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Astron. Astrophys. 321, 907-920 (1997)

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1. Introduction

A large population of planetary nebulae (PNe) are now known to have massive envelopes of molecular gas which have been detected in [FORMULA] (Webster et al. 1988; Zuckerman & Gatley 1988; Kastner et al. 1996) and CO emission (Huggins et al. 1994, 1996). The mass of the molecular envelopes often exceeds the mass of the ionized nebulae, even for the evolved objects, and in the cases which have been mapped (e.g., Forveille & Huggins 1991; Bachiller et al. 1993) the molecular gas is found to be in expanding, fragmented, shell structures which trace the ionization fronts of the PNe. The optical nebulae are clearly forming from the photoionization of the neutral shells.

The physical conditions of the neutral gas are closely linked to the processes which determine the evolution of the molecular envelopes. The envelopes ejected by the red giant progenitors are exposed to extremely high ultraviolet radiation fields from the central star (102 to 106 times the average interstellar field) and are subject to compression and fragmentation from the effects of ionization fronts and/or fast winds. Theoretical understanding of both the gas dynamics and the rapidly evolving photon dominated regions (PDRs) which develop in these envelopes is still incomplete (Tielens 1993), and time-dependent, non-equilibrium models have recently become available (Goldschmidt & Sternberg 1995, Hollenbach & Natta 1995, Bertoldi & Draine 1996). The rapid transition between the Asymptotic Giant Branch (AGB) and the PN phases (the proto-PN phase) is particularly poorly documented because the important physical processes take place in the inner, obscured regions of the envelope. Direct observations of these regions are essential for building realistic models, and they need to be made with the best possible spatial resolution and at wavelengths which are not too severely affected by extinction.

In this paper, we report the first results of a near-infrared program to study the physical conditions of the inner parts of the molecular envelopes associated with proto-PNe and young PNe. A new technique is used in the infrared giving simultaneous access to the emission lines of molecular hydrogen and the recombination lines of hydrogen and helium, combining spatial and spectral resolution over the full object. Images and velocity fields in H2  1-0 S(1) are presented for the well-known proto-PN AFGL 2688 and the young PN NGC 7027. The physical conditions characterizing their inner envelopes are discussed, and a comparison is made between the morphology of the shock-excited [FORMULA] emission in the proto-PN AFGL 2688 and the ultraviolet PDR excitation of [FORMULA] in NGC 7027. A preliminary report of the present results has been given by Cox et al. (1995).

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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