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

Abstract

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

Chemical content of the circumstellar envelope of the oxygen-rich AGB star R Doradus

“…Gobrecht et al (2016) have modelled the inner wind region up to the dust formation zone and found SiO abundances that corresponded well with the observations of Decin et al (2010). Li et al (2016) have modelled the chemistry in the outer wind, using a gas-phas only chemical network. They assumed SiO to be one of the parent species, i.e.…”

“…These profiles contain information on the ongoing circumstellar chemistry of a given molecular species, e.g. depletion of the species onto dust grains, and may be confronted with forward chemistry models that predict the abundance stratification of molecules throughout the stellar wind (see, e.g., Gobrecht et al 2016, Li et al 2016). Such models require certain stellar and dynamical parameters together with a set of parent species and their initial abundances as input, and solve a chemical network.…”

“…The initial abundance in the chemical network was assumed to be that measured by Decin et al (2010). Li et al (2016) found a decrease of approximately an order of magnitude at ∼ 1000 R * , mainly caused by photodissociation. We find that the decline in abundance for R Dor starts at more than an order of magnitude in radius closer to the central star.…”

Circumstellar ammonia in oxygen-rich evolved stars

“…Furthermore, circumstellar chemical models have to 'inject' NH 3 at a high abundance (∼10 −6 -10 −5 ; as derived from observations) in the inner CSE (r < 10 16 cm) in order to explain the observed abundances of NH 3 and other nitrogenbearing molecules (e.g. Nercessian et al 1989;Willacy & Millar 1997;Gobrecht et al 2016;Li et al 2016). …”

“…On the other hand, if the CSE is smooth, nitrogen molecules at the inner radii may be shielded from dissociative photons by dust, H, H 2 , CO, and N 2 itself (Li et al 2013). Recent calculations by Li et al (2016) have shown that self-shielding of N 2 may shift its photodissociation region by about seven times further, to r∼10 17 cm in O-rich CSEs. The CO imaging survey by Castro-Carrizo et al (2010) has indicated a certain degree of clumpiness in AGB stellar winds.…”