Characterization of the Host Binary of the Directly Imaged Exoplanet HD 143811 AB b

HD~143811~AB is the host star to the directly imaged planet HD~143811~AB~b, which was recently discovered using data from the Gemini Planet Imager and Keck NIRC2. A member of the Sco-Cen star-forming region with an age of 13 ± 4 Myr, HD~143811~AB is somewhat rare among hosts of directly imaged planets as it is a close stellar binary, with an ~18 day period. Accurate values for the orbital and stellar parameters of this binary are needed to understand the formation and evolutionary history of the planet in orbit. We utilize archival high-resolution spectroscopy from FEROS on the MPG/ESO 2.2-meter telescope to fit the orbit of the binary, and combine with unresolved photometry to derive the basic stellar properties of the system. From the orbit, we derive precise values of orbital period of 18.59098 ± 0.00007 days, and mass ratio of 0.885 ± 0.003. When combined with stellar evolutionary models, we find masses of both components of MA = 1.30+0.03-0.05 M_ and MB = 1.15+0.03-0.04 M_. While the current data are consistent with the planet and stellar orbits being coplanar, the 3D orientations of both systems are currently poorly constrained, with additional observations required to more rigorously test for coplanarity.