Astron. Astrophys. 332, 939-957 (1998)

Whole Earth Telescope observations of AM Canum Venaticorum - discoseismology at last

J.-E. Solheim ^{1, 14}, J.L. Provencal ^{2, 15}, P.A. Bradley ^{2, 16}, G. Vauclair ³, M.A. Barstow ⁴, S.O. Kepler ⁵, G. Fontaine ⁶, A.D. Grauer ⁷, D.E. Winget ², T.M.K. Marar ⁸, E.M. Leibowitz ⁹, P.-I. Emanuelsen ¹, M. Chevreton ¹⁰, N. Dolez ³, A. Kanaan ⁵, P. Bergeron ⁶, C.F. Claver ^{2, 17}, J.C. Clemens ^{2, 18}, S.J. Kleinman ², B.P. Hine ¹², S. Seetha ⁸, B.N. Ashoka ⁸, T. Mazeh ⁹, A.E. Sansom ^{4, 19}, R.W. Tweedy ⁴, E.G. Meitas ^{11, 13}, A. Bruvold ¹ and C.M. Massacand <sup>1

1</sup> Nordlysobservatoriet, Institutt for Fysikk, Universitetet i Tromso, N-9037 Tromso, Norway
² McDonald Observatory and Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
³ Observatoire Midi-Pyrenees, 14 Avenue E. Belin, F-31400 Toulouse, France
⁴ Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK
⁵ Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91500-970 Porto Alegre - RS, Brazil
⁶ Department de Physique, Université de Montréal, C.P. 6128, Succ A., Montréal, PQ H3C 3J7, Canada
⁷ Department of Physics and Astronomy, University of Arkansas, 2801 S. University Ave, Little Rock, AR 72204, USA
⁸ Technical Physics Division, ISRO Satelite Centre, Airport Rd, Bangalore, 560 017 India
⁹ University of Tel Aviv, Department of Physics and Astronomy, Ramat Aviv, Tel Aviv 69978, Israel
¹⁰ Observatoire de Paris-Meudon, F-92195 Meudon Principal Cedex, France
¹¹ Institute of Material Research and Applied Sciences, Vilnius University, Ciurlionio 29, Vilnius 2009, Lithuania
¹² NASA Ames Research Center, M.S. 244-4, Moffett Field, CA 94035, USA
¹³ Astronomical Observatory of the Institute of Theoretical Physics and Astronomy, Gotauto 12, Vilnius 2600, Lithuania
¹⁴ University of Cape Town, Department of Astronomy (visiting), and McDonald Observatory and Department of Astronomy, The University of Texas at Austin, Texas, USA (visiting)
¹⁵ Department of Physics, University of Delaware, Newark, DE 19716, USA
¹⁶ XTA, MS B-220, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
¹⁷ Kitt Peak National Observatory, Tucson, AZ 85726, USA
¹⁸ California Institute of Technology, 105-24, Pasadena, CA 91125, USA
¹⁹ Center for Astrophysics, University of Central Lancashire, Preston, PR1 2HE, UK

Received 9 May 1997 / Accepted 11 December 1997

Abstract

We report the results of 143.2 hours of time-series photometry over a 12 day period for AM CVn (= HZ 29) as part of the Whole Earth Telescope (WET) project. ¹ This star is believed to be an ultra-short period cataclysmic binary. In the temporal spectrum of the light curve we find a series of 5 harmonically related frequency mo dulations, some with sidebands with a constant frequency spacing of 20.8 µHz always on the high-frequency side. The set of harmonics has a funda- mental frequency of 951 µHz. No modulation is detected at this frequency in the light curve. In addition, modulations with frequencies 972.5 and 988.9 µHz are detected with low amplitudes. The structure of the dominant 1903 µHz modulation explains part of the "phase jitter" observed earlier. The amplitude of this peak is modulated with a period of  hrs. The same period is detected in absorption line shape modulations, most likely arising from variable aspects of the outer parts of the disk (Patterson et al. 1993).

The observed periodic light modulations can be explained as a combination of aspect variations of disk modifications due to tidally induced shocks as described by Savonie et al. (1994), which leads to a two-armed spiral structure, and the mode coupling model of Lubow (1991), which leads to a three-fold azimuthal symmetry in the outer parts of the disk and a prograde precessing wave. The two- and three-fold aszimuthal structures are stationary in the binary frame and explain the higher harmonics of the orbital period we observe in the light curve. This may be the first example of a successful disco- seismological interpretation.

In addition we propose that the variable amplitude modu-lation at 989 µHz may be explained as a g -mode pulsation, which indicates that the central white dwarf may be a DO with a hot envelope.

Key words: stars: individual: AM Canum Venaticorum novae, cataclysmic variables stars: oscillations white dwarfs accretion, accretion disks

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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