arXiv 0805.44592008-05-29astro-ph

主序A星对流带起始的O VI观测

O VI Observations of the Onset of Convection Zones in Main-Sequence A Stars

J. E. Neff, T. Simon

DOI：: 10.1086/590423

摘要：

如果像通常假定的那样，恒星外层大气的磁活动是由于自转和次表面对流之间的相互作用，那么人们就不会期望在T_eff > 8300 K的恒星中观察到活动的迹象。任何来自较热恒星的X射线或紫外线辐射都必须是由于不同的机制，或者是由于一个未解决的、活跃的双星伴星。由于空间分辨率差，X射线仪器特别容易受到源混淆的影响。在波长向1216埃的方向上，比这条假定的分界线更热的恒星的近紫外光谱主要是由光球连续谱控制的。我们已经使用FUSE获得光谱的次日冕O VI发射线，这是在一个波长的光球连续的中期和早期的A星是相对较弱的。我们观测了14颗恒星，T_eff范围从7720到10，000 K。14颗恒星中有11颗显示出OVI发射谱线，其中包括Teff> 8300 K的8颗靶中的6颗。从表面上看，这表明活性不会随着温度的升高而下降。然而，发射线比这些快速旋转恒星的预计旋转速度要窄，这表明发射可能来自未解决的晚期伴星。此外，O VI发射的强度是一致的，从一个看不见的活动K或M矮双星companinon的预期，和高的X射线远紫外线光度比观察表明，这必须是这样的。因此，我们的研究结果是一致的，与传统的恒星结构模型一致，早期的研究表明，在T_eff约8300 K的辐射/对流边界的活动迅速下降。

英文摘要：

If magnetic activity in outer stellar atmospheres is due to an interplay between rotation and subsurface convection, as is generally presumed, then one would not expect to observe indicators of activity in stars with T_eff > 8300 K. Any X-ray or ultraviolet line emission from hotter stars must be due either to a different mechanism or to an unresolved, active, binary companion. Due to their poor spatial resolution, X-ray instruments have been especially susceptible to source confusion. At wavelengths longward of 1216 Angstroms, the near ultraviolet spectra of stars hotter than this putative dividing line are dominated by photospheric continuum. We have used FUSE to obtain spectra of the subcoronal O VI emission lines, which lie at a wavelength where the photospheric continuum of the mid- and early-A stars is relatively weak. We observed 14 stars spanning a range in T_eff from 7720 to 10,000 K. Eleven of the 14 stars showed O VI emission lines, including 6 of the 8 targets with T_eff > 8300 K. At face value, this suggests that activity does not fall off with increasing temperature. However, the emission lines are narrower than expected from the projected rotational velocities of these rapidly-rotating stars, suggesting that the emission could come from unresolved late-type companions. Furthermore, the strength of the O VI emission is consistent with that expected from an unseen active K or M dwarf binary companinon, and the high x-ray to far uv luminosity ratios observed indicate that this must be the case. Our results are therefore consistent with earlier studies that have shown a rapid drop-off in activity at the radiative/convective boundary expected at T_eff about 8300 K, in agreement with conventional stellar structure models.

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