First observations of an R Coronae Borealis star with the space telescope imaging spectrograph: RY Sagittarii near maximum light

We describe the far-UV (1140-1740 Angstrom) spectrum of the hydrogen-defici ent R Coronae Borealis (RCB) star RY Sgr, obtained near maximum light (puls ational phase similar to 0.1) by the Space Telescope Imaging Spectrograph ( STIS) on Hubble Space Telescope. The far-UV spectrum shows a photospheric c ontinuum rising steeply toward longer wavelengths and two prominent emissio n features at the shorter wavelengths: C II lambda 1335 and Cl I lambda 135 1 (the latter is radiatively fluoresced by the 10 times stronger C II multi plet). We also find evidence for CO A-X 4th-positive system absorption band heads and possible weak CO fluorescent emissions pumped by C II lambda 133 5, but the inferred column densities are low (similar to few times 10(16) c m(-2)), consistent with formation in a warm (similar to 5000 K) atmospheric layer. The detection of CO molecules, if confirmed, would be significant, because they are thought to play a key role in the dust ejection episodes o f RCB stars through the initiation of "molecular cooling catastrophes."