[论文解读] X-ray variability of PMS stars - Toward an explanation of the different X-ray properties of CTTS and WTTS

E. Flaccomio, G. Micela+1 more|arXiv (Cornell University)|Oct 25, 2012

Astrophysics and Star Formation Studies被引用 16

一句话总结

该论文提出，原恒星周围结构（如盘面翘曲和吸积流）引起的时变吸收，可解释经典金牛T型变星（CTTSs）相较于弱线金牛T型变星（WTTSs）观测到的更高X射线变异性及更低、更分散的X射线光度。该机制在长达约10天的时间尺度上起作用，假设CTTSs的日冕本身与WTTSs的内在特性相似，可同时解释其增强的变异性与表观的X射线发射抑制。

ABSTRACT

The intense X-ray emission from coronae and accretion shocks in young PMS stars is likely to play an important role in the evolution and dispersal of circumstellar disks. Several aspects of the physics of this X-ray emission remain mysterious, e.g., whether and how much accretion affects coronal emission. We studied the X-ray variability of ~1 Myr old low-mass PMS stars as a function of timescale, stellar rotation, and stellar characteristics, in order to gain insights on the working of PMS coronae, their X-ray emission, and the circumstellar environment in which they are immersed. We have exploited the ~850 ksec long Chandra observation of the Orion Nebula Cluster obtained by the COUP collaboration in Jan. 2003, and statistically analyzed the X-ray lightcurves of low-mass stars in several subsamples. In particular, we characterized the different X-ray behavior of stars with and without circumstellar accretion disks. Accreting stars (Classical T Tauri Stars, CTTSs) are found to be more variable than non accreting ones (Weak-lined T Tauri Stars, WTTSs) at all timescales and in all the X-ray energy bands considered. Variability is seen to increase with time-scale up to ~10 days, i.e. the longest probed. Signatures of rotational modulation are observed for both CTTSs and WTTSs, and most clearly for CTTSs in the soft X-ray band. Lower mass stars are more variable than higher mass ones. We propose that the difference in variability between CTTSs and WTTSs may be explained assuming that the X-ray emission of CTTS is affected by time-variable absorption due circumstellar structures, such as warps in the inner disk and/or accretion streams. This suggestion is appealing because, in the hypothesis that the coronae of CTTSs and WTTSs are similar, it may also explain why CTTSs have lower and more scattered X-ray emission levels with respect to WTTSs.

研究动机与目标

• 调查前主序星中X射线变异性起源，特别是经典金牛T型变星（CTTSs）与弱线金牛T型变星（WTTSs）之间的差异。
• 确定X射线发射的变异性是由日冕的内在过程驱动，还是由周围物质的可变遮蔽引起。
• 评估恒星质量、自转速度及周围原行星盘结构在塑造X射线光曲线中的作用。
• 检验假设：盘面翘曲和吸积流引起的可变吸收可解释CTTSs观测到的X射线特性。

提出的方法

• 对猎户星云星团（COUP巡天）850 ksec的Chandra观测数据中的X射线光曲线进行统计分析。
• 基于吸积特征和光谱分类，将低质量前主序星划分为CTTSs和WTTSs。
• 在不同X射线能量 band（例如，软X射线500–1500 eV）中，计算长达约10天的多个时间尺度上的变异性幅度（VA）。
• 将变异性幅度与恒星质量、总辐射光度和自转周期进行比较，以识别趋势。
• 评估X射线光度-质量关系与X射线光度-总辐射光度关系中的散射，考虑测量不确定性。
• 采用AA Tau型模型作为物理解释框架，以解释软X射线中的周期性调制与长期变异性。

实验结果

研究问题

• RQ1 为何经典金牛T型变星（CTTSs）在所有时间尺度和能量 band 中均表现出比弱线金牛T型变星（WTTSs）更高的X射线变异性？
• RQ2 CTTSs中观测到的X射线变异性是否可由周围结构引起的时变吸收解释，而非日冕本身的内在变异性？
• RQ3 盘面翘曲和吸积流在调制CTTSs的X射线发射（尤其是软X射线波段）中起何作用？
• RQ4 为何CTTSs表现出比WTTSs更低且更分散的X射线光度，尽管其日冕温度相似？
• RQ5 X射线发射的长期变异性（长达约10天）在多大程度上源于周围结构的长期演化？

主要发现

• 在所有时间尺度和所有X射线能量 band 中，CTTSs 的X射线变异性显著高于WTTSs，且变异性幅度在约10天内持续增加。
• 在软X射线波段（500–1500 eV）中，CTTSs 的自转调制最为明显，表明日冕辐射存在周期性遮蔽。
• 低质量恒星的变异性幅度高于高质量恒星，且存在随质量增加而变异性降低的清晰趋势。
• CTTSs 的X射线光度-质量关系中的散射（0.47 dex）超过WTTSs（0.34 dex），且无法完全由测量不确定性解释。
• CTTSs 的X射线光度观测散射显著大于质量、光度和双星污染估计的0.43 dex不确定性，表明存在内在变异性。
• CTTSs 的长期变异性与自转调制最合理的解释是周围结构（如盘面翘曲和吸积流）引起的时变吸收。