A&A 401, 639-654 (2003)
DOI: 10.1051/0004-6361:20030105

The everchanging pulsating white dwarf GD358

S. O. Kepler¹, R. E. Nather², D. E. Winget², A. Nitta³, S. J. Kleinman³, T. Metcalfe^{2, 4}, K. Sekiguchi⁵, Jiang Xiaojun⁶, D. Sullivan⁷, T. Sullivan⁷, R. Janulis⁸, E. Meistas⁸, R. Kalytis⁸, J. Krzesinski⁹, W. Ogloza⁹, S. Zola¹⁰, D. O'Donoghue¹¹, E. Romero-Colmenero¹¹, P. Martinez¹¹, S. Dreizler¹², J. Deetjen¹², T. Nagel¹², S. L. Schuh¹², G. Vauclair¹³, Fu Jian Ning¹³, M. Chevreton¹⁴, J.-E. Solheim¹⁵, J. M. Gonzalez Perez¹⁵, F. Johannessen¹⁵, A. Kanaan¹⁶, J. E. Costa¹, A. F. Murillo Costa¹, M. A. Wood¹⁷, N. Silvestri¹⁷, T. J. Ahrens¹⁷, A. K. Jones¹⁸, A. E. Collins¹⁹, M. Boyer²⁰, J. S. Shaw²¹, A. Mukadam², E. W. Klumpe²², J. Larrison²², S. Kawaler²³, R. Riddle²³, A. Ulla²⁴ and P. Bradley<sup>25

1</sup>  Instituto de Física da UFRGS, Porto Alegre, RS - Brazil
    e-mail: kepler@if.ufrgs.br
²  Department of Astronomy & McDonald Observatory, University of Texas, Austin, TX 78712, USA
³  Sloan Digital Sky Survey, Apache Pt. Observatory, PO Box 59, Sunspot, NM 88349, USA
⁴  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
    e-mail: travis@whitedwarf.org
⁵  Subaru National Astronomical Observatory of Japan
    e-mail: kaz@subaru.naoj.org
⁶  Beijing Astronomical Observatory, Academy of Sciences, Beijing 100080, PR China
    e-mail: jiang@astro.as.utexas.edu
⁷  University of Victoria, Wellington, New Zealand
⁸  Institute of Theoretical Physics and Astronomy, Gostauto 12, Vilnius 2600, Lithuania
⁹  Mt. Suhora Observatory, Cracow Pedagogical University, Ul. Podchorazych 2, 30-084 Cracow, Poland
¹⁰  Jagiellonian University, Krakow, Poland
    e-mail: zola@oa.uj.edu.pl
¹¹  South African Astronomical Observatory
¹²  Universitat Tübingen, Germany
¹³  Université Paul Sabatier, Observatoire Midi-Pyrénées, CNRS/UMR5572, 14 av. E. Belin, 31400 Toulouse, France
¹⁴  Observatoire de Paris-Meudon, DAEC, 92195 Meudon, France
    e-mail: chevreton@obspm.fr
¹⁵  Institutt for fysikk, 9037 Tromso, Norway
¹⁶  Departamento de Física, Universidade Federal de Santa Catarina, CP 476, CEP 88040-900, Florianópolis, Brazil
    e-mail: kanaan@fsc.ufsc.br
¹⁷  Dept. of Physics and Space Sciences & The SARA Observatory, Florida Institute of Technology, Melbourne, FL 32901, USA
¹⁸  University of Florida, 202 Nuclear Sciences Center Gainesville, FL 32611-8300, USA
¹⁹  Johnson Space Center, 2101 NASA Road 1, Mail Code GT2, Houston, TX 77058, USA
²⁰  University of Minnesota, Department of Physics & Astronomy, 116 Church St. S.E., Minneapolis, MN 55455, USA
²¹  University of Georgia at Athens, Department of Physics and Astronomy, Athens, GA 30602-2451, USA
²²  Middle Tennessee State University, Department of Physics and Astronomy Murfreesboro, TN 37132, USA
²³  Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
²⁴  Universidade de Vigo, Depto. de Fisica Aplicada, Facultade de Ciencias, Campus Marcosende-Lagoas, 36200 Vigo (Pontevedra), Spain
    e-mail: ulla@uvigo.es
²⁵  Los Alamos National Laboratory, X-2, MS T-085 Los Alamos, NM 87545, USA

(Received 6 December 2002 / Accepted 21 January 2003)

Abstract
We report 323 hours of nearly uninterrupted time series photometric observations of the DBV star GD 358 acquired with the Whole Earth Telescope (WET) during May 23rd to June 8th, 2000. We acquired more than 232 000 independent measurements. We also report on 48 hours of time-series photometric observations in Aug 1996. We detected the non-radial g-modes consistent with degree and radial order 8 to 20 and their linear combinations up to 6th order. We also detect, for the first time, a high amplitude mode, with a period of 796 s. In the 2000 WET data, the largest amplitude modes are similar to those detected with the WET observations of 1990 and 1994, but the highest combination order previously detected was 4th order. At one point during the 1996 observations, most of the pulsation energy was transferred into the radial order k=8 mode, which displayed a sinusoidal pulse shape in spite of the large amplitude. The multiplet structure of the individual modes changes from year to year, and during the 2000 observations only the k=9 mode displays clear normal triplet structure. Even though the pulsation amplitudes change on timescales of days and years, the eigenfrequencies remain essentially the same, showing the stellar structure is not changing on any dynamical timescale.

Key words: stars: white dwarfs -- stars: variables: general -- stars: oscillations -- stars: individual: GD 358 -- stars: evolution

Offprint request: S. O. Kepler, kepler@if.ufrgs.br

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© ESO 2003

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