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Astron. Astrophys. 333, 1001-1006 (1998)

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4. Conclusions

We have applied the results obtained from the spatial nonlinear evolution of K-H modes in a cylindrical radiative jet, in order to interpret the formation of emission knots in HH jets.

The main differences between K-H and the alternative IWSs mechanisms can be summarized as follows:

  1. For IWSs, the line fluxes from knots show a sudden rise and then a steady decline with distance, while for K-H they show a smooth increase followed by a slow decrease.
  2. In the IWSs scenario, knots are characterized by pattern speeds very close to the jet speed while K-H mechanism produces shocks whose proper motion velocities tend to increase with distance.
  3. K-H scenario predicts intra-knot spacings and line intensity ratios [FORMULA] that depend on the jet Mach number, which is not true for IWSs scenario where they are independent each other with the former depending on the timing of successive outbursts and the latter on the amplitude of the velocity jumps.
  4. Knots in HH jets displaying symmetric intra-knot spacings in jet and counter-jet, and with evidences of intrinsic differences in the respective environments, are not likely to be interpreted as results of K-H instabilities. Instead in the opposite cases, when this symmetry is missing, IWSs can hardly be responsible of the knot origin.

The K-H scenario has the basic advantage of producing morphologies directly from the hydrodynamics of the problem, works under very general conditions, and does not depend on a peculiar behavior of the source but only on the establishment of pressure balance conditions between the jet and its environment. The main problems with K-H are shock morphologies, that only in few cases appear bow-shock like, and spatial distances within the jet are better interpreted by values of M about one half of those derived by observations of jet velocities (but not of jet-to-ambient velocity jumps).

Finally, magnetic fields may have an important impact on the phenomenology of both K-H and IWS scenarios, and the 2-D axial symmetry assumed in the calculations neglects the evolution of higher modes. Radiative 3-D calculations for the nonlinear evolution of K-H instabilities are in progress.

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998