On Chromospheric Heating Mechanisms of “Basal Flux” Stars

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Abstract
Several pieces of evidence have been pieced together over recent years to support the notion that the chromospheric emission measured from stars with convection zones results in part from the upward propagation and dissipation of acoustic waves. One argument, based on a statistical analysis of available UV data of such stars across the H-R diagram, suggests the presence of an omnipresent "basal" level of chromospheric heating, which has been postulated as resulting from nonlinear acoustic wave heating. However, with few exceptions, no studies have been made that test more directly the intrinsically dynamic nature of this shock-heating mechanism. Therefore, in order to search for more direct signatures of such upward-propagating shock waves in lines of C II, we examined Goddard High-Resolution Spectrograph spectra of several evolved stars that have "basal" levels of activity. No evidence is found to support the presence of such waves as a dominant component of the heating mechanism. Instead, behavior reminiscent of the solar transition region is seen, suggesting a magnetic heating mechanism for these stars. We conclude that upward-propagating shock waves do not dominate the observed radiative losses from chromospheres of stars exhibiting typical "basal" behavior, and we suggest that the nonmagnetic origin of the basal components of all convective stars must be called into question. New solar data from the SUMER instrument on SOHO also suggest problems with the acoustic-wave interpretation, although further work is warranted. In the course of this work, we also found a simple explanation for previously noted discrepancies between calculated and observed ratios of C II lines in the spectrum of α Ori.