The Fall and Rise of V854 Centauri: Long-Term Ultraviolet Spectroscopy of a Highly Active R Coronae Borealis Star

Abstract

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 (*V [ 6 mag) 1991, 1992È 1993, and 1994 declines. We also report the optical light curve for the star in the interval 1987È1998, including multicolor photometry obtained during 1989È1998. 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 50È100 days. Other lines, notably ionized C lines, decay on longer timescales ([200 days) or appear to be una †ected 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 Ðrst detection of the transition-region line C IV] j1550 in the spectrum of an RCB star. We Ðt the onset times of all declines from maximum light within the 1987È1998 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.

Pulsation-decline relationships in R Coronae Borealis stars

“…We found that all of the major declines, and most of the minor ones (excluding those for which the AAVSO sampling was poor), could be tied to the respective pulsation periods. Our results, therefore, substantiate the pulsation-decline connections claimed for RY Sgr and V854 Cen by Pugach (1977) and Lawson et al (1992Lawson et al ( , 1999, respectively, and significantly extend the relationship through the introduction of three other RCB stars (UW Cen, R CrB and S Aps) that show the same type of behaviour. This work firmly demonstrates the link between radial pulsations and the ejection of material in the RCB stars.…”

“…RCB stars with similar physical and pulsational properties include RZ Nor, RS Tel, RT Nor, SU Tau, GU Sgr and XX Cam, but unfortunately these objects are not as well observed by the Decline onsets used to determine the decline ephemeris are marked with crosses. Grey crosses identify decline onsets from Lawson et al (1992Lawson et al ( , 1999, whereas black crosses are decline onsets identified in this paper.…”

“…1 shows the AAVSO light curve of this active RCB star that has experienced 18 major declines (declines from maximum light) since its discovery in 1986. The obscuration events listed by Lawson et al (1999) and used in their analysis are marked by grey crosses while the various additional declines, including five major and four minor events (declines that occurred when the star was fainter than maximum light) reported here are indicated in black. These new events include two minor declines during 1992 and 1994 not listed by Lawson et al…”

“…Table 1 contains a complete listing of event times for V854 Cen. For those events listed by Lawson et al (1999), we retained their event times but updated the ephemeris-fitted 'calculated' time to produce revised 'observed − calculated' or O − C residuals using our new solution. Our new O − C values differ from previous values by only ±1 d. The O − C residuals listed in Table 1 have a 1σ scatter of 3.1 d. Only one of the 23 event times falls outside the 2σ limit.…”

“…Eight decline onset times between 1987 and 1991 were fitted with a linear solution with a ≈43.2-d period. Later, Lawson et al (1999) extended this work to fit 15 declines between 1987 and 1998 with a revised ephemeris.…”