Program: GS-2015B-Q-9

We propose to continue our 2015A astrometric monitoring of the directly imaged exoplanet beta Pic b. GPI observations starting in late 2013 show that beta Pic b has passed quadrature and is headed back towards its parent star. These observations have enabled substantial improvements in the determination of the orbital elements (errors reduced by x2-3). 2015B presents the last chance to observe the planet before it is lost in the glare of beta Pic and further improve our knowledge of the orbit. The next opportunity for astrometric measurements will be in late 2019, after beta Pic b reappears. We propose to continue our astrometric monitoring campaign in 2015B to: 1) to explore and understand the interaction between beta Pic b and the inner warped disk, and to measure the mass of beta Pic b by combining orbital information with high-resolution ALMA continuum mapping of the debris disk’s warp, 2) to combine GPI and GAIA astrometry to measure a dynamical mass for beta Pic b, and 3) to enable planning observations of the upcoming conjunction and transit of the planetary Hill sphere in late 2017. This initiative to establish a consistent and sustained observational program to understand systematic errors in astrometry (which currently dominate) will have broad impact and enable other astrometric programs with GPI.