PROTOPLANETARY NEBULAE .2. THE SHOCK-HEATED BIPOLAR NEBULAE GL-618 AND M2-56

The two peculiar, very low ionization proto-planetary nebulae GL 618 and M2-56 are studied using long-slit CCD spectroscopy. Previous optical investigations of these objects have analyzed them in terms of photoionization models, although more recent investigations from the radio to IR have identified much of the emission as shock emission. In this work, improved optical spectra of M2-56 and published spectra of GL 618 are compared with the observed spectra of Herbig-Haro (HH) objects and with theoretical shock model calculations published in the literature. It is shown that the spectra of low-excitation HH objects match very well the spectra of GL 618 and M2-56, with the exception of [S II], which is weaker in the proto-planetary nebulae. The shock models indicate the presence of a range of shock velocities from approximately 40-100 km s-1, again similar to what is seen in low-excitation HH objects. It is argued that the radio brightening seen in GL 618 is due to the advance of an ionization front into the inner parts of the bipolar lobes, while the lack of detected radio flux from M2-56 indicates that it may be at a slightly earlier evolutionary stage. These objects are probably in transition between the OH/IR stars and more fully developed and photoionized planetary nebulae like M2-9 and M1-91. The presence of shock emission and the absence of substantial photoionization in GL 618 and M2-56 allow us a rare opportunity to study the dynamics of the early mass-loss phase in proto-planetary nebulae, free from the effects of the central star's photoionization energy input.