Temperatures of coronae of cool stars, derived from EXOSAT observations

Abstract

We observed the late-type stars Capella, σ² CrB, λ And, AD Leo, GL494, GL569 and Procyon with different combinations of three instruments aboard EXOSAT : the low-energy channel multiplier array (LE), the medium-energy proportional counters (ME), and the transmission grating spectrometer (TGS). We derive crude temperature information on the coronae of these stars from the photometric data, using one- and two-temperature models of optically thin, thermal plasmas. We compare the results with 2-T fits and differential emission measure analyses of three grating observations and with simulations for mixtures of two plasma components with varying emission measure ratios. But for Procyon, all sources have a strong, hot component between 10 and 25 MK. The TGS spectra of Capella and σ² CrB reveal a cooler component with comparable emission measure around 5 MK. The emission from the corona of Procyon is dominated by a very cool (0.6 MK) component, with an additional contribution from a 2-3 MK component.