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UNSOLVED PROBLEMS

Since the discovery of RCrB itself, the mechanism that produces fadings has been elusive. Primary data connecting pulsation phase and the trigger for fadings exists for only two RCBs (V854Cen and RYSgr, [Lawson et al. 1992], [Pugach 1977]), and protracted photometry of several RCBs will be necessary to establish any connection firmly.

A second difficulty is encountered by the physical conditions necessary for dust to condense above the surface of the star. The frequency and duration of fading events implies a geometry in which the dust clouds form within two stellar radii (2 R$_{\star}$, [Clayton et al. 1992]). Under normal conditions, the local temperature would be too high for dust to condense at this distance, and a condensation distance of 20 R$_{\star}$ would be expected ([Fadeyev 1986]). Recent models treat the chemistry, energy balance and dust nucleation in pulsating star atmospheres in considerable detail ([Woitke et al. 1996a], 1996b). They show that excess cooling can occur during adiabatic expansion after the passage of a shock wave, reducing the local temperature to about 1500K within 1.5-3 R$_{\star}$. It remains to be shown that pulsations in all RCB stars provide the necessary conditions for dust nucleation to occur at this distance.

The RCB carbon abundance remains an enigma ([Asplund et al. 2000]). If it is $\sim1\%$ or $\sim10\%$ determines if RCBs are related to the C-rich remnants like the extreme helium stars or to C-strong remnants like the H-deficient central stars of planetary nebulae.

The problem is a natural consequence of stellar atmosphere physics. The strength of a stellar absorption line represents the ratio of line opacity to continuous opacity in the atmosphere; both are related to the number density of the absorbing atoms and so provide the number ratio of line absorbers to continuum absorbers. When the line and continuum absorbers are the same, it becomes impossible to measure the abundance of the absorbing species. In hydrogen-rich stars, hydrogen is normally the main continuum absorber, and since it is also assumed to be the most abundant species, the problem does not arise. Since the predominant continuum absorber in RCB atmospheres is neutral carbon, it has not yet proved possible to measure the carbon abundance from carbon absorption lines.

Finally, the emission line spectrum seen during fading events is difficult to explain, especially the sodium D lines which indicate an expansion (or wind) velocity of several hundred kms$^{-1}$ ([Rao et al. 1999]). Superimposed are a number of narrow absorption components, possibly representing cooling gas from previous ejections. The presence of heavily blue-shifted absorption from infalling material remains a puzzle.