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Detection of Near-IR CO Absorption Bands in R Coronae Borealis Stars
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Detection of Near-IR CO Absorption Bands in R Coronae Borealis Stars
Emily D. Tenenbaum1,2, Geoffrey C. Clayton2,3, Martin Asplund4, C.W. Engelbracht5, Karl D.
Gordon5, M. M. Hanson6, Richard J. Rudy7, David K.Lynch7 , S. Mazuk7, Catherine C.
Venturini7, and R. C. Puetter8
ABSTRACT
R Coronae Borealis (RCB) stars are hydrogen-deficient, carbon-rich pulsating post-
AGB stars that experience massive irregular declines in brightness caused by circum-
stellar 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, 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
1Department of Chemistry, Pomona College, 645 N. College Avenue, Seaver North, Claremont, CA 91711-6338;
emilytenenbaum@
2Maria Mitchell Observatory, 3 Vestal Street, Nantucket , MA 02554
3Departme
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