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

Zorec, J. (1), Cidale, L. (2,3), Arias, M.L. (2,3), Fr'emat, Y. (4), Muratore, M.F. (2),
Torres, A.F. (2,3), Martayan, C. (4,5)

(1) Institut d'Astrophysique de Paris, UMR 7095 du CNRS, Universit'e
Pierre & Marie Curie, 98bis bd. Arago, 75014 Paris, France
(2) Facultad de Ciencias Astron'omicas y Geof'{i}sicas, Universidad Nacional de
La Plata, Paseo del Bosque S/N, La Plata, Buenos Aires, Argentina
(3) Instituto de Astrof'{i}sica de La Plata, (CCT La Plata - CONICET, UNLP),
Paseo del Bosque S/N, La Plata, Buenos Aires, Argentina
(4) Royal Observatory of Belgium, 3 av. Circulaire, 1180 Brussels, Belgium
(5) Observatoire de Paris-Meudon, GEPI, UMR8111 du CNRS, 92195 Meudon Cedex, France

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.
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.
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 |Delta T_eff|/T_eff|=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 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~0.05 for all spectral types and luminosity classes.
The effective temperatures of OB supergiants derived in this work agree on average within some 2,000 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 5,000 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

Reference: Astronomy and Astrophysics
Status: Manuscript has been accepted