The rapidly pulsating subdwarf B star PG 1325+101. II. Structural parameters from asteroseismology

We present a detailed analysis of the rapidly pulsating subdwarf B star (or EC14026 star) PG 1325+101. This analysis exploits the outcome of dedicated multisite observations using time-series photometry that revealed the presence of (at least) 15 periodicities in the luminosity modulation of this star (Silvotti et al. 2006, A&A, 459, 557, Paper I). Based on high-S/N, medium-resolution spectra obtained at the Steward Observatory's 2.3 m Telescope and at the 6.5 m MMT Telescope, new NLTE spectroscopic derivations of the atmospheric parameters of PG 1325+101 are presented. The mean values of T_eff = 35 050 ± 220 K, log g=5.81 ± 0.04, and log N(He)/N(H) = -1.70± 0.02 are in agreement with previous determinations, but are significantly more accurate. These improved spectroscopic parameters are essential for isolating a unique asteroseismic model solution for the observed pulsation spectrum of PG 1325+101. Using the "forward modeling" approach, our combined spectroscopic and asteroseismic analysis leads objectively to the identification of the (k, ?) indices of the 12 independent modes observed in this star, and to the determination of its structural parameters. The periods correspond to low-order acoustic modes with adjacent values of k and having degrees ?=0-4. They define a band of unstable modes, in agreement with nonadiabatic pulsation theory. The average dispersion achieved between the observed periods and the periods of the corresponding theoretical modes of the optimal model is only 0.46% (? 0.54 s), comparable to the results from similar analyses of other EC14026 stars analysed to date. The inferred structural parameters of PG 1325+101 are Teff= 35 050 ± 220 K, log g=5.811±0.004, log M{env}/M*=-4.18±0.10, M*=0.50±0.01 M? (i.e., close to the canonical mass of extreme horizontal branch stars), R/R?=0.145±0.002, and L/L?=28.3±1.5. In addition, by combining detailed model atmosphere calculations with V=14.019±0.012, we estimate that this star has an absolute visual magnitude M_V=4.45±0.04 and is located at a distance of d=820±21 pc. Finally, the presence of fine structure in the observed period spectrum suggests a rotation period of P=1.6±0.2 days, leading to an equatorial velocity of Veq=4.6±0.6 km s-1. Hence, asteroseismic evidence suggests that PG 1325+101 is a slow rotator, a conclusion reinforced by the limit V sin i < 20-30 km s-1, which we determined by modeling the He I 4471 line in our 1 Å-resolution MMT spectrum