It has been recently discovered that spatially separated Herbig-Haro objects,
once considered unrelated, are linked within a chain that may extend for
parsecs in either direction of the embedded protostar forming a "giant
Herbig-Haro jet". Presently, several dozen of these giant flows have been
detected and the best documented example, the HH~34 system, shows a systematic
velocity decrease with distance on either side of the source. In this paper, we
have modeled giant jets by performing fully three-dimensional simulations of
overdense, radiatively cooling jets modulated with long-period
(P ∼ several hundred years) and large amplitude sinusoidal velocity
variability at injection (Δv∼ mean jet flow velocity). Allowing
them to travel over a distance well beyond the source, we have found that
multiple travelling pulses develop and their velocity indeed falls off smoothly
and systematically with distance. This deceleration is fastest if the jet is
pressure-confined, in which case the falloff in velocity is roughly consistent
with the observations. The deceleration occurs as momentum is transferred by
gas expelled sideways from the traveling pulses. The simulation of a
pressure-confined, steady-state jet with similar initial conditions to those of
the pulsed jet shows that the flow in this case experiences acceleration.
This result is thus an additional indication that the primary source of
deceleration in the giant flows cannot be attributed to braking of the jet
head against the external medium.Comment: 25 pages, 10 figures (see higher resolution figures in:
http://www.iagusp.usp.br/~dalpino/giants/apj00.tar.gz); ApJ in pres