Composite spectra: XVII. 12 Comae, a member of the Coma open cluster

The components of binary stars offer the potential to examine the predictions of stellar-evolution theory with particularly tight constraints. Those constraints are further tightened when the binary belongs to a cluster whose properties have been well determined independently. 12 Comae presents both advantages, belonging as it does to the Coma Cluster, Melotte 111. The orbit of 12 Comae has an eccentricity of nearly 0.6 and a period of 13 months. By a process of subtraction we separate the spectra of the component stars, derive a precise double-lined orbit solution, and by modelling the photometry we extract the individual stellar photometric and physical properties, ages and rotation. By fitting theoretical evolutionary tracks to the positions of the stars in the H-R diagram we confirm their individual masses, and derive a ZAMS of ∼0.65 Gyr, which accords well with measurements published for the cluster itself. We show that the primary of 12 Comae is an evolving giant of spectral type ∼G7 and mass 2.6 M_⊙, while its secondary (whose spectrum could be isolated particularly cleanly) is an A3 dwarf which has a mass of 2.05 M_⊙ and has commenced its evolution away from the main sequence. There is evidence that both stars are slightly metal-weak (-0.25 < [Fe/H] < 0.0), well in keeping with analyses of other members of this cluster.