Kinematics of molecular hydrogen emission from pre-planetary nebulae: RAFGL 2688 and RAFGL 618

We present high spectral resolution maps of near-infrared molecular hydrogen emission from the bipolar pre-planetary nebulae RAFGL 2688 and RAFGL 618, obtained with the NOAO Phoenix spectrometer. The measured velocity gradients along the polar axes of both nebulae indicate that the highest velocity gas lies closest to the central stars. These results support the suggestion that the polar H₂ emission regions of both nebulae contain shocked gas formed as fast (∼ 50-150 km s^-1), collimated, post-asymptotic giant branch (AGB) winds collide with slower moving (∼ 10-20 km s^-1) material previously ejected while the central stars were still on the AGB. The kinematics of H₂ emission perpendicular to the polar axis of RAFGL 2688 are consistent with a model combining expansion along the equator at 5-10 km s^-1 with rotation about the polar axis at 5-10 km s^-1. The rapid onset of the common envelope phase of a close binary system may explain both the bipolar structure of RAFGL 2688 and the presence and complex kinematics of its shocked H₂ emission.