The fundamental parameters of the Ap star 78 Virginis
Could 78 Vir be a rapidly oscillating Ap star?

K. Perraut, M. S. Cunha, I. M. Brandão, J. Loridat, D. Mourard, A. Meilland, N. Nardetto, H. McAlister, T. A. ten Brummelaar, J. Sturmann, L. Sturmann, N. Turner, C. Farrington, N. Vargas

Abstract

Context. Determining the effective temperature of Ap stars, including the roAp stellar pulsators, is a difficult task owing to their strong magnetic field and their related spotted surfaces. It is, however, an important step towards constraining models of their complex atmosphere and testing proposed pulsation excitation mechanisms.
Aims. Using the unique angular resolution provided by long-baseline visible interferometry, we aim at deriving accurate angular diameters of a number of Ap targets, so as to determine their unbiased effective temperature (T_eff) and their accurate position in the Hertzsprung-Russell diagram, to estimate their mass and age, and to test non-adiabatic pulsation models. Interferometric results on four Ap stars have been published in earlier works. Here we report the results on a fifth, significantly hotter star.
Methods. We observed 78 Vir with the visible spectrograph VEGA installed at the combined focus of the CHARA long-baseline optical array. We derived the limb-darkened diameter of this Ap star from our interferometric 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. We then used the derived fundamental parameters to perform a non-adiabatic pulsation analysis.
Results. We determined a limb-darkened angular diameter of 0.346 ± 0.006 mas and deduced a linear radius of R = 2.11 ± 0.04 R_⊙. Considering a bolometric flux of 2.73 ± 0.20 10^-7 erg/cm²/s we obtained a luminosity of L/L_⊙ = 27 ± 2 and an effective temperature of T_eff = 9100 ± 190 K. The non-adiabatic pulsation modeling allows us to predict that high overtone pulsations could be excited in 78 Vir at frequencies ranging from 1.2 to 1.9 mHz, provided that the magnetic field is capable of suppressing envelope convection in the polar regions.
Conclusions. Visible long-baseline interferometry is a unique means of deriving accurate fundamental parameters of Ap stars. The Ap star 78 Vir is found to be a promising roAp-star candidate and one that would allow us to extend recent tests on the roAp stars’ excitation mechanism towards the blue edge of the instability strip. Asteroseismic data of this star would, therefore, be of strong interest.

Keywords
methods: observational, techniques: high angular resolution, techniques: interferometric, stars: individual: 78 Vir, stars: fundamental parameters

Astronomy and Astrophysics
Volume 579, Page A85
July 2015

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