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Sibley School of Mechanical and Aerospace Engineering

New article: The Orbit and Transit Prospects for beta Pictoris b Constrained with One Milliarcsecond Astrometry

Article:  Wang, JJ; Graham, JR; Pueyo, L; Kalas, P; Millar-Blanchaer, MA; Ruffio, JB; Rosa, RJ; Ammons, SM; Arriaga, P; Bailey, VP; Barman, TS; Bulger, J; Burrows, AS; Cardwell, A; Chen, CH; Chilcote, JK; Cotten, T; Fitzgerald, MP; Follette, KB; Doyon, R; Duchene, G; Greenbaum, AZ; Hibon, P; Hung, LW; Ingraham, P; Konopacky, QM; Larkin, JE; Macintosh, B; Maire, J; Marchis, F; Marley, MS; Marois, C; Metchev, S; Nielsen, EL; Oppenheimer, R; Palmer, DW; Patel, R; Patience, J; Perrin, MD; Poyneer, LA; Rajan, A; Rameau, J; Rantakyro, FT; Savransky, D; Sivaramakrishnan, A; Song, I; Soummer, R; Thomas, S; Vasisht, G; Vega, D; Wallace, JK; Ward-Duong, K; Wiktorowicz, SJ; Wolff, SG; “The Orbit and Transit Prospects for beta Pictoris b Constrained with One Milliarcsecond Astrometry”, Astronomical Journal, 152 (4)

DOI

Abstract:  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-Loeve 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 to account for correlated noise. In the case of GPI data on beta Pic b, this technique, which we call Bayesian KLIP-FM Astrometry (BKA), outperforms previous techniques and yields 1 sigma errors at or below the one milliarcsecond level. We validate BKA by fitting a Keplerian orbit to 12 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 beta Pic b firmly rule out the possibility of a transit of the planet at 10-sigma significance. However, we confirm that the Hill sphere of beta 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.

Funding Acknowledgement:  Gemini Observatory; National Science Foundation [NSF AST-1518332]; NASA [NNX15AC89G, NNX15AD95G]; U.S. Department of Energy by Lawrence Livermore National Laboratory [DE-AC52-07NA27344]

Funding Text:  The GPI project has been supported by Gemini Observatory, which is operated by AURA, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the NSF (USA), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), MCTI (Brazil) and MINCYT (Argentina). We thank the National Science Foundation (NSF AST-1518332) and NASA (NNX15AC89G and NExSS program NNX15AD95G) for contributing to support of this research. Portions of this work were performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. The posterior distribution plots were made with corner (Foreman-Mackey et al. 2016).

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