The deceleration of relativistic jets by entrainment

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Abstract
Simulations are performed of steady, two-dimensional relativistic jets incorporating the effects of mass entrainment. The form used for the mass source is the stellar wind entrainment model of Komissarov. We present a detailed study of the dynamical effects of entraining ‘cool’ (thermally sub-relativistic) material into ‘hot’ (thermally relativistic) jets by performing a series of pertinent simulations with levels of stellar mass loss compatible with current models and typical stellar distributions for elliptical galaxies. Initially cool jets are shown to heat up as they entrain due to dissipation, whilst initially hot jets cool due to thermal dilution (i.e. the dissipation is not sufficient to heat the entrained material to the temperature of the main flow). All jets decelerate due to entrainment, some to sub-relativistic levels. Similarly, all jets show significantly greater opening angles than associated with adiabatic expansion. The dissipation associated with entrainment causes only modest loss of kinetic energy flux, and we show that relativistic jets are affected much less by dissipation than are classical flows. An explanation for the Fanaroff & Riley division of radio sources is suggested in terms of the initial jet temperature. All else being equal, initially hotter jets are shown to have lower energy fluxes than cooler flows, indicating that the former are more closely related to FR-I sources and the latter to FR-IIs. This distinction is shown to be qualitatively compatible with inferred levels of deceleration in Mach number and Lorentz factor. An estimate of the surface brightness for the simulations is derived using a simple emissivity model. It is found that initially ‘hot’ jets often show a ‘gap’ in surface brightness similar to that observed on kiloparsec scales at the base of many FR-I sources.

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