Detection of Near-Infrared CO Absorption Bands in R Coronae Borealis Stars

R Coronae Borealis (RCB) stars are hydrogen-deficient, carbon-rich, pulsating, post–asymptotic giant branch stars that experience massive irregular declines in brightness caused by circumstellar dust formation. The mechanism of dust formation around RCB stars is not well understood. It has been proposed that CO molecules play an important role in cooling the circumstellar gas so that dust may form. We report on a survey for CO in a sample of RCB stars. We obtained H- and K-band spectra including the first- and second-overtone CO bands for eight RCB stars, the RCB-like star DY Per, and the final-helium-flash star FG Sge. The first- and second-overtone CO bands were detected in the cooler (Teff < 6000 K) RCB stars, Z Umi, ES Aql, SV Sge, and DY Per. The bands are not present in the warmer (Teff > 6000 K) RCB stars, R CrB, RY Sgr, SU Tau, and XX Cam. In addition, first-overtone bands are seen in FG Sge, a final-helium-flash star that is in an RCB-like phase at present. Effective temperatures of the eight RCB stars range from 4000 to 7250 K. The observed photospheric CO absorption bands were compared to line-blanketed model spectra of RCB stars. As predicted by the models, the CO bands are strongest in the coolest RCB stars and not present in the warmest. No correlation was found between the presence or strength of the CO bands and dust formation activity in the stars.