Browsing by Author "Leto, P."
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Item Centrifugal breakout reconnection as the electron acceleration mechanism powering the radio magnetospheres of early-type stars(Monthly Notices of the Royal Astronomical Society, 2022-04-27) Owocki, S. P.; Shultz, M. E.; ud-Doula, A.; Chandra, P.; Das, B.; Leto, P.Magnetic B-stars often exhibit circularly polarized radio emission thought to arise from gyrosynchrotron emission by energetic electrons trapped in the circumstellar magnetosphere. Recent empirical analyses show that the onset and strength of the observed radio emission scale with both the magnetic field strength and the stellar rotation rate. This challenges the existing paradigm that the energetic electrons are accelerated in the current sheet between opposite-polarity field lines in the outer regions of magnetized stellar winds, which includes no role for stellar rotation. Building on recent success in explaining a similar rotation-field dependence of H α line emission in terms of a model in which magnetospheric density is regulated by centrifugal breakout (CBO), we examine here the potential role of the associated CBO-driven magnetic reconnection in accelerating the electrons that emit the observed gyrosynchrotron radio. We show in particular that the theoretical scalings for energy production by CBO reconnection match well the empirical trends for observed radio luminosity, with a suitably small, nearly constant conversion efficiency ϵ ≈ 10−8. We summarize the distinct advantages of our CBO scalings over previous associations with an electromotive force, and discuss the potential implications of CBO processes for X-rays and other observed characteristics of rotating magnetic B-stars with centrifugal magnetospheres.Item MOBSTER – VI. The crucial influence of rotation on the radio magnetospheres of hot stars(Monthly Notices of the Royal Astronomical Society, 2022-04-27) Shultz, M. E.; Owocki, S. P.; ud-Doula, A.; Biswas, A.; Bohlender, D.; Chandra, P.; Das, B.; David-Uraz, A.; Khalack, V.; Kochukhov, O.; Landstreet, J. D.; Leto, P.; Monin, D.; Neiner, C.; Rivinius, Th.; Wade, G. A.Numerous magnetic hot stars exhibit gyrosynchrotron radio emission. The source electrons were previously thought to be accelerated to relativistic velocities in the current sheet formed in the middle magnetosphere by the wind opening magnetic field lines. However, a lack of dependence of radio luminosity on the wind power, and a strong dependence on rotation, has recently challenged this paradigm. We have collected all radio measurements of magnetic early-type stars available in the literature. When constraints on the magnetic field and/or the rotational period are not available, we have determined these using previously unpublished spectropolarimetric and photometric data. The result is the largest sample of magnetic stars with radio observations that has yet been analysed: 131 stars with rotational and magnetic constraints, of which 50 are radio-bright. We confirm an obvious dependence of gyrosynchrotron radiation on rotation, and furthermore find that accounting for rotation neatly separates stars with and without detected radio emission. There is a close correlation between H α emission strength and radio luminosity. These factors suggest that radio emission may be explained by the same mechanism responsible for H α emission from centrifugal magnetospheres, i.e. centrifugal breakout (CBO), however, while the H α-emitting magnetosphere probes the cool plasma before breakout, radio emission is a consequence of electrons accelerated in centrifugally driven magnetic reconnection.Item A scaling relationship for non-thermal radio emission from ordered magnetospheres: from the top of the main sequence to planets Get access Arrow(Monthly Notices of the Royal Astronomical Society, 2021-07-28) Leto, P.; Trigilio, C.; Krtička, J.; Fossati, L.; Ignace, R.; Shultz, M. E.; Buemi, C. S.; Cerrigone, L.; Umana, G.; Ingallinera, A.; Bordiu, C.; Pillitteri, I.; Bufano, F.; Oskinova, L. M.; Agliozzo, C.; Cavallaro, F.; Riggi, S.; Loru, S.; Todt, H.; Giarrusso, M.; Phillips, N. M.; Robrade, J.; Leone, F.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 broad-band 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 broad-band 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 ultracool 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.