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arXiv 1603.09307astro-ph.SR
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TW Hya星协中的M型星：恒星X射线和盘耗散

M Stars in the TW Hya Association: Stellar X-rays and Disk Dissipation

Joel H. Kastner, David A. Principe, Kristina Punzi, Beate Stelzer, Uma Gorti, Ilaria Pascucci, Costanza Argiroffi

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28 pages, 4 figures; to appear in The Astronomical Journal
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摘要：

为了研究年轻低质量恒星的强X射线辐射与其星周行星形成盘的寿命之间的潜在联系，我们编制了$\sim$ 800万年前TW Hya星协（TWA）中M星的X射线光度（$L_X$），目前可获得该星协的X射线数据。我们的调查包括分析档案钱德拉数据的TWA二元系统TWA 8，9和13。虽然我们的研究遭受贫穷的统计晚于M3的恒星，我们发现一个趋势，减少$L_X/L_{bol}$减少$T_{eff}$减少TWA M星，其中最早的类型（M0-M2）的恒星集群附近的$\log{(L_X/L_{bol})} \approx -3.0$，然后$\log{(L_X/L_{bol})}$减少，其分布范围扩大，为M4型及以后。显示残留原始盘材料的证据的TWA星的分数也急剧增加，在这个相同的（中M）光谱类型制度。低质量TWA星的相对X射线光度与其周星盘的长度之间的这种明显的关系表明，在TWA中围绕早期M型星运行的原始盘由于其持续的大X射线通量而迅速分散。相反地，星协中围绕着质量最低的前MS恒星和前MS棕矮星的星盘可能幸存下来，因为它们的X射线光度和星盘光蒸发率开始时非常低，然后在前MS演化的相对早期进一步下降。

英文摘要：

To investigate the potential connection between the intense X-ray emission from young, low-mass stars and the lifetimes of their circumstellar, planet-forming disks, we have compiled the X-ray luminosities ($L_X$) of M stars in the $\sim$8 Myr-old TW Hya Association (TWA) for which X-ray data are presently available. Our investigation includes analysis of archival Chandra data for the TWA binary systems TWA 8, 9, and 13. Although our study suffers from poor statistics for stars later than M3, we find a trend of decreasing $L_X/L_{bol}$ with decreasing $T_{eff}$ for TWA M stars wherein the earliest-type (M0--M2) stars cluster near $\log{(L_X/L_{bol})} \approx -3.0$ and then $\log{(L_X/L_{bol})}$ decreases, and its distribution broadens, for types M4 and later. The fraction of TWA stars that display evidence for residual primordial disk material also sharply increases in this same (mid-M) spectral type regime. This apparent anticorrelation between the relative X-ray luminosities of low-mass TWA stars and the longevities of their circumstellar disks suggests that primordial disks orbiting early-type M stars in the TWA have dispersed rapidly as a consequence of their persistent large X-ray fluxes. Conversely, the disks orbiting the very lowest-mass pre-MS stars and pre-MS brown dwarfs in the Association may have survived because their X-ray luminosities and, hence, disk photoevaporation rates are very low to begin with, and then further decline relatively early in their pre-MS evolution.

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