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G.C. Clayton (LSU), M.M. Hanson (U. Cincinnati), K.D. Gordon (Steward Obs.), T.R. Ayres (U. Colorado)
It has been known for 60 years that the declines of R Coronae Borealis (RCB) stars are caused by circumstellar dust formation. The declines occur suddenly and without warning. No spectroscopic changes have yet been noted near the beginning of the decline, which might be linked to the cooling gas that must be condensing to form the dust. The mechanism of mass loss and dust formation is still a mystery. New observational evidence along with models of the carbon chemistry around RCB stars suggest that dust may condense close to the star. Shocks propagating through the outer atmospheres encourage non-equilibrium conditions where the conditions for carbon nucleation may be present. CO is thought to be a critical gas coolant in this process. We report detections of the CO 4th positive system lines in STIS UV spectra of two RCB stars and of the CO v=2 bands in the near-IR for several RCB stars. These observations are being used to model the temperature and density of condensing gas around these stars.