Assessing the capability of a model-based stellar XUV estimation

TL;DR

This study validates a numerical model for estimating stellar XUV spectra by comparing it with observations of the Sun and young solar-type stars, highlighting its accuracy and limitations across different activity levels.

Contribution

The paper extends the validation of a stellar XUV estimation model to multiple stars, demonstrating its effectiveness and identifying its underestimation of high-energy X-rays in active stars.

Findings

Model accurately reproduces observed spectra within a factor of 3 for stars with magnetic flux up to 100 times solar.

High-energy X-rays (< 1 nm) are systematically underestimated in active stars.

Model's accuracy depends on known surface magnetic flux and elemental abundance.

Abstract

Stellar XUV (X-ray and extreme ultraviolet) emission drives the heating and chemical reactions in planetary atmospheres and protoplanetary disks, and therefore, a proper estimation of a stellar XUV spectrum is required for their studies. One proposed solution is to estimate stellar atmospheric heating using numerical models, although the validation was restricted to the Sun over a limited parameter range. In this study, we extend the validation of the model by testing it with the Sun and three young, nearby solar-type stars with available XUV observational data. We first test the model with the solar observations, examining its accuracy in activity minimum and maximum phases, its dependence on loop length, the effect of loop length superposition, and its sensitivity to elemental abundance. We confirm that the model spectrum is mostly accurate both in activity minimum and maximum, although the high-energy X-rays (< 1 nm) are underestimated in the activity maximum. Applying the model to young solar-type stars, we find that it can reproduce the observed XUV spectra within a factor of 3 in the range of 1-30 nm for stars with magnetic flux up to 100 times that of the Sun. For a star with 300 times the solar magnetic flux, although the raw numerical data show a systematically lower spectrum than observed, the spectra are in good agreement once corrected for the effect of insufficient resolution in the transition region. For all young solar-type stars, high-energy X-rays (< 1 nm) are significantly underestimated, with the deviation increasing with stellar magnetic activity. Our findings indicate that the stellar XUV spectrum can be reasonably estimated through a numerical model, given that the essential input parameters (surface magnetic flux and elemental abundance) are known.