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

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.

Partly based on VEGA/CHARA observations.Appendix A is available in electronic form at http://www.aanda.org