2024A&A...692A.189R

The emission lines of the Ca II H&K doublet present one of the most important channels of radiative cooling for the chromospheres of cool stars. Although most other line emissions (Mg II h&k and numerous iron lines) populate the far-UV and require a very competitive space-born observing time, the Ca II H&K lines in the optical UV are easily accessible by ground-based spectroscopy with relatively cheap instrumentation, and the observational data are plentiful. At the same time, in order to include realistic mechanical and magnetic heating, advanced chromospheric models now require radiative cooling losses in absolute terms and therefore call for a precise surface flux scale, which could be provided by matching a photospheric Ca II H&K line profile computed by photospheric models. However, a major obstacle here is the significant ambiguity in parameter space in the face of a very sensitive dependence of cool stellar optical UV surface fluxes on the effective temperature. Consequently, we have developed a rigorous method by which precise physical parameters, most notably the effective temperature, are first determined by ISPEC tools working on high signal-to-noise spectra and based on a suitable line list and reference continuum. However, crosstalk and navigation of multiple local chi-square minima (best solutions) in parameter space must be considered. Only with the optimal set of parameters is a single PHOENIX model calculated, which defines the spectral surface flux scale in the Ca II H&K line region. In this paper, we discuss the results of the absolute measurements of Ca II K line fluxes for 32 representative cool stars and the accuracy of this approach. Finally, we show a comparison of the Ca II K chromospheric fluxes with line widths, and a relation of chromospheric flux with effective temperature and gravity following the logarithmic form log F_Ca II_ = α' log g + β' log T_eff + C'.