FIP, FIT or MAD. Analysis of High Signal-to-Noise ASCA Spectra of Coronal Stars

摘要

ASCA (Advanced Satellite for Cosmology and Astrophysics) and EUVE (Extreme Ultraviolet Explorer) spectra of active late-type stars imply that Fe and other medium-Z elements may be 2-10 times less abundant in the coronae of these stars than in their photo-spheres (the MAD effect). These deficiencies may be related to the solar FIP (First Ionization Potential) effect, in which Fe and other low First Ionization Potential elements appear enriched in the solar corona over their photospheric values. The FIP effect is time variable. As part of this proposal, the K0-2 III star, 29 Draconis, was observed in X rays with the ASCA spacecraft in order to measure the coronal abundances of this star at three different stellar longitudes over its 31-day rotation cycle. The goal of the observations was to learn whether coronal abundances, and hence coronal magnetic structure, vary across the surface of 29 Draconis in phase with the motion of dark star-spots across its disk. A second task included in this project was a systematic reanalysis of 18-20 deep exposures of active coronal stars, which were extracted from the ASCA public archives. New thermal models were computed for each spectrum in order to derive coronal metal abundances for each star. The goal of this survey was to search for possible trends in coronal abundance with various stellar parameters such as rotation, chromospheric activity levels at ultraviolet and optical wavelengths, or evolutionary stage.