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.
© 2019. The American Astronomical Society.
de Rosa, Robert J.; Nielsen, Eric L.; Rameau, Julien; Duchêne, Gaspard; Greenbaum, Alexandra Z.; Wang, Jason J.; Mark Ammons, S.; 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; Ward-Duong, Kimberly; and al, et, "Detection of a Low-Mass Stellar Companion to the Accelerating A2IV Star HR 1645" (2019). Astronomy: Faculty Publications, Smith College, Northampton, MA.