A Physical Orbit for the High Proper Motion Binary HD 9939

We report spectroscopic and interferometric observations of the high proper motion double-lined binary system HD 9939, with an orbital period of approximately 25 days. By combining our radial-velocity and visibility measurements, we estimate the system physical orbit and derive dynamical masses for the components of M_A = 1.072 ± 0.014 M_☉ and M_B = 0.8383 ± 0.0081 M_☉, with fractional errors of 1.3% and 1.0%, respectively. We also determine a system distance of 42.23 ± 0.21 pc, corresponding to an orbital parallax of π_orb = 23.68 ± 0.12 mas. The system distance and the estimated brightness difference between the stars in V, H, and K yield component absolute magnitudes in these bands. By spectroscopic analysis and spectral energy distribution modeling, we also estimate the component effective temperatures and luminosities as T = 5050 ± 100 K and T = 4950 ± 200 K and L_A = 2.451 ± 0.041 L_☉ and L_B = 0.424 ± 0.023 L_☉. Both our spectral analysis and comparison with stellar models suggest that HD 9939 has elemental abundances near solar values. Further, comparison with stellar models suggests that the HD 9939 primary has evolved off the main sequence and appears to be traversing the Hertzsprung gap as it approaches the red giant phase of its evolution. Our measurements of the primary properties provide new empirical constraints on stellar models during this particularly dynamic evolutionary phase. That HD 9939 is currently in a relatively short lived evolutionary state allows us to estimate the system age as 9.12 ± 0.25 Gyr. In turn, the age and abundance of the system place a potentially interesting, if anecdotal, constraint on star formation in the Galactic disk.