The infrared [WC] stars

Abstract

A number of late [WC] stars have unique infrared properties, not found among the non-[WC] planetary nebulae, and together define a class of IR-[WC] stars. They have unusual IRAS colours, resembling stars in the earliest post-AGB evolution and possibly related to PAH formation. Most or all show a double chemistry, with both a neutral (molecular) oxygen-rich and an inner carbon-rich region. Their dense nebulae indicate recent evolution from the AGB, suggesting a fatal-thermal-pulse (FTP) scenario. Although both the colours and the stellar characteristics predict fast evolution, it is shown that this phase must last for 10^4 yr. The morphologies of the nebulae are discussed. For one object in Sgr, the progenitor mass (1.3 solar masses) is known. The stellar temperatures of the IR-[WC] stars appear much higher in low metallicity systems (LMC, Sgr). This may be indicative of an extended 'pseudo' photosphere. It is proposed that re-accretion of ejected gas may slow down the post-AGB evolution and so extend the life time of the IR-[WC] stars.