Self-consistent and Time-dependent Magnetohydrodynamic Chromosphere Models for Magnetically Active Stars

Abstract

We present self-consistent and time-dependent MHD heating models for chromospheres of magnetically active stars. We investigate the propagation and dissipation of longitudinal flux-tube waves in K2 V stars with different rotation rates implying different photospheric and chromospheric magnetic filling factors. These filling factors are critical for determining the number of flux tubes on the stellar surface and the spreading of the tubes with height, which is relevant for the propagation and dissipation of the magnetic energy as well as the generated radiative emission losses. The filling factors used in this Letter are estimated using a relationship between the photospheric values for B0f0 and Prot in accord with very recent magnetic field measurements by Ruedi et al. We also consider revised computations of magnetic energy fluxes by Ulmschneider & Musielak generated by turbulent motions. Our models show increased shock strengths and energy dissipation rates in stars of faster rotation because of the narrower spreading of the tubes. This also leads to increased chromospheric emission, particularly in Mg II in stars of faster rotation. We consider these results as a first step toward a theoretical derivation of chromospheric emission—stellar rotation relationships for stars of different masses and evolutionary status.