The formation radius of HCN in O-rich asymptotic giant branch stars
Molecules detected in circumstellar envelopes around asymptotic giant branch (AGB) stars are generally well explained in terms of formation under chemical equilibrium in the stellar atmosphere or photochemistry in the outer expanding layers. However, several molecules are detected in the inner circumstellar regions with abundances orders of magnitude above the predictions of chemical equilibrium. Anomalously abundant molecules comprise H2O, NH3, SiH4, and PH3 in C-rich envelopes and HCN, CS, and NH3 in O-rich sources. These molecules are formed by some yet unknown nonequilibrium processes. Aims: We aim to shed light on the nonequilibrium process that enhances the abundance of anomalously abundant molecules in the inner regions of circumstellar envelopes. Here, we focus on HCN in O-rich stars to constrain its formation radius. Method: We observed the O-rich AGB stars RCrt and IKTau with ALMA at high angular resolution (HPBW=~0.05") in the J=4-3 line of HCN in both the ground and the \nu_2=1 vibrational states. The radial distribution of the emission was modeled using a large velocity gradient radiative transfer code in which we considered an abundance profile with a central hole. Results: The models with a hole in the abundance distribution of HCN reproduce the specific shape of the radial emission distribution around the two studied O-rich stars. The best models locate the formation radius of HCN at 4-6 Rstar in RCrt and 3-5 Rstar in IKTau. Conclusion: The formation radius of HCN inferred from observations is consistent with models that invoke shocked-induced chemistry but also with models where photochemistry acts as the solely disequilibrium process. High angular resolution observations of more O-rich stars and tailored shocked-induced and/or photochemical models are needed to unambiguously unveil the underlying mechanism behind anomalously abundant molecules in AGB envelopes.