Spectroscopic characterization of LOFAR radio-emitting M dwarfs

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
² ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, Dwingeloo 7991 PD, The Netherlands
³ Department of Astronomy & Astrophysics, 525 Davey Laboratory, Penn State, University Park, PA 16802, USA
⁴ Center for Exoplanets and Habitable Worlds, 525 Davey Laboratory, Penn State, University Park, PA 16802, USA
⁵ Astrobiology Research Center, 525 Davey Laboratory, Penn State, University Park, PA 16802, USA
⁶ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁷ Department of Physics & Astronomy, The University of California, Irvine, Irvine, CA 92697, USA
⁸ Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA
⁹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁰ Electrical, Computer & Energy Engineering, 440 UCB, University of Colorado, Boulder, CO 80309, USA
¹¹ Department of Physics, 390 UCB, University of Colorado, Boulder, CO 80309, USA
¹² Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08540, USA
¹³ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
¹⁴ Millennium Institute for Astrophysics, Santiago, Chile
¹⁵ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA
¹⁶ Earth and Planets Laboratory, Carnegie Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
¹⁷ Department of Astronomy, California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
¹⁸ School of Mathematical & Physical Sciences, 12 Wally’s Walk, Macquarie University, NSW 2113, Australia
¹⁹ Astrophysics & Space Institute, Schmidt Sciences, New York, NY 10011, USA
²⁰ Carleton College, One North College St., Northfield, MN 55057, USA
²¹ Penn State Extraterrestrial Intelligence Center, 525 Davey Laboratory, Penn State, University Park, PA 16802, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 July 2025
Accepted: 16 November 2025

Abstract

Recent observations with the LOw Frequency ARray (LOFAR) have revealed 19 nearby M dwarfs showing bright circularly polarized radio emission. One of the possible sources of such emission is through magnetic star-planet interactions (MSPIs) with unseen close-in planets. We present the 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 characterized the stellar properties, constraining their effective temperatures (T_eff), surface gravities (log g), metallicities ([Fe/H]), projected rotational velocities (v sin i_⋆), rotation periods (P_rot), stellar radii (R_⋆), and stellar inclinations (i_⋆) where possible. Further, from a homogeneous analysis of the HPF spectra, we inferred their chromospheric activity and spectroscopic multiplicity states. From this, we identified GJ 625, GJ 1151, and LHS 2395 as single, quiescent stars amenable to precise radial velocity 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 refined 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 identified GJ 3861 as a spectroscopic binary with an orbital period of P = 14.841181_−0.00010^+0.00011 d and with a mass ratio of q = 0.7663_−0.0018^+0.0020, 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 MSPIs.