Absolute Properties of the Main-Sequence Eclipsing Binary Star GX Geminorum: Constraints on Convective Core Overshooting

We present 12,155 new differential photometric observations in the V filter made with the Undergraduate Research Studies in Astronomy and NF\Observatory WebScopes, as well as 33 radial velocities from spectroscopic observations of the detached, 4.04 day double-lined eclipsing binary star GX Gem. Absolute dimensions of the components are determined from these new, and also from previously published, data with high precision (less than 0.7% in the masses and radii) for the purpose of testing various aspects of theoretical modeling. We obtain 1.488 ± 0.011 solar masses and 2.327 ± 0.012 solar radii for the primary, and 1.467 ± 0.010 solar masses and 2.238 ± 0.012 solar radii for the secondary. The effective temperatures and interstellar reddening of the stars are accurately determined from the spectra, uvbyβ, and UBV photometry: 6195 ± 100 K for the primary, 6165 ± 100 K for the secondary, corresponding to spectral types of F7 and F7 V, and 0.068 mag for the color excess E_{b - y} . Spectral line widths give rotational velocities that are synchronous with the mean orbital motion in a circular orbit. The components of GX Gem are relatively old main-sequence stars with an age of about 2.8 billion years according to models. Because the components are located right at the end of their main-sequence evolutionary paths, we can place good limits on the convective core overshooting parameter. The lowest value of α_ov consistent with the observations would appear to be approximately α_ov = 0.18. Two different theories of tidal synchronization and circularization agree that the components should be synchronously rotating, but disagree about whether or not the orbits should be circular.