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Session 12 - Stellar Evolution - Theory.
Display session, Wednesday, January 07
The vast amount of observational data collected at wavelengths ranging from X-rays to radio waves have indicated the ubiquity of stellar chromospheres among late-type stars. In addition, there is growing observational evidence for inhomogeneous and locally strong magnetic fields in stellar atmospheres. It is reasonable to assume that stellar magnetic inhomogeneities may be similar to the `flux tube' structures observed in the solar atmosphere outside sunspots. If so, two distinct components of stellar chromospheres must be recognized, namely, non-magnetic component, where acoustic waves are responsible for the heating, and magnetic component, where MHD tube waves supply energy for the heating. To construct theoretical models of stellar chromospheres (see paper by Cuntz et al. presented at this meeting), it is necessary to know the amount of non-radiative energy generated in stellar convective zones and carried by acoustic and MHD tube waves through stellar photospheres. In this paper, we discuss the correct status of computing acoustic and MHD wave energy fluxes for the Sun and late-type dwarfs. Our calculations are based on grey LTE mixing-length convection zone models and both linear and non-linear theories of wave generation are used. New acoustic and MHD wave energy fluxes are presented for stars of population I and II in the range of effective temperatures T_eff 2000 - 10000 K and gravities log g = 1 - 8. The turbulent flow field is represented by an extended Kolmogorov spatial and modified Gaussian temporal energy spectrum. The mixing-length parameter is varied in the range \alpha = 1 - 2. We find that the obtained acoustic wave energy strongly depend on stellar chemical composition and that MHD fluxes show wide variations for a given spectral type, variations which can be attributed to changes in the stellar flux tube filling factor. We discuss the range of the filling factor for which the calculated MHD fluxes may account for the observed levels of chromospheric activity.
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