MOST light-curve analysis of the γ Doradus pulsator HR 8799, showing resonances and amplitude variations ^⋆

¹ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussel, Belgium
e-mail: adam.sodor@oma.be
² Konkoly Observatory, MTA CSFK, Konkoly Thege M. u. 15–17, 1121 Budapest, Hungary
e-mail: sodor@konkoly.hu
³ Gemini Observatory, Northern Operations Center, 670 North A’ohoku Place, Hilo HI 96720, USA
⁴ Department of Astrophysics and Optics, School of Physics, University of New South Wales, 2052 Sydney, Australia
⁵ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁶ 1234 Hewlett Place, Victoria, BC V8S 4P7, Canada
⁷ Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
⁸ Institute for Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁹ NASA Ames Research Park, MS 244-30, Building 244, Room 107A, Moffett Field CA 94035-1000, USA
¹⁰ Institute for Computational Astrophysics, Department of Astronomy and Physics, Saint Marys University, Halifax, NS B3H 3C3, Canada
¹¹ Département de physique, Université de Montréal, CP 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, Canada
¹² Department of Astronomy and Astrophysics, David Dunlap Observatory, University of Toronto, PO Box 360, Richmond Hill, ON L4C 4Y6, Canada
¹³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA

Received: 10 April 2014
Accepted: 30 June 2014

Abstract

Context. The central star of the HR 8799 system is a γ Doradus-type pulsator. The system harbours four planetary-mass companions detected by direct imaging, and is a good solar system analogue. The masses of the companions are not accurately known because the estimation depends greatly on the age of the system, which is also not known with sufficient accuracy. Asteroseismic studies of the star might help to better constrain the age of HR 8799. We organized an extensive photometric and multi-site spectroscopic observing campaign to study the pulsations of the central star.

Aims. The aim of the present study is to investigate the pulsation properties of HR 8799 in detail via the ultra-precise 47 d nearly continuous photometry obtained with the Microvariability and Oscillations in STars (MOST) space telescope, and to find as many independent pulsation modes as possible, which is the prerequisite for an asteroseismic age determination.

Methods. We carried out Fourier analysis of the wide-band photometric time series.

Results. We find that resonance and sudden amplitude changes characterize the pulsation of HR 8799. The dominant frequency is always at f₁ = 1.978 d^-1.Many multiples of one-ninth of the dominant frequency appear in the Fourier spectrum of the MOST data: n/9 f₁, where n = {1,2,3,4,5,6,7,8,9,10,13,14,17,18}. Our analysis also reveals that many of these peaks show strong amplitude decrease and phase variations even on the 47 d time scale. The dependencies between the pulsation frequencies of HR 8799 make the planned subsequent asteroseismic analysis rather difficult. We point out some resemblance between the light curve of HR 8799 and the modulated pulsation light curves of Blazhko RR Lyrae stars.