The Effective Temperature Scale of FGK Stars. I. Determination of Temperatures and Angular Diameters with the Infrared Flux Method

Abstract

The infrared flux method (IRFM) has been applied to a sample of 135 dwarf and 36 giant stars covering the following regions of the atmospheric parameter space: (1) the metal-rich ([Fe/H] ≳ 0) end (consisting mostly of planet-hosting stars), (2) the cool ( T eff ≲ 5000 K) metal-poor (-1 ≲ [Fe/H] ≲ -3) dwarf region, and (3) the very metal-poor ([Fe/H] ≲ -2.5) end. These stars were especially selected to cover gaps in previous works on T eff versus color relations, particularly the IRFM T eff scale of A. Alonso and collaborators. Our IRFM implementation was largely based on the Alonso et al. study (absolute infrared flux calibration, bolometric flux calibration, etc.) with the aim of extending the ranges of applicability of their T eff versus color calibrations. In addition, in order to improve the internal accuracy of the IRFM T eff scale, we recomputed the temperatures of almost all stars from the Alonso et al. work using updated input data. The updated temperatures do not significantly differ from the original ones, with few exceptions, leaving the T eff scale of Alonso et al. mostly unchanged. Including the stars with updated temperatures, a large sample of 580 dwarf and 470 giant stars (in the field and in clusters), which cover the ranges 3600 K ≲ T eff ≲ 8000 K and -4.0 ≲ [Fe/H] ≲ +0.5, have T eff homogeneously determined with the IRFM. The mean uncertainty of the temperatures derived is 75 K for dwarfs and 60 K for giants, which is about 1.3% at solar temperature and 4500 K, respectively. It is shown that the IRFM temperatures are reliable in an absolute scale given the consistency of the angular diameters resulting from the IRFM with those measured by long baseline interferometry, lunar occultation, and transit observations. Using the measured angular diameters and bolometric fluxes, a comparison is made between IRFM and direct temperatures, which shows excellent agreement, with the mean difference being less than 10 K for giants and about 20 K for dwarf stars (the IRFM temperatures being larger in both cases). This result was obtained for giants in the ranges 3800 K < T eff < 5000 K and -0.7 < [Fe/H] < 0.2 and dwarfs in the ranges 4000 K < T eff < 6500 K and -0.55 < [Fe/H] < 0.25; thus, the zero point of the IRFM T eff scale is essentially the absolute one (that derived from angular diameters and bolometric fluxes) within these limits. The influence of the bolometric flux calibration adopted is explored and it is shown that its effect on the T eff scale, although systematic, is conservatively no larger than 50 K. Finally, a comparison with temperatures derived with other techniques is made. Agreement is found with the temperatures from Balmer line profile fitting and the surface brightness technique. The temperatures derived from the spectroscopic equilibrium of Fe I lines are differentially consistent with the IRFM, but a systematic difference of about 100 and 65 K (the IRFM temperatures being lower) is observed in the metal-rich dwarf and metal-poor giant T eff scales, respectively.