On coronal structures and their variability in active stars: The case of Capella observed with Chandra/LETGS

In this paper we present a detailed analysis of two X-ray spectra of Capella, taken eleven months apart with the Low Energy Transmission Grating Spectrometer (LETGS) of the Chandra Observatory. We have studied variability of the coronal emission over different time scales, both in the whole X-ray band and in narrow temperature ranges identified by lines. The comparison of the two observations shows that the whole coronal emission of Capella in March 2000 was 3% higher than in February 2001; there also appears to be a tendency, albeit a marginal one, for the hottest lines to show the largest changes between the two observations. A detailed search for short-term variability (on time scales ranging from 10(2) to 10(4) s) in the emission of individual lines shows that in all cases the emission is compatible with a constant source; the firm upper limits of 5%-10% to the source variability on short time scales suggests that the intense X-ray emission is due to stable coronal structures and not to flaring activity. We have also determined the coronal thermal structure, as described with the emission measure distribution vs. temperature and with the help of plasma density, derived from the analysis of the O VII, Ne IX, Mg XI and Si XIII He-like ion triplets. The emission measure distribution, em(T), and the element abundances, have been reconstructed with the Markov-Chain Monte Carlo method by Kashyap Drake; the em(T) presents a previously known sharp peak around log T=6.8-6.9, but we have also found evidence of a small amount of plasma at T>10(7) K. With the help of the em(T) and the density values we have estimated the pressure and volume of the emitting plasma at different temperatures, and we have derived information about the structure of individual loops and about the population of loops having different maximum temperatures. Our results indicate that loops with higher maximum temperature have higher pressure and smaller volume than lower temperature loops.