Forty eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance

• Published in: Monthly notices of the Royal Astronomical Society 2005-02, Vol.357 (1), p.304-324
• Main Authors: Hilditch, R. W., Howarth, I. D., Harries, T. J.
• Format: Article
• Language: English
• Subjects: binaries: close; binaries: eclipsing; binaries: general; binaries: spectroscopic; Magellanic Clouds
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2005.08653.x

We have conducted a programme to determine the fundamental parameters of a substantial number of eclipsing binaries of spectral types O and B in the Small Magellanic Cloud (SMC). New spectroscopic data, obtained with the two-degree-field (2dF) multi-object spectrograph on the 3.9-m Anglo-Australian Telescope, have been used in conjunction with photometry from the Optical Gravitational Lens Experiment (OGLE-II) data base of SMC eclipsing binaries. Previously we reported results for 10 systems; in this second and concluding paper we present spectral types, masses, radii, temperatures, surface gravities and luminosities for the components of a further 40 binaries. The uncertainties are typically ±10 per cent on masses, ±4 per cent on radii and ±0.07 on log L. The full sample of 50 OB-type eclipsing systems is the largest single set of fundamental parameters determined for high-mass binaries in any galaxy. We find that 21 of the systems studied are in detached configurations, 28 are in semidetached post-mass-transfer states, and one is a contact binary. The overall properties of the detached systems are consistent with theoretical models for the evolution of single stars with SMC metal abundances (Z ≃ 0.004); in particular, observed and evolutionary masses are in excellent agreement. Although there are no directly applicable published models, the overall properties of the semidetached systems are consistent with them being in the slow phase of mass transfer in case A. About 40 per cent of these semidetached systems show photometric evidence of orbital-phase-dependent absorption by a gas stream falling from the inner Lagrangian point on the secondary star towards the primary star. This sample demonstrates that case-A mass transfer is a common occurrence amongst high-mass binaries with initial orbital periods P ≲ 5 d, and that this slow phase has a comparable duration to the detached phase preceding it. Each system provides a primary distance indicator. We find a mean distance modulus to the SMC of 18.91 ± 0.03 ± 0.1 (internal and external uncertainties; D = 60.6 ± 1.0 ± 2.8 kpc). This value represents one of the most precise available determinations of the distance to the SMC.