Volume 480, Number 1, March II 2008
|Page(s)||223 - 232|
|Section||Stellar structure and evolution|
|Published online||17 January 2008|
MOST photometry and modeling of the rapidly oscillating (roAp) star γ Equulei *
Institute for Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria e-mail: email@example.com
2 Astronomical Institute, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan e-mail: saio @ astr.tohoku.ac.jp
3 Department of Astronomy and Physics, St. Mary's University Halifax, NS B3H 3C3, Canada e-mail: firstname.lastname@example.org
4 Department Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada e-mail: email@example.com; firstname.lastname@example.org; email@example.com
5 Dépt. de physique, Univ. de Montréal CP 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, Canada e-mail: firstname.lastname@example.org
6 Dept. Astronomy & Astrophysics, David Dunlop Obs., Univ. Toronto PO Box 360, Richmond Hill, ON L4C 4Y6, Canada e-mail: email@example.com
7 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA e-mail: firstname.lastname@example.org
Accepted: 6 January 2008
Aims.Despite photometry and spectroscopy of its oscillations obtained over the past 25 years, the pulsation frequency spectrum of the rapidly oscillating Ap (roAp) star γ Equ has remained poorly understood. Better time-series photometry, combined with recent advances to incorporate interior magnetic field geometry into pulsational models, enable us to perform improved asteroseismology of this roAp star.
Methods.We obtained 19 days of continuous high-precision photometry of γ Equ with the Most (Microvariability & Oscillations of STars) satellite. The data were reduced with two different reduction techniques and significant frequencies were identified. Those frequencies were fitted by interpolating a grid of pulsation models that include dipole magnetic fields of various polar strengths.
Results.We identify 7 frequencies in γ Equ that we associate with 5 high-overtone p-modes and 1st and 2nd harmonics of the dominant p-mode. One of the modes and both harmonics are new discoveries for this star. Our best model solution (1.8 , log Teff ~3.882; polar field strength ~8.1 kG) leads to unique mode identifications for these frequencies ( = 0, 1, 2 and 4). This is the first purely asteroseismic fit to a grid of magnetic models. We measure amplitude and phase modulation of the primary frequency due to beating with a closely spaced frequency that had never been resolved. This casts doubts on theories that such modulation – unrelated to the rotation of the star – is due to a stochastic excitation mechanism.
Key words: stars: chemically peculiar / stars: oscillations / stars: individual: γ Equ / stars: magnetic fields / methods: data analysis
© ESO, 2008
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