The unusual ultraviolet chromospheric spectrum of the R coronae borealis star, V854 centauri (NSV 6708), at minimum light
The first IUE observations of a decline of the recently discovered R CrB star, V854 Cen, are reported. These observations are of particular interest because they include spectra taken at 7 mag below maximum light This is the deepest minimum ever observed in an R CrB star with the IUE. The "chromospheric" emission spectra show striking differences from UV spectra of other R CrB stars in decline. In fact, the emission spectrum of V854 Cen at minimum light does not seem to have an analogue in any known emission-line object Strong emission appears in several lines not normally seen in R CrB emission-line spectra, in particular at C II] λ2326, Mg I λ2852, and C i λλ2965, 2967. Spectral evolution similar to that seen in visible spectra of R CrB stars is clearly seen in the ultraviolet for the first time. The differences between V854 Cen and other R CrB stars may be related to known abundance differences or to different temperature and density conditions in the emission-line regions of the various R CrB stars. The emission may also be significantly affected by the presence of dust in or near the emitting region.