Asteroseismological constraints on the pulsating planetary nebula nucleus (PG1159-type) RX J2117.1+3412

Abstract

We present asteroseismological inferences on RX J2117.1+3412, the hottest known pulsating PG1159 star. Our results are based on full PG1159 evolutionary models recently presented by Miller Bertolami & Althaus (2006). We performed extensive computations of adiabatic g-mode pulsation periods on PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 Mo. PG1159 stellar models are extracted from the complete evolution of progenitor stars started from the ZAMS, through the thermally pulsing AGB and born-again phases to the domain of the PG 1159 stars. We constrained the stellar mass of RX J2117.1+3412 by comparing the observed period spacing with the asymptotic period spacing and with the average of the computed period spacings. We also employed the individual observed periods to find a representative seismological model. We derive a stellar mass of 0.56-0.57 Mo from the period spacing data alone. In addition, we found a best-fit model representative for RX J2117.1+3412 with an effective temperature of 163,400 K, a stellar mass of 0.565 Mo, and a surface gravity log g= 6.61. The derived stellar luminosity and radius are log(L/Lo)= 3.36 and log(R/Ro)= -1.23, respectively, and the He-rich envelope thickness is Menv= 0.02 Mo. We derive a seismic distance of 452 pc and a linear size of the planetary nebula of 1.72 pc. These inferences seem to solve the discrepancy between the RX J2117.1+3412 evolutionary timescale and the size of the nebula. All of the seismological tools we use concur to the conclusion that RX J2117.1+3412 must have a stellar mass of 0.565 Mo much in agreement with recent asteroseismology studies and in clear conflict with the predictions of spectroscopy plus evolutionary tracks.