We describe the far-UV (1140-1740 Å) spectrum of the hydrogen-deficient R Coronae Borealis (RCB) star RY Sgr, obtained near maximum light (pulsational phase ≯0.1) by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope. The far-UV spectrum shows a photospheric continuum rising steeply toward longer wavelengths and two prominent emission features at the shorter wavelengths: C II λ1335 and Cl I λ1351 (the latter is radiatively fluoresced by the 10 times stronger C II multiplet). We also find evidence for CO A-X 4th-positive system absorption band heads and possible weak CO fluorescent emissions pumped by C II λ1335, but the inferred column densities are low (∼few times 1016 cm-2), consistent with formation in a warm (∼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 of RCB stars through the initiation of "molecular cooling catastrophes.".
Publication Source (Journal or Book title)
Clayton, G., Ayres, T., Lawson, W., Drilling, J., Woitke, P., & Asplund, M. (1999). First observations of an R Coronae Borealis star with the space telescope imaging spectrograph: RY sagittarii near maximum light. Astrophysical Journal, 515 (1 PART 1), 351-355. https://doi.org/10.1086/307028