Empirical Prediction of Plasma Emission Measure Distributions and X-Ray/Extreme-ultraviolet Spectra of Late-type Stars

Abstract High-energy emission spectra from the outer atmospheres of late-type stars represent an important feature of the stellar activity in several contexts, such as the photoevaporation and photochemistry of planetary atmospheres or the modeling of irradiated circumstellar disks in young objects. An accurate determination of these spectra in the EUV and soft X-ray (XUV) band requires high-resolution spectroscopy, which is rarely feasible with current instrumentation. We employed a relatively large set of plasma emission measure distributions (EMDs) as a function of temperature, derived from far-ultraviolet and X-ray emission line spectra acquired with the Hubble Space Telescope and Chandra or XMM-Newton, in order to devise a relatively simple recipe for predicting EMDs and XUV spectra of stars of different spectral types, activity levels, and plasma metallicities. We show that the EMDs in the range of temperatures between 10 4 and 10 7.5 K can be described using the stellar surface X-ray flux as a control parameter, but this parameterization also depends on the spectral type. In particular, we find that M-type stars show slightly lower emission measure at temperatures below ∼10 5 K and higher emission measures for T ≳ 10 6.5 K with respect to FGK stars with similar surface X-ray fluxes. We evaluated the uncertainties in the broadband EUV and X-ray fluxes derived from synthetic EMDs and spectra, considering in the error budget also the limited knowledge on the chemical abundances in stellar outer atmospheres.