ISOPHOT observations of R CrB: A star caught smoking

R CrB is a very unusual star, being extremely hydrogen deficient and undergoing irregular deep minima in its visible light. R CrB started to undergo a fading episode in October 1995. The infrared Space Observatory (ISO) observed the star after it had faded by 7 magnitudes. The inner, warm dust shell has been observed by ISOPHOT, using the long wavelength camera mode (from 60 mu m to 200 mu m). The dust shell was observed using the low resolution spectrometer (from 2.5 mu m to 5 mu m and 5.8 mu m to 11.6 mu m). The energy distribution peaks around 6 - 8 mu m, indicative of a 650K dust shell with an unusual shape to the observed spectrum. The long wavelength photometry, when compared to IRAS data, shows the temperature of the warm dust shell is unaffected by the ejection of a new cloud of carbon from the central star. The dust cloud is probably composed of pure carbon, but other materials cannot be excluded. There is no evidence of hydrogenated carbon molecules.