长，近似斜率为 1.6，散射比早期一些研究小。
• 在 L_star 约小于 0.1 L_sun 的区域，缺少强吸积体，其中 L_acc < 0.01 L_star。
• log Mdot_acc–log M_star 关系显示断点：对于 M_star ≲ 0.2 M_sun 更陡峭，对于更高质量则更平缓。
• 在用于推导 M_star 的四种演化模型中，Mdot_acc–M_star 关系的双峰性持续存在，支持在早期盘演化中的自重力效应，以及可能的其他消散机制。
• 扩大后的样本使具有同质 L_acc 和多条线的 L_line 测量的 YSOs 数量翻倍以上，精炼了 L_acc–L_line 的经验关系。
• 该研究确认了更广泛的 Lupus 样本中显著的 L_acc–L_line 相关性，并为大多数 Lupus class II/transition 群体提供自洽的吸积诊断。