Coronal Structure and Abundances of Capella from Simultaneous EUVE and ASCA Spectroscopy
Coronal Structure and Abundances of Capella from Simultaneous EUVE and ASCA Spectroscopy
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DOI：
10.1086/308350
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发表时间：
2000-02
期刊：
影响因子：
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通讯作者：
N. Brickhouse;A. Dupree;Richard J. Edgar;D. Liedahl;Stephen A. Drake;N. E. White;K. P. Singh
中科院分区：
文献类型：
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作者：
N. Brickhouse;A. Dupree;Richard J. Edgar;D. Liedahl;Stephen A. Drake;N. E. White;K. P. Singh
我们报告了1996年3月对光谱双星Capella的EUVE和ASCA同时观测的分析。EUVE光谱以高度电离的铁线为主，需要在广泛的温度范围内进行连续的发射测量分布。ASCA谱显示了S、硅和镁的类He谱线发射特征，以及Fe、Ni、类H和类He Ne谱线的未分辨L壳层发射线。这些线特征中的通量不能独立于连续谱通量来确定。ASCA谱相对较软，在4keV以上的计数很少。由极紫外铁谱线强度的线基分析确定的发射测量分布在Te~6×105~2×107K之间有很好的约束，但不超过这个范围，因为FeXXIV是EUVE观测到的最高温度线。由于EUVE对Capella的反复观测表明，最热的EUV发射材料(FeXXI至XXIV)的发射线强度变化最大可达4倍，因此ASCA光谱对于扩大温度覆盖范围非常重要。因此，ASCA光谱的高能截止对最高温度排放措施提供了限制。原则上，元素丰度是从全局拟合到ASCA谱确定的；然而，还没有找到适合Capella高信噪比ASCA性能验证谱的模型(1993年9月2日)。新的ASCA谱(1996年3月3-4日)显示出类似的拟合困难的模式。使用EUVE测量(1996年3月3-7日)来约束模型，我们对原子数据、源物理和仪器校准进行了敏感性研究。1.2keV附近的等离子体光谱发射模型(Raymond-Smith，Mekal，SPEX)相对于观测到的ASCA计数谱似乎有通量亏损。Liedahl和Brickhouse用HULLAC代码计算的新原子模型提供了一组现有等离子体代码中缺失的行，以填补这一通量赤字。结合这些额外的谱线，极大地改善了光谱模型对数据的拟合，允许可靠地确定元素丰度。新的原子模型在卡佩拉问题上的成功应用可能会产生广泛的影响，影响星系的光谱模型、星团冷却流动、超新星遗迹以及其他恒星日冕。用新的原子模型同时获得的ASCA和EUVE数据的分析证实了以前的EUVE结果，在Te~6×106K，Capella的连续发射测量分布有很大的增强，而双温模型实际上比EUVE导出的连续模型提供了更好的ASCA谱拟合，但EUVE数据在两个温度下不能很好地拟合。我们发现，Mg、Si、S和Fe的丰度与太阳光球值一致，而Ne的贫度约为太阳光球值的3~4倍。
We report analysis of the simultaneous 1996 March EUVE and ASCA observations of the spectroscopic binary Capella. The EUVE spectrum is dominated by lines of highly ionized Fe, requiring a continuous emission-measure distribution over a wide range of temperatures. The ASCA spectrum shows He-like line emission features of S, Si, and Mg, as well as unresolved L-shell emission lines of Fe and Ni and H-like and He-like Ne lines. The flux in these line features cannot be determined independently from the continuum flux. The ASCA spectrum is relatively soft, with few counts above 4 keV. The emission-measure distribution determined by Line-Based Analysis of the EUV Fe line intensities is well constrained from Te ~ 6 × 105 to 2 × 107 K, but it is not constrained above this range since Fe XXIV is the highest temperature line observed with EUVE. Since repeated observations of Capella by EUVE have shown that emission-line intensities of the hottest EUV-emitting material (Fe XXI to XXIV) vary by factors up to 4, the ASCA spectrum is important for extending the temperature coverage. Thus, the high-energy cut-off of the ASCA spectrum provides a constraint on the highest temperature emission measures. In principle, elemental abundances are determined from global fits to the ASCA spectrum; however, no well-fitting model has been found for the high signal-to-noise ASCA performance verification spectrum of Capella (1993 September 2). The newer ASCA spectrum of Capella (1996 March 3-4) shows a similar pattern of fitting difficulties. Using the EUVE measurements (1996 March 3-7) to constrain models, we have conducted sensitivity studies of the atomic data, source physics, and instrument calibration. The plasma spectral emission models (Raymond-Smith, MEKAL, SPEX) around 1.2 keV appear to have flux deficits relative to the observed ASCA count spectrum. New atomic models by Liedahl and Brickhouse, calculated with the HULLAC code, provide a set of lines—missing from the existing plasma codes—to fill in this flux deficit. Incorporating these additional lines dramatically improves the spectral model fits to the data, allowing reliable determination of elemental abundances. The successful application of the new atomic models to the Capella problem can have widespread implications, affecting spectral models of galaxies, cluster cooling flows, and supernova remnants, as well as other stellar coronae. Analysis with the new atomic models of the simultaneous ASCA and EUVE data confirms the previous EUVE results that the continuous emission-measure distribution of Capella has a strong enhancement at Te ~ 6 × 106 K. While a two-temperature model actually provides a better fit to the ASCA spectrum than the EUVE-derived continuous model, the EUVE data are not well fitted with only two temperatures. We find that the abundances of Mg, Si, S, and Fe are consistent with solar photospheric values, while Ne appears to be underabundant by a factor of ~3 to 4.