A scaling relationship for non-thermal radio emission from ordered magnetospheres: from the top of the Main Sequence to planets

TL;DR

This paper investigates non-thermal radio emissions from magnetic stars across spectral types, proposing a new model involving radiation belts inside magnetospheres that explains observed emissions better than wind-based models.

Contribution

It introduces a scaling relationship linking radio luminosity to magnetospheric electric voltage, applicable from stars to planets, and challenges previous wind-based emission theories.

Findings

Radio emissions are independent of stellar wind mass-loss rates.

A radiation belt model explains broadband radio spectra.

A universal scaling law relates radio luminosity to magnetospheric voltage.

Abstract

In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broadband radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner-magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broadband radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere's co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultra-cool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons.