Chemistry and distribution of daughter species in the circumstellar envelopes of O-rich AGB stars

摘要: Context. Thanks to the advent of Herschel and ALMA, new high-quality observations molecules present in circumstellar envelopes asymptotic giant branch (AGB) stars are being reported that reveal large differences from existing chemical models. New molecular data more comprehensive models chemistry now available. Aims. The aims determine study important formation destruction pathways O-rich AGB provide reliable predictions abundances, column densities, radial distributions for potentially detectable species with physical conditions applicable envelope surrounding IK Tau. Methods. We use a gas-phase model an including effects CO N 2 self-shielding spherical geometry newly compiled list inner-circumstellar parent derived detailed modeling observations. trace dominant expanding investigate as probe physics phase by studying variations abundances mass-loss rates expansion velocities. Results. find pattern daughter forming photodissociation products contributions ion-neutral abstraction dissociative recombination. outer zones differs traditional PDRs photoionization plays significant role. With proper treatment self-shielding, → C + transitions shifted outward factors 7 2, respectively, compared earlier An upper limit on abundance CH 4 (≲2.5 × 10 -6 respect H ) is found Tau, several observable relatively simple links other determined. assumed stellar rate, particular, has impact calculated cations peak-abundance radius both neutrals: rate increases, peak generally decreases all moves outwards. varying velocity cosmic-ray ionization not significant. Conclusions.