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

TL;DR

This paper introduces a new photometric method to accurately determine the effective temperatures of G- and K-type giants and supergiants using indices unaffected by interstellar absorption, validated against infrared flux measurements.

Contribution

It presents empirical relations linking photometric indices to effective temperatures, enabling simple and accurate temperature estimation within specified ranges.

Findings

Established relations between Teff and photometric indices Q and [c1].

Identified metallicity-dependent discrepancies in Teff estimations.

Provided polynomial approximations for Teff determination.

Abstract

A method for determining the effective temperature Teff 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 [c1] index in the uvby system. Empirical relations between the Teff 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 [c1] indices for these stars, on the other hand, are plotted. A systematic discrepancy between the dependences of Teff 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 Teff value in the range of 3800 < Teff < 5100 K (based on the Q index) or 4900 < Teff < 5500 K (based on the [c1] index).