Pulsation-decline relationships in R Coronae Borealis stars

Abstract

Decline onset times were measured in long‐term visual light curves for five R Coronae Borealis (RCB) variable stars. These included RY Sgr and V854 Cen, the two RCB stars previously reported to have a relationship between dust formation events and pulsational variations. Analysis of the decline epochs showed that all decline onsets for a given star obey a linear ephemeris tied to the object's dominant radial pulsation period. Thus, in addition to confirming the pulsation–decline correlation for RY Sgr and V854 Cen, this same behaviour was demonstrated in UW Cen, R CrB and S Aps for the first time. This general result firmly establishes the connection between radial pulsation and mass loss in the RCB stars. The dominant pulsation period of ≈40 d for all of these objects therefore represents the characteristic time‐scale on which these stars produce dust.

Very Large Telescope Interferometer observations of the dust geometry around R Coronae Borealis stars★

“…The onset of dust formation has been found to be correlated with the pulsational phase in five RCB stars, including RY Sgr and V854 Cen (Pugach 1977; Lawson et al 1999; Crause et al 2007). RCB stars’ pulsation amplitudes are at most a few tenths of a magnitude and their periods are 40–100 d (Lawson et al 1990; Lawson & Cottrell 1997).…”

Mid-infrared variations of R Coronae Borealis stars

“…In the absence of traditional (i.e., trigonometric) methods of estimating luminosity, we rely on the pulsation-period relation for RCBs. Pulsation is also of direct interest to the puzzle of dust formation in that a link, first suggested by Pugach (1977) and confirmed by Crause et al (2007) ties onset of dust formation to a preferred phase in a RCBs radial pulsation. However, copious dust formation appears not to occur at every passage through the preferred phase.…”

Indicator of a forthcoming light minimum in a star with R Coronae Borealis type variability

“…and other stars [4], it has been found that a global light minimum begins after a pulsation light maximum in an interval of pulsation phase of 0.2-0.4. For RY Sgr and FG Sge it has also been found that the end of a minimum is also related to the pulsations:…”

“…Condensation of dust along the line of sight is recognized as the cause of the decline in brightness. Observations have shown that the condensation process is associated with pulsations of the star [3,4]. The leading role of pulsations in the global variability of the star has stimulated research on the effect of pulsations on the parameters of the star's atmosphere.…”