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Nonthermal phenomena in stellar coronae: Evidence from radio observations

  • George A. Dulk
Nonthermal Phenomena in Stellar Coronae
Part of the Lecture Notes in Physics book series (LNP, volume 291)

Abstract

In the past few years a number of different kinds of stars have been detected at radio wavelengths, the majority of them cool stars. In most cases the radio emission is indicative of high energy, nonthermal processes: acceleration of electrons to some 10's of keV or, in some circumstances, several MeV or more. These electrons then emit radio waves by the incoherent processes of gyroresonance and gyrosynchrotron radiation, and the coherent processes of cyclotron maser and plasma radiation.

The source of energy for the electron acceleration in single stars and some binaries is probably the generation of strong magnetic fields in the stars' interiors (aided, in close binaries, by rapid rotation enforced by synchronous rotation), their emergence above the photospheres, and their subsequent dissipation. For some stars, RS CVn's and cataclysmic variables, interactions between the magnetic fields of two close companions may also be important. For evolved binaries the energy probably is mainly gravitational, coming from the transfer of matter onto a compact star.

Whatever the basic energy source, some energy appears in the form of fast electrons that are capable of generating intense radio waves. If the energy of the electrons is greater than ∼ 1 keV they may emit gyroresonance radiation of brightness temperature T B ∼ 107 K, and if ≿ 100 keV they may emit gyrosynchrotron radiation with T B ≿ 108 K (maximum so far observed: a few ×1010 K). For the coherent radiation mechanisms of cyclotron maser and plasma radiation, the electrons need not be more energetic than ∼ 10 keV, but they must have free energy, probably in the form of a loss cone distribution (for the cyclotron maser) or a bump-in-the-tail distribution (for plasma radiation). Then they are able to emit radiation with T B greater than the maximum so far observed, ∼ 1015 K.

Keywords

Radio Emission Plasma Radiation Single Star Radio Flux Radio Luminosity 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer-Verlag 1987

Authors and Affiliations

  • George A. Dulk
    • 1
  1. 1.Department of Astrophysical, Planetary and Atmospheric SciencesUniversity of ColoradoBoulderUSA

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