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 CH 3 OH and C 2 H in two oxygen-rich stars (IK Taurus and TX Camelopardalis) carried out with the Owens Valley Millimeter Array. Neither CH 3 OH nor C 2 H 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 C 2 H of 9:7 ; 10 À9 in IK Tau and 3:2 ; 10 À8 in TX Cam and also place upper limits on the fractional abundance of CH 3 OH of 4:5 ; 10 À9 in IK Tau and 1:1 ; 10 À8 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 00 . 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 AE 0:4 mJy for IK Tau and 3:7 AE 0:4 mJy for TX Cam. The continuum emission is unresolved at the %3 00 level, as expected from a model for the emission mechanism proposed by Reid and Menten.

Circumstellar molecular composition of the oxygen-rich AGB star IK Tauri

“…Chemical abundance stratifications are coupled to theoretical non-equilibrium (non-TE) predictions in the outer envelope by Willacy & Millar (1997) and compared to the shock-induced non-TE inner wind predictions by Duari et al (1999) and Cherchneff (2006). The star IK Tau has been chosen for study because of the wealth of observations which are available for this target and because its envelope is thought to be (roughly) spherically symmetric (Lane et al 1987;Marvel 2005;Hale et al 1997;Kim et al 2010). …”

Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen byHerschel/HIFI

“…In the first instance, the kinetic temperature and velocity structure of the envelope were calculated by solving the equations of motion of gas and dust and the energy balance simultaneously. To get insight into the structure in the inner wind region, the HCN J = 3-2 and J = 4-3 transitions were used, since observational evidence exists that HCN is formed close to the star ( < ∼ 3.85 , Marvel 2005). The Gaussian HCN line profiles indeed point toward line formation partially in the inner wind where the stellar wind has not yet reached its full terminal velocity (Bujarrabal & Alcolea 1991).…”

Circumstellar molecular composition of the oxygen-rich AGB star IK Tauri

“…To get the mean brightness temperature estimates, the spectra were corrected by the beam-filling factors assuming a CO source size of 17 (Bujarrabal & Alcolea 1991), an HCN source size of 3.85 (Marvel 2005) and source sizes for the other molecules of 2.2 (Lucas et al 1992). The CO size may be uncertain, probably underestimated, since the signal-to-noise (S/N) ratios of the profiles obtained by Bujarrabal & Alcolea (1991) are much lower than those of the CO profiles presented in this paper.…”

“…Interpreting the molecular emission profiles of VY CMa is therefore a very complex task, subject to many uncertainties. To enlarge our insight into the chemical structure in the envelopes of oxygen-rich low and intermediate-mass stars, we therefore have started a submillimeter survey of the oxygen-rich AGB star IK Tau, which is thought to be (roughly) spherically symmetric (Lane et al 1987;Marvel 2005). We thereby hope to advance understanding of the final stages of stellar evolution of the majority of stars in galaxies like our Milky Way and their resulting impact on the interstellar medium and the cosmic cycle.…”