On the sizes of stellar X-ray coronae
On the sizes of stellar X-ray coronae
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DOI：
10.1051/0004-6361:20040504
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发表时间：
2004-07
影响因子：
6.5
通讯作者：
J. Ness;M. Güdel;J. H. M. M. Schmitt-J.-H.-M.-M.-Schmitt-2244038850;M. Audard;A. Telleschi
中科院分区：
文献类型：
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作者：
J. Ness;M. Güdel;J. H. M. M. Schmitt-J.-H.-M.-M.-Schmitt-2244038850;M. Audard;A. Telleschi
来自恒星X射线日冕的空间信息无法直接评估，但来自太阳日冕的标度定律使得有可能从温度和密度等物理参数估计恒星日冕的大小。虽然日冕等离子体的温度早已可用，我们集中在新的可用的密度测量线通量的X射线线测量的大样本的恒星日冕的钱德拉和XMM-牛顿光栅。我们收集了42个恒星日冕的64个光栅光谱。用Ness & Wichmann（2002）的CORA单用途线拟合工具测量强H-样离子和He-样离子以及Fe离子线的线计数。密度估计从氦类f /i流量比的O和Ne代表较冷（1-6 MK）的等离子体成分。密度分布在来自O三重态的logne = 9.5−11和来自Ne三重态的logne = 10.5−12之间，但我们要注意的是，后一种三重态可能会受到Fe和Fe线的污染。我们发现，低活动恒星（由来自H和He类线通量比的特征温度参数化）倾向于显示来自O的密度不超过10 10 cm −3的几倍，而没有发现更活跃的恒星的明确趋势。调查密度的较热的等离子体与不同的Fe的电子线比，我们发现，没有一个光谱一致地表明存在非常高的密度。我们认为，我们的测量是兼容的低密度限制的相应比率（105 × 10 12 cm −3）。这些上限与发射有源区中的恒定压力一致。我们专注于常用的Rosner等人（1978年）的标度律推导环的长度从温度和密度假设环状结构相同的积木。我们从直接测量的离子特定的发射措施和密度的发射体积。可用的体积计算从环的长度和恒星半径，并与发射体积进行比较，以推断填充因子。对于所有阶段的活动，我们发现类似的填充因子高达0.1。
Spatial information from stellar X-ray coronae cannot be assessed directly, but scaling laws from the solar corona make it possible to estimate sizes of stellar coronae from the physical parameters temperature and density. While coronal plasma temperatures have long been available, we concentrate on the newly available density measurements from line fluxes of X-ray lines measured for a large sample of stellar coronae with the Chandra and XMM-Newton gratings. We compiled a set of 64 grating spectra of 42 stellar coronae. Line counts of strong H-like and He-like ions and Fe lines were measured with the CORA single-purpose line fitting tool by Ness & Wichmann (2002). Densities are estimated from He-like f /i flux ratios of O and Ne representing the cooler (1-6 MK) plasma components. The densities scatter between logne ≈ 9.5−11 from the O triplet and between logne ≈ 10.5−12 from the Ne triplet, but we caution that the latter triplet may be biased by contamination from Fe and Fe lines. We find that low-activity stars (as parameterized by the characteristic temperature derived from H- and He-like line flux ratios) tend to show densities derived from O of no more than a few times 10 10 cm −3 , whereas no definitive trend is found for the more active stars. Investigating the densities of the hotter plasma with various Fe line ratios, we found that none of the spectra consistently indicates the presence of very high densities. We argue that our measurements are compatible with the low-density limit for the respective ratios (≈5 × 10 12 cm −3 ). These upper limits are in line with constant pressure in the emitting active regions. We focus on the commonly used Rosner et al. (1978) scaling law to derive loop lengths from temperatures and densities assuming loop-like structures as identical building blocks. We derive the emitting volumes from direct measurements of ion- specific emission measures and densities. Available volumes are calculated from the loop-lengths and stellar radii, and are compared with the emitting volumes to infer filling factors. For all stages of activity we find similar filling factors up to 0.1.