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Pulsations

Most RCB stars are small-amplitude variables at maximum light. Light variations are typically a few tenths of a magnitude with periods between 40 and 100 days. In well-studied cases, radial-velocity variations have been found with amplitudes proportional to the light variations. The visual light curve of RYSgr (period $\sim38$ days, amplitude 0.2-0.3 mag.) correlates well with the radial velocity curve (amplitude $40\,{\rm km\,s^{-1}}$) and indicates that RCBs are radially PULSATING STARS.

The light curves of RCB stars are not strictly periodic. Some authors suggest that this is due to interference between a number of radial modes with different periods. It is more likely due to extreme non-adiabacity in the stellar envelope which can lead to chaotic behaviour in the pulsation cycle length. Since pulsation periods are directly related to the mean density of a star, evidence for period changes was thought to imply secular changes in RCB radii. It now seems likely that the observed period changes are due to chaotic behaviour.

The driving mechanism for pulsations in RCB stars is not the classical $\kappa$ mechanism found in CEPHEID VARIABLES; RCB stars do not lie in any of the classical instability strips. Their pulsations are due to `strange-mode' oscillations whereby waves trapped in parts of the acoustic cavity of the stellar envelope are amplified. Such pulsation modes are also seen in other HIGH LUMINOSITY STARS, such as luminous blue variables and $\alpha$Cyg variables.

A consequence of the high L/M ratios of RCB stars is that densities in their atmospheres are low. The action of the pulsations on the photosphere resembles that of a piston imparting an outward impulse at regular intervals. In between these impulses the photosphere is virtually in free-fall. During the impulse, highly non-linear processes can generate shock waves within the photosphere. Evidence for such non-linearity has been observed in RYSgr where, at minimum radius, absorption lines are seen to double. A red-shifted component is due to infalling material and a blue-shifted component is due to lower-lying plasma that has already being accelerated outwards.


next up previous
Next: Frequency of fading events Up: Physical Characteristics Previous: Atmospheres
Simon Jeffery
2000-03-16