Theoretical Basal Ca Ii and Mg Ii Fluxes for Late-Type Stars: Results From Acoustic Wave Spectra With Time-Dependent Ionization and Multilevel Radiation Treatments

Abstract

Computations of chromospheric models and the resulting spectral line emission fluxes are presented for late-type stars exhibiting very low level of chromospheric activity, referred to as a basal flux stars or low activity stars. The computations are self-consistent, and consider the entire acoustic wave energy spectra generated in the stellar convection zones. We consider multilevel atomic models, take into account departures from local thermodynamic equilibrium and also consider the time-dependent ionization processes of hydrogen. We employ the new finding of the mixing-length parameter alpha = 1.8. The Ca II H+K and Mg II h+k line fluxes are computed assuming pseudo-partial redistribution. The results show the importance of time-dependent ionization in modelling the middle and high chromospheres. Models without considering time-dependent ionization overestimate the emitted Ca II fluxes by factors between 1.1 and 5.6 for F8V and M0V stars, respectively, while factors between 1.8 for G0V and 17.4 for M0V stars have been obtained for the MgII fluxes. The theoretically computed basal fluxes in Ca II and MgII, respectively, follow simple linear formulae depending on the effective temperature log T-eff. The obtained results for Ca II fluxes show reasonable agreement with observations.