2025AJ....170..230Q

The carrier of the 21 μm emission feature discovered in a handful of protoplanetary nebulae (PPNe) is one of the most intriguing enigmas in circumstellar chemistry. Investigating the gas-phase molecules in PPNe could yield important hints for understanding the 21 μm feature. In this paper, we report observations of the carbon monosulfide (CS) J = 5 → 4 line at 245 GHz and the CO J = 1 → 0 line at 115 GHz toward seven PPNe exhibiting the 21 μm feature. We find that CS is extremely scarce in these PPNe and the CS line is only detected in one source, IRAS Z02229+6208. Based on the assumption of local thermal equilibrium and negligible optical depth, we derive that the CS column densities and fractional abundances relative to H₂ are N(CS) < 9.1 × 10¹³ cm^–2 and f(CS) < 8.1 × 10^–7. A comparison of the CS abundances across different circumstellar envelopes reveals that the variations in CS abundance are complex, depending not only on the evolutionary stages but also on the properties of individual objects.