Coronal Abundances in Orion Nebula Cluster Stars
Coronal Abundances in Orion Nebula Cluster Stars
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猎户座星云星团恒星的日冕丰度
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发表时间：
2007
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通讯作者：
P. S. University
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作者：
A. Maggio;E. Flaccomio;F. Favata;G. Micela;S. Sciortino;E. Feigelson;K. V. G. I. ;Astrophysics Division ;Space Science Department of Esa;P. S. University
继钱德拉猎户座超深计划 (COUP) 观测之后，我们研究了猎户座星云团 (ONC) 中 146 颗 X 射线明亮的前主序星样本中热等离子体的化学成分。我们使用从 Chandra ACIS 仪器获得的低分辨率 X 射线光谱报告了 86 个轻微吸收和明亮 X 射线源子样本的单个元素丰度的测量结果。 X射线发射源自等离子体，其温度和元素丰度与老恒星中的活跃日冕非常相似。如果假设太阳光球丰度作为参考，则丰度与第一电离势（FIP）的清晰模式是显而易见的。结果通过广泛的模拟得到验证。观测到的丰度分布与所有恒星的单一丰度模式兼容，尽管可能存在对耀斑环大小的弱依赖性。钙的丰度是唯一一种在恒星之间存在显着差异的元素，但这一数量受到相对较大的不确定性的影响。 X 射线明亮的 COUP 源的整体特性证实，就太阳成分和平均恒星光球值而言，发射等离子体中的铁含量不足。将当前等离子体丰度与恒星光球层和星云气态成分的等离子体丰度进行比较，表明所有其他元素与可用测量结果具有良好的一致性，特别是高 FIP 元素（Ne、Ar、O 和 S）和低 FIP 元素 Si。我们得出的结论是，有证据表明仅铁具有显着的化学分馏效应，相对于恒星成分而言，铁似乎被消耗了 1.5-3 倍。
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 (ONC). 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 versus 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 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.