Photospheric and coronal abundances in solar-type stars: the peculiar case of τ Bootis
Photospheric and coronal abundances in solar-type stars: the peculiar case of τ Bootis
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太阳型恒星的光球和日冕丰度：τ Bootis 的特殊情况
DOI：
10.1051/0004-6361/201015587
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发表时间：
2010
影响因子：
6.5
通讯作者：
L. Scelsi
中科院分区：
文献类型：
--
作者：
Antonio Maggio;J. Sanz;L. Scelsi
目标。太阳型恒星的化学丰度在许多方面仍然是一个备受争议的话题。特别是，已知行星宿主恒星富含金属，但这种特性是否也适用于外层恒星大气的化学成分仍有待澄清。更一般地说，晚型恒星的日冕和感光赫里克丰度在许多情况下似乎是不同的，但要了解恒星大气中的化学分层效应如何起作用，需要比目前更大的观测基础。方法.我们获得了牧夫座的XMM-牛顿高分辨率X射线谱，牧夫座是一颗著名的附近星星，有一颗木星质量的近距离行星。我们分析了这些数据，目的是进行详细的线为基础的发射测量分析，并得出单独的元素在日冕中的丰度与两种不同的方法独立应用。我们已经将Bootis的日冕丰度与基于高分辨率光谱的已公布的光球丰度进行了比较，并与其他具有不同磁活动水平的晚型恒星（包括太阳）进行了比较。结果我们发现，两种方法对热等离子体的发射测量分布和日冕丰度的测量结果在统计不确定性范围内是一致的，在2 μ水平上的差异仅限于3-4 MK温度下的等离子体量和O和Ni丰度。在这两种情况下，日冕和光球测量都可用的元素（C，N，O，Si，Fe和Ni）在日冕中的丰度系统地减少了3倍或更多，除了日冕Ni丰度与光球值相似。与其他类似活动水平的晚型恒星的比较表明，这些日冕/光球丰度比是牧夫座特有的，可能与这颗行星宿主星星的特征性过金属量有关。
Aims. Chemical abundances in solar-type stars are still a much debated topic in many respects. In particular, planet-hosting stars are known to be metal-rich, but whether or not this peculiarity applies also to the chemical composition of the outer stellar atmospheres is still to be clarified. More in general, coronal and photosp heric abundances in late-type stars appear to be different in many cases, but understanding how chemical stratification e ffects work in stellar atmospheres requires an observational base larger than currently available. Methods. We obtained XMM-Newton high-resolution X-ray spectra ofBootis, a well known nearby star with a Jovian-mass close-in planet. We analyzed these data with the aim to perform a detailed line-based emission measure analysis and derive the abundances of individual elements in the corona with two different methods applied independently. We have compared the coronal abundances of � Bootis with published photospheric abundances based on high-resolution optical spectra and with those of other late-t ype stars with different magnetic activity levels, including the Sun. Results. We find that the two methods provide consistent results withi n the statistical uncertainties for both the emission measu re distribution of the hot plasma and for the coronal abundances, with discrepancies at the 2�level limited to the amount of plasma at temperatures of 3-4 MK and to the O and Ni abundances. In both cases, the elements for which both coronal and photospheric measurements are available (C, N, O, Si, Fe, and Ni) result systematically less abundant in the corona by a factor 3 or more, with the exception of the coronal Ni abundance which is similar to the photospheric value. Comparison with other late-type stars of similar activity level shows that these coronal/photospheric abundance ratios are peculiar toBootis and possibly related to the characteristic over-metallicity of this planet-hosting star.