Integrated mechanism that both removes accretion disk angular momentum and drives astrophysical jets

Astrophysical jets are long collimated plasma columns shooting out from certain stars and always appear in pairs and are always associated with accretion disks. Critical outstanding issues are how angular momentum is removed from the material accreting to an accretion disk and why jets require existence of accretion disks.

 

A mechanism [1,2] that simultaneously removes accretion disk angular momentum and drives astrophysical jets is proposed. The mechanism depends on the extreme difference between the governing physics in the weakly-ionized, highly collisional accretion disk and the governing physics in the completely-ionized, nearly collisionless region outside the disk.

 

In the completely-ionized exterior region, axisymmetric Hamiltonian mechanics constrain charged particles to move on nested poloidal magnetic flux surfaces. In contrast, fluid elements in the weakly-ionized, highly-collisional accretion disk behave as collisionless meta-particles having an effective charge to mass ratio reduced from than that of an ion by the extremely small disk fractional ionization. This extremely small charge to mass ratio allows meta-particles to have an effective cyclotron frequency comparable to the Kepler frequency in which case there is a direct competition between gravitational and magnetic forces.

 

For meta-particles in a stratum having a critical ionization fraction, the charge to mass ratio is such that the meta-particles have zero canonical angular momentum; i.e., the mechanical and magnetic parts of the canonical angular momentum are equal and opposite. Hamiltonian mechanics shows that these meta-particles experience no centrifugal force and so they spiral in towards the central body while conserving canonical angular momentum.  Because these inward spiraling meta-particles contain positive charge, their accumulation near the central body produces a radially outward electric field. This electric field drives an out-of-plane poloidal electric current in the completely-ionized region exterior to the disk.  This current and its associated toroidal magnetic field produce J x B forces that drive bi-directional astrophysical jets flowing normal to and away from the disk. The increase in linked toroidal magnetic flux as the jet lengthens is associated with the radial voltage drop at the disk in accordance with Faraday’s law.

 

[1] Bellan, P.M., Monthly Notices Royal Astronomical Society 458, 4400 (2016)

[2] Bellan, P.M., Plasma Physics and Controlled Fusion, online at https://doi.org/10.1088/1361-6587/aa85f9 (2017)