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

Abstract

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.

Evolution of solar-type activity: acoustic and magnetic energy generation and propagation in $\beta $ Hydri (G2 IV)

“…For the determination of the magnetic field strength at height z = 0 km, we first compute the maximum allowed field of a void tube assuming the inner gas pressure p i = 0 dyne cm −2 ; this values also known as the equipartition field strength given by p e /8π = B eq taken at 0 km, the photospheric reference height. According to the initial atmospheric model, the resulting equipartition magnetic field strength is given as B eq = 1505 G. A comparison to the solar case, as well as in alignment to previous magneto-acoustic model simulations for other stars (e.g., Fawzy & Cuntz 2018Cuntz et al 2021), we choose a surface magnetic field strength given as 0.85 B eq . Nevertheless, as part of our parameter study of magnetic energy generation based on longitudinal flux-tube waves, we consider a larger set of parameters η = B/B eq , given as 0.75, 0.85, and 0.95.…”