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

TL;DR

This study models hydrogen cyanide formation in the inner envelopes of TP-AGB stars by coupling shock dynamics with chemo-kinetics, successfully matching observed HCN abundances and revealing the importance of shock proximity and chemistry in these environments.

Contribution

It introduces a coupled shock and chemo-kinetic model for TP-AGB stars, calibrated with observations, to explain HCN formation and its dependence on stellar evolution and shock properties.

Findings

Models match observed HCN abundances across different mass-loss rates.

HCN concentration increases along the chemical sequence of TP-AGB stars.

Shocks likely originate near the photosphere and are predominantly isothermal.

Abstract

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