MOST discovers a multimode δ Scuti star in a triple system: HD 61199^*

¹ Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Vienna, Austria e-mail: hareter@astro.univie.ac.at
² Department of Physics and Astronomy, Uppsala University, Box 515, 751 20 Uppsala, Sweden
³ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
⁴ Tavrian National University, Dep. Astronomy, Simferopol, Ukraine
⁵ Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, B.C., V6T 1Z1, Canada
⁶ David Dunlap Observatory, Department of Astronomy, University of Toronto, Toronto, Ontario, L4C 4Y6, Canada
⁷ Department of Astronomy and Physics, St. Mary's University, Halifax, Nova Scotia, NS B3H 3C3, Canada
⁸ Département de physique, Université de Montréal, Montréal, Québec, QC H3C 3J7, Canada
⁹ Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, MA 02138, USA
¹⁰ University of Technology, Institute of Communications and Radio-Frequency Engineering, Gusshausstrasse 25/389, 1040 Vienna, Austria

Received: 17 April 2008
Accepted: 3 October 2008

Abstract

Context. A field star, HD 61199 (), simultaneously observed with Procyon by the MOST (Microvariability & Oscillations of STars) satellite in continuous runs of 34, 17, and 34 days in 2004, 2005, and 2007, was found to pulsate in 11 frequencies in the δ Scuti range with amplitudes from 1.7 down to 0.09 mmag. The photometry also showed variations with a period of about four days. To investigate the nature of the longer period, 45 days of time-resolved spectroscopy was obtained at the Thüringer Landessternwarte Tautenburg in 2004. The radial velocity measurements indicate that HD 61199 is a triple system.

Aims. A δ Scuti pulsator with a rich eigenspectrum in a multiple system is promising for asteroseismology. Our objectives were to identify which of the stars in the system is the δ Scuti variable and to obtain the orbital elements of the system and the fundamental parameters of the individual components, which are constrained by the pulsation frequencies of the δ Scuti star.

Methods. Classical Fourier techniques and least-squares multi-sinusoidal fits were applied to the MOST photometry to identify the pulsation frequencies. The groundbased spectroscopy was analysed with least-squares-deconvolution (LSD) techniques, and the orbital elements derived with the KOREL and ORBITX routines. Asteroseismic models were also generated.

Results. The photometric and spectroscopic data are compatible with a triple system consisting of a close binary with an orbital period of 3.57 days and a δ Scuti companion (HD 61199 A) as the most luminous component. The δ Scuti star is a rapid rotator with about  = 130   and an upper mass limit of about 2.1 . For the close binary components, we find they are of nearly equal mass, with lower mass limits of about 0.7 . Comparisons to synthetic spectra indicate these stars have a late-F spectral type. The observed oscillation frequencies are compared to pulsation models to further constrain the evolutionary state and mass of HD 61199  A. The orbit frequency of the close binary corresponds to the difference of the two δ Scuti frequencies with the highest amplitudes – a coincidence that is remarkable, but not explained.