Extreme Ultraviolet Line Emission from Cool Stars: Resonantly Scattered or Not—That Is the Question!

Abstract

A challenge to the classical assumption that the radiative losses from stellar coronae are optically thin has been raised by Schrijver, van den Oord, & Mewe, who argued that some of the stronger emission lines detected in the high-resolution spectra of cool stars observed with the Extreme Ultraviolet Explorer (EUVE) satellite are optically thick. If they assume all radiation optically thin, an explanation of the short-wavelength (SW) part of the EUVE spectrum requires large amounts of emission measure at very high temperatures (T ≈ 108 K), which is unlikely the case for rather inactive stars. We show that the soft X-ray pulse height spectrum obtained with the Position Sensitive Proportional Counter (PSPC) on board ROSAT is inconsistent with such high-temperature emission and inconsistent with the assumption that the EUV line emission is optically thick. We further demonstrate via an analysis of fit residuals that the observed count fluctuations in the EUVE SW spectrum are inconsistent with the hypothesis that the bulk of the observed flux arises from a continuum. Therefore, resonance scattering does not appear to be required for the interpretation of the EUV and X-ray spectra of inactive cool stars.