Winds of Binary AGB Stars as Observed by Herschel

We present Herschel/PACS observations of the large-scale environments of binary AGB stars as part of the Mass-loss of Evolved StarS (MESS) sample. From the literature we found 18 of the objects to be members of physically bound multiple systems. Several show a large-scale far-IR emission which differs significantly from spherical symmetry. A probable cause is the gravitational force of the companion on the stellar AGB wind and the mass-losing star itself. A spiral pattern is thereby imprinted in the dusty stellar wind.

The most remarkable structures are found around o Ceti, W Aquilæ, R Aquarii, and π¹ Gruis. The environments of o Cet and W Aql show a spiral pattern while the symbiotic nature of R Aqr is revealed as two opposing arms which reflect a nova outburst. The emission around π¹ Gru is dominated by two structures, a disk and an arc, which are presumably not caused by the same companion. We found evidence that π¹ Gru is a hierarchical triple system in which a close companion attracts the AGB wind onto the orbital plane and the outer companion forms a spiral arm.

These far-IR observations underline the role of a companion as a major external influence in creating asymmetric winds in the AGB phase, even before the star becomes a planetary nebula (PN).