Large-scale α2-dynamo in low-mass stars and brown dwarfs

We develop a model based on three dimensional mean-field magnetohydrodynamics computations for the generation of large scale magnetic fields in fully convective objects like low-mass stars, brown dwarfs and possibly gaseous planets. The dynamo process is of alpha(2) type and thus differs from the shell-dynamo at work in more massive stars. The alpha(2) dynamo is found to become supercritical for Coriolis numbers. Omega(*) >= 1, i.e. Rossby numbers Ro <= 10. It generates a large-scale, non-axisymmetric, steady field that is symmetric with respect to the equatorial plane. Saturation of the alpha(2)-generated field at the equipartition field strength yields strengths of several kiloGauss, in agreement with observations of active M dwarfs, and provides a qualitative explanation for the observed activity saturation in late M stars. For brown dwarfs with a conductive core, as occurs at the center of the most massive and oldest of these objects, we have also studied an alpha(2)omega dynamo, i.e. the effect of differential rotation. In this case the field is predominantly toroidal, axisymmetric, and oscillatory, like the solar magnetic field. The topology of the field in the fully convective objects exhibits a high order multipole character that differs from the aligned dipole field generated by the alpha,omega dynamo. The strong reduction of the dipolar component due to the non-axisymmetry of the field should considerably reduce the Alfven radius and thus the efficiency of magnetic braking, providing an appealing explanation for the decreasing angular momentum loss rate observed in low-mass stars and brown dwarfs. This may have also implications for cataclysmic variables below the period gap. In spite of this large-scale field, the decreasing conductivity in the dominantly neutral atmosphere of these objects may prevent the current generation necessary to support a chromosphere and thus activity. An observational signature of the present model would be (i) asymmetry of the chromospheric activity, contrary to the spatially uniform activity expected from small-scale turbulent dynamo and (ii) the absence of cycles in uniformly rotating (fully convective) low-mass objects.