New intrinsic-colour calibration for uvby- β photometry

Abstract

A new intrinsic-colour calibration ((b-y)_o-β) is presented for the uvby- β photometric system, making use of re-calibrated Hipparcos parallaxes and published reddening maps. This new calibration for (b-y)_o-β, our Eq. (1), has been based upon stars with d_Hip<70pc in the photometric catalogues of Schuster and Nissen (1988), Schuster et al. (2004, 2006), provides a small dispersion, ±0.009, and has a positive "standard" +2.239Δβ coefficient, which is not too different from the coefficients of Crawford (1975a, +1.11) and of Olsen (1988, +1.34). For 61 stars with spectra from CASPEC, UVES/VLT, and FIES/NOT databases, without detectable Na I lines, the average reddening value <E(b-y)>=-0.001±0.002 shows that any zero-point correction to our intrinsic-colour equation must be minuscule.