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

CORSICO, A. H.; MILLER BERTOLAMI, M. M; ALTHAUS, L. G.; VAUCLAIR, G.; WERNER, K.

ASTRONOMY AND ASTROPHYSICS

EDP Sciences

Año: 2007 vol. 475 p. 619 - 627

0004-6361

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 Msolar. 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 Msolar from a comparison between the observed period spacing and the computed asymptotic period spacing, and a stellar mass of 0.567 Msolar 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 T_eff = 81 500 K, a stellar mass M* = 0.556 Msun, a surface gravity log g = 7.65, a stellar luminosity and radius of log(L/Lsolar) = 1.14 and log(R/Rsolar) = -1.73, respectively, and a He-rich envelope thickness of M_env = 1.9 × 10-2 Msolar. We derive a seismic distance d 614 pc and a parallax of 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.