An accretion model for the X-ray emission of classical T Tauri stars

众所周知，经典的和弱线的金牛座T星都是丰富的X射线发射体。由于其深对流区和快速自转，磁活动被认为低估了观测到的X射线活动现象。然而，最近获得的经典T金牛座恒星(CTTS)的高分辨率X射线光谱显示，与正常恒星相比，CTTS的一些关键光谱特征明显不同，这表明存在其他或至少是额外的X射线产生机制(S)。由于CTT仍在吸积来自恒星周盘的物质，因此在磁性漏斗吸积柱底部的吸积激波显然是这种额外的X射线产生机制的候选。我们建立了这样一个吸积激波柱的一维模型来计算其出射谱，并与实际的X射线观测结果进行了对比。通过这种方式，我们可以仅从X射线观测中得出流入速度、冲击波密度、流入物质的化学丰度、质量吸积率和填充因子。通过拟议的项目，我们计划通过加入额外的物理来改进这一模型，并将其范围扩展到紫外线中的线通量和线轮廓。这将极大地增强我们将观测到的辐射划分为日冕和吸积源，并描述更多天体的能力。

Both classical as well as weak-line T Tauri stars are known to be copious X-ray emitters. Because of their deep convection zones and rapid rotation magnetic activity was thought to underly the observed X-ray activity phenomena. Recently obtained high-resolution X-ray spectroscopy of classical T Tauri stars (cTTS) shows, however, clear differences in some key spectroscopic features between the X-ray spectra of cTTS as compared to normal stars, suggesting other or at least additional X-ray production mechanism(s). Since cTTS are still accreting material from a circumstellar disk, an accretion shock at the bottom of a magnetically funnelled accretion column is an obvious candidate for such an additional X-ray production mechanism. We have developed a one-dimensional model of such an accretion shock column to compute its emergent spectrum and confronted this model with actual X-ray observations. In this fashion we can to derive infall velocities, shock densities, chemical abundances of the infalling material, mass accretion rates and filling factors from X-ray observations alone. With the proposed project we plan to improve this model by incorporating additional physics and extend its scope to line fluxes and line profiles in the UV. This will greatly enhance our ability to divide the observed emission between coronal and accretion dominated sources and to characterise further objects.