The fall and rise of V854 Centauri: Long-term ultraviolet spectroscopy of a highly active R Coronae Borealis star

We examine long-term low-dispersion International Ultraviolet Explorer SWP and LWP spectroscopy of the R Coronae Borealis (RCB) star V854 Cen, obtaine d across the deep (Delta V > 6 mag) 1991, 1992-1993, and 1994 declines. We also report the optical light curve for the star in the interval 1987-1998, including multicolor photometry obtained during 1989-1998. The light curve includes at least eight major declines where the amplitude exceeds 5 mag, many of which appear to be multiple decline events. Analysis of the UV emis sion-line spectra indicates most lines decay during the deep declines on ch aracteristic timescales comparable to that reported for optical features. F e, Mg, and neutral C lines decay on timescales of typically 50-100 days. Ot her lines, notably ionized C lines, decay on longer timescales (>200 days) or appear to be unaffected by the declines. The general nature of the UV em ission lines and other UV features during the declines is consistent with t he E1/E2/BL line region model developed from the behavior of optical spectr al features during declines. However, the detailed line behavior indicates large intrinsic variability between decline events inconsistent with the si mple E1/E2/BL model. Limited temporal coverage prevents detailed examinatio n of the geometry of the emission-line region or the obscuring dust. We als o report the first detection of the transition-region line C IV] lambda 155 0 in the spectrum of an RCB star. We fit the onset times of all declines fr om maximum light within the 1987-1998 interval, irrespective of decline amp litude, with a 43.23 day linear solution, thus improving the decline epheme ris. The linear term is probably the pulsation period of V854 Cen, further supporting the suspected link between radial pulsations and mass loss in th ese types of stars.