THE ULTRAVIOLET EXTINCTION CURVE FOR CIRCUMSTELLAR DUST FORMED IN THEHYDROGEN-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 Co ronae Borealis star, V348 Sgr, from its 1992 minimum. These spectra ha ve been combined with spectra from the IUE archives to obtain a defini tive extinction curve for dust recently ejected from this star, and ma ke available for the first time an extinction curve of high accuracy i n the far-UV for circumstellar dust formed in a hydrogen-poor environm ent. We find that a Drude profile with a central wavelength of 2425 An gstrom and a full width at half-maximum of 1.342 mu m(-1) is able to s atisfactorily represent the circumstellar dust ejected from V348 Sgr, not just during one ejection episode, but in general. We calculated th eoretical extinction curves for comparison with the observed data usin g 400 Angstrom graphite grains and 300 Angstrom amorphous carbon grain s. 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.