Stellar Coronae, Solar Flares: a Detailed Comparison of sigma Gem, HR 1099, and the Sun in High-resolution X-rays

Abstract

Chandra HETG spectra of the coronally active binary stars sigma Gem and HR 1099 are among the highest fluence observations for such systems taken at high spectral resolution in x-rays with this instrument. We compare their properties to solar flare spectra obtained with the Russian CORONAS-F RESIK instrument at similar resolution in an overlapping bandpass. We emphasize the comparisons of the 3.3-6.1 A region from solar flare spectra to the corresponding sigma Gem and HR 1099 spectra. We also model the the HETG spectra from 1.7-25 A to determine coronal temperatures and elemental abundances. Sigma Gem is a single lined coronally active long period binary which has a very hot corona. HR 1099 is a similar, but shorter period, double lined system. In the deep exposures we study emission from some of the weaker species, such as K, Na, and Al, which have the lowest first ionization potentials (FIP). The solar flare temperatures reach ~20 MK, comparable to the sigma Gem and HR 1099 coronae. During the Chandra exposures, sigma Gem was slowly decaying from a flare and its spectrum is well characterized by a collisional ionization equilibrium plasma with a broad temperature distribution ranging from 2-60 MK, peaking near 25 MK, but with substantial emission from 50 MK plasma. We have detected K XVIII and Na XI emission which allow us to set limits on their abundances. HR 1099 was also in a flare state but had no detectable K XVIII. These measurements provide new comparisons of solar and stellar coronal abundances, especially at the lowest FIP values. The low FIP elements do not show enhancement in the stellar coronae as they do in the Sun, except for K in sigma Gem. Sigma Gem and HR 1099 differ in their emission measure distributions but have very similar elemental abundances.

Accepted by the Astrophysical Journal, 19 pages, 10 figures, 9 tables

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