The \sim 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 catalogues .
This signal is consistent with either a stellar companion with a moderate mass ratio ( q \sim 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 modelling 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 .