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Possible Astrometric Signature of a Planetary-mass Companion to the Nearby Young Star TW Piscis Austrini (Fomalhaut B): Constraints from Astrometry, Radial Velocities, and Direct Imaging

De Rosa, Robert J. and Esposito, Thomas M. and Hirsch, Lea A. and Nielsen, Eric L. and Marley, Mark S. and Kalas, Paul and Wang, Jason J. and Macintosh, Bruce (2019) Possible Astrometric Signature of a Planetary-mass Companion to the Nearby Young Star TW Piscis Austrini (Fomalhaut B): Constraints from Astrometry, Radial Velocities, and Direct Imaging. Astronomical Journal, 158 (6). Art. No. 225. ISSN 1538-3881.

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We present constraints on the presence of substellar companions to the nearby (d ~ 7.6 pc) young (440 ± 40 Myr) K4Ve star TW Piscis Austrini, the wide (~0.3 pc) companion to the A4V star Fomalhaut. We combined absolute astrometry from Hipparcos and Gaia with literature radial velocity measurements and dedicated high-contrast imaging observations obtained with Keck/NIRC2 to achieve sensitivity to brown dwarf and planetary-mass companions (≳2 M_(Jup)) over many decades of orbital period (≾10^3 yr). The significant astrometric acceleration measured between the Hipparcos and Gaia catalogs, reported previously in the literature, cannot be explained by the orbital motion of TW PsA around the barycenter of the Fomalhaut triple system. Instead, we find that it is consistent with the reflex motion induced by an orbiting substellar companion. The combination of astrometry, radial velocities, and a deep L' imaging data set leads to a constraint on the companion mass of 1.2^(+0.7)_(-0.6) M_(Jup). However, the period of the companion is poorly constrained, with a highly multimodal period posterior distribution due to aliasing with the 24.25 yr baseline between Hipparcos and Gaia. If confirmed through continued astrometric or spectroscopic monitoring or via direct detection, the companion to TW PsA would represent a choice target for detailed atmospheric characterization with high-contrast instruments on the upcoming James Webb Space Telescope and Wide Field Infrared Survey Telescope.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
De Rosa, Robert J.0000-0002-4918-0247
Esposito, Thomas M.0000-0002-0792-3719
Hirsch, Lea A.0000-0001-8058-7443
Nielsen, Eric L.0000-0001-6975-9056
Marley, Mark S.0000-0002-5251-2943
Kalas, Paul0000-0002-6221-5360
Wang, Jason J.0000-0003-0774-6502
Macintosh, Bruce0000-0003-1212-7538
Additional Information:© 2019 The American Astronomical Society. Received 2019 May 25; revised 2019 October 7; accepted 2019 October 8; published 2019 November 13. The authors wish to thank Dmitry Savransky and Vanessa Bailey for useful discussions relating to this work. The authors were supported in part by NSF AST-1411868 (R.D.R., E.L.N., B.M.), AST-1518332 (R.D.R., T.M.E., P.K., J.J.W.), NASA NNX14AJ80G (R.D.R., E.L.N., B.M.), NNX15AC89G (R.D.R., T.M.E., P.K., J.J.W), NNX15AD95G (R.D.R., T.M.E., P.K., J.J.W.), NSSC17K0535 (R.D.R., E.L.N., B.M.), and NNG16PJ24C (R.D.R., B.M., E.L.N.). J.J.W. is supported by the Heising-Simons Foundation 51 Pegasi b postdoctoral fellowship. This work benefited from NASA's Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA's Science Mission Directorate. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work has made use of data from the European Space Agency (ESA) mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC; Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research has made use of the SIMBAD database and the VizieR catalog access tool, both operated at the CDS, Strasbourg, France. This research has made use of the Imaging Mission Database, which is operated by the Space Imaging and Optical Systems Lab at Cornell University. Facility: Keck:II (NIRC2). - Software: Astropy (The Astropy Collaboration et al. 2013), Matplotlib (Hunter 2007), orbitize (Blunt et al. 2019), emcee (Foreman-Mackey et al. 2013), radvel (Fulton et al. 2018).
Funding AgencyGrant Number
Heising-Simons Foundation51 Pegasi b Fellowship
W. M. Keck FoundationUNSPECIFIED
Gaia Multilateral AgreementUNSPECIFIED
Subject Keywords:astrometry – stars: individual (TW PsA) – techniques: high angular resolution – techniques: radial velocities
Issue or Number:6
Record Number:CaltechAUTHORS:20191114-082931007
Persistent URL:
Official Citation:Robert J. De Rosa et al 2019 AJ 158 225
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99827
Deposited By: Tony Diaz
Deposited On:14 Nov 2019 17:04
Last Modified:14 Nov 2019 17:04

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