2006A&A...458..293P

With the Δa photometric system, it is possible to study very distant galactic and even extragalactic clusters with a high level of accuracy. This can be done with a classical color-magnitude diagram and appropriate isochrones. The new calibration presented in this paper is a powerful extension. For open clusters, the reddening is straightforward for an estimation via Isochrone fitting and is needed in order to calculate the reddening-free, temperature sensitive, index (g₁-y)₀. As a last step, the calibration can be applied to individual stars. Because no a priori reddening-free photometric parameters are available for the investigated spectral range, we have applied the dereddening calibrations of the Stroemgren uvbyβ system and compared them with extinction models for the Milky Way. As expected from the sample of bright stars, the extinction is negligible for almost all objects. As a next step, already established calibrations within the Stroemgren uvbyβ, Geneva 7-color, and Johnson UBV systems were applied to a sample of 282 normal stars to derive a polynomial fit of the third degree for the averaged effective temperatures to the individual (g₁-y)₀ values. We derived an empirical temperature calibration for the Δa photometric system for A-type to mid F-type with a mean of the error for the whole sample of ΔT_eff is 134K, which is lower than the value in Paper I for hotter stars. No statistically significant effect of the rotational velocity on the precision of the calibration was found. We have derived a new intrinsically consistent, empirical, effective temperature calibration for a spectral range from early B-type to mid F-type, luminosity class V to III stars within the photometric Δa system. The statistical mean error over the complete spectral range of about 140 to 240K will allow to individual objects of far distant galactic be studied as well as extragalactic clusters with high accuracy.