Abstract
Due to the strong magnetic field and related abnormal surface layers existing in rapidly oscillating Ap stars, systematic errors are likely to be present when determining their effective temperatures, which potentially compromises asteroseismic studies of these pulsators. Using long-baseline interferometry, our goal is to determine accurate angular diameters of a number of roAp targets to provide a temperature calibration for these stars. We obtained interferometric observations of 10 Aql with the visible spectrograph VEGA at the CHARA array. We determined a limb-darkened angular diameter of 0.275+/-0.009 mas and deduced a linear radius of 2.32+/-0.09 R_sun. 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. For two data sets of bolometric flux we derived an effective temperature of 7800+/-170 K and a luminosity of 18+/-1 L_sun or of 8000+/-210 K and 19+/-2 L_sun. We used these fundamental parameters together with the large frequency separation 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 of 1.92 M_sun and an age of 780 Gy or a mass of 1.95 M_sun and an age of 740 Gy, depending on the considered bolometric flux. For the first time, 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.
Highlights
Oscillating Ap stars are a subgroup of Ap stars that are chemically peculiar main-sequence magnetic stars of spectral type B8 to F0
We present here the results we obtained on 10 Aql (HD 176232; F0p), which is one of the brightest rapidly oscillating Ap (roAp) stars of the northern hemisphere
Where u(R) denotes the limb-darkening coefficient in the R band. It is obtained by the adjustment of the radial intensity distribution on the stellar disk using a stellar atmosphere model characterized by the effective temperature (Teff), the gravity, and the metallicity ([Fe/H])
Summary
Oscillating Ap (roAp) stars are a subgroup of Ap stars that are chemically peculiar main-sequence magnetic stars of spectral type B8 to F0. The roAp stars exhibit strong large-scale organized magnetic fields (between a hundred of G up to 24.5 kG for HD 154708 – Hubrig et al (2005)), abundance anomalies of Sr, Cr and rare earths, small rotation speeds (≤100 km s−1 and often ≤30–40 km s−1), and pulsations with a range of amplitudes from about 10 millimagnitudes (Kurtz et al 2006) to 10 micromagnitudes (Balona et al 2011) and periods ranging from 6 min for HD 134214 (Saio et al 2012) up to 24 min for HD 177765 (Alentiev et al 2012). We discuss how our results compare with those published in the literature (Sect. 6)
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