A deep search for C2H in the oxygen-rich post-AGB star OH 231.8+4.2

In carbon-rich environment, C 2 H emission is seen to be associated with HC 3 N, but whether it can trace the HC 3 N molecule in oxygen-rich environment is unknown. Here, we have searched for rotational emission from ethynyl radical (C 2 H) in the oxygen-rich circumstellar envelope (CSE) of the post-asymptotic giant branch (post-AGB) star OH 231.8+4.2, a renowned galactic proto-planetary nebula (PPN) known to display cyanoacetylene (HC 3 N) emission. Our observations were conducted using the James Clerk Maxwell Telescope (JCMT), and the total on-source time is 11.9 h. Base on local thermodynamic equilibrium (LTE) excitation analysis, we have calculated the column density and the abundance relative to H 2 for C 2 H. The calculated column density for C 2 H is less than 1.4 × 10 13 cm -2 , which corresponds to a fractional abundance, f (C 2 H), less than 4.5 × 10 –9 . This inference suggests that the typical transformation pathway from C 2 H and C 2 H 2 to HC 3 N may not play a significant role in O-rich environments. It indicates the presence of alternative, unidentified pathways that contribute to the formation of HC 3 N in O-rich circumstellar envelopes.