(News & views on the paper by Ron Schreier & Noam Soker, RAA, 2016, vol.16, 70)
News and Views on "Launching jets from accretion belts"
(National Technical Institute for the Deaf, Rochester Institute of Technology, Rochester, NY USA)
Collimated mass outflows are ubiquitous in astrophysics. Observations from a myriad of systems as diverse as young stellar objects, micro quasars, active galactic nuclei, gamma-ray bursts, planetary nebulae, and evolved stars show jets and highly-collimated winds (Lada 1985, Urry & Padovani 1995, Livio 1999). The formation mechanisms invoked to explain these outflows often involve some combination of central engine rotation, accretion and dynamically-relevant magnetic fields.
In traditional accretion disks, shear and turbulence act to amplify magnetic fields while angular momentum is transported outward, allowing mass to fall inward from where magneto-centrifugal launching can occur (Stepanovs et al. 2014). However, astrophysical systems with significant rotation may not always possess sufficient conditions for robust disk formation. Schreier & Soker ( 2016) suggest collimated outflows and jets may still form in inflowing systems if the rotation rates are slightly sub-Keplerian and strong shear flows remain present. Such accretion belts might drive collimated outflows from main-sequence companions in common envelopes where short inspiral times and ram pressure effects may prevent disk formation (Ivanova et al. 2013). For massive stars, the collapse of the iron core in supernova progenitors, followed by the stalling of the bounce shock, represents another astrophysical system in which accreting shear flows occur (Nordhaus et al. 2010).
In Schreier & Soker ( 2016), analytic estimates are presented for the accretion-belt scenario that suggest future study is warranted. Numerical experiments will be necessary to establish the viability of the scenario in different astrophysical contexts. In particular, the details of angular momentum transport, magnetic field amplification, and magnetic reconnection processes in sub-Keplerian shear flows are needed. However, if it can be established that accretion belts can efficiently extract and launch collimated outflows, the breadth of astrophysical systems that incur accretion and aspherical ejection will be significantly broadened.
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