The fundamental parameters of the roAp star HD 24712

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.

Towards reliable uncertainties in IR interferometry: the bootstrap for correlated statistical and systematic errors

“…It is well known that a naïve error propagation in presence of high correlations can lead, and indeed leads as we discovered in our data, to a least-squares fit that falls far off the data and yields a parameter estimation inconsistent with a more careful analysis. The paradox, known as Peelle's pertinent puzzle (Peelle 1987), has an easy remedy in the case of a fit by a constant value (Neudecker et al 2012) or a set of constant values (Neudecker et al 2014). In their analytic derivation of the covariances, the weighted average of the measurements replaces the individual measurements.…”

Imaging at High Angular Resolution of Stellar Surfaces and Close Environment

“…The problem comes down to compare the temperatures derived by spectroscopy and interferometry, respectively. This is what is currently done on roAp type stars, which have also spots due to their magnetism [74], which are known to alter the derived temperature. Since the roAp have extremely small angular diameters, the VEGA/CHARA instrument, whose spatial resolution is one of the largest in the world (see Figure 104) has been used to measure their angular diameter.…”

“…Since the roAp have extremely small angular diameters, the VEGA/CHARA instrument, whose spatial resolution is one of the largest in the world (see Figure 104) has been used to measure their angular diameter. For instance, it allowed to measure the angular diameter of 78 Vir and HD24712, which have indeed the smallest diameters ever measured, respectively θ LD = 0.346± 0.006 mas [75] and θ LD = 0.335 ± 0.009 mas [74] (see Fig. 115).…”

Benchmarking the fundamental parameters of Ap stars with optical long-baseline interferometric measurements

“…Such large bandwidths lead to a mean spectral resolution of about 40, and our visibility measurements are thus mainly sensitive to the stellar photosphere. We followed the same processing as described in our previous papers (Perraut et al 2011(Perraut et al , 2013(Perraut et al , 2015(Perraut et al , 2016 to compute raw squared visibility for each block of 2500 individual frames. Even though we recorded data with telescope triplets, we cannot retrieve closure phases on our targets since the signal-to-noise ratio on this observable is strongly limited by the saturation limit of the VEGA photon-counting detectors (see Fig.…”

“…We follow the same approach as in our previous papers to derive the bolometric fluxes from observational spectral energy distributions (SEDs; e.g. Perraut et al 2011Perraut et al , 2013Perraut et al , 2015Perraut et al , 2016. So, in addition to the six Ap targets presented in the last section, we also computed the SED of HD 188041 and HD 204411, which was not done in Romanovskaya et al (2019b).…”