Astron. Astrophys. 320, 799-810 (1997)

An improved calibration of Cepheid visual and infrared surface brightness relations from accurate angular diameter measurements of cool giants and supergiants

P. Fouqué ^{1, 2} and W.P. Gieren <sup>2, 3, 4

1</sup> Observatoire de Paris-Meudon DESPA F-92195 Meudon Cedex, France
² European Southern Observatory, Casilla 19001, Santiago 19, Chile
³ Universidad de Concepción, Departamento de Física, Casilla 4009, Concepción, Chile
⁴ P. Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Casilla 104, Santiago 22, Chile
(pfouque@eso.org and wgieren@astro.puc.cl)

Received 2 August 1996 / Accepted 7 October 1996

Abstract

We have calibrated optical and near-infrared surface brightness - colour relations for cool giant and supergiant stars using high-precision angular diameters of these stars determined from Michelson interferometry. We find that the giant and supergiant relations are undistinguishable over a wide range of intrinsic colours. We independently determine the slopes of these relations obeyed by Cepheid variables and find that in all the diagrams considered, these agree very well with the slopes derived from the stable giants and supergiants. Forcing the slopes to the values derived from the Cepheids, we determine a very precise value of the zero point of the surface brightness - colour relations valid for Cepheid variables, which is . This value is in agreement with the one derived from the Cepheid effective temperature scale of Pel (1978), and from the lunar occultation angular diameter of the Cepheid Gem (Ridgway et al. 1982).

We apply our newly calibrated surface brightness - colour relations to the cluster Cepheid U Sgr to find its radius and distance from the optical V, and the infrared V, and K, Barnes-Evans technique. While the numerical values derived from the three different versions of the technique do agree within , the near-infrared distance and radius values are times more accurate than the optical one; in particular, the distance and radius of the star derived from the V, solution are pc and solar radii, respectively, with errors less than 1 percent. We briefly discuss the potential of the near-infrared versions of the Barnes-Evans technique to set a very accurate (0.02 mag) zero point to Cepheid period-luminosity relations and thus make a very important contribution to the absolute calibration of the extragalactic distance scale.

Key words: cepheids stars: distances stars: fundamental parameters stars: individual: U Sgr infrared: stars

Send offprint requests to: P. Fouqué

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Contents

• 1. Introduction
• 2. Surface brightness relations for cool giants and supergiants
  • 2.1. Selection criteria
  • 2.2. The giants surface brightness relations
  • 2.3. Adopted reddening law
  • 2.4. The supergiants surface brightness relations
• 3. Surface brightness relations for Cepheid variables
  • 3.1. The slope on the vs. diagram
  • 3.2. The slope on the vs. diagram
  • 3.3. The slope on the vs. diagram
  • 3.4. Adopted surface brightness relations for Cepheid variables
  • 3.5. Validity of the adopted reddening law
• 4. The distance and radius of the cluster Cepheid U Sgr
  • 4.1. Observational data
  • 4.2. The method
  • 4.3. Results
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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