ON CHROMOSPHERIC HEATING MECHANISMS OF BASAL FLUX STARS

Several pieces of evidence have been pieced together over recent years to support the notion that the chromospheric emission measured from s tars with convection zones results in part from the upward propagation and dissipation of acoustic waves. One argument, based on a statistic al analysis of available UV data of such stars across the H-R diagram, suggests the presence of an omnipresent ''basal'' level of chromosphe ric heating, which has been postulated as resulting from nonlinear aco ustic 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 direc t signatures of such upward-propagating shock waves in lines of C II, we examined Goddard High-Resolution Spectrograph spectra of several ev olved stars that have ''basal'' levels of activity. No evidence is fou nd to support the presence of such waves as a dominant component of th e heating mechanism. Instead, behavior reminiscent of the solar transi tion region is seen, suggesting a magnetic heating mechanism for these stars. We conclude that upward-propagating shock waves do not dominat e the observed radiative losses from chromospheres of stars exhibiting typical ''basal'' behavior, and we suggest that the nonmagnetic origi n of the basal components of all convective stars must be called into question. New solar data from the SUMER instrument on SOHO also sugges t problems with the acoustic-wave interpretation, although further wor k is warranted. In the course of this work, we also found a simple exp lanation for previously noted discrepancies between calculated and obs erved ratios of C II lines in the spectrum of alpha Ori.