Gas-phase Molecules in Protoplanetary Nebulae with the 21 m Emission Feature

It has been more than 30 years since the enigmatic 21 mu m emission feature was first discovered in protoplanetary nebulae (PPNs). Although dozens of different dust carrier candidates have been proposed, there is as yet no widely accepted one. We present the results of molecular observations toward 21 mu m objects using the 10 m Submillimeter Telescope of Arizona Radio Observatory in the 1.3 mm band and the 13.7 m telescope of Purple Mountain Observatory in the 3 mm band, aiming to investigate whether the gas-phase environments of these unusual sources have some peculiarities compared to normal PPNs. We detect 31 emission lines belonging to seven different molecular species, most of which are the first detection in 21 mu m PPNs. The observations provide clues to the identification of the 21 mu m feature. We report a correlation study between the fractional abundance of gas-phase molecules and the strengths of the 21 mu m emission. Our study shows that, given the small sample size, the 21 mu m feature has weak or no correlations with the gas-phase molecules. Future radio observations of high spatial and spectral resolution toward a large sample are desirable to elucidate the 21 mu m emission phenomena.