Episodic dust formation in the wind of HD 193793

Abstract

In 1985 April, the WC7+abs star HD 193793 was observed, using UKIRT, to have brightened significantly in the infrared owing to the formation of a new dust shell. Examination of infrared photometry of this star since 1979 and previously published data indicates that the dust formation occurs at intervals of 7.9 years. Phasing the published radial velocities of the absorption line component with this period confirms that it is a member of an eccentric (e = 0.7−0.8) binary system having periastron passage shortly before dust formation. The X-ray spectrum also changed between 1984 and 1985 in becoming significantly “harder” while the non-thermal radio source disappeared, both changes indicating greater extinction. This suggests a model wherein the source of the non-thermal radio and X-ray emission moves deep into the Wolf-Rayet wind.