Stromgren (b-y) Color Versus Effective Temperature for AFGK Main Sequence Stars

Boyajian et al. (2013) present an "Angular Diameter Anthology" of derived angular diameters and effective temperatures (Teffs) for stars of luminosity class V and IV (on or near the main sequence). While Boyajian et al. (2013) provided several useful color vs. Teff relations in a wide range of photometric systems (including Johnson, Cousins, Kron, Sloan, 2MASS, and WISE), no relation was given for the Stromgren (b-y) color. Stromgren (b-y) colors have been measured for tens of thousands of the brightest stars in the sky, and were compiled by Hauck & Mermilliod (1998). I match the 131 reported Teffs from Table 3 of Boyajian et al. (2013) to 90 stars with reported (b-y) colors in the Hauck & Mermilliod (1998) catalog. I omitted several stars whose (b-y) colors were reported to be unresolved photometry for 2 or more components. I did not take into account metallicity in the calibration. For those stars with reported metallicities, the median [Fe/H] is 0.00, and the 68.3% range is +-0.20 dex, hence the calibration should be appropriate for stars of approximately solar composition. The best low-order fit to the color-Teff data was a trinomial with rms scatter of 0.011 dex in log(Teff), or approximately +-150K for stars of solar Teff (5772 K; Mamajek 2012). As the mean (b-y) uncertainty is only +-0.002 mag, and the mean Teff uncertainty is only +-60K, it seems likely that much of the remaining Teff scatter is due to the effects of metallicity and surface gravity. The trinomial listed in the figure is only appropriate for dwarf stars with intrinsic (b-y) colors between -0.01 and 0.56.