EDP Sciences
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Volume 459, Number 2, November IV 2006
Page(s) 565 - 576
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20065316

A&A 459, 565-576 (2006)
DOI: 10.1051/0004-6361:20065316

The rapidly pulsating subdwarf B star PG 1325+101

II. Structural parameters from asteroseismology
S. Charpinet1, R. Silvotti2, A. Bonanno3, G. Fontaine4, P. Brassard4, P. Chayer5, 6, E. M. Green7, P. Bergeron4, S. Bernabei8, S. Leccia3, H. Kjeldsen9, R. Janulis10, A. Frasca3, R. Østensen11, S.-L. Kim12, B.-G. Park12, X. Jiang13, M. D. Reed14, R. S. Patterson14, K. M. Gietzen14, P. J. Clark14, G. W. Wolf14, Y. Lipkin15, L. Formiggini15, E. Leibowitz15, T. D. Oswalt16, M. Rudkin16 and K. Johnston16

1  UMR 5572, Université Paul Sabatier et CNRS, Observatoire Midi-Pyrénées, 14 Av. E. Belin, 31400 Toulouse, France
    e-mail: scharpin@ast.obs-mip.fr
2  INAF - Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
    e-mail: silvotti@na.astro.it
3  INAF - Osservatorio Astronomico di Catania, via S. Sofia 78, 95123 Catania, Italy
    e-mail: [alfio;leccia;afr]@ct.astro.it
4  Département de Physique, Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal, QC, H3C 3J7, Canada
    e-mail: $\!$[ fontaine$\!$;brassard$\!$;bergeron] @astro.umontreal.ca
5  Dept. of Physics and Astronomy, Johns-Hopkins University, 3400 North Charles St., Baltimore, MD 21218-2686, USA
    e-mail: chayer@pha.jhu.edu
6  Primary affiliation: Dept. of Physics and Astronomy, University of Victoria, PO Box 3055, Victoria, BC V8W 3P6, Canada
7  Steward Observatory, University of Arizona, 933 North Cherry Av., Tucson, AZ 85721, USA
    e-mail: bgreen@as.arizona.edu
8  INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
    e-mail: bernabei@astbo3.bo.astro.it
9  Danish Asteroseismology Center, Institut for Fysik og Astronomi, Aarhus Universitet, Ny Munkegade, Bygn. 1520, 8000 Aarhus C., Denmark
    e-mail: kjeldsen@phys.au.dk
10  Institute of Theoretical Physics and Astronomy, Vilnius University, Lithuania
    e-mail: jr@itpa.lt
11  Isaac Newton Group, S. Cruz de la Palma, Canary Islands, Spain
    e-mail: roy@ing.iac.es
12  Korea Astronomy and Space Science Institute, South Korea
    e-mail: [slkim;bgpark]@kasi.re.kr
13  National Astronomical Observatories, Beijing, 100012, PR China
    e-mail: xjjiang@bao.ac.cn
14  Missouri State University and Baker Observatory, 901 S. National, Springfield, MO 65897, USA
    e-mail: [MikeReed;gww836f;rsp814f]@missouristate.edu
15  Wise Observatory, Israel
    e-mail: [yiftah;lili;elia]@wise.tau.ac.il
16  Florida Institute of Technology, USA
    e-mail: [toswalt;rudkin;kyjohnst]@fit.edu

(Received 29 March 2006 / Accepted 11 May 2006)

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 $T_{\rm eff} = 35\,050 \pm 220$ K, $\log g=5.81 \pm 0.04$, and $\log N({\rm He})/N({\rm H}) = -1.70\pm 0.02$ 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, $\ell$) 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 $\ell=0{-}4$. 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% ($\sim $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 $T_{\rm eff} = 35\,050 \pm 220$ K, $\log g=5.811\pm0.004$, $\log M_{{\rm
env}}/M_{*}=-4.18\pm0.10$, $M_{*}=0.50\pm0.01$ $M_{\odot}$ (i.e., close to the canonical mass of extreme horizontal branch stars), $R/R_{\odot}=0.145\pm0.002$, and $L/L_{\odot}=28.3\pm1.5$. In addition, by combining detailed model atmosphere calculations with $V=14.019\pm0.012$, we estimate that this star has an absolute visual magnitude $M_{\rm V}=4.45\pm0.04$ and is located at a distance of $d=820\pm21$ pc. Finally, the presence of fine structure in the observed period spectrum suggests a rotation period of $P=1.6\pm0.2$ days, leading to an equatorial velocity of $V_{{\rm eq}}=4.6\pm0.6$ 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.

Key words: stars: interiors -- stars: oscillations -- stars: horizontal-branch -- stars: individual: PG 1325+101

© ESO 2006

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