The systematics and distribution of molecular hydrogen in planetary nebulae.

Twenty-two planetary nebulae have been searched for the infrared lines of excited molecular hydrogen, and strong H2 lines have been detected and imaged in 11 of these. From these and published data, it has been demonstrated that all planetaries with strong excited molecular hydrogen belong to a particular subclass of the Type I planetary nebulae. This subclass is characterized morphologically by an equatorial toroid with faint bipolar extensions. Furthermore, nearly all planetaries with these characteristics have strong molecular lines. In most cases, the distribution of the excited H2 lines is indistinguishable from the red images which delineate singly ionized nitrogen (and ionized hydrogen), suggesting that the emitting molecules lie close to the transition region between ionized and neutral gas. If the excitation is by shocks, which is consistent with the line ratios and the distribution, then the observations suggest that the morphology of these planetaries has been controlled by a fast stellar wind interacting with a disk of gas concentrated in the equatorial plane.