No Methane Here. The HCN Puzzle: Searching for CH3OH and C2H in Oxygen-rich Stars

Abstract

I present results from a sensitive imaging search for CH3OH and C2H in two oxygen-rich stars (IK Taurus and TX Camelopardalis) carried out with the Owens Valley Millimeter Array. Neither CH3OH nor C2H are detected in these stars, in agreement with single-dish observations by Charnley and Latter. The observations place upper limits on the fractional abundance of C2H of 9.7 × 10-9 in IK Tau and 3.2 × 10-8 in TX Cam and also place upper limits on the fractional abundance of CH3OH of 4.5 × 10-9 in IK Tau and 1.1 × 10-8 in TX Cam. A chemical model developed by Charnley and coworkers explaining the presence of HCN in these objects is clearly ruled out by these observations. The HCN distribution in these oxygen-rich stars has been mapped with a resolution of ≈3''. The HCN emission is marginally resolved, centrally concentrated, and does not appear to be well fitted by a thin-shell model. Continuum fluxes for the sources have been measured. At 96.7 GHz, the observed continuum fluxes were 6.1 ± 0.4 mJy for IK Tau and 3.7 ± 0.4 mJy for TX Cam. The continuum emission is unresolved at the ≈3'' level, as expected from a model for the emission mechanism proposed by Reid and Menten.