The remarkably high excitation planetary nebula GC 6537

NGC 6537 is an unusually high excitation point symmetric planetary nebula w ith a rich spectrum. Its kinematical structures are of special interest. We are here primarily concerned with the high resolution spectrum as revealed by the Hamilton echelle Spectrograph at Lick Observatory (resolution appro ximate to 0.2 Angstrom) and supplemented by UV and near-UV data. These exte nsive data permit a determination of interstellar extinction, plasma diagno stics, and ionic concentrations. The photoionization models that have been used successfully for many planetary nebulae are not entirely satisfactory here. The plasma electron temperature of a photoionization model cannot muc h exceed 20,000 K, but plasma diagnostics show that regions emitting radiat ion of highly ionized atoms such as [NeIV] and [NeV] are much hotter, showi ng that shock excitation must be important, as suggested by the remarkable kinematics of this object. Hence, instead of employing a strict photoioniza tion model, we are guided by the nebular diagnostics, which reveal how elec tron temperature varies with ionization potential and accommodates density effects. The predictions of the photoionization model may be useful in esti mating ionization correction factor, In effect, we have estimated the chemi cal composition by using both photoionization and shock considerations.