Chromospheric Heating in K & M Dwarfs

Abstract

An empirical lower limit is found in the Mg II flux extending down to the latest spectral type for a sample of K & M dwarfs. It is a magnetic component that dominates the emission from the atmosphere of most cool dwarfs. However it co-exists with an acoustically heated component, which can only be identified in the stars with the lowest fluxes in the flux-color diagram (these stars possibly being the slowest rotators). Data for the dM(e) stars, i.e. those stars with zero Hα, show that these are divided into two classes: (i) an in-active M dwarf star with very weak chromospheric heating, perhaps only by acoustic waves and (ii) an intermediate chromospheric activity star dominated by magnetic heating.