Improving the surface brightness-color relation for early-type stars using optical interferometry^⋆,⋆⋆

¹ Laboratoire Lagrange, UMR 7293, UNS/CNRS/OCA, 06300 Nice, France
e-mail: mounir.challouf@oca.eu
² Laboratoire Dynamique Moléculaire et Matériaux Photoniques, UR11ES03, Université de Tunis/ESSTT, Tunisie
³ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Paraguay
⁴ Warsaw University Observatory, AL. Ujazdowskie 4, 00-478 Warsaw, Poland
⁵ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
⁶ CNRS, IPAG, 38000 Grenoble, France
⁷ Université Lyon 1, Observatoire de Lyon, 9 avenue Charles André, 69230 Saint Genis Laval, France
⁸ CNRS/UMR 5574, Centre de Recherche Astroph. de Lyon, École Normale Supérieure, 69007 Lyon, France
⁹ Georgia State University, PO Box 3969, Atlanta GA 30302-3969, USA
¹⁰ CHARA Array, Mount Wilson Observatory, 91023 Mount Wilson CA, USA

Received: 7 March 2014
Accepted: 2 September 2014

Abstract

Context. The method of distance determination of eclipsing binaries consists in combining the radii of both components determined from spectro-photometric observations with their respective angular diameters derived from the surface brightness-color relation (SBC). However, the largest limitation of the method comes from the uncertainty on the SBC relation: about 2% for late-type stars (or 0.04 magnitude) and more than 10% for early-type stars (or 0.2 mag).

Aims. The aim of this work is to improve the SBC relation for early-type stars in the −1 ≤ V − K ≤ 0 color domain, using optical interferometry.

Methods. Observations of eight B- and A-type stars were secured with the VEGA/CHARA instrument in the visible. The derived uniform disk angular diameters were converted into limb darkened angular diameters and included in a larger sample of 24 stars, already observed by interferometry, in order to derive a revised empirical relation for O, B, A spectral type stars with a V − K color index ranging from −1 to 0. We also took the opportunity to check the consistency of the SBC relation up to V − K ≃ 4 using 100 additional measurements.

Results. We determined the uniform disk angular diameter for the eight following stars: γ Ori, ζ Per, 8 Cyg, ι Her, λ Aql, ζ Peg, γ Lyr, and δ Cyg with V − K color ranging from −0.70 to 0.02 and typical precision of about 1.5%. Using our total sample of 132 stars with V − K colors index ranging from about − 1 to 4, we provide a revised SBC relation. For late-type stars (0 ≤ V − K ≤ 4), the results are consistent with previous studies. For early-type stars (− 1 ≤ V − K ≤ 0), our new VEGA/CHARA measurements combined with a careful selection of the stars (rejecting stars with environment or stars with a strong variability), allows us to reach an unprecedented precision of about 0.16 magnitude or ≃7% in terms of angular diameter.

Conclusions. We derive for the first time a SBC relation for stars between O9 and A3, which provides a new and reliable tool for the distance scale calibration.