Magnetic field effects on the structure and evolution of overdense radiatively cooling jets

We investigate the effect of magnetic fields on the propagation dynamics an d the morphology of overdense, radiatively cooling, supermagnetosonic jets, with the help of fully three-dimensional smoothed particle magnetohydrodyn amic simulations. Evaluated for a set of parameters that are mainly suitabl e for protostellar jets (with density ratios between those of the jet and t he ambient medium. eta approximate to 3-10, and ambient Mach number M-a app roximate to 24), these simulations are also compared with baseline nonmagne tic and adiabatic calculations. Two initial magnetic field topologies tin a pproximate equipartition with the gas, beta = p(th)/p(B) similar or equal t o 1) are considered: (1) a helical held and (2) a longitudinal held, both o f which permeate both the jet and the ambient medium. We find that, after a mplification by compression and reorientation in nonparallel shocks at the working surface, the magnetic held that is carried backward with the shocke d gas into the cocoon improves the jet collimation relative to the purely h ydrodynamic (HD) systems, but this effect is larger in the presence of the helical field. In both magnetic configurations, low-amplitude, approximatel y equally spaced (lambda approximate to 2 - 4R(j)) internal shocks (which a re absent in the HD systems) are produced by magnetohydrodynamic (MHD) Kelv in-Helmholtz reflection pinch modes. The longitudinal field geometry also e xcites nonaxisymmetric helical modes that cause some beam wiggling. The str ength and amount of these modes are, however, reduced (by about 2 times) in the presence of radiative cooling relative to the adiabatic cases; Besides , a large density ratio, eta, between the jet and the ambient medium also r educes, in general, the number of the internal shocks. As a consequence, th e weakness of the induced internal shocks makes it doubtful that the magnet ic pinches could by themselves produce the bright knots observed in the ove rdense, radiatively cooling protostellar jets. Magnetic fields may leave al so important signatures on the head morphologies of the radiative cooling j ets. The amplification of the nonparallel components of the magnetic fields , particularly in the helical field geometry, reduces the postshock compres sibility and increases the postshock cooling length. This may lead to stabi lization of the cold shell of shocked material that develops at the head ag ainst both the Rayleigh-Taylor and global thermal instabilities. As a conse quence, the clumps that develop by fragmentation of the shell in the ND jet s tend to be depleted in the helical field geometry. The jet immersed in th e longitudinal field, on the other hand, still retains the clumps, although they have their densities decreased relative to the I-ID counterparts. As stressed in our previous work, since the fragmented shell structure resembl es the knotty pattern commonly observed in HH objects behind the bow shocks of protostellar jets, this result suggests that, as long as (equipartition ) magnetic fields are present, they should probably be predominantly longit udinal at the heads of these jets.