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The fundamental parameters of the roAp star HD 24712. A rapidly oscillator at the red edge of the instability strip

K. Perraut, I. M. Brandão, M. S. Cunha, D. Shulyak, D. Mourard, N. Nardetto, T. A. ten Brummelaar

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 Teff = 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.Based on observations made with the VEGA/CHARA spectro-interferometer.

Keywords
stars: fundamental parameters, stars: individual: HD 24712, techniques: high angular resolution, techniques: interferometric

Astronomy and Astrophysics
Volume 590
May 2016

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