X-ray lines in stellar coronae

Abstract

Because the emission line spectra and continua from optically thin plasmas are fairly well known, high-resolution X-ray spectroscopy has its most obvious application in the measurement of optically thin sources such as the coronae of stars. In particular X-ray observations with the EINSTEIN observatory have demonstrated that soft X-ray emitting coronae are a common feature among stars on the cool side of the Hertzsprung-Russell diagram, with the probable exception of single very cool giant and supergiant stars and A-type dwarfs. Observations with the spectrometers aboard EINSTEIN and EXOSAT have shown that data of even modest spectral resolution (λ/Δλ= 10–100 permit the identification of coronal material at different temperatures whose existence may relate to a range of possible magnetic loop structures in the outer atmospheres of these stars. The improved spectral resolution of the next generation of spectrometers aboard XMM and AXAF is needed to fully resolve the temperature structure of stellar coronae and to enable density and velocity diagnostics. In this paper spectral results from EINSTEIN and EXOSAT are discussed. A few simulations of high-resolution re L, K, and 2s–2p spectra with AXAF, XMM, and several detectors such as calorimeter and Nb-junction are shown to demonstrate the capabilities for plasma diagnostics.