Coronal activity in F-, G-, and K-type stars. IV - Evidence for expanding loop geometries in stellar coronae

Abstract

High-resolution soft X-ray spectra of Capella and a 2 CrB are analyzed to constrain basic properties of stellar magnetic loops. The spectra were obtained with the EXOSAT transmission grating spectrometer (TGS), and permit the study of individual spectral lines and line complexes between 10 Â and 200 Â, with approximately 3 Â resolution. The spectra of Capella and a 2 CrB are compatible with coronae consisting of two distinct ensembles of static loops with different maximum temperatures (T m) and ratios (F) of the cross sectional areas at the loop top and at the footpoint. Fits to the observed specra are made using a two-loop model to derive T m and F for each loop component. The cool (5 MK) loop component in both stars shows evidence of a relatively strong expansion with height: the best-fit expansion factors F for the cool loops on Capella and a 2 CrB are 30 and 50, respectively, being larger than 5 and 2 at the 99% confidence level. The loops appear to be much smaller than the stellar radius. The hot (30 MK) components appear to expand much less: best-fit values for F are 2.5 and 4, respectively. At the 90% confidence level the expansion factors for the cool and hot loop ensembles may be the same. If this is the case, the fits indicate a best value of F around 10. The values obtained for the expansion factors are underestimated if the maximum temperatures are not identical for all loops, but instead cover a small range for each of the two loop families.