Research Note

The coronal temperatures of low-mass main-sequence stars

University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: colin.johnstone@univie.ac.at

Received: 5 November 2014
Accepted: 29 April 2015

Abstract

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 T̅_cor = 0.11 F_X^0.26, where T̅_cor is in MK and F_X is in erg s^-1 cm^-2.