Fundamental parameters of B supergiants from the BCD system I. Calibration of the (lambda(1), D) parameters into T-eff

Context. Effective temperatures of early-type supergiants are important to test stellar atmosphere- and internal structure-models of massive and intermediate mass objects at different evolutionary phases. However, these T-eff values are more or less discrepant depending on the method used to determine them. Aims. We aim to obtain a new calibration of the T-eff parameter for early-type supergiants as a function of observational quantities that are: a) highly sensitive to the ionization balance in the photosphere and its gas pressure; b) independent of the interstellar extinction; c) as much as possible model-independent. Methods. The observational quantities that best address our aims are the (lambda(1), D) parameters of the BCD spectrophotometric system. They describe the energy distribution around the Balmer discontinuity, which is highly sensitive to T-eff and log g. We perform a calibration of the (lambda(1), D) parameters into T-eff using effective temperatures derived with the bolometric-flux method for 217 program stars, whose individual uncertainties are on average vertical bar Delta T-eff vertical bar/T-eff(f) = 0.05. Results. We obtain a new and homogeneous calibration of the BCD (lambda(1), D) parameters for OB supergiants and revisit the current calibration of the (lambda(1), D) zone occupied by dwarfs and giants. The final comparison of calculated with obtained T-eff values in the (lambda(1), D) calibration show that the latter have total uncertainties, which on average are epsilon T-eff/T-eff(f) similar or equal to +/- 0.05 for all spectral types and luminosity classes. Conclusions. The effective temperatures of OB supergiants derived in this work agree on average within some 2000 K with other determinations found in the literature, except those issued from wind-free non-LTE plane-parallel models of stellar atmospheres, which produce effective temperatures that can be overestimated by up to more than 5000 K near T-eff = 25 000 K. Since the stellar spectra needed to obtain the (lambda(1), D) parameters are of low resolution, a calibration based on the BCD system is useful to study stars and stellar systems like open clusters, associations or stars in galaxies observed with multi-object spectrographs and/or spectro-imaging devices.