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Frequency of fading events

The most obvious and spectacular properties of RCB stars are their dramatic fading events. They spend the majority of their time at maximum light. Initial decline is sudden and steep, the star can fade by up to 8 magnitudes in a few weeks. The decline may show a series of standstills, partial recoveries and subsequent declines. The final recovery to maximum light may take several months to a year.

The first outline of a mechanism for the fading events was proposed in the 1930's when it was ``shown that the shape of the light curve of R CrB and its spectral variations at minimum can be accounted for by supposing it to eject matter which condenses at a considerable distance and forms obscuring clouds. The solid matter is believed to be principally carbon.''

From nearly two centuries of observations of RCrB, a characteristic interval of 1100 days between fading events has been deduced. Several searches have consistently failed to find any regular patterns in RCB declines which might help to identify a causal factor such as rotation or binarity. For two cases only, RYSgr and V854Cen, it has been shown that the onset of the decline episodes may be closely correlated with a particular phase of the pulsation cycle. It is attractive to relate the onset of a dust formation episode to the pulsation of the stellar envelope, but the evidence for such a connection is not yet compelling.

The ratio between the characteristic interval between fades, and the typical pulsation period of RCBs implies that a decline occurs about once every twenty pulsation cycles.

Figure 2: RCB dust-cloud ejection and evolution (based on a figure by Clayton G C 1996, Publs. Astr. Soc. Pac. 108 225-241, and reproduced by permission)
\begin{figure}
\epsfig{file=rcbcloud.ps, width=85mm}\end{figure}


next up previous
Next: Anatomy of a decline Up: Physical Characteristics Previous: Pulsations
Simon Jeffery
2000-03-16