X-ray spectroscopy of stars

摘要

Non-degenerate stars of essentially all spectral classes are soft X-raysources. Their X-ray spectra have been important in constraining physical processesthat heat plasma in stellar environments to temperatures exceeding one million degrees.Low-mass stars on the cooler part of the main sequence and their pre-main sequencepredecessors define the dominant stellar population in the galaxy by number. TheirX-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solarcorona. The Sun itself as a typical example of a main-sequence cool star has been a piv-otal testbed for physical models to be applied to cool stars. X-ray emission from coolstars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronalplasma, although plasma parameters such as temperature, density, and element abun-dances vary widely. Coronal structure, its thermal stratification and geometric extentcan also be interpreted based on various spectral diagnostics. New features have beenidentified in pre-main sequence stars; some of these may be related to accretion shockson the stellar surface, fluorescence on circumstellar disks due to X-ray irradiation, orshock heating in stellar outflows. Massive, hot stars clearly dominate the interactionwith the galactic interstellar medium: they are the main sources of ionizing radia-tion, mechanical energy and chemical enrichment in galaxies.