Searching for radio emission from stellar wind-magnetosphere interaction or co-rotation breakdown in brown dwarfs

TL;DR

This work conducts a deep, low-frequency radio search for emission from wide-orbit brown dwarfs using the uGMRT and JVLA across 0.3–2 GHz. No detections are found, yielding stringent 3σ upper limits on Stokes V and isotropic luminosity that are consistent with a low occurrence rate of ECM-driven radio emission in such systems. By applying wind-magnetosphere, rotation-powered, and flux-normalization models, the authors derive tentative constraints on magnetic field strengths, wind properties, and rotation, while stressing large uncertainties and beaming effects that could hide signals. The results imply that detections are challenging with current facilities but remain plausible with next-generation instruments like the SKA, LOFAR, and ASKAP, motivating broader surveys and coordinated, multi-epoch campaigns to fully probe the magnetospheric physics of brown dwarfs and exoplanetary companions.

Abstract

With the improvements in radio interferometry sensitivity, the quest for coherent radio emission from exoplanets and ultra-cool dwarfs, which is indicative of their magnetic fields, has gained significant momentum in recent years. We investigated the relatively unexplored possibility of radio emission from wide-orbit brown dwarf companions, which may radiate through rapid rotation, as in isolated ultra-cool dwarfs, or via interactions between their extended magnetospheres and the host star's wind. We analysed $\sim 60$ hours of Upgraded Giant Metrewave Radio Telescope and Karl G. Jansky Very Large Array data for a set of well-characterized systems previously unobserved at 0.3-2 GHz. The targets include companions orbiting the G-type stars HD 26161 and BD-004475, the K-type HD 153557A and $ν$ Oph, and the M dwarfs GJ 3626 and 2MJ01225093-2439505. No detections were obtained with 3$σ$ upper limits down to $\sim 25\,μ$Jy/beam in Stokes V in the best cases. The light-curve analysis also revealed no evidence of short ($\gtrsim$ minutes), intense ($\gtrsim$ mJy) radio bursts. The upper limits provide tentative constraints on model parameters. However, the effects of model uncertainties, limited observational coverage, and intrinsic variability or beaming of the emission must be considered. The improvement in sensitivity of the next-generation radio interferometers will likely allow to go below the expected flux range over a much larger range of free parameters.

Figures (3)

• Figure 1: Flux estimates of BD-00 4475 B for the different physically based models, from top to bottom: as a function of wind velocity $v_{\rm sw}$ and density $n_{\rm sw}$ for the the kinetic model, wind velocity and stellar surface magnetic field for the magnetic model, and $P_{\rm rot}$ and $\Sigma_p\dot M_{\rm bd}$ for the rotation-powered model. The black dashed lines in the plots correspond to 10, 50 and 100 $\mu$Jy.
• Figure 2: Tentative constraints on the parameters of models, from top to bottom: kinetic model as a function of mass loss rate and wind density (eqs. (\ref{['eq:pinput_kin']}), (\ref{['eq:stevens']})), magnetic model (eq. \ref{['eq:pinput_mag']}), and rotation-powered auroral model (eq. \ref{['eq:poval']}). Lines in the space parameters corresponding to the $3\sigma_I$ upper limits of the combined images for each target (different colours). Assuming that the models are correct, a favourable beaming and no time variability, the regions above the lines are nominally excluded. The dashed lines indicate the observations made with VLA and the solid lines with uGMRT.
• Figure 3: Stokes I maps of each source, combining all data for a given instrument and band. The colour bar spans from $-3\sigma_{I}$ to the maximum peak flux density of the field sources. The white contours mark the $-3, 3, 5\,\sigma_{I}$ levels. The clean beam is shown in the lower left corner of each image.