The radius and effective temperature of the binary Ap starβ CrB from CHARA/FLUOR and VLT/NACO observations

Abstract

<i>Context. <i/>The prospects for using the asteroseismology of rapidly oscillating Ap (roAp) stars are hampered by the large uncertainty in fundamental stellar parameters. Results in the literature for the effective temperature (<i>T<i/><sub>eff<sub/>) often span a range of 1000 K.<i>Aims. <i/>Our goal is to reduce systematic errors and improve the <i>T<i/><sub>eff<sub/> calibration of Ap stars based on new interferometric measurements.<i>Methods. <i/>We obtained long-baseline interferometric observations of <i>β<i/> CrB using the CHARA/FLUOR instrument. To disentangle the flux contributions of the two components of this binary star, we obtained VLT/NACO adaptive optics images.<i>Results. <i/>We determined limb-darkened angular diameters of 0.699<i>±<i/>0.017 mas for <i>β<i/> CrB A (from interferometry) and 0.415<i>±<i/>0.017 mas for <i>β<i/> CrB B (from surface brightness-colour relations), corresponding to radii of 2.63<i>±<i/>0.09 (3.4% uncertainty) and 1.56<i>±<i/>0.07 (4.5%). The combined bolometric flux of the A+B components was determined from satellite UV data, spectrophotometry in the visible, and broadband data in the infrared. The flux from the B component constitutes 16<i>±<i/>4% of the total flux and was determined by fitting an ATLAS9 model atmosphere to the broad-band NACO <i>J<i/> and <i>K<i/> magnitudes. By combining the flux of the A component with its measured angular diameter, we determined the effective temperature <i>T<i/><sub>eff<sub/> (A) = 7980<i>±<i/>180 K (2.3%).<i>Conclusions. <i/>Our new interferometric and imaging data enable nearly model-independent determination of the effective temperature of <i>β<i/> CrB A. Including our recent study of <i>α<i/> Cir, we now have direct <i>T<i/><sub>eff<sub/> measurements of two of the brightest roAp stars, providing a strong benchmark for improved calibration of the <i>T<i/><sub>eff<sub/> scale for Ap stars. This will support the use of potentially strong constraints imposed by asteroseismic studies of roAp stars.