A radio molecular line search for evidence of an O-rich environment around binary in silicate carbon stars

We report observations of oxygen-rich (O-rich) radio molecular line tracers, including H₂CO, H₂O, and HCO⁺ toward 28 silicate carbon stars (SCSs). SCSs are special evolved star systems that consist of a carbon star associated with peculiar O-rich circumstellar material. Absorption in the 6 cm H2CO transition was detected from seven SCSs using the Shanghai 65 m Tianma radio telescope. In order to further investigate their O-rich material, several 3 mm molecular lines including HCO⁺ and CO were also observed with the Purple Mountain Observatory 13.7 m telescope. Double-peak profiles in some of these lines suggest the possible presence of a companion in the SCS system. A negative association between H₂O masers and H₂CO absorption was found in our sample of SCSs. The H₂O maser sources may correspond to an earlier evolutionary stage when the circumstellar envelope (CSE) material is O-rich and limited to a compact and denser region. As the star evolves the CSE gradually expands and carbon abundance is enhanced, resulting in the H₂O masers disappearing and H₂CO absorption arising.