Hipparcos, Gaia, and RVs reveal that the radio emitting F star HD 220242 has an M dwarf companion, a likely source of the radio emission

The detection of circularly polarized, low frequency radio emission offers the tantalizing possibility of the observation of 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 nineteen M dwarfs were detected by LOFAR in circular polarization as part of the V-LoTSS survey, HD220242 is the only F star to have a circularly polarized low frequency radio detection in the same survey. We conducted radial velocity follow up with the Habitable-zone Planet Finder and combined these observations with additional archival RVs and \textit{Hipparcos}-\textit{Gaia} proper motion accelerations to determine that HD220242 has a stellar companion with P=16.79$\pm$0.04,yrs and a mass of $0.619\pm0.014$,M$_\odot$. We use Spectral Energy Distribution fitting and lack of any UV excess to rule out a co-evolved 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.

💡 Research Summary

This paper presents a comprehensive follow-up study of the F5-type main-sequence star HD 220242, which was uniquely identified as a source of circularly polarized low-frequency radio emission in the LOFAR V-LoTSS survey, unlike the majority of such sources which are M dwarfs. The central mystery addressed is the origin of this coherent radio emission from a seemingly inactive F star, which typically lacks the strong magnetic coronal activity associated with such signals.

To solve this puzzle, the authors employed a multi-pronged observational strategy. They obtained new high-precision radial velocity (RV) measurements in the near-infrared using the Habitable-zone Planet Finder (HPF) spectrograph. These new data were combined with archival RV data from the Tautenberg Coude Echelle Spectrograph and from Nordstrom et al. (1997), significantly extending the temporal baseline. Crucially, they also incorporated astrometric data in the form of proper motion accelerations derived from the positional differences between the Hipparcos and Gaia satellite missions.

A joint analysis of this combined dataset (RVs + proper motion accelerations) using orbit-fitting codes revealed the presence of a previously unknown companion orbiting HD 220242. The companion’s orbit has a period of 16.79 ± 0.04 years and a minimum mass of 0.619 ± 0.014 solar masses.

The next critical step was characterizing the nature of this companion. The mass alone allowed for possibilities ranging from a K dwarf, an M dwarf, to a white dwarf. The team performed Spectral Energy Distribution (SED) fitting on photometric data from the ultraviolet to the infrared. This analysis showed a complete lack of ultraviolet (UV) excess, which effectively rules out a hot white dwarf companion. Additionally, high-contrast speckle imaging ruled out bright stellar companions at wider separations.

The derived companion mass places it firmly within the realm of early M dwarfs. Given that F-type stars like HD 220242 lack the requisite coronal properties to generate the observed type of coherent radio emission, and that M dwarfs are known prolific sources of such emission (often via the electron-cyclotron maser instability mechanism), the logical conclusion is that the radio waves detected by LOFAR most likely originate from the M dwarf companion, not the primary F star.

In summary, this research demonstrates the power of synthesizing data across multiple wavelengths and techniques—radio detection, precision radial velocities, astrometry, and SED analysis—to uncover hidden companions and definitively attribute complex astrophysical signals to their correct source. It resolves the anomaly of HD 220242 by revealing it is not a radio-loud F star, but rather a binary system where the radio emission comes from a lower-mass, magnetically active stellar companion.