Absolute Dimensions of the G7+K7 Eclipsing Binary Star IM Virginis: Discrepancies with Stellar Evolution Models

We report extensive spectroscopic and differential photometric BVRI observations of the active, detached, 1.309-day double-lined eclipsing binary IM Vir, composed of a G7-type primary and a K7 secondary. With these observations, we derive accurate absolute masses and radii of M ₁ = 0.981 ± 0.012 M _sun, M ₂ = 0.6644 ± 0.0048 M _sun, R ₁ = 1.061 ± 0.016 R _sun, and R ₂ = 0.681 ± 0.013 R _sun for the primary and secondary, with relative errors under 2%. The effective temperatures are 5570 ± 100 K and 4250 ± 130 K, respectively. The significant difference in mass makes this a favorable case for comparison with stellar evolution theory. We find that both stars are larger than the models predict, by 3.7% for the primary and 7.5% for the secondary, as well as cooler than expected, by 100 K and 150 K, respectively. These discrepancies are in line with previously reported differences in low-mass stars, and are believed to be caused by chromospheric activity, which is not accounted for in current models. The effect is not confined to low-mass stars: the rapidly rotating primary of IM Vir joins the growing list of objects of near-solar mass (but still with convective envelopes) that show similar anomalies. The comparison with the models suggests an age of 2.4 Gyr for the system, and a metallicity of [Fe/H] ≈-0.3 that is consistent with other indications, but requires confirmation.