Relations between magnetic activity and stellar properties

  • Cornelis Zwaan
Session I: Dynamos and Stellar Activity: Observations
Part of the Lecture Notes in Physics book series (LNP, volume 254)


This paper discusses the basal emission found in strong chromospheric resonance lines, the activity emission, and the tight power-law relations that exist for the majority of cool stars between the flux densities measured in the various coronal, transition-region and excess chromospheric emissions (the “flux-flux relations”). These relations do not depend on effective temperature and surface gravity.

There is also a clear “activity-rotation” relation. The simplest empirical activity-rotation relations are obtained plotting excess flux densities in chromospheric or transition-region lines against rotation period. All such relations depend on effective temperature, but are virtually independent of surface gravity. A group of “overactive” evolved stars depart from the main activity-rotation relation; this departure has not yet been related to any stellar property. The other stars, whether they are components in close spectroscopic binaries or single stars, obey the activity-rotation and flux-flux relations.

The Vaughan-Preston gap in the FCaII(B-V) diagram for dwarfs corresponds to a knee in the activity-rotation relation.

The basal emission is suggested to originate in the “intranetwork chromosphere”, i.e. the part of the chromosphere not dominated by strong magnetic field.

The Linsky-Haisch dividing line is explained as a statistical effect caused by the distribution of rotation rates of single stars across the Hertz sprung-Russell diagram.

The pronounced non-linearity in the relation between soft X-ray emission and chromospheric emission, both for solar active regions, and for the radiation averaged over stellar disks, points to a large-scale organization of magnetic structure in stellar atmospheres.


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Copyright information

© Springer-Verlag 1986

Authors and Affiliations

  • Cornelis Zwaan
    • 1
  1. 1.UtrechtThe Netherlands

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