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SwSt 1: an O-rich planetary nebula around a C-rich central star

ABSTRA C T The hydrogen-deficient [WCL] type central star HD 167362 and its planetary nebula (PN) SwSt 1 are investigated. The central star has a carbon-rich emission-line spectrum, and yet the nebula exhibits a 10-mm emission feature from warm silicate dust, perhaps indicating a recent origin for the carbon-rich stellar spectrum. Its stellar and nebular properties might therefore provide further understanding as to the origin of the [WCL] central star class. The central star optical and UV spectra are modelled with state-of-the art non-LTE codes for expanding atmospheres, from which the stellar parameters are determined. Using the Sobolev approximation code ISA-Wind, we find T effa 40000 K, logO _ M=M( yr 21 Ua26:72, La 8900 L( (for a distance of 2.0 kpc), and v1 . 900 km s 21 . The abundance mass fractions for helium, carbon and oxygen are determined to be 37, 51 and 12 per cent, respectively. From this we derive C=O a 4:3 (by mass), confirming that the star suffered efficient third dredgeup. The nitrogen abundance is close to zero, while an upper limit of ,10 per cent by mass is established for H. The model uses a composite beta velocity law which allows us to reproduce the optical line profiles. The overall shape of the dereddened spectrum agrees with the V-scaledâmV a 11:48 mag, EOB 2 VUa0:46 maga model atmosphere, showing the nebularderived reddening to be consistent with the reddening indicated by the stellar analysis. We confirm our model results by using the comoving frame code CMFGEN, although a few differences remain.