Estimating stellar spots from Ca II H&K activity proxy in solar-like stars

The magnetic phenomena on the solar surface have been the subject of several investigations over the last 400 years. The early indicator of the solar magnetic activity was the sunspot counting. The main indicators now are the International Sunspot Number, the Group Number, the Total Sunspot Area, and Photometric Sunspot Index. Several improvements in observational techniques have allowed measuring the magnetic activity using the solar/stellar spectra. The standard spectroscopic activity indicator is the S-index, based on the Ca II H&K emission lines, jointly the chromospheric component R'_HK. Observations to stellar spots in other stars remain unclear, it is worth proposing a theoretical estimate to spot amount on other solar-type stars using the Sun as a star. We report a calibration between the spectroscopic chromospheric activity proxies (S-index and R'_HK) and the sunspot indicators over the last decades. Our results indicate a quadratic power-law relationship.