MOLECULAR-HYDROGEN MORPHOLOGY, KINEMATICS AND EXCITATION IN AFGL-2688AND NGC-7027

We report the first results of spectro-imaging post-AGB objects in the near-infrared K'-band using a new instrument called BEAR. This instru ment has been used to obtain high spatial (0.6'') and spectral(similar to 40 km s(-1)) 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 H-2 in their circumstellar envelopes. In AFGL 2688, the H-2 emission shows four bright clumps forming a remarkable cross-like pattern with weaker H-2 emission connecting the north and east, and th e south and west clumps, respectively. No trace of lines or continuum from ionized gas is seen in the spectra. The continuum emission seen i n the central parts of the nebula. is stellar light scattered by nebul ar dust. The velocity field shows that the northern and eastern clumps are blue-shifted whereas the southern and western clumps an red-shift ed with respect to the systemic velocity. There is also evidence for s ignificant velocity gradients across the H-2 structures. The emission of the H-2 lines in AFGL 2688 is consistent with shock excitation. In NGC 7027 new morphological details are revealed by the observations. T he emission from the inner envelope is dominated by continuum and line emission from the ionized nebula, including strong Br gamma, HeII and HeI lines. The H-2 emission is distributed at the periphery of the io nized gas, along a four-lobed clover pattern with an equatorial torus which is seen for the first time in its entirety. The H-2 velocity dis tribution demonstrates that the kinematics of the hut gas traced by H- 2 is firmly linked to the outer molecular envelope. The H-2 emission i n NGC 7027 is consistent with excitation in the UV photon dominated re gion (PDR) at the interface of the ionized and molecular gas. The morp hology and excitation of H-2 in AFGL 2688 and NGC 7027 is consistent w ith an evolutionary scheme in which fast winds during the proto-planet ary nebula phase develop strong shocks in the slower expanding AGE env elope and are able to excite the H-2 emission, and in the young planet ary nebula phase ultraviolet radiation from the hot, central star ioni zes the inner cavity and excites the molecular hydrogen in the PDR.