Magnetic Field Effects on the Structure and Evolution of Herbig-Haro Jets

In this work, the structure and evolution of Herbig-Haro (HH) jets is analyzed with the help of three-dimensional (3-D), Smoothed Particle Hydrodynamics simulations. We have carried out the first 3-D magneto-hydrodynamics (MHD) numerical simulations of steady-state and pulsed, radiativelly cooling jets, adopting parameters that are suitable for HH jets in the literature (e.g., de Gouveia Dal Pino & Cerqueira, Astro. Lett. and Commun. 1996, 34, 303). We show that the inclusion of the magnetic field, for which we have used different initial configurations and intensities (β = 8 pi p / B² ~eq 0.1, 1 and infinity; or B ~eq 260 μG, 83 μG and 0), enhances the jet collimation, particularly in the case of helical and toroidal fields. In the case of steady-state simulations, we show that the appearance of internal crossing shocks, which are excited by the MHD Kelvin-Helmholtz (K-H) instability is inhibited when radiative cooling is taken into account. Therefore, MHD K-H instabilities seem to play a secondary role in the formation of internal emission knots along the jet beam of HH jets, where the radiative cooling is very important (Cerqueira & de Gouveia Dal Pino 1999, ApJ, 510, 828). We show that the presence of magnetic fields can significantly affect the morphology of the steady-state jets close to the head (Cerqueira, de Gouveia Dal Pino & Herant, 1997, ApJ, L185). In particular, both the helical and the toroidal fields, unlike the longitudinal field, can inhibit the formation of the cold, dense structures that are found to develop in this region in purely hydrodynamic simulations (which are due to Rayleigh-Taylor and global thermal instabilities). We have also performed 3-D simulations of pulsed jets. In this case, the bright emission knots observed along the HH jets are naturally explained by the pulses which quickly steepen into double shock structures as they propagate downstream (forming the so-called internal working surfaces; IWS's). We show that the presence of different magnetic field topologies in close equipartition with the gas does not introduce relevant effects on the overall morphology of the HH jets. However, the magnetic fields can significantly affect the detailed structure and emission properties of the head and internal knots with respect to purely hydrodynamic calculations, particularly for helical and toroidal geometries (Cerqueira & de Gouveia Dal Pino 2001a, ApJ Letters, 550, in press). We have estimated the Hα emission within the knots and found that the intensity in this line could be up to 4-5 times larger than that found in the pure HD case. For the different adopted magnetic field configurations, we find that the toroidal component steepens within the IWS's, while the longitudinal component attains its maximum intensity between the IWS's. Our simulations of pulsed jet with an initially helical magnetic field topology show the development of the kink mode of the MHD Kelvin-Helmholtz instability which promotes a gentle wandering of the jet axis. Some features previously detected in 2-D MHD simulations seem to be smoothed out in the 3-D flows. For example, we find no evidence for the appearance of the so-called nose-cones which are often detected in two-dimensional numerical simulations involving toroidal fields (see Cerqueira & de Gouveia Dal Pino 2001b, ApJ, submitted). Since there is no direct observational evidence of such structures in the HH jets, the present findings stress the importance of fully 3-D studies of these objects.