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Bipolar outflow on the asymptotic giant branch - the case of IRC+10011

Near-infrared imaging of the asymptotic giant branch (AGB) star IRC+10011 (= CIT3) reveals the presence of a bipolar structure within the central ~0.1 arcsec of a spherical dusty wind. We show that the image asymmetries originate from ~10-4 Msolar of swept-up wind material in an elongated cocoon whose expansion is driven by bipolar jets. We perform detailed 2D radiative transfer calculations with the cocoon modelled as two cones extending to ~1100 au within an opening angle of ~30°, embedded in a wind with the standard r-2 density profile. The cocoon expansion started <~200 yr ago, while the total lifetime of the circumstellar shell is ~5500 yr. Similar bipolar expansion, at various stages of evolution, has been recently observed in a number of other AGB stars, culminating in jet breakout from the confining spherical wind. The bipolar outflow is triggered at a late stage in the evolution of AGB winds, and IRC+10011 provides its earliest example thus far. These new developments enable us to identify the first instance of symmetry breaking in the evolution from AGB to planetary nebula.

radiative transfer; stars: AGB and post-AGB; circumstellar matter; stars: imaging; stars: individual: IRC+10011; infrared: stars

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