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Preprint E/02

The temporal spectrum of the sdB pulsating star HS 2201+2610 at 2 ms resolution

R. Silvotti (1), R. Janulis (2,3), S. L. Schuh (4), S. Charpinet (5), T. Oswalt (6), N. Silvestri (6), J. M. Gonzalez Perez (7), R. Kalytis (3), E. Meistas (3,2), D. Alisauskas (3), S. Marinoni (8), X. J. Jiang (9), M. D. Reed (10), R. L. Riddle (11), S. Bernabei (12), U. Heber (13), O. Bärnbantner (14), O. Cordes (15), S. Dreizler (4), E. Göhler (4), R. Østensen (16,7), J. Bochanski (17), G. Carlson (6)

(1) Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
(2) Institute of Theoretical Physics and Astronomy, Gostauto 12, Vilnius 2600, Lithuania
(3) Astronomical Observatory of Vilnius University, Ciurlionio 29, Vilnius 2009, Lithuania
(4) Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany
(5) Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France
(6) Department of Physics and Space Sciences and the SARA Observatory, Florida Institute of Technology, Melbourne, FL 32901-6975, USA
(7) Institutt for Fysikk, Universitetet i Tromsø, 9037 Tromsø, Norway
(8) Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
(9) National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012, PR China
(10) Southwest Missouri State University, Department of Physics, Astronomy, and Material Science, 901 S. National, Springfield, MO. 65804, USA
(11) Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
(12) Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
(13) Dr. Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, 96049 Bamberg, Germany
(14) Universitäts-Sternwarte München, Scheinerstrasse 1, 81679 München, Germany
(15) Sternwarte der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
(16) Isaac Newton Group of Telescopes, 37800 Santa Cruz de La Palma, Canary Islands, Spain
(17) Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085, USA

Astronomy & Astrophysics, 389, 180-190 (2002)

Abstract. In this article we present the results of more than 180 hours of time-series photometry on the low gravity (log g=5.4, T_eff=29 300 K, log He/H=-3.0 by number) sdB pulsating star HS 2201+2610, obtained between September 2000 and August 2001. The temporal spectrum is resolved and shows 5 close frequencies: three main signals at 2860.94, 2824.10 and 2880.69 mu Hz, with amplitudes of about 1%, 0.5% and 0.1% respectively, are detected from single run observations; two further peaks with very low amplitude (<0.07%) at 2738.01 and 2921.82 mu Hz are confirmed by phase analysis on several independent runs. Due to the small number of detected frequencies, it is not possible to obtain a univocal identification of the excited modes and perform a detailed seismological analysis of the star. No clear signatures of rotational splitting are seen. Nevertheless, the observed period spectrum is well inside the excited period window obtained from pulsation calculations with nonadiabatic models having effective temperature and surface gravity close to the spectroscopic estimates. Due to its relatively simple temporal spectrum, HS 2201+2610 is a very good candidate for trying to measure the secular variation of the pulsation periods in time. With this purpose a long-term monitoring of the star was started. The results of the first 11 months show amplitude variations up to ~ 20% on time-scales of months, which are probably real, and allow us to measure the pulsation frequencies with an unprecedented 0.02 mu Hz resolution. Based on observations obtained at the following telescopes: Loiano 1.5 m (Bologna Astronomical Observatory), Moletai 1.65 m (Institute of Theoretical Physics and Astronomy, Vilnius), Calar Alto 2.2 and 1.2 m (German-Spanish Astronomical Center operated by the Max-Plank-Institute für Astronomie Heidelberg jointly with the Spanish National Commission for Astronomy), SARA 0.9 m (Southeastern Association for Research in Astronomy, at Kitt Peak, Arizona), Tenerife 0.8 m (Instituto de Astrofisica de Canarias), NOT 2.6 m (operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias), Beijing 0.85 m (Beijing Astronomical Observatory), Fick 0.6 m (Iowa State University), Wendelstein 0.8 m (University of Munich).

Key words: stars: subdwarfs - stars: oscillations - stars: individual: HS 2201+2610

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