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, obtained 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 emission-line spectra indicates most lines decay during the deep declines on characteristic timescales comparable to that reported for optical features. Fe, Mg, and neutral C lines decay on timescales of typically 50-100 days. Other 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 emission lines and other UV features during the declines is consistent with the E1/E2/BL line region model developed from the behavior of optical spectral features during declines. However, the detailed line behavior indicates large intrinsic variability between decline events inconsistent with the simple E1/E2/BL model. Limited temporal coverage prevents detailed examination of the geometry of the emission-line region or the obscuring dust. We also report the first detection of the transition-region line C IV] lambda 1550 in the spectrum of an RCB star. We fit the onset times of all declines from maximum light within the 1987-1998 interval, irrespective of decline amplitude, with a 43.23 day linear solution, thus improving the decline ephemeris. The linear term is probably the pulsation period of V854 Cen, further supporting the suspected link between radial pulsations and mass loss in these types of stars.