Asteroseismological constraints on the coolest GW Virginis variable star (PG 1159-type) PG 0122+200

¹ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, (1900) La Plata, Argentina e-mail: [acorsico;althaus;mmiller]@fcaglp.unlp.edu.ar
² Instituto de Astrofísica La Plata, IALP, CONICET-UNLP, Argentina
³ Université Paul Sabatier, Observatoire Midi-Pyrénées, CNRS/UMR5572, 14 Av. Belin, 31400 Toulouse, France e-mail: gerardv@ast.obs-mip.fr
⁴ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: werner@astro.uni-tuebingen.de

Received: 22 June 2007
Accepted: 29 August 2007

Abstract

Aims.We present an asteroseismological study of PG 0122+200, the coolest known pulsating PG 1159 (GW Vir) star. Our results are based on an augmented set of the full PG 1159 evolutionary models recently presented by Miller Bertolami & Althaus (2006).

Methods.We perform extensive computations of adiabatic g-mode pulsation periods on PG 1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 . These models take into account the complete evolution of progenitor stars, through the thermally pulsing asymptotic giant branch phase and born-again episode. We constrain the stellar mass of PG 0122+200 by comparing the observed period spacing with the asymptotic period spacing and with the average of the computed period spacings. We also employ the individual observed periods to find a representative seismological model for PG 0122+200.

Results.We derive a stellar mass of 0.626 from a comparison between the observed period spacing and the computed asymptotic period spacing, and a stellar mass of 0.567 by comparing the observed period spacing with the average of the computed period spacing. We also find, on the basis of a period-fit procedure, an asteroseismological model representative of PG 0122+200 that is able to reproduce the observed period pattern with an average of the period differences of = 0.88 s and a root-mean-square residual of = 1.27 s. The model has an effective temperature = 81 500 K, a stellar mass = 0.556 , a surface gravity log g = 7.65, a stellar luminosity and radius of = 1.14 and = -1.73, respectively, and a He-rich envelope thickness of M_env = 1.9 10^-2 . We derive a seismic distance d ~ 614 pc and a parallax π ~ 1.6 mas. The results of the period-fit analysis carried out in this work suggest that the asteroseismological mass of PG 0122+200 could be ~6-20% lower than hitherto thought, and in closer agreement (to within ~5%) with the spectroscopic mass. This result suggests that a reasonable consistency between the stellar mass values obtained from spectroscopy and asteroseismology can be expected when detailed PG 1159 evolutionary models are considered.