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

R Coronae Borealis (RCB) stars are hydrogen-deficient, carbon-rich pulsating postAGB 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 Hand 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, 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. Subject headings: R Coronae Borealis stars — circumstellar matter — astrochemistry — dust Department of Chemistry, Pomona College, 645 N. College Avenue, Seaver North, Claremont, CA 91711-6338; [email protected] Maria Mitchell Observatory, 3 Vestal Street, Nantucket , MA 02554 Department of Physics & Astronomy, Louisiana State University, Baton Rouge, LA 70803; [email protected] Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australia; [email protected] Steward Observatory, University of Arizona, Tucson, AZ 85721; cengelbracht, [email protected] Department of Physics, University of Cincinnati, Cincinnati, OH, 45221-0011; [email protected] The Aerospace Corporation, M2/266, P.O. Box 92957, Los Angeles, CA 90009; david.k.lynch, richard.j.rudy, steve.mazuk, [email protected] UC, San Diego, CASS 0424, 9500 Gilman Dr., La Jolla, CA 92093-0424; [email protected]