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

¹ Universitäts-Sternwarte München, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 München, Germany
² IRAP, Université de Toulouse, CNRS, UPS, 9 avenue colonel Roche, BP 44346, 31028 Toulouse cedex 4, France

^⋆ Corresponding authors; enyi.zhu@irap.omp.eu; preibisch@usm.uni-muenchen.de

Received: 30 August 2024
Accepted: 12 December 2024

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 ≤ 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_X/L_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_X/L_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_X/L_bol less than 10 times the solar value.

Conclusions. The X-ray activity levels of the majority (≳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.