Rotation, convection, and magnetic activity in lower main-sequence stars

Rotation periods are reported for 14 main-sequence stars, bringing the total number of such stars with well determined rotation periods to 41. It is found that the mean level of their Ca II H and K emission (averaged over 15 years) is correlated with rotation period, as expected. However, there is a further dependence of the emission on spectral type. When expressed as the ratio of chromospheric flux to total bolometric flux, the emission is well correlated with the parameter P(obs)/tau(c), where P(obs) is the observed rotation period and tau(c)(B - V) is a theoretically-derived convective overturn time, calculated assuming a mixing length to scale height ratio alpha of about 2. This finding is consonant with general predictions of dynamo theory, if the relation between chromospheric emission and dynamo-generated magnetic fields is essentially independent of rotation rate and spectral type for the stars considered. The dependence of mean chromospheric emission on rotation and spectral type is essentially the same for stars above and below the Vaughan-Preston (1980) 'gap', thus casting doubt on explanations of the gap in terms of a discontinuity in dynamo characteristics.