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 (zJP_>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.

Acoustic and magnetic wave heating in stars

“…The similar case for the total Mg II h+k emission fluxes is shown in Fig. 9 where the observational limits are given by the Rutten et al (1991) and Vilhu (1987).…”

“…Here "Min" and "Max" indicate the observed range of chromospheric emission displayed in Fig. 2 and Rutten et al (1991) and Vilhu (1987).…”

“…11. Total observed Mg II h+k line emission of stars by Vilhu (1987) and Rutten et al (1991) (dots), compared with theoretical fluxes for pure acoustic wave heating (dashed) and for magnetic wave heating in flux tubes, which at photospheric height have an area filling factor of f = 0.4 (solid).…”

“…The Mg II emission fluxes Figure 11 shows the observed emission fluxes in the Mg II h+k lines for late-type stars taken from Vilhu (1987) and Rutten et al (1991). Similarly as for the Ca II fluxes, it is seen that the observed minimum flux limit agrees within about a factor of two with the emission flux of non-magnetic stars that are heated purely by acoustic waves (see also Buchholz et al 1998).…”

Microflaring of a solar bright point

“…Comparing the sun and solar type active stars, Vilhu (1987Vilhu ( , 1994 suggested a correlation between the CIV 1550 line intensity (10 5 K) and the magnetic filling factor. Lines formed at 10 5 K arise close to the loop foot points and are good indicators of their surface area.…”