Fundamental Parameters of B Supergiants from the BCD System I. Calibration of the (lambda_1, D) parameters into Teff

ZOREC, J.; CIDALE, L.; ARIAS, M. L.; FRÉMAT, Y.; MURATORE, M. F.; TORRES, A. F.; MARTAYAN, C.

ASTRONOMY AND ASTROPHYSICS

EdP Sciences

Lugar: Les Ulis ; Año: 2009 vol. 501 p. 297 - 320

0004-6361

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 Teff values are more or less discrepant depending on the method used to determine them. We aim at obtaining a new calibration of the Teff 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. The observational quantities that better respond to 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 Teff and log g. We perform the calibration of the (lambda_1, D) parameters into Teff using effective temperatures derived with the bolometric-flux method for all program stars, whose individual uncertainties are on average |Delta Teff|/Teff}^f =0.05. 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 effective temperatures of OB supergiants derived in this work agree on average within some 2000 K with other determinations found in the literature, except with those issued from wind-free non-LTE plane-parallel models of stellar atmospheres, which produce effective temperatures that can be overestimated up to more than 5000 K near Teff=25000 K.par Since the stellar spectra needed to obtain the (lambda_1, D) parameters are of low resolution, the 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.