A Physical Orbit for the High Proper Motion Binary HD 9939

We report spectroscopic and interferometric observations of the high-propermotion double-lined binary system HD 9939, with an orbital period ofapproximately 25 days. By combining our radial-velocity and visibilitymeasurements we estimate the system physical orbit and derive dynamical massesfor the components of $M_A = 1.072 \pm 0.014$ M$_{\sun}$ and $M_B = 0.8383 \pm0.0081$ M$_{\sun}$; fractional errors of 1.3% and 1.0%, respectively. We alsodetermine a system distance of $42.23 \pm 0.21$ pc, corresponding to an orbitalparallax of $\pi_{\rm orb} = 23.68 \pm 0.12$ mas. The system distance and theestimated brightness difference between the stars in $V$, $H$, and $K$ yieldcomponent absolute magnitudes in these bands. By spectroscopic analysis andspectral energy distribution modeling we also estimate the component effectivetemperatures and luminosities as $T_{\rm eff}^A = 5050 \pm 100$ K and $T_{\rmeff}^B = 4950 \pm 200$ K and $L_A$ = 2.451 $\pm$ 0.041 $L_{\sun}$ and $L_B$ =0.424 $\pm$ 0.023 $L_{\sun}$. Both our spectral analysis and comparison with stellar models suggest that HD9939 has elemental abundances near solar values. Further, comparison withstellar models suggests the HD 9939 primary has evolved off the main sequenceand appears to be traversing the Hertzsprung gap as it approaches the red giantphase of its evolution. Our measurements of the primary properties provide newempirical constraints on stellar models during this particularly dynamicevolutionary phase. That HD 9939 is currently in a relatively short-livedevolutionary state allows us to estimate the system age as 9.12 $\pm$ 0.25 Gyr.In turn the age and abundance of the system place a potentially interesting, ifanecdotal, constraint on star formation in the galactic disk.Comment: 19 pages, 6 figures, ApJ in press; tables 1 and 3 to appear in ApJ on-line versio