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

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.

Centrifugal breakout reconnection as the electron acceleration mechanism powering the radio magnetospheres of early-type stars

“…3). Note that the second equality recovers the empirical scaling L rad ∝ B 2 R 4 * /P 2 rot found by Leto et al (2021) and verified in Paper I.…”

“…The empirical scaling relationship discovered by Leto et al (2021) and confirmed in paper I, L rad ∝ B 2 d R 4 * /P 2 rot = (Φ/Prot) 2 , is explained above as a consequence of electron acceleration via centrifugal breakout. However, Leto et al pointed out that Φ/Prot has the physical dimension of an electromotive force E (EMF), which they speculated may be suggestive of an underlying theoretical mechanism.…”

“…The alternative theoretical scalings explored here were motivated by a more recent companion empirical analysis by Shultz et al (2021, hereafter Paper I), which confirms the basic results of Leto et al (2021), but within a significantly extended sample that allows further exploration of potential empirical trends and scalings. In particular, we show below (section 3) that these empirical scalings for nonthermal radio emission can be well fit by models grounded in the same CBO paradigm that has been so successful for Hα emission.…”

“…An initially favoured model by Trigilio et al (2004) proposed that these electrons could be accelerated in the current sheet between field lines of opposite polarity that have been stretched outward by the stellar wind, as illustrated in the left panel of Figure 1. But a recent empirical analysis by Leto et al (2021) has shown that the observed radio emission has a clear dependence on stellar rotation, providing strong evidence against this currentsheet model, which includes no role for rotation. Instead Leto et al (2021) noted that their fits to the radio luminosity scale in proportion to a quantity that has the physical dimension of an electromotive force (EMF), which they speculated may be suggestive of an underlying mechanism.…”

“…But a recent empirical analysis by Leto et al (2021) has shown that the observed radio emission has a clear dependence on stellar rotation, providing strong evidence against this currentsheet model, which includes no role for rotation. Instead Leto et al (2021) noted that their fits to the radio luminosity scale in proportion to a quantity that has the physical dimension of an electromotive force (EMF), which they speculated may be suggestive of an underlying mechanism. Indeed, the EMF is invoked (Hill 2001) to model auroral emission from the interaction of high-energy magnetospheric particles with planetary atmospheres.…”