磁多普勒成像考虑大气结构的修改，由于当地的丰度：奢侈品或必要性？
Magnetic Doppler imaging considering atmospheric structure modifications due to local abundances: a luxury or a necessity?
摘要：
磁多普勒成像是目前解释恒星高分辨率分光偏振观测的最有力的方法。这项技术揭示了Ap星表面存在意想不到的小尺度磁场。这些研究最近受到了Stift等人（2012年）的批评，他们声称磁反转并不稳健，并且由于忽略了金属丰度增加区域的局部大气结构对斯托克斯线轮廓的反馈而受到破坏。我们表明，Stift等人错误地解释了已发表的磁多普勒成像结果，忽略了磁映射背后的一些最基本的原则。我们表明，大气结构的变化与化学斑点的恒星表面的影响当地的连续强度，但可以忽略不计的归一化本地斯托克斯配置文件。对于磁盘集成光谱的Ap星与极端的丰度变化，我们发现，平均模型大气的假设导致适度的错误斯托克斯我，但偏振光谱可以忽略不计。利用一种新的磁反演编码，结合大气结构的水平变化，我们重建了Ap星α ^2 CVn的新的磁场和铁丰度图。与以前基于单一模型大气的建模相比，化学斑点的分布变化不大，而磁场几何形状根本没有变化。这表明，由于不合理的假设和推断，以及方法上的缺陷和分析的不一致性，Stift等人的断言被夸大了。我们的讨论证明，发表的磁性反转的基础上，平均恒星大气是强大的和可靠的，并存在小尺度磁场结构的表面上的Ap星是真实的。
英文摘要：
Magnetic Doppler imaging is currently the most powerful method of interpreting high-resolution spectropolarimetric observations of stars. This technique has revealed the presence of unexpected small-scale magnetic fields on the surfaces of Ap stars. These studies were recently criticisied by Stift et al. (2012), who claimed that magnetic inversions are not robust and are undermined by neglecting a feedback on the Stokes line profiles from the local atmospheric structure in the regions of enhanced metal abundance. We show that Stift et al. misinterpreted published magnetic Doppler imaging results and neglected some of the most fundamental principles behind magnetic mapping. We demonstrate that the variation of atmospheric structure across the surface of a star with chemical spots affects the local continuum intensity but is negligible for the normalised local Stokes profiles. For the disk-integrated spectra of an Ap star with extreme abundance variations, we find that the assumption of a mean model atmosphere leads to moderate errors in Stokes I but is negligible for polarisation spectra. Employing a new magnetic inversion code, which incorporates the horizontal variation of atmospheric structure, we reconstructed new maps of magnetic field and Fe abundance for the Ap star alpha^2 CVn. The resulting distribution of chemical spots changes insignificantly compared to the previous modelling based on a single model atmosphere, while the magnetic field geometry does not change at all. This shows that the assertions by Stift et al. are exaggerated as a consequence of unreasonable assumptions and extrapolations, as well as methodological flaws and inconsistencies of their analysis. Our discussion proves that published magnetic inversions based on a mean stellar atmosphere are robust and reliable, and that the presence of small-scale magnetic field structures on the surfaces of Ap stars is real.
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