Empirical prediction of plasma emission measure distributions and X-EUV spectra of late-type stars

TL;DR

This study develops a method to predict the EUV and X-ray spectra of late-type stars using plasma emission measure distributions derived from observational data, accounting for spectral type and activity level variations.

Contribution

It introduces a simple recipe to estimate stellar XUV spectra based on surface X-ray flux and spectral type, improving spectral predictions without high-resolution spectroscopy.

Findings

EMDs can be modeled using stellar surface X-ray flux as a control parameter.

M-type stars differ in emission measure distributions compared to FGK stars at similar flux levels.

Uncertainties in flux estimates include chemical abundance knowledge and spectral modeling limitations.

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, that 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 FUV 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 metallicity. We show that the EMDs in the range of temperatures between 10^4 K 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 broad-band EUV and X-ray fluxes derived from synthetic EMDs and spectra, considering in the error budget also the limited knowledge of the chemical abundances in stellar outer atmospheres.