Testing the forward modeling approach in asteroseismology

II. Structure and internal dynamics of the hot B subdwarf component in the close eclipsing binary system PG 1336-018

¹ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 Av. E. Belin, 31400 Toulouse, France e-mail: [stephane.charpinet;valerie.vangrootel]@ast.obs-mip.fr
² Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, QC H3C 3J7, Canada e-mail: [fontaine;brassard]@astro.umontreal.ca
³ Department of Applied Mathematics, University of Sheffield, Hounsfield Road, Sheffield, S3 7RH, UK e-mail: D.Reese@sheffield.ac.uk
⁴ Steward Observatory, University of Arizona, 933 North Cherry avenue, Tucson, AZ 85721, USA e-mail: bgreen@as.arizona.edu
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: chayer@stsci.edu

Received: 4 April 2008
Accepted: 17 July 2008

Abstract

Aims. We present a stringent test on the forward modeling technique in asteroseismology by confronting the predictions of a detailed seismic analysis of the pulsating subdwarf component in the unique close eclipsing binary system PG 1336-018 with those derived independently from modeling the binary light curve of the system. We also take advantage of the observed rotationally-split rich period spectrum to investigate the internal dynamics of the pulsating component in this system expected to be tidally locked.

Methods. We carry out numerical exercises based on the double optimization technique that we developed within the framework of the forward modeling approach in asteroseismology. We use a recently updated version that now incorporates the effects of stellar rotation on the pulsation properties. We thus search in parameter space for the optimal model that objectively leads to the best simultaneous match of the 25 periods (including rotationally-split components) observed in PG 1336-018. For the first time, we also attempt to precisely reconstruct the internal rotation profile of the pulsator from its oscillations.

Results. Our principal result is that our seismic model, which closely reproduces the observed periods, is remarkably consistent with one of the best-fitting possible solutions uncovered independently from the binary light curve analysis, in effect pointing to the correct one. The latter indicates a mass of = 0.466±0.006 and a radius of = 0.15±0.01 for the sdB star. In comparison, our seismic analysis, combined to high-quality time-averaged spectroscopy, leads to the following estimates of the basic structural parameters of the sdB component: = 0.459±0.005 , = 0.151±0.001 , log g = 5.739±0.002, = 32 740 ± 400 K, and = -4.54±0.07. We also find strong evidence that the sdB star has reached spin-orbit synchronism and rotates as a solid body down to at least r ~ 0.55 . We further estimate that higher-order perturbation effects due to rotation and tidal deformation of the star are insufficient to alter in a significant way the proposed asteroseismic solution itself (i.e., the derived structural parameters and rotation properties). Future efforts to improve further the accuracy of the seismic models will clearly have to incorporate such effects, however.

Conclusions. We conclude that our approach to the asteroseismology of sdB stars has passed a fundamental test with this analysis of PG 1336-018. The structural parameters and inferences about the internal dynamics of this star derived in the present paper through this approach should rest on very solid grounds. More generally, our results underline the power and usefulness of the forward modeling method in asteroseismology, despite historical misgivings about it.