The filling factor of active regions on the surfaces of the dM stars

Abstract

Results from M dwarf chromospheric models combined with observational considerations suggest that the surfaces of those dM stars that exhibit chromospheric Ha absorption can be characterized by a significant area coverage of, presumably, magnetic field-related active (plagelike) regions. Lower limits to the area coverage , or filling factor, of active regions on the surfaces of the non-dMe stars based on the equivalent width of the observed chromospheric Ha absorption line are derived. Results inferred from data provided by J. R. Stauffer, in advance of publication, yield a range in lower limits to the active region filling factor of 0.31-0.67 for a sample of dM stars characterized by (R-I) K = 0.9. The upper limit is 1.0. These area constraints can, in turn, be used to infer upper limits to the intrinsic chromospheric Ca n and Mg n resonance line emission in active regions on the dM stars. The implications of the results for dynamo theory and theories of chromo-spheric heating are discussed. In particular, dynamo theory must now account for the occurrence of a high filling factor of magnetic field regions on the surfaces of the relatively slowly rotating dM stars while theories concerning atmospheric heating must account for the relatively low chromospheric and coronal emission of dM stars despite the fact that their surfaces can have a high filling factor of magnetic field regions. In order to account for the latter phenomenon, a hypothesis involving open magnetic field geometries combined with downwardly directed coronal X-ray emission as the source of chromospheric heating in M dwarfs, as suggested in 1982 by Cram, is discussed. Finally, an extension of the conceptual approach presented herein to more nearly solar-type dwarfs is described. In particular, the triplet He i lines, 25876 and 210830, can potentially be utilized as diagnostics of the filling factor of active regions on solar-type stars.