1997A&A...321..907C

We report the first results of spectro-imaging post-AGB objects in the near-infrared K'-band using a new instrument called BEAR. This instrument has been used to obtain high spatial (0.6") and spectral (∼40km/s) resolution observations of the proto-planetary nebula AFGL 2688 and the young planetary nebula NGC 7027. The current paper is devoted to a detailed study of the morphology, velocity field, and excitation of H₂ in their circumstellar envelopes. In AFGL 2688, the H₂ emission shows four bright clumps forming a remarkable cross-like pattern with weaker H₂ emission connecting the north and east, and the south and west clumps, respectively. No trace of lines or continuum from ionized gas is seen in the spectra. The continuum emission seen in the central parts of the nebula is stellar light scattered by nebular dust. The velocity field shows that the northern and eastern clumps are blue-shifted whereas the southern and western clumps are red-shifted with respect to the systemic velocity. There is also evidence for significant velocity gradients across the H₂ structures. The emission of the H₂ lines in AFGL 2688 is consistent with shock excitation. In NGC 7027 new morphological details are revealed by the observations. The emission from the inner envelope is dominated by continuum and line emission from the ionized nebula, including strong Brγ, HeII and HeI lines. The H₂ emission is distributed at the periphery of the ionized gas, along a four-lobed clover pattern with an equatorial torus which is seen for the first time in its entirety. The H₂ velocity distribution demonstrates that the kinematics of the hot gas traced by H₂ is firmly linked to the outer molecular envelope. The H₂ emission in NGC 7027 is consistent with excitation in the UV photon dominated region (PDR) at the interface of the ionized and molecular gas. The morphology and excitation of H₂ in AFGL 2688 and NGC 7027 are consistent with an evolutionary scheme in which fast winds during the proto-planetary nebula phase develop strong shocks in the slower expanding AGB envelope and are able to excite the H₂ emission, and in the young planetary nebula phase ultraviolet radiation from the hot, central star ionizes the inner cavity and excites the molecular hydrogen in the PDR.