Long-slit CCD spectroscopy has been used to study the two peculiar, very low ionization protoplanetary nebulae, GL 618 and M2-56, comparing the results obtained with observed Herbig-Haro (HH) objects' spectra as well as with theoretical shock-model calculations from the literature. It is found that the spectra of low-excitation HH objects match the spectra of GL 618 and M2-56; in addition, the shock models indicate the presence of shock velocities in the 40-100 km/sec range, which is again congruent with low excitation HH object results.
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Robert W. Goodrich (1991) studied this question.