[论文解读] X-ray deficiency on strong accreting T Tauri stars - Comparing Orion with Taurus

Ignacio Bustamante, B. Merín+4 more|arXiv (Cornell University)|Dec 30, 2015

Astrophysics and Star Formation Studies参考文献 45被引用 5

一句话总结

本研究利用钱德拉和哈勃望远镜的数据，调查了在猎户座和奥里昂星云两个星形成区中，强吸积的T Tauri恒星的X射线光度与质量吸积率之间的反相关性。研究结果证实，更高的吸积率与较低的剩余X射线发射相关，表明吸积过程会抑制X射线输出，这一现象是T Tauri恒星的固有特性，与环境无关。

ABSTRACT

Depending on whether a T Tauri star accretes material from its circumstellar disk or not, different X-ray emission properties can be found. The accretion shocks produce cool heating of the plasma, contributing to the soft X-ray emission from the star. Using X-ray data from the Chandra Orion Ultra-deep Project and accretion rates that were obtained with the Hubble Space Telescope/WFPC2 photometric measurements in the Orion Nebula Cluster, we studied the relation between the accretion processes and the X-ray emissions of a coherent sample of T Tauri sources in the region. We performed regression and correlation analyses of our sample of T Tauri stars between the X-ray parameters, stellar properties, and the accretion measurements. We find that a clear anti-correlation is present between the residual X-ray luminosity and the accretion rates in our samples in Orion that is consistent with that found on the XMM-Newton Extended Survey of the Taurus molecular cloud (XEST) study. We provide a catalog with X-ray luminosities (corrected from distance) and accretion measurements of an Orion Nebula Cluster (ONC) T Tauri stars sample. Although Orion and Taurus display strong differences in their properties (total gas and dust mass, star density, strong irradiation from massive stars), we find that a similar relation between the residual X-ray emission and accretion rate is present in the Taurus molecular cloud and in the accreting samples from the Orion Nebula Cluster. The spread in the data suggests dependencies of the accretion rates and the X-ray luminosities other than the stellar mass, but the similarity between Orion and Taurus hints at the environment not being one of them. The anti-correlation between the residual X-ray luminosity and mass accretion rate is inherent to the T Tauri stars in general, independent of their birthplace and environment, and intrinsic to early stellar evolution.

研究动机与目标

• 检验猎户座与奥里昂星云之间的环境差异是否影响吸积T Tauri恒星的X射线发射特性。
• 检验在两个不同的星形成区中，剩余X射线光度与质量吸积率之间反相关性的稳健性。
• 利用多波段数据评估猎户星云星团中吸积率分类（经典型与弱线型T Tauri恒星）的可靠性。
• 评估测量不确定性和异常值检测对X射线与吸积率分析中回归结果的影响。
• 确定强吸积者中X射线不足是否为原行星质量恒星的内在属性，还是受外部环境因素影响。

提出的方法

• 从钱德拉猎户星云超深场项目获取X射线光度数据，并利用哈勃空间望远镜/WFPC2的测光数据获取猎户星云星团中的吸积率。
• 使用Python中的ODRPACK执行最小二乘回归，以建立剩余X射线光度与质量吸积率之间的关系模型。
• 通过astroML MCMC软件包应用异常值检测与剔除，以评估对测量不确定性的敏感性。
• 采用两种吸积率指标：Hα等效宽度和U波段过量，分别分析各子样本。
• 对X射线光度进行距离修正，并与奥里昂星云中的XEST调查结果进行比较，以检验环境独立性。
• 评估提高不确定度估计值对回归稳定性的影响，特别是避免将变异性吸积状态误判为异常值。

实验结果

研究问题

• RQ1 在猎户星云星团中，是否如在奥里昂分子云中先前观测到的那样，存在剩余X射线光度与质量吸积率之间的反相关性？
• RQ2 与奥里昂星云相比，猎户星云中更高的恒星密度和更强的辐射场是否改变了吸积T Tauri恒星的X射线发射特性？
• RQ3 经典型与弱线型T Tauri恒星之间分类的不确定性在多大程度上影响了观测到的X射线-吸积相关性？
• RQ4 回归结果对测量不确定性和异常值剔除方法的敏感性如何，特别是考虑到吸积过程的内在变异性？
• RQ5 强吸积者中的X射线不足是T Tauri恒星的内在属性，还是受外部环境因素调制？

主要发现

• 在猎户星云星团中发现剩余X射线光度与质量吸积率之间存在显著的反相关性，与奥里昂分子云中的先前结果一致。
• 该反相关性在Hα和U波段过量吸积率子样本中均持续存在，尽管基于Hα的结果统计显著性较弱。
• U波段过量子样本的结果在数值上与奥里昂星云中的XEST调查结果一致，支持了该相关性的稳健性。
• 对低测量不确定度敏感的异常值检测方法可能扭曲回归结果，但提高不确定度估计值可恢复原始的OLS回归结果。
• 尽管气体质量、尘埃含量和恒星密度存在差异，猎户星云与奥里昂星云中X射线-吸积行为的相似性表明，该反相关性是吸积T Tauri恒星的内在属性。
• 观测到的X射线不足可能源于吸积激波对软X射线的热化或吸收，而非环境效应。