Absolute dimensions of eclipsing binaries. XXVIII. BK Pegasi and other F-type binaries: Prospects for calibration of convective core overshoot

Context. Double-lined, detached eclipsing binaries are our main source for accurate stellar masses and radii. In this paper we focus on the 1.15-1.70 M_⊙ interval where convective core overshoot is gradually ramped up in theoretical evolutionary models.
Aims: We aim to determine absolute dimensions and abundances for the F-type detached eclipsing binary BK Peg, and to perform a detailed comparison with results from recent stellar evolutionary models, including a sample of previously studied systems with accurate parameters.
Methods: uvby light curves and uvbyβ standard photometry were obtained with the Strömgren Automatic Telescope, ESO, La Silla, and high-resolution spectra were acquired with the FIES spectrograph at the Nordic Optical Telescope, La Palma.
Results: The 5 fd 49 period orbit of BK Peg is slightly eccentric (e = 0.053). The two components are quite different with masses and radii of (1.414 ± 0.007 M_⊙, 1.988 ± 0.008 R_sun) and (1.257 ± 0.005 M_⊙, 1.474 ± 0.017 R_sun), respectively. The measured rotational velocities are 16.6 ± 0.2 (primary) and 13.4 ± 0.2 (secondary) km s^-1. For the secondary component this corresponds to (pseudo)synchronous rotation, whereas the primary component seems to rotate at a slightly lower rate. We derive an iron abundance of [Fe/H] = -0.12 ± 0.07 and similar abundances for Si, Ca, Sc, Ti, Cr and Ni. The stars have evolved to the upper half of the main-sequence band. Yonsei-Yale and Victoria-Regina evolutionary models for the observed metal abundance reproduce BK Peg at ages of 2.75 and 2.50 Gyr, respectively, but tend to predict a lower age for the more massive primary component than for the secondary. We find the same age trend for three other upper main-sequence systems in a sample of well studied eclipsing binaries with components in the 1.15-1.70 M_⊙ range. We also find that the Yonsei-Yale models systematically predict higher ages than the Victoria-Regina models. The sample includes BW Aqr, and as a supplement we have determined a [Fe/H] abundance of -0.07 ± 0.11 for this late F-type binary.
Conclusions: We propose to use BK Peg, BW Aqr, and other well-studied 1.15-1.70 M_⊙ eclipsing binaries to fine-tune convective core overshoot, diffusion, and possibly other ingredients of modern theoretical evolutionary models.

Based on observations carried out at the Strömgren Automatic Telescope (SAT) and the 1.5m telescope (63.H-0080) at ESO, La Silla, and the Nordic Optical Telescope at La PalmaTables 13-17 are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/516/A42