The radius and effective temperature of the binary Ap star β CrB from CHARA/FLUOR and VLT/NACO observations^*

¹ LESIA, CNRS UMR 8109, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France e-mail: bruntt@phys.au.dk
² Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
⁴ Universidade do Porto, Centro de Astrofísica, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ Departamento de Matemática Aplicada, Faculdade de Ciências, Universidade do Porto, 4169 Porto, Portugal
⁶ Center for High Angular Resolution Astronomy, Georgia State University, PO Box 3965, Atlanta, Georgia 30302-3965, USA
⁷ Konkoly Observatory of the Hungarian Academy of Sciences, Budapest, Hungary
⁸ National Optical Astronomy Observatory, PO box 26732, Tucson, AZ 85726, USA

Received: 5 October 2009
Accepted: 10 December 2009

Abstract

Context. 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 (T_eff) often span a range of 1000 K.

Aims. Our goal is to reduce systematic errors and improve the T_eff calibration of Ap stars based on new interferometric measurements.

Methods. We obtained long-baseline interferometric observations of β 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.

Results. We determined limb-darkened angular diameters of 0.699±0.017 mas for β CrB A (from interferometry) and 0.415±0.017 mas for β CrB B (from surface brightness-colour relations), corresponding to radii of 2.63±0.09  (3.4% uncertainty) and 1.56±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±4% of the total flux and was determined by fitting an ATLAS9 model atmosphere to the broad-band NACO J and K magnitudes. By combining the flux of the A component with its measured angular diameter, we determined the effective temperature T_eff  (A) = 7980±180 K (2.3%).

Conclusions. Our new interferometric and imaging data enable nearly model-independent determination of the effective temperature of β CrB A. Including our recent study of α Cir, we now have direct T_eff measurements of two of the brightest roAp stars, providing a strong benchmark for improved calibration of the T_eff scale for Ap stars. This will support the use of potentially strong constraints imposed by asteroseismic studies of roAp stars.