The ultraviolet extinction curve for circumstellar dust formed in the hydrogen-poor environment of V348 Sagittarii

New International Ultraviolet Explorer (IUE) spectra with high signal-to-noise ratios have been obtained during the recovery of the hot R Coronae Borealis star, V348 Sgr, from its 1992 minimum. These spectra have been combined with spectra from the IUE archives to obtain a definitive extinction curve for dust recently ejected from this star, and make available for the first time an extinction curve of high accuracy in the far-UV for circumstellar dust formed in a hydrogen-poor environment. We find that a Drude profile with a central wavelength of 2425 Å and a full width at half-maximum of 1.342 μm-1 is able to satisfactorily represent the circumstellar dust ejected from V348 Sgr, not just during one ejection episode, but in general. We calculated theoretical extinction curves for comparison with the observed data using 400 Å graphite grains and 300 Å amorphous carbon grains. It is clear that scattering is very important at short wavelengths. Neither type of grain works very well when scattering is not included. The amorphous carbon fits are somewhat better than the graphite fits. The data seem to be more consistent with a cloud covering the entire star, and not just lying in our line of sight. This contradicts other empirical evidence that points toward the formation of patchy clouds around R Coronae Borealis stars. © 1997. The American Astronomical Society. All rights reserved