A DEEP CHANDRA X-RAY SPECTRUM OF THE ACCRETING YOUNG STAR TW HYDRAE
A DEEP CHANDRA X-RAY SPECTRUM OF THE ACCRETING YOUNG STAR TW HYDRAE
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DOI：
10.1088/0004-637x/710/2/1835
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发表时间：
2010-02-20
影响因子：
4.9
通讯作者：
Wolk, S.
中科院分区：
文献类型：
--
作者：
Brickhouse, N. S.;Cranmer, S. R.;Wolk, S.
我们用钱德拉高能传输光栅对489K观测到的吸积年轻恒星长蛇座TW的X射线能谱进行了分析。光谱为电子温度T(E)、电子密度N(E)、氢柱密度N(H)、相对元素丰度和速度提供了一套丰富的诊断方法，并揭示了其来源在恒星大气的三个不同区域：恒星日冕、吸积激波和非常大的扩展后热等离子体体积。光谱中存在的Mg XII、Si XIII和Si XIV发射线需要在10mK附近的日冕结构。在2.5MK形成的较低温度线(例如，来自O VIII、Ne IX和Mg XI)似乎与吸积激波的发射更一致。类氦Ne_x线比诊断表明，激波中T(E)=2.50+/-0.25mK，N(E)=3.0+/-0.2×10(12)cm(-3)。这些值与标准的磁吸积模型符合得很好。然而，钱德拉的观测结果与目前对震后等离子体的模型预测大相径庭。这种气体预计会以辐射方式冷却，在流入密度越来越高的恒星大气时产生Ovii。令人惊讶的是，Ovii表明N(E)=5.7(-1.2)(+4.4)×10(11)cm(-3)，比吸积激波本身的N(E)低5倍，与模型预测的N(E)低7倍。我们估计，产生Ovii的震后区域的体积大约是地震本身的300倍，发射质量大约是地震本身的30倍。显然，受冲击的等离子体会将周围的恒星大气加热到发射软X射线的温度，并将这种物质提供给附近的大型磁性结构--这些结构可能是闭合的磁环，也可能是导致质量外流的开放磁场。我们的模型解释了在许多吸积系统中发现的软X射线过剩，以及在一些恒星中未能观测到高N(E)信号的原因。这种吸积形成的日冕可能在吸积年轻恒星的大气层中随处可见。
We present X-ray spectral analysis of the accreting young star TW Hydrae from a 489 ks observation using the Chandra High Energy Transmission Grating. The spectrum provides a rich set of diagnostics for electron temperature T(e), electron density N(e), hydrogen column density N(H), relative elemental abundances, and velocities, and reveals its source in three distinct regions of the stellar atmosphere: the stellar corona, the accretion shock, and a very large extended volume of warm postshock plasma. The presence of Mg XII, Si XIII, and Si XIV emission lines in the spectrum requires coronal structures at similar to 10 MK. Lower temperature lines (e. g., from O VIII, Ne IX, and Mg XI) formed at 2.5 MK appear more consistent with emission from an accretion shock. He-like Ne IX line ratio diagnostics indicate that T(e) = 2.50 +/- 0.25 MK and N(e) = 3.0 +/- 0.2 x 10(12) cm(-3) in the shock. These values agree well with standard magnetic accretion models. However, the Chandra observations significantly diverge from current model predictions for the postshock plasma. This gas is expected to cool radiatively, producing O VII as it flows into an increasingly dense stellar atmosphere. Surprisingly, O VII indicates N(e) = 5.7(-1.2)(+4.4) x 10(11) cm(-3), 5 times lower than N(e) in the accretion shock itself and similar to 7 times lower than the model prediction. We estimate that the postshock region producing O VII has roughly 300 times larger volume and 30 times more emitting mass than the shock itself. Apparently, the shocked plasma heats the surrounding stellar atmosphere to soft X-ray emitting temperatures and supplies this material to nearby large magnetic structures-which may be closed magnetic loops or open magnetic field leading to mass outflow. Our model explains the soft X-ray excess found in many accreting systems as well as the failure to observe high N(e) signatures in some stars. Such accretion-fed coronae may be ubiquitous in the atmospheres of accreting young stars.