Ultraviolet observations of the hot R Coronae Borealis-type star V348 sagittarii during a deep minimum
We obtained three high-quality ultraviolet spectra of the hot R Coronae Borealis type star V348 Sgr using the Faint Object Spectrograph of the Hubble Space Telescope (HST). The first of these spectra was obtained when the star was in a shallow minimum (ΔV less than 1 mag), whereas the other two were obtained during a deep minimum (ΔV ≈ 6 mag). While strong carbon and helium emission lines are seen in the shallow minimum, during the deep minimum these lines are apparently eclipsed by a dust cloud. In the first deep minimum spectrum a few weak nebular emission lines are present, but in the second (a month later) these lines have mostly disappeared. This is consistent with a progressive eclipse caused by an expanding dust cloud. Comparison with International Ultraviolet Explorer (IUE) spectra obtained at maximum light allowed three extinction curves to be calculated for the newly formed dust. The extinction curves obtained using the spectra at deep minimum are quite different from those obtained using spectra at shallow minima. Extinction curves from shallow minima (which have been previously calculated using IUE data) show stellar flux attenuated by amorphous carbon dust in front of the star at wavelengths above 2000 Å, while at wavelengths below 2000 Å they show contributions from light scattered by dust outside the line of sight. During the deep minimum, however, nearly all of the light seen in the ultraviolet below 3200 Å is due to scattering into the field of view. The data are consistent with (1) an optically thick cloud of amorphous carbon dust in front of the star and (2) either a few additional optically thick clouds that are outside a direct line from the star to the earth, or an optically thin cloud that surrounds a small fraction of the star and scatters light into the HST field of view. The data taken together are consistent with the dust forming close to the star.