MOST photometry of the roAp star 10 Aquilae^*

¹ Institute for Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria e-mail: huber@astro.univie.ac.at
² Astronomical Institute, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
³ Dept. of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
⁴ Dept. of Astronomy and Physics, St. Mary's University, Halifax, NS B3H 3C3, Canada
⁵ Dépt. de physique, Univ. de Montréal, CP 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, Canada
⁶ Dept. of Astronomy & Astrophysics, David Dunlop Obs., Univ. Toronto, PO Box 360, Richmond Hill, ON L4C 4Y6, Canada
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 8 December 2007
Accepted: 18 February 2008

Abstract

Context. We present 31.2 days of nearly continuous MOST photometry of the rapidly oscillating Ap star 10 Aql.

Aims. The goal was to provide an unambiguous frequency identification for this little studied star, as well as to discuss the detected frequencies in the context of magnetic models and analyze the influence of the magnetic field on the pulsation.

Methods. Using traditional Fourier analysis techniques on three independent data reductions, intrinsic frequencies for the star are identified. Theoretical non-adiabatic axisymmetric modes influenced by a magnetic field having polar field strengths = 0–5 kG were computed to compare the observations to theory.

Results. The high-precision data allow us to identify three definite intrinsic pulsation frequencies and two other candidate frequencies with low . Considering the observed spacings, only one ( = 50.95 μHz) is consistent with the main sequence nature of roAp stars. The comparison with theoretical models yields a best fit for a 1.95  model having solar metallicity, suppressed envelope convection, and homogenous helium abundance. Furthermore, our analysis confirms the suspected slow rotation of the star and sets new lower limits to the rotation period ( month) and inclination ().

Conclusions. The observed frequency spectrum is not rich enough to unambiguously identify a model. On the other hand, the models hardly represent roAp stars in detail due to the approximations needed to describe the interactions of the magnetic field with stellar structure and pulsation. Consequently, errors in the model frequencies needed for the fitting procedure can only be estimated. Nevertheless, it is encouraging that models which suppress convection and include solar metallicity, in agreement with current concepts of roAp stars, fit the observations best.