BEHAVIOR OF JETS FROM YOUNG STELLAR OBJECTS IN LARGE-SCALE INTERSTELLAR MAGNETIC-FIELDS - MHD MODEL OF HERBIG-HARO OBJECTS IN 2.5-D SIMULATIONS

We describe the results of 2.5-D magnetohydrodynamic numerical simulations of the behavior of YSO (young stellar objects) jets - so-called "optical jets" - in a magnetized interstellar medium. Our aim is to understand the behavior of such jets under circumstances in which the large-scale interstellar magnetic field is twisted helically by the rotation of the protostar and the protostellar disk through which these fields thread. Herbig-Haro objects are interpreted by our magnetic model as being the shocked gas caused by the injection of YSO jets into an interstellar medium pervaded by a large-scale helical magnetic field. Our results show that the bow shock and the jet-terminal shock are both magnetohydrodynamic fast shocks, accompanied by a slow shock, respectively. For weak magnetic fields, the shocked gas expands in the lateral direction and forms a cocoon around the jet by stretching the magnetic field. For strong magnetic fields, fast shocks are either weak or not formed, and the kinetic energy is mainly dissipated at slow shocks. In the presence of strong magnetic fields, the separation between the bow shock and the jet-terminal shock becomes much larger because of the effect of the magnetic pressure; further, Kelvin-Helmholtz instability is entirely suppressed. The cocoon structure in the strong-field case is either very slender, or not formed at all, because the shocked gas cannot expand perpendicular to the magnetic field. Although the weak helical magnetic fields in the ambient medium reduce the propagation velocity of the bow shock, the bow shock can nevertheless propagate faster in magnetic fields exceeding a certain critical value. We discuss the relation of our results to Herbig-Haro objects in general, while focusing particularly on HH47A and HH47D, which we contend can be explained by a single jet with a strong helical magnetic field.