Change in the activity character of the coronae of low-mass stars of various spectral types

Abstract

We study the dependence of the coronal activity index on the star's rotation rate. This question was considered earlier for 824 late-type stars on the basis of a consolidated catalogue of the soft X-ray fluxes. We carry out a more refined analysis separately for G, K and M dwarfs. They distinctively exhibit two modes of activity. The first one is the saturation mode, it is characteristic of young stars and is practically not related to their rotation. The second one refers to the solar-type activity the level of which strongly depends on the rotation period. We show that the transition from one mode to another takes place at the rotation periods of 1.1, 3.3 and 7.2 days for the stars of spectral types G2, K4 and M3 respectively. In the light of the discovery of superflares on G and K stars on the Kepler spacecraft there arises a question of how these objects differ from other active late-type stars. We analyse the location of superflare stars relative to the stars observed by Kepler on the "amplitude of rotational brightness modulation (ARM) -- rotation period" diagram. The value of ARM reflects the relative spots area on a star and characterises the activity level in the whole atmosphere. It is shown that G and K superflare stars are basically fast rotating young objects, but some of them belong to the stars with the solar type of activity.