The Entrainment-Limited Evolution of FR II Sources: Maximum Sizes and A Possible Connection to FR Is

TL;DR

This paper presents a theoretical model showing how entrainment limits the size of FR II radio sources and explores the potential evolution of FR IIs into FR Is based on jet erosion processes.

Contribution

It introduces a mixing-layer model for FR II jets that explains size limitations and suggests a possible evolutionary link to FR I sources.

Findings

Maximum sizes of FR II sources are constrained by entrainment effects.

Jet bulk velocities predicted by the model align with observations.

FR IIs may evolve into FR Is after reaching size limits.

Abstract

We construct a simple theoretical model to investigate how entrainment gradually erodes high-speed FR II jets. This process is described by embedding a mixing-layer model developed originally to describe FR I objects in a self-similar model for the lobe structure of classical FR II sources. Following the classical FR II models, we assume that the lobe is dominated by the particles injected from the central jet. The entrainment produces a boundary shear layer which acts at the interface between the dense central jet and the less denser surrounding lobe, and the associated erosion of the jet places interesting limits on the maximum size of FR II sources. The model shows that this limit depends mainly on the initial bulk velocity of the relativistic jet triggered. The bulk velocities of FR IIs suggested by our model are in good agreement with that obtained from direct pc-scale observations on ordinary radio galaxies and quasars. Finally, we discuss how FR IIs may evolve into FR Is upon reaching their maximum, entrainment-limited sizes.