Determining the effective temperatures of G- and K-type giants and supergiants based on observed photometric indices

A method for determining the effective temperature T _eff of G- and K-type giants and supergiants is proposed. The method is based on the use of two photometric indices free from the interstellar absorption influence: the Q index in the UBV photometric system and the c ₁] index in the uvby system. Empirical relations between the T _eff values found for nearby and bright G- and K-type giants and supergiants with the use of the infrared fluxes method (IRFM), on the one hand, and the observed Q and c ₁] indices for these stars, on the other hand, are plotted. A systematic discrepancy between the dependences of T _eff on Q for the stars with standard and reduced metallicities is found. Approximating the plotted dependences with second-order polynomials, one can obtain a relatively simple and a rather accurate method for determining the T _eff value in the range of 3800 ≤ T ≤ 5100 K (based on the Q index) or 4900 ≤ T _eff ≤ 5500 K (based on the c ₁] index).