Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt.......232b&link_type=abstract
PhD Thesis, Astronomy Dept., Manchester Univ., U. K., 1994.
Astronomy and Astrophysics
Astronomy
1
Scientific paper
Recent observations of Herbig-Haro jets (or optical jets) show evidence that these highly collimated, supersonic flows associated with young stellar objects are not steady, but rather show many types of variations. The string of shock excited nebulae that make up such jets have highly supersonic proper motions, substantial variations of radial velocity along the length of the jet and often several working surfaces. Several models (both analytic and numerical) exist which explain some of the observed properties of optical jets. However, these jets have a wide variety of shapes, sizes and velocities, so a general model for them is not possible. Three simple time-dependent effects have been simulated for this thesis using the approximate Riemann solver, Flux-Vector-Splitting. A periodic variation in the magnitude of the velocity of the flow produces a straight jet with a head (or working surface) and several regularly spaced pairs of shocks (internal working surfaces, IWS). This structure is similar to that of the HH34 and HH111 jets. Special attention is given to a single IWS. The evolution and emission properties of this structure are determined. A periodic variation in the direction of the flow produces a jet with a sinuous body and a complex working surface. Pairs of shocks appear at the bends of this sinuous beam. An adiabatic simulation of the complete jet serves to illustrate the general structure of the flow. The sinuous body and knotty working surface are similar to the HH46/47 jet. A non-adiabatic simulation of the body of the jet (excluding the head) permits the calculation of emission maps which show a structure similar to that observed in HH46/47. By combining the variations in magnitude and direction of the velocity, a jet is created which eventually breaks-up into independent knots (or ``bullets''). Maps of the emission obtained from this simulation present a structure reminiscent of the flow in L1551+IRS5. In general, groups of bow-shaped HH objects flowing away from one source may be explained in this way. The qualitative agreement between observations and the results of these simulations seems to indicate that some properties of HH jets can be explained as a result of a variability in the ejection mechanism which generates them.
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