Photometric Solutions for Semidetached Eclipsing Binaries: Selection of Distance Indicators in the Small Magellanic Cloud

Estimation of distances to nearby galaxies by the use of eclipsing binaries as standard candles has recently become feasible because of new large-scale instruments and the discovery of thousands of eclipsing binaries as spin-off from Galactic microlensing surveys. Published measurements of distances to detached eclipsing binaries in the Large Magellanic Cloud combine stellar surface areas (in absolute units) determined from photometric light and radial velocity curves with surface brightnesses from model atmospheres and observed spectra. The method does not require the stars to be normal or undistorted and is not limited in its applicability to the well-detached systems that have traditionally been considered. We discuss the potential usefulness of semidetached vis a vis detached eclipsing binaries for distance determination and examine and quantify criteria for their selection from large catalogs. Following our earlier paper on detached binaries in the Small Magellanic Cloud (SMC), we carry out semidetached light-curve solutions for SMC binaries discovered by the OGLE collaboration, identify candidates for SMC distance estimation that can be targets of future high-quality observations, and tabulate results of OGLE light-curve solutions. We point out that semidetached binaries have important advantages over well-detached systems as standard candles, although this idea runs counter to the usual view that the latter are optimal distance indicators. Potential advantages are that (1) light-curve solutions can be strengthened by exploiting lobe-filling configurations, (2) only single-lined spectra may be needed for radial velocities because the mass ratio can be determined from photometry in the case of complete eclipses, and (3) nearly all semidetached binaries have sensibly circular orbits, which is not true for detached binaries. We carry out simulations with synthetic data to see if semidetached binaries can be reliably identified and to quantify the accuracy of solutions. The simulations were done for detached as well as semidetached binaries so as to constitute a proper controlled study. The simulations demonstrate two additional advantages for semidetached distance determination candidates; (4) the well-known difficulty in distinguishing solutions with interchanged radii (aliasing) is much less severe for semidetached than for detached binaries and (5) the condition of complete eclipse (which removes a near degeneracy between inclination and the ratio of the radii) is identified with improved reliability. In many cases we find that parameters are accurately determined (e.g., relative errors in radii smaller than 10%) and that detached and semidetached systems can be distinguished. We select 36 candidate semidetached systems (although seven of these are doubtful because of large mass ratios or periods) from the OGLE SMC eclipsing binary catalog. We expected that most semidetached candidates would have light curves similar to those of common Algol binaries, but that turned out not to be the case, and we note that fully Algol-like light curves are nearly absent in the OGLE sample. We discuss possible explanations for the near absence of obvious Algols in OGLE, including whether their paucity is real or apparent.