Oxygen Chemistry in the Circumstellar Envelope of the Carbon‐Rich Star IRC +10216

Abstract

In this paper we study the oxygen chemistry in the C-rich circumstellar shells of IRC +10216. Therecent discoveries of oxygen-bearing species (water, hydroxyl radical, and formaldehyde) toward thissource challenge our current understanding of the chemistry in C-rich circumstellar envelopes. Thepresence of icy comets surrounding the star or catalysis on iron grain surfaces have been invoked toexplain the presence of such unexpected species. This detailed study aims at evaluating the chancesof producing O-bearing species in the C-rich circumstellar envelope only by gas-phase chemicalreactions. For the hot inner envelope we show that although most of the oxygen is locked in CO nearthe photosphere (as expected for a C/O ratio greater than 1), for radial distances larger than ~15stellar radii, species such as H 2 O and CO 2 have a large abundance under the assumption ofthermochemical equilibrium. It is also shown how non-LTE chemistry makes the CO ? H 2 O, CO 2transformation predicted in LTE very difficult. Concerning the chemistry in the colder, outerenvelope, we show that formaldehyde can be formed through gas-phase reactions. However, in order toform water vapor, it is necessary to include a radiative association between atomic oxygen andmolecular hydrogen with quite a high rate constant. The chemical models explain the presence of HCO+ and predict the existence of SO and H 2 CS (which has been detected in a 3 mm line survey to bepublished). We have modeled the line profiles of H 2 CO, H 2 O, HCO + , SO, and H 2 CS using anonlocal radiative transfer model and the abundance profiles predicted by our chemical model. Theresults have been compared to the observations and discussed.