* Astronomy

Title: Evidence that the Directly-Imaged Planet HD 131399 Ab is a Background Star
Author: Eric L. Nielsen, Robert J. De Rosa, Julien Rameau, Jason J. Wang, Thomas M. Esposito, Maxwell A. Millar-Blanchaer, Christian Marois, Arthur Vigan, S. Mark Ammons, Etienne Artigau, Vanessa P. Bailey, Sarah Blunt, Joanna Bulger, Jeffrey Chilcote, Tara Cotten, René Doyon, Gaspard Duchêne, Daniel Fabrycky, Michael P. Fitzgerald, Katherine B. Follette, Benjamin L. Gerard, Stephen J. Goodsell, James R. Graham, Alexandra Z. Greenbaum, Pascale Hibon, Sasha Hinkley, Li-Wei Hung, Patrick Ingraham, Rebecca Jensen-Clem, Paul Kalas, Quinn Konopacky, James E. Larkin, Bruce Macintosh, Jerome Maire, Franck Marchis, Stanimir Metchev, Katie M. Morzinski, Ruth A. Murray-Clay, Rebecca Oppenheimer, David Palmer, Jennifer Patience, Marshall Perrin, Lisa Poyneer, Laurent Pueyo, Roman R. Rafikov, Abhijith Rajan, et al. (12 additional authors not shown)

We present evidence that the recently discovered, directly-imaged planet HD 131399 Ab is a background star with non-zero proper motion. From new JHK1L' photometry and spectroscopy obtained with the Gemini Planet Imager, VLT/SPHERE, and Keck/NIRC2, and a reanalysis of the discovery data obtained with VLT/SPHERE, we derive colours, spectra, and astrometry for HD 131399 Ab. The broader wavelength coverage and higher data quality allow us to re-investigate its status. Its near-infrared spectral energy distribution excludes spectral types later than L0 and is consistent with a K or M dwarf, which are the most likely candidates for a background object in this direction at the apparent magnitude observed. If it were a physically associated object, the projected velocity of HD 131399 Ab would exceed escape velocity given the mass and distance to HD 131399 A. We show that HD 131399 Ab is also not following the expected track for a stationary background star at infinite distance. Solving for the proper motion and parallax required to explain the relative motion of HD 131399 Ab, we find a proper motion of 12.3 mas/yr. When compared to predicted background objects drawn from a galactic model, we find this proper motion to be high, but consistent with the top 4% fastest-moving background stars. From our analysis we conclude that HD 131399 Ab is a background K or M dwarf.

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