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Z.E. Musielak (Uni. Texas, Arlington), D. Fawzy, P. Ulmschneider, W. Rammacher (Uni. Heidelberg, Germany), K. Stepien (Warsaw Uni. Observatory, Poland)
To identify the main heating mechanisms operating in atmospheres of late-type stars, we have constructed purely theoretical, two-component and time-dependent models of stellar chromospheres. Our models depend only on four basic stellar parameters: effective temperature, gravity, metallicity, and filling factor, which determines the coverage of these stars by surface magnetic fields and is treated as a free parameter. They consist of non-magnetic regions heated by acoustic waves and magnetic flux tubes heated by longitudinal and transverse tube waves. At each height in stellar atmospheres, the time-dependent energy balance between the dissipated wave energy and the most prominent radiative losses is calculated. By specifying the filling factor, theoretical models of stellar atmospheres with different chromospheric activity are computed. We have used these models to simulate the emerging Ca II and Mg II chromospheric emission fluxes and compare them with observations. The comparison shows that the wave heating alone can explain most but not all of the observed range of chromospheric activity. In addition, the obtained results clearly imply that the base of stellar chromospheres is heated by acoustic waves, the heating of the middle and upper chromospheric layers is dominated by magnetic waves associated with magnetic flux tubes, and that other non-wave (e.g., reconnective) heating mechanisms are required to explain the structure of the highest layers of stellar chromospheres.
This work was supported by NSF, NATO, DFG, KBN and The Alexander von Humboldt Foundation.