The Orbit and Transit Prospects for $β$ Pictoris b constrained with One Milliarcsecond Astrometry
Creators: name: Wang, Jason J.
name: Graham, James R.
name: Pueyo, Laurent
name: Kalas, Paul
name: Millar-Blanchaer, Maxwell A.
name: Ruffio, Jean-Baptiste
name: De Rosa, Robert J.
name: Ammons, S. Mark
name: Arriaga, Pauline
name: Bailey, Vanessa P.
name: Barman, Travis S.
name: Bulger, Joanna
name: Burrows, Adam S.
name: Cardwell, Andrew
name: Chen, Christine H.
name: Chilcote, Jeffrey K.
name: Cotten, Tara
name: Fitzgerald, Michael P.
name: Follette, Katherine B.
name: Doyon, René
name: Duchêne, Gaspard
name: Greenbaum, Alexandra Z.
name: Hibon, Pascale
name: Hung, Li-Wei
name: Ingraham, Patrick
name: Konopacky, Quinn M.
name: Larkin, James E.
name: Macintosh, Bruce
name: Maire, Jérôme
name: Marchis, Franck
name: Marley, Mark S.
name: Marois, Christian
name: Metchev, Stanimir
name: Nielsen, Eric L.
name: Oppenheimer, Rebecca
name: Palmer, David W.
name: Patel, Rahul
name: Patience, Jenny
name: Perrin, Marshall D.
name: Poyneer, Lisa A.
name: Rajan, Abhijith
name: Rameau, Julien
name: Rantakyrö, Fredrik T.
name: Savransky, Dmitry
name: Sivaramakrishnan, Anand
name: Song, Inseok
name: Soummer, Remi
name: Thomas, Sandrine
name: Vasisht, Gautam
name: Vega, David
name: Wallace, J. Kent
name: Ward-Duong, Kimberly
name: Wiktorowicz, Sloane J.
name: Wolff, Schuyler G.
Publisher: name: arXiv
Year: 2016
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: A principal scientific goal of the Gemini Planet Imager (GPI) is obtaining milliarcsecond astrometry to constrain exoplanet orbits. However, astrometry of directly imaged exoplanets is subject to biases, systematic errors, and speckle noise. Here we describe an analytical procedure to forward model the signal of an exoplanet that accounts for both the observing strategy (angular and spectral differential imaging) and the data reduction method (Karhunen-Loève Image Projection algorithm). We use this forward model to measure the position of an exoplanet in a Bayesian framework employing Gaussian processes and Markov chain Monte Carlo (MCMC) to account for correlated noise. In the case of GPI data on $β$ Pic b, this technique, which we call Bayesian KLIP-FM Astrometry (BKA), outperforms previous techniques and yields 1$σ$-errors at or below the one milliarcsecond level. We validate BKA by fitting a Keplerian orbit to twelve GPI observations along with previous astrometry from other instruments. The statistical properties of the residuals confirm that BKA is accurate and correctly estimates astrometric errors. Our constraints on the orbit of $β$ Pic b firmly rule out the possibility of a transit of the planet at 10-$σ$ significance. However, we confirm that the Hill sphere of $β$ Pic b will transit, giving us a rare chance to probe the circumplanetary environment of a young, evolving exoplanet. We provide an ephemeris for photometric monitoring of the Hill sphere transit event, which will begin at the start of April in 2017 and finish at the end of January in 2018.
description: 18 pages, 7 figures. Accepted to AJ. v2 corrects an error in plotting the full size of the Hill sphere in Figure 6