Observations of High Rotational CO Lines in Post–Asymptotic Giant Branch Stars and Planetary Nebulae

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Abstract
We have observed bright emission in high rotational CO lines (J = 13-12, 17-16, and 22-21) in the post-AGB object GL 2688 and the planetary nebulae, GL 618 and NGC 7027. These stars represent different evolutionary stages, as reflected by their effective temperatures. The observations reveal copious amounts of warm, dense molecular gas (n ~ 107 cm-3; T ~ 103 K), implying that the AGB is terminated by a short-lived phase of high mass-loss rate. We investigate heating mechanisms for these objects. For NGC 7027, UV photons from the central star produce a photodissociation region and the gas cools via CO rotational transitions, as well as via fine-structure atomic lines. A shock is thought to heat the gas in GL 2688, which cools mainly via the [O I] 63 μm line and molecular rotational lines. For GL 618, both mechanisms may coexist.