Solar-like oscillations in the metal-poor subgiant ν Indi

II. Acoustic spectrum and mode lifetime

¹ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium e-mail: fabien@ster.kuleuven.be
² Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
³ Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark e-mail: [hans;fgj;tbn]@phys.au.dk
⁴ School of Physics A28, University of Sydney, NSW 2006, Australia e-mail: [bedding;brewer]@physics.usyd.edu.au
⁵ Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA e-mail: [paul;chris]@dtm.ciw.edu
⁶ Institut d'Astrophysique et de Géophysique, Université de Liège, 17 allée du 6 Août, Bât. B5c, 4000 Liège, Belgium e-mail: eggenberger@astro.ulg.ac.be
⁷ Department of Astronomy and Astrophysics, PennSylvania State University, 525 Davey Lab, University Park, PA 16802-6305, USA e-mail: retter@astro.psu.edu
⁸ Anglo-Australian Observatory, PO Box 296, Epping, NSW 1710, Australia e-mail: cgt@aaoepp.aao.gov.au

Received: 8 December 2006
Accepted: 18 May 2007

Abstract

Context.Convection in stars excites resonant acoustic waves which depend on the sound speed inside the star, which in turn depends on properties of the stellar interior. Therefore, asteroseismology is an unrivaled method to probe the internal structure of a star.

Aims.We made a seismic study of the metal-poor subgiant star ν Indi with the goal of constraining its interior structure.

Methods.Our study is based on a time series of 1201 radial velocity measurements spread over 14 nights obtained from two sites, Siding Spring Observatory in Australia and ESO La Silla Observatory in Chile.

Results.The power spectrum of the high precision velocity time series clearly presents several identifiable peaks between 200 and 500 μHz showing regularity with a large and small spacing of = 25.14±0.09 μHz and = 2.96±0.22 μHz at 330 μHz. Thirteen individual modes have been identified with amplitudes in the range 53 to 173 cm s^-1. The mode damping time is estimated to be about 16 days (1-σ range between 9 and 50 days), substantially longer than in other stars like the Sun, the α Cen system or the giant ξ Hya.