On nonaxisymmetric modes in rotating astrophysical jets

Adiabatic helical disturbances in a cylindrical jet of radius a and constan t speed U embedded in an ambient fluid and rotating with constant angular v elocity Omega about its axis, which have been investigated by Bodo and coll aborators, are reconsidered. The numerical solution constructed covers the case where the parameter omega = Omega a/c(i)(a) much less than 1, c(i)(a) being the sound speed at the periphery of the jet. It is shown that the com parable study by the other authors omits one term in one of the two governi ng equations of the problem. Stability characteristics in terms of omega, t he dimensionless axial wavenumber b, and the Mach number M of the flow for b much less than 1 are established. Our detailed results are compared, as f ar as possible, with those of the other authors. We find a new set of unsta ble modes due to rotation with small growth rates. They set off where the a xial velocity of the disturbance equals the sound speed in the ambient gas. We also find substantial differences between our results and those of the other authors, concerning the boundaries of the stability regions of the pr oblem. There is a transformation that converts a stable solution for the ca se where the gas density rho at the jet interface is continuous to a soluti on where it is discontinuous. The transformation also converts part of the stability boundary for the former case into part of the stability boundary for the latter case.