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We investigate the formation of hydrogen cyanide (HCN) in the innercircumstellar envelopes of thermally pulsing asymptotic giant branch (TP-AGB)stars. A dynamic model for periodically shocked atmospheres, which includes anextended chemo-kinetic network, is for the first time coupled to detailedevolutionary tracks for the TP-AGB phase computed with the COLIBRI code. Wecarried out a calibration of the main shock parameters (the shock formationradius and the effective adiabatic index) using the circumstellar HCNabundances recently measured for a populous sample of pulsating TP-AGB stars.Our models recover the range of the observed HCN concentrations as a functionof the mass-loss rates, and successfully reproduce the systematic increase ofHCN moving along the M-S-C chemical sequence of TP-AGB stars, that traces theincrease of the surface C/O ratio. The chemical calibration brings along twoimportant implications: i) the first shock should emerge very close to thephotosphere, and ii) shocks are expected to have a dominant isothermalcharacter in the denser region close to the star (within ~ 3-4 R), implyingthat radiative processes should be quite efficient. Our analysis also suggeststhat the HCN concentrations in the inner circumstellar envelopes are criticallyaffected by the H-H2 chemistry during the post-shock relaxation stages.

monthly notices of the royal astronomical society

Connecting the evolution of thermally pulsing asymptotic giant branch  stars to the chemistry in their circumstellar envelopes -- I. The case of  hydrogen cyanide