The chromospheric Ca II and Mg II radiative losses in late-type stars: a computational comparison between two-level and multi-level atomic models

In this study, we consider chromospheric heating models for 55 Cancri in conjunction with observations. The theoretical models, previously discussed in Paper I, are self-consistent, nonlinear and time-dependent ab-initio computations encompassing the generation, propagation, and dissipation of waves. Our focus is the consideration of both acoustic waves and longitudinal flux tube waves amounting to two-component chromosphere models. 55 Cancri, a K-type orange dwarf, is a star of low activity, as expected by its age, which also implies a relatively small magnetic filling factor. The Ca II K fluxes are computed (multi-ray treatment) assuming partial redistribution and time-dependent ionization. The theoretical Ca II H+K fluxes are subsequently compared with observations. It is found that for stages of lowest chromospheric activity the observed Ca II fluxes are akin, though not identical, to those obtained by acoustic heating, but agreement can be obtained if low levels of magnetic heating — consistent with the assumed photospheric magnetic filling factor — are considered as an additional component; this idea is in alignment with previous proposals conveyed in the literature.

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Chromospheric activity in 55 Cancri – I. Results from theoretical wave studies

Fawzy

Cuntz

2021

Monthly Notices of the Royal Astronomical Society

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We present theoretical models of chromospheric heating for 55 Cancri, an orange dwarf of relatively low activity. Self-consistent, nonlinear and time-dependent ab-initio numerical computations are pursued encompassing the generation, propagation, and dissipation of waves. We consider longitudinal waves operating among arrays of flux tubes as well as acoustic waves pertaining to nonmagnetic stellar regions. Additionally, flux enhancements for the longitudinal waves are also taken into account as supplied by transverse tube waves. The Ca II K fluxes are computed (multi-ray treatment) assuming partial redistribution as well as time-dependent ionization. The self-consistent treatment of time-dependent ionization (especially for hydrogen) greatly impacts the atmospheric temperatures and electron densities (especially behind the shocks); it also affects the emergent Ca II fluxes. Particularly, we focus on the influence of magnetic heating on the stellar atmospheric structure and the emergent Ca II emission, as well as the impact of nonlinearities. Our study shows that a higher photospheric magnetic filling factor entails a larger Ca II emission; however, an increased initial wave energy flux (e.g., associated with mode coupling) is of little difference. Comparisons of our theoretical results with observations will be conveyed in forthcoming Paper II.

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The chromospheric Ca II and Mg II radiative losses in late-type stars: a computational comparison between two-level and multi-level atomic models

Cited by 4 publications

References 29 publications

Theoretical basal Ca II fluxes for late-type stars: results from magnetic wave models with time-dependent ionization and multi-level radiation treatments

Theoretical basal Ca II fluxes for late-type stars: results from magnetic wave models with time-dependent ionization and multi-level radiation treatments

Chromospheric activity in 55 Cancri: II. Theoretical wave studies versus observations

Chromospheric activity in 55 Cancri – I. Results from theoretical wave studies

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