Cool Stars On and Off the Main Sequence 1021 Stellar Interiors and Magnetic Dynamos

Stars on the main-sequence derive their energy from nuclear burning in their hot and dense cores. Convection and radiation are the main processes transporting the energy produced in the stars' cores to the photosphere, where it is radiated into space. According to stellar structure theory early-type stars (spectral type O and B) have convective cores and radiative exteriors, A-type stars are purely radiative, and cool main-sequence stars with spectral types F to mid-M have radiative cores and outer convection zones. These stars are - presumably - the nearest analogs to the Sun in terms of their interior structure and in terms of their activity. According to theory, solar-like dynamos (aQ-dynamos) are localized at the interface region connecting the outer convection zones with the radiative interiors [35]. Therefore, solar-type dynamo action and related manifestations of solar-type magnetic activity ought to be restricted to moderately cool main-sequence stars.

Stars of spectral type «M3 or later are expected by theory to be fully convective and should, therefore, lack any boundary layer between convective and radiative zones. Consequently, dynamos operating in their interiors should be different from the "classical" aQ -dynamo thought to be operating in the Sun. Nuclear burning is expected to persist for stars down to a mass of approximately 0.075Me. Stars with lower masses do not reach central temperatures large enough to start main-sequence hydrogen burning and can, therefore, by definition not be on the "main-sequence." Such "substellar" objects are called brown dwarfs. The X-ray properties of fully convective stars and brown dwarfs are discussed in Sect. 10.3. In this section, we focus on solar-like stars, by which we mean stars with an outer convection zone and an interior radiative core, i.e., stars, where presumably the same dynamo mechanism can operate as on the Sun.

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