Stellar Coronae, Solar Flares: a Detailed Comparison of sigma Gem, HR 1099, and the Sun in High-resolution X-rays. (arXiv:1304.0408v1 [astro-ph.SR])

Stellar Coronae, Solar Flares: a Detailed Comparison of sigma Gem, HR 1099, and the Sun in High-resolution X-rays. (arXiv:1304.0408v1 [astro-ph.SR]):
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.