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

In this paper we study the oxygen chemistry in the C-rich circumstellarshells of IRC+10216. The recent discoveries of oxygen bearing species (water,hydroxyl radical and formaldehyde) toward this source challenge our currentunderstanding of the chemistry in C-rich circumstellar envelopes. The presenceof icy comets surrounding the star or catalysis on iron grain surfaces havebeen invoked to explain the presence of such unexpected species. This detailedstudy aims at evaluating the chances of producing O-bearing species in theC-rich circumstellar envelope only by gas phase chemical reactions. For theinner hot envelope, it is shown that although most of the oxygen is locked inCO near the photosphere (as expected for a C/O ratio greater than 1), somestellar radii far away species such as H2O and CO2 have large abundances underthe assumption of thermochemical equilibrium. It is also shown how non-LTEchemistry makes very difficult the CO-->H2O,CO2 transformation predicted inLTE. Concerning the chemistry in the outer and colder envelope, we show thatformaldehyde can be formed through gas phase reactions. However, in order toform water vapor it is necessary to include a radiative association betweenatomic oxygen and molecular hydrogen with a quite high rate constant. Thechemical models explain the presence of HCO+ and predict the existence of SOand H2CS (which has been detected in a 3 mm line survey to be published). Wehave modeled the line profiles of H2CO, H2O, HCO+, SO and H2CS using anon-local radiative transfer model and the abundance profiles predicted by ourchemical model. The results have been compared to the observations anddiscussed.Comment: 20 pages, 9 figures, accepted for publication in the Astrophysical Journa