Absolute dimensions of eclipsing binaries

XXIX. The Am-type systems SW Canis Majoris and HW Canis Majoris^{⋆,⋆⋆,⋆⋆⋆}

¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
e-mail: gtorres@cfa.harvard.edu
² Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
³ School of Physics A28, University of Sydney, 2006 NSW, Australia
⁴ Instituto de Astrofísica de Andalucía, Apartado 3004, 18080 Granada, Spain

Received: 29 July 2011
Accepted: 29 November 2011

Abstract

Context. Accurate physical properties of eclipsing stars provide important constraints on models of stellar structure and evolution, especially when combined with spectroscopic information on their chemical composition. Empirical calibrations of the data also lead to accurate mass and radius estimates for exoplanet host stars. Finally, accurate data for unusual stellar subtypes, such as Am stars, also help to unravel the cause(s) of their peculiarities.

Aims. We aim to determine the masses, radii, effective temperatures, detailed chemical composition and rotational speeds for the Am-type eclipsing binaries SW CMa (A4-5m) and HW CMa (A6m) and compare them with similar normal stars.

Methods. Accurate radial velocities from the Digital Speedometers of the Harvard-Smithsonian Center for Astrophysics were combined with previously published uvby photometry to determine precise physical parameters for the four stars. A detailed abundance analysis was performed from high-resolution spectra obtained with the Nordic Optical Telescope (La Palma).

Results. We find the masses of the (relatively evolved) stars in SW CMa to be 2.10 and 2.24 M_⊙, with radii of 2.50 and 3.01 R_⊙, while the (essentially zero-age) stars in HW CMa have masses of 1.72 and 1.78 M_⊙, radii of 1.64 and 1.66 R_⊙ – all with errors well below 2%. Detailed atmospheric abundances for one or both components were determined for 14 elements in SW CMa ([Fe/H] = +0.49/+0.61 dex) and 16 in HW CMa ([Fe/H] = +0.33/+0.32 dex); both abundance patterns are characteristic of metallic-line stars. Both systems are well fit by current stellar evolution models for assumed bulk abundances of [Fe/H] = +0.05 and +0.23, respectively ([α/Fe] = 0.0), and ages of  ~700 Myr and 160 Myr.