MAGNETIC ACTIVITY IN DWARF STARS WITH SHALLOW CONVECTIVE ENVELOPES

The magnetic activity in dwarf stars with shallow convective envelopes is addressed by studying the C II (1335 angstrom) and soft X-ray surface flux densities in relation to the stellar rotation velocity. The C II emission from the acoustically heated basal atmospheric component increases with increasing effective temperature up to at least B-V almost-equal-to 0.25, reaching a level of about 8 10(4) erg cm-2 s-1 for early F-type dwarf stars, i.e. about half of their typical C II emission. A relationship between rotation rate and level of magnetic activity is found to extend down to at least B-V almost-equal-to 0.25 (A9V), although the C II emission associated with the magnetic activity at B-V=0.25 is suppressed by a factor of about 100 as compared to stars cooler than B-V almost-equal-to 0.60 (GIV) with the same rotation rate. The suppression of activity is strongly colour-dependent below B-V almost-equal-to 0.60, with the dynamo efficiency leveling off for cooler stars. The radiative losses associated with magnetic activity do not show a noticeable dependence on colour in diagrams relating fluxes from chromosphere and corona for F-, G-, and K-type dwarf stars, suggesting comparable atmospheric structures for the magnetic component. The observed mean rotational velocities as a function of spectral type are shown to be consistent with the assumptions of (a) a reduced dynamo efficiency (as measured by the mean magnetic surface flux density at a given rotation rate) and (b) a comparable mechanism for angular momentum loss through a stellar wind for all magnetically active cool stars. Consequently this study does not support the idea that a strong stellar wind originates from F-type dwarfs as suggested in the literature.