Hipparcos, Gaia, and Radial Velocities Reveal that the Radio-emitting F Star HD 220242 Has an M Dwarf Companion, a Likely Source of the Radio Emission*

Abstract The detection of circularly polarized, low-frequency radio emission offers the tantalizing possibility of observing interactions between stars and their possible substellar companions, as well as direct emission from exoplanets. Additional follow-up of systems with radio emission is key to understanding the true origin of the emission, since multiple astrophysical mechanisms can plausibly lead to such signals. While 19 M dwarfs were detected by the LOw-Frequency ARray in circular polarization as part of the V-LoTSS survey, HD 220242 is the only F star to have a circularly polarized, low-frequency radio detection in the same survey. We conducted radial velocity (RV) follow-up with the Habitable-zone Planet Finder and combined these observations with additional archival RVs and Hipparcos-Gaia proper motion accelerations to determine that HD 220242 has a stellar companion with P = 16.79 ± 0.04 yr and a mass of 0.619 ± 0.014 M ⊙ . We use spectral energy distribution fitting and lack of any UV excess to rule out a coevolved white dwarf companion, and confirm that the companion is an M dwarf star. Given that F stars lack the coronal properties to produce such coherent emission, and the companion mass and lack of UV excess are consistent with an M dwarf, the radio emission is most plausibly associated with the companion.