Heating of stellar chromospheres and coronae: Evidence for non-magnetic heating

Abstract

A detailed analysis of the radiative losses from the outer atmospheres of magnetically active stars suggests that two emission components are present. These two components may correspond to two distinct mechanisms of non-radiative heating that operate in the outer atmospheres of cool stars. One of these mechanisms depends on the stellar rotation rate, and is associated with stellar magnetic activity. The other component varies with stellar effective temperature, appears to be insensitive to surface gravity, and independent of the stellar rotation rate. The radiative energy loss of this second component results in the observed lower boundaries in diagrams of flux density versus colour, hence the name “basal chromosphere.” The associated heating mechanism may be dissipation of acoustic waves. The temperatures of the basal part of the atmosphere are limited to a few tens of thousands of degrees or less in G- and K-type stars. The basal atmosphere in F-type dwarfs may reach up to transition-region temperatures, or higher.