The fundamental parameters of the roAp star 10 Aquilae

Context. Owing to the strong magnetic field and related abnormal surface layers existing in rapidly oscillating Ap (roAp) stars, systematic errors are likely to be present when determining their effective temperatures, which potentially compromises asteroseismic studies of this class of pulsators. Aims. Using the unique angular resolution provided by long-baseline visible interferometry, our goal is to determine accurate angular diameters of a number of roAp targets, so as to derive unbiased effective temperatures (T-eff) and provide a T-eff calibration for these stars. Methods. We obtained long-baseline interferometric observations of 10 Aql with the visible spectrograph VEGA at the combined focus of the CHARA array. We derived the limb-darkened diameter of this roAp star from our visibility measurements. Based on photometric and spectroscopic data available in the literature, we estimated the star's bolometric flux and used it, in combination with its parallax and angular diameter, to determine the star's luminosity and effective temperature. Results. We determined a limb-darkened angular diameter of 0.275 +/- 0.009 mas and deduced a linear radius of R = 2.32 +/- 0.09 R-circle dot. For the bolometric flux we considered two datasets, leading to an effective temperature of T-eff = 7800 +/- 170 K and a luminosity of L/L-circle dot = 18 +/- 1 or T-eff = 8000 +/- 210 K and L/L-circle dot = 19 +/- 2. Finally we used these fundamental parameters together with the large frequency separation determined by asteroseismic observations to constrain the mass and the age of 10 Aql, using the CESAM stellar evolution code. Assuming a solar chemical composition and ignoring all kinds of diffusion and settling of elements, we obtained a mass M/M-circle dot similar to 1.92 and an age of similar to 780 Gy or a mass M/M-circle dot similar to 1.95 and an age of similar to 740 Gy, depending on the derived value of the bolometric flux. Conclusions. For the first time, thanks to the unique capabilities of VEGA, we managed to determine an accurate angular diameter for a star smaller than 0.3 mas and to derive its fundamental parameters. In particular, by only combining our interferometric data and the bolometric flux, we derived an effective temperature that can be compared to those derived from atmosphere models. Such fundamental parameters can help for testing the mechanism responsible for the excitation of the oscillations observed in the magnetic pulsating stars.