CHROMOSPHERIC HEATING IN LATE-TYPE DWARFS - ACOUSTIC OR MAGNETIC

Mg II h & k fluxes for 69 K and 88 M dwarfs have been analysed. An empirical lower limit is found in the Mg II flux, extended down to the latest spectral types. Based on the data presented here it is the magnetic component that dominates the emission from the atmosphere of cool dwarfs, especially the M dwarfs. It however 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 H-alpha, 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.