Whole Earth telescope observations of the ZZ Ceti star HL Tau 76

¹ Université Paul Sabatier, Observatoire Midi-Pyrénées, CNRS/UMR5572, 14 Av. E. Belin, 31400 Toulouse, France
² Sloan Digital Sky Survey, Apache Point Observatory, PO Box 59, Sunspot, NM 88349, USA
³ Observatoire de Paris-Meudon, LESIA, 92195 Meudon, France
⁴ Astronomy Group, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussel, Belgium
⁵ Department of Physics, University of Tromsø, 9037 Tromsø, Norway
⁶ Institute of Theoretical Astrophysics, University of Oslo, P. Box 1029, 0315 Oslo, Norway
⁷ Present address: Instituto de Astrofisica de Canarias, Tenerife, Spain
⁸ Departamento de Fisica Aplicada, Universidade de Vigo, 36200 Vigo, Spain
⁹ Universidade Federal de Santa Catarina, Florianopolis, SC, Brazil
¹⁰ Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
¹¹ Present address: Department of Physics, Astronomy, and Material Science, SW Missouri State University, Springfield, MO 65804, USA
¹² Department of Astronomy and McDonald Observatory, Texas University at Austin, Austin, TX 78712, USA
¹³ Present address: High Altitude Observatory, PO Box 3000, Boulder, CO 80307-3000, USA
¹⁴ Present address: Department of Physics and Astronomy, Middle Tennessee State University, Murfreesboro, TN 37132, USA
¹⁵ Department of Physics and Space Sciences and SARA Observatory, Florida Institute of Technology, Melbourne, FL 32901, USA
¹⁶ Present address Department of Astronomy, Box 351580, University of Washington, Seattle, WA 98195-1580, USA
¹⁷ School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington, New-Zealand
¹⁸ National Astronomical Observatories, Chinese Academy of Sciences, 20A, Datun Road, Beijing 100012, PR China
¹⁹ Indian Space Research Organization, Airport Road, Vimanapura PO, Bangalore 560 017, India
²⁰ Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel
²¹ South African Astronomical Observatory, PO Box 9, Cape Town, 7935 South-Africa
²² Vilnius University Research Institute of Theoretical Physics and Astronomy, A. Gostauto 12, 01108 Vilnius, Lithuania
²³ Copernicus Astronomical Center, Ul. Bartycka 18, 00-716 Warsaw, Poland
²⁴ Mt. Suhora Observatory, Cracow Pedagogical University, Ul. Podchorazych 2, 30-084 Cracow, Poland
²⁵ Institut für Astronomie Universität Wien, Tükenschanzstrasse 17, 1180, Austria
²⁶ Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131, Napoli, Italy

Received: 29 March 2005
Accepted: 12 September 2005

Abstract

This paper analyses the Whole Earth Telescope observations of HL Tau 76, the first discovered pulsating DA white dwarf. The star was observed during two Whole Earth Telescope campaigns. It was a second priority target during the XCOV13 campaign in 1996 and the first priority one during the XCOV18 campaign in 1999. The 1999 campaign reached 66.5% duty cycle. With a total duration of 18 days, the frequency resolution achieved is 0.68 μHz. With such a frequency resolution, we were able to find as many as 78 significant frequencies in the power spectrum, of which 34 are independent frequencies after removal of all linear combinations. In taking into account other frequencies present during the 1996 WET campaign and those present in earlier data, which do not show up in the 1999 data set, we find a total of 43 independent frequencies. This makes HL Tau 76 the richest ZZ Ceti star in terms of number of observed pulsation modes. We use those pulsation frequencies to determine as much as possible of the internal structure of HL Tau 76. The pulsations in HL Tau 76 cover a wide range of periods between 380 s and 1390 s. We propose an identification for 39 of those 43 frequencies in terms of and non-radial g-modes split by rotation. We derive an average rotation period of 2.2 days. The period distribution of HL Tau 76 is best reproduced if the star has a moderately “thick” hydrogen mass fraction log . The results presented in this paper constitute a starting point for a detailed comparison of the observed periods with the periods calculated for models as representative as possible of HL Tau 76.