Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow

Abstract

Publisher rights © 2018 ESO. This work is made available online in accordance with the publisher's policies. Please refer to any applicable terms of use of the publisher. General rightsCopyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. ABSTRACTContext. In the inner regions of asymptotic giant branch (AGB) outflows, several molecules have been detected with abundances much higher than those predicted from thermodynamic equilibrium chemical models. The presence of the majority of these species can be explained by shock-induced non-equilibrium chemical models, where shocks caused by the pulsating star take the chemistry out of equilibrium in the inner region. Moreover, a non-uniform density structure has been detected in several AGB outflows. Both largescale structures, such as spirals and disks, and small-scale density inhomogeneities or clumps have been observed. These structures may also have a considerable impact on the circumstellar chemistry. A detailed parameter study on the quantitative effects of a non-homogeneous outflow has so far not been performed. Aims. We examine the effects of a non-uniform density distribution within an AGB outflow on its chemistry by considering a stochastic, clumpy density structure. Methods. We implement a porosity formalism for treating the increased leakage of light associated with radiation transport through a clumpy, porous medium. We then use this method to examine the effects from the altered UV radiation field penetration on the chemistry, accounting also for the increased reaction rates of two-body processes in the overdense clumps. The specific clumpiness is determined by three parameters: the characteristic length scale of the clumps at the stellar surface, the clump volume filling factor, and the inter-clump density contrast. In this paper, the clumps are assumed to have a spatially constant volume filling factor, which implies that they expand as they move outward in the wind.Results. We present a parameter study of the effect of clumping and porosity on the chemistry throughout the outflow. Both the higher density within the clumps and the increased UV radiation field penetration have an important impact on the chemistry, as they both alter the chemical pathways throughout the outflow. The increased amount of UV radiation in the inner region leads to photodissociation of parent species, releasing the otherwise deficient elements. We find an increased abundance in the inner region of all species not expected to be present assuming thermodynamic equilibrium chemistry, such as HCN in O-rich outflows, H 2 O in C-rich outflows, and NH 3 in both. Conclusions. A non-uniform density distribution directly influences the chemistry throughout the AGB outflow, both through the density structure itself and through its effect on the U...

The Role of Internal Photons on the Chemistry of the Circumstellar Envelopes of AGB Stars

“…We have presented the first model calculations that include the chemical effects of internal UV photons, here assumed to be blackbody radiation from cool AGB stars using the porosity formalism introduced by Van de Sande et al (2018b) and the CSE chemistry code from McElroy et al (2013). For the radiation fields used here, we find that internal photons are essentially unimportant at 10 −5 M ⊙ yr −1 due to the large value of dust extinction.…”

“…At every radial distance r, we calculate the effective dust extinction in the porosity formalism, A eff V , in terms of the optical depth for a uniform outflow, A V . The equations for both a one-component model, where the inter-clump medium is void, and a two-component model, where mass is distributed in both a clump and inter-clump medium, are given in Appendix C of Van de Sande et al (2018b). The internal photon flux is diluted geometrically and extinguished by dust.…”

The mass-loss, expansion velocities, and dust production rates of carbon stars in the Magellanic Clouds

“…Some of the observed stars are likely to be characterised by a clumpy structure (Sloan et al 2016). Van de Sande et al (2018) showed that the optical depth is lower for clumpy mediums than in the homogeneous case for the same value of massloss rate. Thus, in case the medium is non-homogeneous, the DPR is probably underestimated under our assumptions.…”

An ALMA view of CS and SiS around oxygen-rich AGB stars

“…It is also possible that a clumpy circumstellar medium accounts for some of the differences we see between the stars in our sample. For example, Van de Sande et al (2018b) show that clumpy outflows can produce a significant amount of CS. Various channel maps of other molecules towards R Dor show arc-like structures in the CSE rather than a smooth outflow, indicating a level of clumpiness in the outflow.…”