Distinguishing Propagation vs. Launch Physics of Astrophysical Jets and the Role of Experiments

The absence of other viable momentum sources for collimated flows leads to the likelihood that magnetic fields play a fundamental role in jet launch and/or collimation in astrophysical jets. To best understand the physics of jets, it is useful to distinguish between the launch region where the jet is accelerated and the larger scales where the jet propagates as a collimated structure. Observations presently resolve jet propagation, but not the launch region. Simulations typically probe the launch and propagation regions separately, but not both together. Here, I identify some of the physics of jet launch vs. propagation and what laboratory jet experiments to date have probed. Reproducing an astrophysical jet in the lab is unrealistic, so maximizing the benefit of the experiments requires clarifying the astrophysical connection. 1. Distinguishing Jet Launch vs. Jet Propagation Physics Jets in astrophysics emanate from accretion disk engines. The available jet mechanical luminosity is inversely proportional to the radius from the central engine, so the jet power is drawn from the inner most regions of the disk. Material must be accelerated to outflow speeds comparable to the escape speeds at the launch point. Radiation pressure is typically incapable of providing the directed momentum and instead various flavors of magnetic launch models remain the most promising. (see Livio 2004, Pudritz 2004 for reviews; also Lynden-Bell 2003). Magnetic models take different forms. In steady-state “fling” models (e.g. Blandford & Payne 1982), mass flux is sustained by centrifugal and toroidal magnetic pressure forces along the poloidal field. Explosive “spring” models (e.g. Wheeler, Meier, & Wilson 2002; Matt et al. 2004; Moiseenko et al. 2006) also thrive on a gradient of magnetic field pressure, but are time dependent and do not require an initially imposed mass flux. Such “springs” may operate in gamma-ray bursts (GRB) and maybe supernovae. In both spring and fling models, the launch region is Poynting flux dominated but on scales ∼ < 50Rin, (where Rin is the scale of the inner engine) the jet becomes flow dominated.