Composite spectra Paper 11: α Equulei, an astrometric binary with an Am secondary

The spectrum of the secondary component of the bright composite-binary system α Equ, whose visual orbit is already known accurately, is isolated by the method of spectrum subtraction and classified accurately for the first time. The primary is a normal giant of type ~G7, while the secondary is an Am star of type ~kA3hA4mA9. The system's mass ratio, q, is determined to be 1.15+/-0.03 from measurements of the relative radial-velocity displacements between the components. Random and systematic errors in q are evaluated on the basis of the scatter of results derived from sets of spectra obtained from three different sources, and from tests conducted on independent versions of the secondary's spectrum. A spectroscopic analysis of a composite system such as α Equ is strongly challenged by the blending of a great many lines that are common to both spectra. Even when the primary spectrum is thought to have been subtracted adequately, a seemingly unavoidable ghost spectrum of faint residuals can bias wavelength measurements of the secondary's lines. That blending was the principal cause of a history of puzzling and discrepant measurements of q in α Equ. The derived masses of M₁=2.3M_solar, M₂=2.0M_solar for the giant and dwarf, respectively, constrain the choice of models for fitting evolutionary tracks in the (logT_eff, logL) plane; the stellar points fit a single isochrone (for 0.74Gyr). Both components are found to be slightly over-luminous compared to normal for their supposed luminosity classes. The giant appears to be commencing its first ascent of the red-giant branch. The dwarf has started to evolve away from the main sequence; its M_V is similar to that of a sub-giant.