The coronal temperatures of low-mass main-sequence stars (Research Note)

Aims. We study the X-ray emission of low-mass main-sequence stars to derive a reliable general scaling law between coronal temperature and the level of X-ray activity. Methods. We collect ROSAT measurements of hardness ratios and X-ray luminosities for a large sample of stars to derive which stellar X-ray emission parameter is most closely correlated with coronal temperature. We calculate average coronal temperatures for a sample of 24 low-mass main-sequence stars with measured emission measure distributions (EMDs) collected from the literature. These EMDs are based on high-resolution X-ray spectra measured by XMM-Newton and Chandra. Results. We confirm that there is one universal scaling relation between coronal average temperature and surface X-ray flux, F-X, that applies to all low-mass main-sequence stars. We find that coronal temperature is related to F-X by T-cor = 0.11F(X)(0.26), where T-cor is in MK and F-X is in erg s(-1) cm(-2).