Spectroscopic Characterization of LOFAR Radio-emitting M dwarfs

Recent observations with the LOw Frequency ARray (LOFAR) have revealed 19 nearby M dwarfs showing bright circularly polarised radio emission. One of the possible sources of such emission is through magnetic star-planet interactions (MSPI) with unseen close-in planets. We present initial results from a spectroscopic survey with the Habitable-zone Planet Finder (HPF) and NEID spectrographs designed to characterize this sample and further investigate the origin of the radio emission. We provide four new insights into the sample. I) We uniformly characterize the stellar properties, constraining their effective temperatures, surface gravities, metallicities, projected rotational velocities, rotation periods, stellar radii, and stellar inclinations where possible. Further, from a homogenous analysis of the HPF spectra, we infer their chromospheric activity and spectroscopic multiplicity states. From this, we identify GJ 625, GJ 1151, and LHS 2395 as single, quiescent stars amenable to precise RV follow-up, making them strong MSPI candidates. II) We show that the distribution of stellar inclinations are compatible with an isotropic distribution, providing no evidence for a preference to pole-on configurations. III) We refine the radial velocity solution for GJ 625 b, the only currently known close-in planet in the sample, reducing the uncertainty in its orbital period by a factor of three, to facilitate future phase-dependent radio analysis. IV) Finally, we identify GJ 3861 as a spectroscopic binary with an orbital period of $P=14.841181_{-0.00010}^{+0.00011}$ d, making it the only confirmed binary with a relatively short orbit in the sample, where we surmise the radio emission is likely related to magnetospheric interactions between the two stars. These results advance our understanding of radio-emitting M dwarfs and establish an observational foundation for identifying MSPI.

💡 Research Summary

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This paper presents the first spectroscopic characterization of the 19 nearby M‑dwarfs that exhibit bright, circularly polarized low‑frequency radio emission in LOFAR surveys. The authors obtained near‑infrared spectra with the Habitable‑zone Planet Finder (HPF) on the 10 m Hobby‑Eberly Telescope and optical spectra with the NEID spectrograph on the WIYN 3.5 m telescope. Using a homogeneous pipeline (HPF‑SERVAL and NEID‑SERVAL) they derived precise radial velocities (median precision ≈ 1 m s⁻¹ for HPF and 0.5 m s⁻¹ for NEID) and extracted stellar atmospheric parameters (effective temperature, surface gravity, metallicity) by fitting PHOENIX model grids to the spectra. Projected rotational velocities (v sin i) were measured from line broadening, and rotation periods were inferred from activity indicators (Hα, Ca II) and photometric variability. Combining v sin i, rotation period, and stellar radius (derived from empirical relations) allowed the authors to estimate stellar inclination angles (i★). The distribution of i★ is statistically consistent with an isotropic (cos i uniform) distribution, providing no evidence that the radio‑bright sample is preferentially pole‑on, contrary to some theoretical expectations for magnetic star‑planet interactions (MSPI).

A key focus is the only known close‑in planet in the sample, GJ 625 b, a super‑Earth with a minimum mass of ~2.8 M⊕ and an orbital period near 14.6 days. The authors combined new NEID radial velocities with archival HARPS‑N, CARMENES, and HIRES data (all re‑processed with SERVAL for consistency) to refine the orbital solution. An MCMC analysis yields a period of 14.628 ± 0.004 days, reducing the period uncertainty by a factor of three relative to previous work, while the semi‑amplitude and eccentricity remain consistent with a circular orbit. This improved ephemeris will enable phase‑resolved LOFAR studies to test whether the radio emission is modulated at the planetary orbital period, a decisive test for MSPI.

The spectroscopic survey also uncovered a new short‑period spectroscopic binary: GJ 3861. Radial‑velocity monitoring reveals a clear sinusoid with P = 14.841181 ± 0.00011 days and a mass ratio q = 0.766 ± 0.002. The authors argue that the radio emission from this system is likely driven by magnetospheric interaction between the two M‑dwarfs rather than a star‑planet interaction.

From the activity and multiplicity analysis, three stars—GJ 625, GJ 1151, and LHS 2395—appear single and magnetically quiet (low Hα and Ca II emission), making them optimal targets for ultra‑precise radial‑velocity follow‑up and prime MSPI candidates. The remaining objects show higher activity levels or evidence of binarity, suggesting alternative radio emission mechanisms such as flares, co‑rotation breakdown, or star‑star magnetic interaction.

Overall, the paper delivers a comprehensive, homogeneous set of stellar parameters, rotation and inclination estimates, and refined orbital solutions that lay the groundwork for discriminating among possible radio emission mechanisms in low‑frequency M‑dwarf emitters. The authors conclude that while the inclination distribution does not support a pole‑on bias, the identification of quiet, single stars and the precise ephemeris for GJ 625 b provide a solid observational foundation for future MSPI searches with LOFAR, the upcoming Square Kilometre Array, and complementary high‑precision radial‑velocity campaigns.