EDP Sciences
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Volume 432, Number 1, March II 2005
Page(s) 219 - 224
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20041125

A&A 432, 219-224 (2005)
DOI: 10.1051/0004-6361:20041125

Whole Earth Telescope observations of BPM 37093: A seismological test of crystallization theory in white dwarfs

A. Kanaan1, A. Nitta2, D. E. Winget3, S. O. Kepler4, M. H. Montgomery5, 3, T. S. Metcalfe6, 3, H. Oliveira1, L. Fraga1, A. F. M. da Costa4, J. E. S. Costa4, B. G. Castanheira4, O. Giovannini7, R. E. Nather3, A. Mukadam3, S. D. Kawaler8, M. S. O'Brien8, M. D. Reed8, 9, S. J. Kleinman2, J. L. Provencal10, T. K. Watson11, D. Kilkenny12, D. J. Sullivan13, T. Sullivan13, B. Shobbrook14, X. J. Jiang15, B. N. Ashoka16, S. Seetha16, E. Leibowitz17, P. Ibbetson17, H. Mendelson17, E. G. Meistas18, R. Kalytis18, D. Alisauskas19, D. O'Donoghue12, D. Buckley12, P. Martinez12, F. van Wyk12, R. Stobie12, F. Marang12, L. van Zyl12, W. Ogloza20, J. Krzesinski20, S. Zola20, 21, P. Moskalik22, M. Breger23, A. Stankov23, R. Silvotti24, A. Piccioni25, G. Vauclair26, N. Dolez26, M. Chevreton27, J. Deetjen28, S. Dreizler28, 29, S. Schuh28, 29, J. M. Gonzalez Perez30, R. Østensen31, A. Ulla32, M. Manteiga32, O. Suarez32, M. R. Burleigh33 and M. A. Barstow33

1    Departamento de Física Universidade Federal de Santa Catarina, CP 476, 88040-900 Florianópolis, SC, Brazil
    e-mail: kanaan@astro.ufsc.br
2  Apache Point Observatory, 2001 Apache Point Road, PO Box 59, Sunspot, NM 88349, USA
3  Department of Astronomy, 1 University Station Stop C1400, University of Texas, Austin, TX 78712, USA
4  Instituto de Física, Universidade Federal de Rio Grande do Sul, CP 10501, 91501-970 Porto Alegre, RS, Brazil
5  Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
6  Harvard-Smithsonian Center for Astrophysics, USA
7  Departamento de Fisica e Quimica, UCS, Brazil
8  Dept. of Physics & Astronomy, Iowa State University, USA
9  Astronomy Dept., Southwest Missouri State University, USA
10  Dept. of Physics & Astronomy, University of Delaware, USA
11  Southwestern University, Georgetown, TX, USA
12  South African Astronomical Observatory, South Africa
13  Victoria University of Wellington, New Zealand
14  Chatterton Astronomy Dept., University of Sydney, Australia
15  Astronomical Observatory, Academy of Sciences, PR China
16  Indian Space Research Organization, India
17  Wise Observatory, Tel-Aviv University, Israel
18  Institute of Theoretical Physics & Astronomy, Lithuania
19  Vilnius University, Lithuania
20  Mt. Suhora Observatory, Cracow Pedagogical University, Poland
21  Astronomical Observatory, Jagiellonia University, Poland
22  Copernicus Astronomical Center, Poland
23  Institut für Astronomie, Universität Wien, Austria
24  INAF - Osservatorio Astronomico di Capodimonte, Italy
25  Dipartimento di Astronomia, Università di Bologna, Italy
26  Université Paul Sabatier, Observatoire Midi-Pyrénées, France
27  Observatoire de Paris-Meudon, France
28  Institut für Astronomie und Astrophysik, Germany
29  Universitätssternwarte Göttingen, Germany
30  University of Tromsø, Norway
31  Isaac Newton Group, Spain
32  Departamento de Física Aplicada, Universidad de Vigo, Spain
33  Dept. of Physics & Astronomy, University of Leicester, UK

(Received 20 April 2004 / Accepted 31 October 2004)

BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass (~1.1  $M_\odot$) to theoretically crystallize while still inside the ZZ Ceti instability strip ( $T_{\rm
eff}\sim12\,000$  K). As a consequence, this star represents our first opportunity to test crystallization theory directly. If the core is substantially crystallized, then the inner boundary for each pulsation mode will be located at the top of the solid core rather than at the center of the star, affecting mainly the average period spacing. This is distinct from the "mode trapping" caused by the stratified surface layers, which modifies the pulsation periods more selectively. In this paper we report on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999. Based on a simple analysis of the average period spacing we conclude that a large fraction of the total stellar mass is likely to be crystallized.

Key words: stars: evolution -- stars: individual: BPM 37093 -- stars: interiors -- stars: oscillations -- white dwarfs

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