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

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.

Flattening and surface-brightness of the fast-rotating star δ Persei with the visible VEGA/CHARA interferometer

“…3. This bias of 0.05 mag found for δ Per is much lower than the 1σ intrinsic dispersion (or 0.16 mag) found on the SBCR by Challouf et al (2014b), and is consistent with the bias on the SBCR (offset and dispersion) due to rotation estimated in Challouf et al (2015b).…”

“…Fast rotating stars and their environments are known to have an impact on the calibration of SBCR of early-type stars, limiting the precision on the relation to around 0.16 mag (Challouf et al 2014b). The SBCR is necessary to derive the distance of early-type eclipsing binaries (O, B, A), which are much brighter and thus easier to detect than late-type eclipsing binaries (Pietrzyński et al 2009;Mochejska et al 2001;Vilardell et al 2006;Pawlak et al 2013).…”

Theoretical impact of fast rotation on calibrating the surface brightness-color relation for early-type stars

“…The only limitation is currently the precision of the surface-brightness relation. Recently, Challouf et al (2014b; hereafter Paper I) derived the SBCR (as a function of the V − K color) for the first time and with a precision of about 0.16 mag using interferometric measurements (Challouf et al , 2014a. However, to achieve 0.02 mag of precision on the SBCR (or 1% in terms of distance), one has to consider the stellar activity in early-type Article published by EDP Sciences A107, page 1 of 8 stars, such as wind, mass-loss, pulsation, and the rotation, which together represent the most important effect on the SBCR.…”

The Araucaria project. Precise physical parameters of the eclipsing binary IO Aquarii

“…The mass ratio was fixed at the value derived from the spectroscopy described below. We consulted various tabulations of linear limb-darkening coefficients (van Hamme 1993;Díaz-Cordovés et al 1995;Claret et al 1995;Claret 2000) and then set the value of x LD for both stars to be equal to a fixed average value from these tabulations. We performed independent least-squares fits to investigate the sensitivity of the other parameters to the assumed value of x LD and accounted for this additional uncertainty when calculating the standard errors on these parameters.…”

“…It has been proven that such parameters combined with empirical stellar surface brightness -colour (SBC) relations derived from interferometric measurements (e.g. Kervella et al 2004;Challouf et al 2014) result in accurate distance determinations even to extragalactic eclipsing binaries. Indeed, very accurate distances to the two Magellanic Clouds determined with the eclipsing binary method have recently been reported (Pietrzyński et al 2013;Graczyk et al 2014).…”