Ah. Cerqueira et Emd. Dal Pino, Magnetic field effects on the structure and evolution of overdense radiatively cooling jets, ASTROPHYS J, 510(2), 1999, pp. 828-845
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.