X-ray activity of nearby G-, K-, and M-type stars and implications for planet habitability around M stars

TL;DR

This study analyzes the X-ray activity of nearby G, K, and M stars, revealing that most M dwarfs have X-ray emission levels comparable to G stars, suggesting limited impact on habitability of planets around M stars.

Contribution

It provides the first unbiased, volume-limited comparison of X-ray luminosities across G, K, and M stars, highlighting that M dwarfs generally do not exhibit higher X-ray activity than G stars.

Findings

Majority of M dwarfs have X-ray activity levels similar to G stars.

X-ray activity peaks differ between early and late M stars.

Most M stars' X-ray emission is not excessive for planetary habitability.

Abstract

Context. The intense X-ray and UV emission of some active M stars has raised questions about the habitability of planets around M-type stars. Aims. We aim to determine the unbiased distribution of X-ray luminosities in complete, volume-limited samples of nearby M dwarfs, and compare them to those of K and G dwarfs. Methods. We constructed volume-complete samples of 205 M stars with a spectral type $\leq$ M6 within 10 pc of the Sun, 129 K stars within 16 pc, and 107 G stars within 20 pc. We used X-ray data from Chandra, XMM-Newton, eROSITA, and ROSAT to obtain the X-ray luminosities of the stars. Results. Our samples reach an X-ray detection completeness of 85%, 86%, and 80% for M, K, and G stars, respectively. The fractional X-ray luminosities relative to the bolometric luminosities, $\log(L_\mathrm{X}/L_\mathrm{bol})$, of the M stars show a bimodal distribution, with one peak at around -5, mostly contributed by early M stars (M0--M4), and another peak around -3.5, contributed mainly by M4--M6 stars. The comparison of the different spectral classes shows that 63% of all M stars in our sample (80% of the M stars with a spectral type $<$ M4) have $L_\mathrm{X}/L_\mathrm{bol}$ values that are within the central 80% quantile of the distribution function for G stars. In addition, 55% of all M stars in our sample (and 72% of the M stars with a spectral type $<$ M4) have $L_\mathrm{X}/L_\mathrm{bol}$ less than 10 times the solar value. Conclusions. The X-ray activity levels of the majority ($\ge 60\%$) of nearby M dwarfs no later than M6 are actually not higher than the typical (80% quantile) levels for G-type stars. The X-ray irradiation of habitable-zone planets around these stars should therefore not present a specific problem for their habitability.