H.J. Walker 1, I. Heinrichsen 2, G. C. Clayton 3, & A. E. Rosenbush 4
1 RAL, Chilton, Didcot, Oxon, UK 2 IPAC, Caltech, Pasadena, USA 3 LSU, Louisiana, USA 4 MAO, Kiev, Ukraine
R CrB is a very unusual type of variable star, in that the variability is caused by the formation of a cloud of carbon dust in the line of sight. The cloud is formed near the stellar photosphere, condenses (dimming the star's visual brightness by several magnitudes) and then dissipates as it moves away from the star. An observation of R CrB with the low resolution spectrometer in the photometer on ISO (ISOPHOT) early in the mission, revealed a very unusual carbon dust feature, a plateau of emission between around 5 microns and 8 microns. A further three observations during the ISO mission have shown that the plateau changes strength and shape with time. These data are combined with other observations (from ISO and elsewhere) to investigate the behaviour of the warm dust shell. There are indications that molecules are responsible for the variability.
Another member of the small group of R CrB stars, RY Sgr, has been observed three times during the ISO mission. The dust has the same plateau emission feature, but it is not as strong as in R CrB, or as variable. A third member of the group, V854 Cen, has some hydrogen in its stellar atmosphere (R CrB and RY Sgr have very little) and the spectrum of this dust has PAH emission features as well as the plateau emission. The PAH molecules containing hydrogen are important chains in the dust condensation process.
The dust shells were observed with ISOPHOT to determine the dust temperature, which is typically several hundred degrees. In addition to the warm dust shell, several R CrB stars were found to have very large, cool `fossil' dust shells, from IRAS data. Flux densities were measured using ISOPHOT at various locations in the fossil shells, to confirm the IRAS detections and to determine the temperature more precisely. The IRAS data implied the fossil shells may be parsecs across. These data can give some insight into how the R CrB type of star (and the fossil shell) was created, whether the helium flash generated the shell, or whether it occurred earlier.