Abstract
Context. There is still a debate about the nature of the mechanism that causes the pulsation excitation of the rapidly oscillating Ap stars that oscillate above the highest theoretically acoustic frequency. HD 24712 is a good test case for such a study because it is bright, its parallax accurately determined, and its frequency spectrum is well known.Aims. Visible long-baseline interferometry is a unique technique for measuring accurate angular diameters of targets as small as the brightest roAp stars, and thus estimating accurate radii by a method as independent as possible of atmosphere models.Methods. We used the visible spectrograph VEGA at the CHARA long-baseline optical array to observe HD 24712, and we derived its limb-darkened diameter. We also estimated its bolometric flux from spectroscopic data in the literature and determined its radius, luminosity, and effective temperature.Results. We determined a limb-darkened angular diameter of 0.335 ± 0.009 mas for HD 24712 and derived a radius of R = 1.772 ± 0.057 R ⊙ , a luminosity of L = 7.2 ± 1.8 L ⊙ , and an effective temperature of T eff = 7235 ± 280 K, which is in very close agreement with the values provided by the self-consistent stratified model developed for this star. We used these fundamental parameters to set HD 24712 in the Hertzsprung-Russell diagram. Its position is marginally consistent with the region where high radial order modes are predicted to be excited by the κ -mechanism. Conclusions. We conclude that oscillations in this star are most likely not driven by the κ -mechanism.
Highlights
HD 24712 (HR 1217; DO Eri) is a cool rapidly oscillating Ap star whose 6.15 min light variation was discovered by Kurtz (1981) and whose radial velocity (RV) variations with an amplitude of 400 ± 50 m/s were detected by Matthews et al (1988)
The uncertainty associated with the bolometric flux was estimated by considering a 10% uncertainty on the flux computed from the combined IUE spectra, and a conservative 15% was considered both on the Kurucz model flux and on the flux derived from interpolation
Taking advantage of the unique angular resolution of the CHARA array when coupled with a visible instrument, we clearly resolved the roAp star HD 24712
Summary
HD 24712 (HR 1217; DO Eri) is a cool rapidly oscillating (roAp) Ap star whose 6.15 min light variation was discovered by Kurtz (1981) and whose radial velocity (RV) variations with an amplitude of 400 ± 50 m/s were detected by Matthews et al (1988). All the modes are damped, and the κ-mechanism in the hydrogen ionization area that is generally invoked to explain the excitation of pulsation modes in roAp stars (Balmforth et al 2001; Cunha 2002) seems to be too weak to be able to excite supercritical high-order p modes. To better study the mechanism that is able to excite pulsations in such cool roAp stars, the radius is of primordial importance since the frequency region where modes are expected to be excited is very sensitive to the stellar radius (Cunha et al 2013). We compare our results with those published in the literature (Sect. 5)
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