Heating of stellar chromospheres and coronae observational constraints and evidence for saturation

Abstract

Chromospheric-coronal emission of cool stars has a clear observed (spectral type dependent) upper limit (saturation). This boundary is seen in the surface flux vs. color (and flux vs. period) diagrams for different types of cool stars, excluding T Tauri-stars. The limit is occupied by rapid rotators (τ _c /P≥3), either very young stars or components in close binaries with tidally forced rotation. Saturated stars seem to be almost totally filled with equipartition magnetic fields. This is one of the reason for the saturation: super-active regions with B= 1–2 kG fill the stellar surface, and no more magnetic flux can get out. This is due to the feed-backs between magnetic fields, convection and differential rotation.

For saturated stars the observed average fluxes (over the disc) could also represent the local surface fluxes, if the many filling factors involved are really close to unity. Then a comparison with local heating is possible. The mechanical flux of subphotospheric convective motions is not very much larger than estimated total radiative losses from the chromospheric coronal plasma of saturated stars. Hence, the power of this mechanical driver of coronal heating sets another natural constraint to the extent of this hot plasma. We conclude that there is an ubiquitous need for hydrodynamic simulations of convection with magnetic fields and for future high resolution X-EUV-UV spectroscopic missions.