Atmospheric activity in red dwarf stars

Abstract

Radiation losses from quiescent and flaring regions of red dwarf atmospheres are compared and found to be equally important. The flare emission on all wavelengths approaches 1% of the stellar bolometric luminosity for active dMe stars, but is 100 times less for some dM stars. The discriminating parameter is flare frequency, which may vary with stellar mass, age, rotation rate or other quantities. Radiation losses from chromospheric emission lines are larger than losses from transition region lines. H I is the most important element in late dMe stars, while Ca II dominates the chromospheric radiation loss from early dMe and dKe stars. The corona is less significant in dKe stars, but dominates over chromospheric radiation losses in dMe stars. A sharp drop in coronal importance is seen near dM5e, where stars become fully convective. All forms of radiation losses have a common cause, probably to be found in the convection zone. The magnetic field may be the instrument that brings mechanical energy into the outer atmosphere, where it is radiated into space.