KELVIN-HELMHOLTZ INSTABILITY OF STRATIFIED JETS

We investigate the Kelvin-Helmholtz instability of stratified jets. Th e internal component (core) is made of a relativistic gas moving with a relativistic bulk speed. The second component (sheath or envelope) f lows between the core and external gas with a nonrelativistic speed. S uch a two-component jet describes a variety of possible astrophysical jet configurations like e.g. (1) a relativistic electron-positron beam penetrating a classical electron-proton disc wind or (2) a beam-cocoo n structure. We perform a linear stability analysis of such a configur ation in the hydrodynamic, plane-parallel, vortex-sheet approximation. The obtained solutions of the dispersion relation show very apparent differences with respect to the single-jet solutions. Due to the refle ction of sound waves at the boundary between sheet and external gas, t he growth rate as a function of wavenumber presents a specific oscilla tion pattern. Overdense sheets can slow down the growth rate and contr ibute to stabilize the configuration. Moreover, we obtain the result t hat even for relatively small sheet widths the properties of sheet sta rt to dominate the jet dynamics. Such effects could have important ast rophysical implications, for instance on the origin of the dichotomy b etween radio-loud and radio-quiet objects.