Color Temperatures of Red Giants and their Relation to the Effective Temperature

Abstract

Color temperatures of red giant stars can be calibrated in terms of effective temperature if the colors are measured at wavelengths of minimal discrete absorption and nearly equal continuous opacity. The problem, of selecting suitable wavelengths in the complex spectra of M stars is discussed; measurements in the near infrared are recommended for early M stars, and the inclusion of a measurement at 4.00 µm is very helpful for stars later than about M5. Color temperatures based on infrared continuum points appear to confirm the new scale of effective temperatures derived by Ridgway et al., in that line-blanketed models of those effective temperatures are able to reproduce the observed color temperatures. A program of direct measurements of absolute infrared monochromatic fluxes is now in progress.