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

无论是经典的还是弱线的金牛座T星，都被认为是丰富的x射线发射器。由于它们的深对流区和快速旋转，磁活动被认为是观测到的x射线活动现象的基础。然而，最近获得的经典金牛座T星（cTTS）的高分辨率x射线光谱显示，与正常恒星相比，cTTS的x射线光谱在某些关键光谱特征上存在明显差异，这表明其他或至少是额外的x射线产生机制。由于cTTS仍在从星周盘吸积物质，磁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者。我们已经开发了这种吸积激波柱的一维模型来计算其涌现光谱，并将该模型与实际的x射线观测结果进行了比较。通过这种方式，我们可以仅从x射线观测中推导出坠落速度、激波密度、坠落物质的化学丰度、质量吸积率和填充因子。在拟议的项目中，我们计划通过纳入额外的物理学来改进该模型，并将其范围扩展到紫外线中的线通量和线轮廓。这将大大提高我们将观测到的辐射区分为日冕源和吸积源的能力，并进一步描述物体的特征。

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.