# Pointing Chandra Toward the Extreme Ultraviolet Fluxes of Very Low-Mass Stars

## TL;DR

This paper presents a novel method using Chandra X-ray data to estimate the EUV emission of very low-mass stars, crucial for understanding planetary atmosphere evolution, especially for stars too faint for direct EUV observation.

## Contribution

The study introduces a new approach to infer EUV fluxes of ultra-cool dwarf stars from X-ray observations, enabling better modeling of their atmospheric impact.

## Key findings

- The method can estimate EUV spectra within a factor of 2-4 for wavelengths below 400-500 Å.
- A shallow emission measure distribution slope ($DEM \,\propto T^{3/2}$) fits the data well.
- Model uncertainties increase significantly at longer wavelengths and lower temperatures.

## Abstract

The X-ray and EUV emission of stars plays a key role in the loss and evolution of the atmospheres of their planets. The coronae of dwarf stars later than M6 appear to behave differently to those of earlier spectral types and are more X-ray dim and radio bright. Too faint to have been observed by the Extreme Ultraviolet Explorer, their EUV behavior is currently highly uncertain. We have devised a method to use the Chandra X-ray Observatory High Resolution Camera to provide a measure of EUV emission in the 50-170 \AA\ range and have applied it to the M6.5 dwarf LHS 248 in a pilot 10 ks exposure. Analysis with model spectra using simple, idealised coronal emission measure distributions inspired by an analysis of Chandra HETG spectra of the M5.5 dwarf Proxima Cen and results from the literature, finds greatest consistency with a very shallow emission measure distribution slope, $DEM \propto T^{3/2}$ or shallower, in the range $\log T=5.5$-$6.5$. Within $2\sigma$ confidence, a much wider range of slopes can be accommodated. Model spectra constrained by this method can provide accurate (within a factor of 2-4) synthesis and extrapolation of EUV spectra for wavelengths $<400$-500 \AA. At longer wavelengths models are uncertain by an order of magnitude or more, and depend on the details of the emission measure distribution at temperatures $\log T < 5.5$. The method is sensitive to possible incompleteness of plasma radiative loss models in the 30 170 \AA\ range for which re-examination would be warranted.