The ZZ Ceti star G 185-32: new insight from asteroseismology

Free access article

Issue		A&A Volume 453, Number 1, July I 2006


Page(s)		219 - 227
Section		Stellar structure and evolution
DOI		http://dx.doi.org/10.1051/0004-6361:20054370

A&A 453, 219-227 (2006)
DOI: 10.1051/0004-6361:20054370

The ZZ Ceti star G 185-32: new insight from asteroseismology

D. Pech and G. Vauclair

Université Paul-Sabatier, Observatoire Midi-Pyrénées, CNRS/UMR5572, 14 Av. Edouard Belin, 31400 Toulouse, France
e-mail: gerardv@ast.obs-mip.fr

(Received 18 October 2005 / Accepted 29 January 2006 )

Abstract
One of the brightest pulsating DA white dwarfs (ZZ Ceti stars), G 185-32, shows 19 periods in its power spectrum, from 71 s to 651 s. Its short periods are among the shortest ones observed in a ZZ Ceti star. The one at 141.9 s shows an amplitude which does not vary with wavelength, in contrast with what the linear theory predicts for $\ell=1$ or $\ell=2$ non-radial g-modes. The mass and $T_{\rm eff}$ of G 185-32 are well constrained by spectroscopic observations and parallax measurement. The aim of the present paper is to take advantage of this rich observational background to derive the fundamental parameters of the star from asteroseismology. We computed a grid of realistic models for G 185-32 and the periods of their adiabatic $\ell=1$ and $\ell=2$ non-radial g-modes. We determined the model which fits best the observations. We identify 14 periods as real modes and determine their $\ell$ , k, and m values. We show that the remaining 5 periods are linear combinations.We find that G 185-32 oscillates mostly according to $\ell=2$ modes. We present a new hypothesis to account for the peculiar behaviour of the 141.9 s mode's amplitude and suggest that it is a true mode interfering with higher $\ell$ degree modes, involving a possible triplet resonance. We determine the main parameters of G 185-32: its total mass M_*=0.638 ( $\pm$ 0.007) $M_{\odot}$ , its hydrogen mass fraction $M_{\rm H}= 1.70$ ( $\pm$ 0.10) $\times~10^{-4}~M_{*}$ , which implies a helium mass fraction of $\approx$ 10^-2 M*, its $T_{\rm eff}= 12\,280$ ( $\pm$ )80 K, and estimate its rotation period to be 14.5 h.

Key words: stars: fundamental parameters -- stars: white dwarfs -- stars: oscillations -- stars: individul: ZZ Ceti G 185-32

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