EDP Sciences
Free access
Volume 501, Number 1, July I 2009
Page(s) 297 - 320
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361/200811147
Published online 27 March 2009
A&A 501, 297-320 (2009)
DOI: 10.1051/0004-6361/200811147

Fundamental parameters of B supergiants from the BCD system

I. Calibration of the ( $\mathsf{\lambda_1}$, D) parameters into T $\mathsf{_{eff}}$
J. Zorec1, L. Cidale2, 3, M. L. Arias2, 3, Y. Frémat4, M. F. Muratore2, A. F. Torres2, 3, and C. Martayan4, 5

1  Institut d'Astrophysique de Paris, UMR 7095 du CNRS, Université Pierre & Marie Curie, 98bis bd. Arago, 75014 Paris, France
    e-mail: zorec@iap.fr
2  Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, La Plata, Buenos Aires, Argentina
3  Instituto de Astrofísica 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

Received 14 October 2008 / Accepted 6 March 2009

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 $T_{\rm eff}$ values are more or less discrepant depending on the method used to determine them.
Aims. We aim to obtain a new calibration of the $T_{\rm eff}$ 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 $T_{\rm eff}$ and $\log g$. We perform a calibration of the ( $\lambda_1, D$) parameters into $T_{\rm eff}$ using effective temperatures derived with the bolometric-flux method for 217 program stars, whose individual uncertainties are on average $\vert\Delta T_{\rm eff}\vert/T_{\rm eff}^f$ = 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 $T_{\rm eff}$ values in the $(\lambda_1,D)$ calibration show that the latter have total uncertainties, which on average are $\epsilon_{T_{\rm eff}}/T_{\rm eff}^f$ $\simeq$ $\pm0.05$ 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 $T_{\rm eff}=25\,000$ 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.

Key words: stars: early-type -- stars: fundamental parameters

© ESO 2009