Mining Planet Search Data for Binary Stars: The ψ1 Draconis system

Several planet-search groups have acquired a great deal of data in the form of time-series spectra of several hundred nearby stars with time baselines of over a decade. While binary star detections are generally not the goal of these long-term monitoring efforts, the binary stars hiding in existing planet search data are precisely the type that are too close to the primary star to detect with imaging or interferometry techniques. We use a cross-correlation analysis to detect the spectral lines of a new low-mass companion to ψ¹ Draconis A, which has a known roughly equal-mass companion at ∼680 AU. We measure the mass of ψ¹ Draconis C as M₂ = 0.70 ± 0.07M_⊙, with an orbital period of ∼20 years. This technique could be used to characterize binary companions to many stars that show large-amplitude modulation or linear trends in radial velocity data.