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

6. H₂ emission in the proto-PN to PN transition

Both AFGL 2688 and NGC 7027 are surrounded by massive envelopes of molecular gas, and clearly belong to the class of objects which retain their neutral envelopes well into the PN phase (see Huggins et al. 1996). The surface brightness of H₂ is quite similar in the two cases, but the observations reported here indicate that the distribution of the emission and the excitation mechanisms are different, and this can be partly understood in terms of their different stages of evolution. According to Jura & Kroto (1990), AFGL 2688 evolved beyond the AGB phase about 200 years ago, and the spectral type is now F5. NGC 7027 is significantly more evolved. The kinematic age of the ionized nebula is  600 yr (Masson 1989) and the central star has a surface temperature of 200,000 K.

In AFGL 2688 the central star is still too cool to ionize the inner cavity, but high-velocity winds are present and must impinge on the more slowly moving material ejected during the AGB phase. The molecular hydrogen emission in the bipolar lobes and the localized regions near the equator presumably trace the inner surfaces of the envelope material where the winds interact.

The further evolution of such an object will lead to rapid expansion of the fast outflows in the directions of least density (along the polar axis), and as the temperature of the central star increases the inner cavity will be ionized. The ionization will also propagate most rapidly along the directions of least density. In addition, the UV radiation of the central star will develop PDRs in the interface regions between the ionized and neutral gas and will give rise to emission. The equatorial-polar density contrast will lead to morphologies of the ionized nebula and the surrounding emission region similar to that observed in NGC 7027.

There is now no fast neutral gas observed in NGC 7027, which is consistent with observations of other objects which show that the fast neutral winds are most prominent in the proto-PN phase. It is not unlikely that there was a fast wind at an earlier phase, although the effects on the morphology seem to be modest, in the sense that the gas in the polar directions has not been substantially swept away as appears to be the case in the extreme butterfly nebulae. However, the cavity in the neutral gas discussed in Sect. 4 may have resulted from wind activity. In AFGL 2688 the polar lobes seem to be well defined already, and it may well develop into a butterfly type planetary nebulae. Objects at intermediate stages of evolution between AFGL 2688 and NGC 7027 are likely to have emission from both PDR and shock excited gas and will need careful study to sort out the details.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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