Evidence for bipolar jets in late stages of AGB winds

Bipolar expansion at various stages of evolution has been recently observed in a number of AGB stars. The expansion is driven by bipolar jets that emerge late in the evolution of AGB winds. The wind traps the jets, resulting in an expanding, elongated cocoon. Eventually the jets break-out from the confining spherical wind, as recently observed in W43A. This source displays the most advanced evolutionary stage of jets in AGB winds. The earliest example is IRC+10011, where the asymmetry is revealed in high-resolution near-IR imaging. In this source the jets turned on only ∼ 200 years ago, while the spherical wind is ∼ 4000 years old. The premise that asymmetric planetary nebulae (PNe) evolve from spherically symmetric Asymptotic Giant Branch (AGB) winds raises one of the most intriguing questions of stellar astrophysics: when and how does the stellar wind break its spherical symmetry? Since many proto-planetary nebulae (PPNe) show distinct large-scale asymmetries (e.g. Su, Hrivnak & Kwok 2001, Sahai 2002), the symmetry breaking process has to operate already during the late stages of AGB evolution. This implies that AGB winds are not always spherical. Indeed, in recent years, high-resolution imaging has yielded conclusive evidence of asymmetry for several AGB objects (V Hya: Plez & Lambert 1994, Sahai et al. 2003; X Her: Kahane & Jura 1996; IRC+10011: Hofmann et al. 2001; IRC+10216: Weigelt et al. 1998 & 2002, Haniff & Buscher 1998, Skinner et al. 1998, Osterbart et al. 2000; RV Boo: Bergman et al. 2000, Biller et al. 2003; CIT6: Lindqvist et al. 2000, Monnier et al. 2000, Schmidt et al. 2002). There have been suggestions that asymmetries could be even prevalent in AGB winds (Plez & Lambert 1994, Kahane at al. 1997). 1. Jet-driven asymmetry evolution in AGB stars Concurrently, an increasing number of jet and jet-like features has been identified in various PNe and PPNe, prompting a suggestion that jets are also responsible School of Natural Sciences, Institute for Advanced Study, Princeton, NJ 08540