Two‐Component Theoretical Chromosphere Models for K Dwarfs of Different Magnetic Activity: Exploring the CaiiEmission–Stellar Rotation Relationship

Abstract

We compute two-component theoretical chromosphere models for K2 V stars with diUerent levels of magnetic activity. The two components are a nonmagnetic component heated by acoustic waves and a magnetic component heated by longitudinal tube waves. The —lling factor for the magnetic component is determined from an observational relationship between the measured magnetic area coverage and the stellar rotation period. We consider stellar rotation periods between 10 and 40 days. We investigate two diUerent geometrical distributions of magnetic —ux tubes: uniformly distributed tubes, and tubes arranged as a chromospheric network embedded in the nonmagnetic region. The chromosphere models are constructed by performing state-of-the-art calculations for the generation of acoustic and magnetic energy in stellar convection zones, the propagation and dissipation of this energy at the diUerent atmo- spheric heights, and the formation of speci—c chromospheric emission lines that are then compared to the observational data. In all these steps, the two-component structure of stellar photospheres and chromospheres is fully taken into account. We —nd that heating and chromospheric emission is signi—- cantly increased in the magnetic component and is strongest in —ux tubes that spread the least with height, expected to occur on rapidly rotating stars with high magnetic —lling factors. For stars with very slow rotation, we are able to reproduce the basal —ux limit of chromospheric emission previously identi- —ed with nonmagnetic regions. Most importantly, however, we —nd that the relationship between the Ca II H)K emission and the stellar rotation rate deduced from our models is consistent with the relationship given by observations. Subject headings: line: formationMHDstars: activitystars: chromospheres ¨ stars: late-typestars: rotation