The non-active stellar chromosphere: Ca ii basal flux

We analyse high-resolution, high-s/n European Southern Observatories (ESO)-archive spectra (from UVES, the UV echelle spectrograph) of 76 inactive or modestly active stars of spectral type G to M, main sequence and giants. Using phoenix model photospheres with Ca ii K lines that match the observed line profiles, we (i) revise the effective temperatures, (ii) obtain a precise surface flux scale for each star and (iii) directly determine the exact surface fluxes of each Ca ii K chromospheric emission with respect to the photospheric line profile. We find that our stellar sample exhibits a lower boundary to its chromospheric surface flux distribution with an unprecedented definition. From a subsample of the 25 least active stars, we obtain a simple empirical formula for the basal Ca ii flux as a function of effective temperature: $\log {F^{\rm basal}_{\rm Ca\,\small {II}(H+K)}} = 7.05(\pm 0.31) \log {T_{\rm eff}} - 20.86(\pm 1.15)$ . This is in good agreement with the Mg ii basal flux. In a direct comparison with the large body of Mt Wilson S-measurements of the chromospheric Ca ii emission and its well-defined cut-off, excellent agreement is achieved as well. A new result, however, is the small scatter of the least active star's fluxes about the basal flux. It is about 25 per cent and equals the residual uncertainties of our approach. At the same time, we do not find any evidence for a gravity dependence within these limits. This strongly confirms the basal flux as a well-defined and universal phenomenon, which characterizes every inactive chromosphere.