Detection of a Low-mass Stellar Companion to the Accelerating A2IV Star HR 1645

December 2019 • 2019AJ....158..226D

Authors • De Rosa, Robert J. • Nielsen, Eric L. • Rameau, Julien • Duchêne, Gaspard • Greenbaum, Alexandra Z. • Wang, Jason J. • Ammons, S. Mark • Bailey, Vanessa P. • Barman, Travis • Bulger, Joanna • Chilcote, Jeffrey • Cotten, Tara • Doyon, Rene • Esposito, Thomas M. • Fitzgerald, Michael P. • Follette, Katherine B. • Gerard, Benjamin L. • Goodsell, Stephen J. • Graham, James R. • Hibon, Pascale • Hom, Justin • Hung, Li-Wei • Ingraham, Patrick • Kalas, Paul • Konopacky, Quinn • Larkin, James E. • Macintosh, Bruce • Maire, Jérôme • Marchis, Franck • Marley, Mark S. • Marois, Christian • Metchev, Stanimir • Millar-Blanchaer, Maxwell A. • Oppenheimer, Rebecca • Palmer, David • Patience, Jennifer • Perrin, Marshall • Poyneer, Lisa • Pueyo, Laurent • Rajan, Abhijith • Rantakyrö, Fredrik T. • Ren, Bin • Ruffio, Jean-Baptiste • Savransky, Dmitry • Schneider, Adam C. • Sivaramakrishnan, Anand • Song, Inseok • Soummer, Remi • Tallis, Melisa • Thomas, Sandrine • Wallace, J. Kent • Ward-Duong, Kimberly • Wiktorowicz, Sloane • Wolff, Schuyler

Abstract • The ∼500 Myr A2IV star HR 1645 has one of the most significant low-amplitude accelerations of nearby early-type stars measured from a comparison of the Hipparcos and Gaia astrometric catalogs. This signal is consistent with either a stellar companion with a moderate mass ratio (q ∼ 0.5) on a short period (P < 1 yr), or a substellar companion at a separation wide enough to be resolved with ground-based high-contrast imaging instruments; long-period equal-mass ratio stellar companions that are also consistent with the measured acceleration are excluded with previous imaging observations. The small but significant amplitude of the acceleration made HR 1645 a promising candidate for targeted searches for brown dwarf and planetary-mass companions around nearby, young stars. In this paper we explore the origin of the astrometric acceleration by modeling the signal induced by a wide-orbit M8 companion discovered with the Gemini Planet Imager, as well as the effects of an inner short-period spectroscopic companion discovered a century ago but not since followed up. We present the first constraints on the orbit of the inner companion, and demonstrate that it is a plausible cause of the astrometric acceleration. This result demonstrates the importance of vetting of targets with measured astrometric acceleration for short-period stellar companions prior to conducting targeted direct imaging surveys for wide-orbit substellar companions.

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Alexandra Greenbaum

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