2007A&A...475..619C

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 (2006A&A...454..845M). We perform extensive computations of adiabatic g-mode pulsation periods on PG 1159 evolutionary models with stellar masses ranging from 0.530 to 0.741M_☉. 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. We derive a stellar mass of 0.626M_☉ from a comparison between the observed period spacing and the computed asymptotic period spacing, and a stellar mass of 0.567M_☉ 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 <δΠ_i≥0.88s and a root-mean-square residual of σ_δΠi=1.27s. The model has an effective temperature T_eff=81500K, a stellar mass M_*=0.556M_☉, a surface gravity logg=7.65, a stellar luminosity and radius of log(L_*/L_☉)=1.14 and log(R_*/R_☉)=-1.73, respectively, and a He-rich envelope thickness of M_env=1.9x10^–2M_☉. We derive a seismic distance d∼614pc and a parallax π∼1.6mas. 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.