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

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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Journal articles

https://hal-insu.archives-ouvertes.fr/insu-03699406
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Submitted on : Monday, June 20, 2022 - 11:24:16 AM
Last modification on : Saturday, June 25, 2022 - 3:06:35 AM

### Citation

Robert J. de Rosa, Eric L. Nielsen, Julien Rameau, Gaspard Duchêne, Alexandra Z. Greenbaum, et al.. Detection of a Low-mass Stellar Companion to the Accelerating A2IV Star HR 1645. The Astronomical Journal, 2019, 158, ⟨10.3847/1538-3881/ab4ef7⟩. ⟨insu-03699406⟩

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