Mass entrainment in astrophysical jets

Abstract

There is evidence that astrophysical jets interact with their environment by transferring momentum to the surrounding gas, and this interaction may have significant effects upon their evolution. This process is studied through a series of numerical calculations which follow the evolution of supersonic jets. The simulations cover a factor of 10 in Mach number and in jet to environment density ratio, as well as variations in the jet radius. Entrainment is found to occur at the leading end of the propagating jet and also along the jet well behind the initial shock. In terms of the absolute amount of mass entrained, intermediate Mach numbers (5-10) are the most effective for both processes. In terms of entrainment efficiency as measured by mass entrained per unit momentum input, the efficiency declines with increasing Mach number. Hot, low-density jets are very efficient but do not entrain much total mass. Application of the results to astrophysical jets shows that 10 to the 7th - 10 to the 9th solar masses of ambient material can readily be entrained by jets found in extragalactic radio sources.