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

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access.

An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account.
    In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.