Activity in late-type stars. VIII. The nature of the dM(e) or "zero" H-alpha stars.

Abstract

We present the results of ultraviolet spectroscopy of three M dwarf stars which, according to Stauffer & Hartmann (1986), have no Hα, either in absorption or emission, in their visible spectra. Using chromospheric and transition region emission line fluxes derived from these spectra, we demonstrate that these stars undergo lower rates of chromospheric non-radiative heating than dM (Hα absorption) stars. Therefore, the hypothesis that the dM(e) stars, as we have called the zero-Hα objects, have a predominantly Hα absorption atmosphere, filled in by the emission from a small number of active regions, is not tenable in these cases. We also discuss the applicability of the Schrijver (1987) basal flux relationship to the dM(e) stars, particularly to G1 105B, which has the weakest chromospheric Mg II emission yet observed and point out that the convective zone energy flux of Bohn (1984) may not be sufficient to acoustically heat basal chromospheres in the latest spectral type M stars.