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

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

Abstract

BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass (~1.1 ) to theoretically crystallize while still inside the ZZ Ceti instability strip ( 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.