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

Monthly Notices of the Royal Astronomical Society Volume 456, Issue 1, 2016, Pages 23-46 Connecting the evolution of thermally pulsing asymptotic giant branch stars to the chemistry in their circumstellar envelopes - I. Hydrogen cyanide (Article) Marigo, P.a , Ripamonti, E.a, Nanni, A.a, Bressan, A.b, Girardi, L.c a Department of Physics and Astronomy G. Galilei, University of Padova, Vicolo dell'Osservatorio 3, Padova, Italy b Astrophysics Sector, SISSA, Via Bonomea 265, Trieste, Italy c Astronomical Observatory of Padova - INAF, Vicolo dell'Osservatorio 5, Padova, Italy View additional affiliations View references (104) 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 rs,0 and the effective adiabatic index γeff ad) 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, which traces the increase of the surface C/O ratio. The chemical calibration brings along two important implications for the physical properties of the pulsation-induced shocks: (i) the first shock should emerge very close to the photosphere (rs,0 ≃ 1R), and (ii) shocks are expected to have a dominant isothermal character (γ eff ad ≃ 1) in the denser region close to the star (within ~ 3-4R), 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. Given the notable sensitiveness of the results to stellar parameters, this paper shows that such chemodynamic analyses may indeed provide a significant contribution to the broader goal of attaining a comprehensive calibration of the TP-AGB evolutionary phase.