P. Cox'
title: A CO Survey of Young Planetary Nebulae 
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Introduction
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Observations of molecular gas have played a key role in developing our current understanding of stellar evolution from the Asymptotic Giant Branch (AGB) through the planetary nebula (PN) phase. Stars evolving on the upper AGB undergo an increasing rate of mass loss which forms a dense, slowly expanding circumstellar envelope of dust and molecular gas. The mass loss eventually terminates the evolution on the AGB, and when the surface temperature of the star becomes high enough to emit a significant flux of ultraviolet radiation ($T\ga 30,000$ K), it begins to photo-ionize the circumstellar material and form an optically visible PN.

Observations of neutral gas around bona fide PNe have been crucial in the development of this general scenario, and numerous atomic and molecular species have been observed. These include H[i]{}, C[i]{}, O[i]{}, and other neutral atoms (e.g., Rodriguez, Goss, & Williams 2002; Bachiller et al. 1994; Liu et al. 2001; Dinerstein, Sneden, & Uglum 1995), as well as H$_2$ (e.g., Kastner et al. 1996), OH (e.g., Zijlstra et al. 2001), CO (e.g., Huggins et al. 1996), and other, rarer molecular species such as HCO$^+$ and CN (e.g., 