标题： 猎户座星云团恒星的日冕丰度 显示英文标题

标题： Coronal Abundances in Orion Nebula Cluster Stars

摘要： 在钱德拉猎户座超深项目（COUP）观测之后，我们研究了猎户座星团中146颗X射线明亮的前主序星中热等离子体的化学成分。 利用钱德拉ACIS仪器获得的低分辨率X射线光谱，我们报告了对86个略微吸收且明亮的X射线源子样本中各个元素丰度的测量结果。 X射线发射来源于等离子体，其温度和元素丰度与年老恒星活跃的日冕非常相似。 如果假设太阳光球层丰度作为参考值，则可以清楚地看出丰度随First Ionization Potential (FIP)的变化模式。 通过广泛的模拟验证了这些结果。 观测到的丰度分布与所有恒星具有单一丰度模式一致，尽管可能存在弱依赖于耀斑环尺寸的情况。 钙的丰度似乎是唯一在恒星之间变化显著的，但该数值受到相对较大的不确定性影响。 X射线明亮的COUP源的整体性质确认，发射等离子体中的铁相对于太阳成分以及平均恒星光球层值都处于不足状态。 将当前等离子体的丰度与恒星光球层以及星云气体组分的丰度进行比较，表明其他所有元素（尤其是高FIP元素Ne、Ar、O和S以及低FIP元素Si）均与现有测量值一致。 我们得出结论，只有铁显示出显著的化学分离效应，其丰度相对于恒星组成减少了1.5到3倍。 显示英文摘要

摘要： Following the Chandra Orion Ultradeep Project (COUP) observation, we have studied the chemical composition of the hot plasma in a sample of 146 X-ray bright pre-main sequence stars in the Orion Nebula Cluster. We report measurements of individual element abundances for a subsample of 86 slightly-absorbed and bright X-ray sources, using low resolution X-ray spectra obtained from the Chandra ACIS instrument. The X-ray emission originates from a plasma with temperatures and elemental abundances very similar to those of active coronae in older stars. A clear pattern of abundances vs. First Ionization Potential (FIP) is evident if solar photospheric abundances are assumed as reference. The results are validated by extensive simulations. The observed abundance distributions are compatible with a single pattern of abundances for all stars, although a weak dependence on flare loop size may be present. The abundance of calcium is the only one which appears to vary substantially between stars, but this quantity is affected by relatively large uncertainties. The ensemble properties of the X-ray bright COUP sources confirm that the iron in the emitting plasma is underabundant with respect to both the solar composition and to the average stellar photospheric values. Comparison of the present plasma abundances with those of the stellar photospheres and those of the gaseous component of the nebula, indicates a good agreement for all the other elements with available measurements, and in particular for the high-FIP elements (Ne, Ar, O, and S) and for the low-FIP element Si. We conclude that there is evidence of a significant chemical fractionation effect only for iron, which appears to be depleted by a factor 1.5--3 with respect to the stellar composition.