Correlating the interstellar magnetic field with protostellar jets and its sources

This article combines new CCD polarimetric data with previous information about protostellar objects in a search for correlations involving the interstellar magnetic field. Specifically, we carried out an optical polarimetric study of a sample of 28 fields of 10 X 10 arcmin^2 located in the neighborhood of protostellar jets and randomly spread over the Galaxy. The polarimetry of a large number of field stars is used to estimate both the average and dispersion of the interstellar magnetic field (ISMF) direction in each region. The results of the applied statistical tests are as follows. Concerning the alignment between the jet direction and the interstellar magnetic field, the whole sample does not show alignment. There is, however, a statistically significant alignment for objects of Classes 0 and I. Regarding the interstellar magnetic field dispersion, our sample presents values slightly larger for regions containing T Tauri objects than for those harboring younger protostars. Moreover the ISMF dispersion in regions containing high-mass objects tends to be larger than in those including only low-mass protostars. In our sample, the mean interstellar polarization as a function of the average interstellar extinction in a region reaches a maximum value around 3% for A(V) = 5, after which it decreases. Our data also show a clear correlation of the mean value of the interstellar polarization with the dispersion of the interstellar magnetic field: the larger the dispersion, the smaller the polarization. Based on a comparison of our and previous results, we suggest that the dispersion in regions forming stars is larger than in quiescent regions.Comment: ApJ accepte

Magnetic Field Effects on the Head Structure of Protostellar Jets

We present the results of 3-D SPMHD numerical simulations of supermagnetosonic, overdense, radiatively cooling jets. Two initial magnetic configurations are considered: (i) a helical and (ii) a longitudinal field. We find that magnetic fields have important effects on the dynamics and structure of radiative cooling jets, especially at the head. The presence of a helical field suppresses the formation of the clumpy structure which is found to develop at the head of purely hydrodynamical jets. On the other hand, a cooling jet embedded in a longitudinal magnetic field retains clumpy morphology at its head. This fragmented structure resembles the knotty pattern commonly observed in HH objects behind the bow shocks of HH jets. This suggests that a strong (equipartition) helical magnetic field configuration is ruled out at the jet head. Therefore, if strong magnetic fields are present, they are probably predominantly longitudinal in those regions. In both magnetic configurations, we find that the confining pressure of the cocoon is able to excite short-wavelength MHD K-H pinch modes that drive low-amplitude internal shocks along the beam. These shocks are not strong however, and it likely that they could only play a secondary role in the formation of the bright knots observed in HH jets.Comment: 14 pages, 2 Gif figures, uses aasms4.sty. Also available on the web page http://www.iagusp.usp.br/preprints/preprint.html. To appear in The Astrophysical Journal Letter

Magnetic Field Effects on the Structure and Evolution of Overdense Radiatively Cooling Jets

We investigate the effect of magnetic fields on the propagation dynamics and morphology of overdense, radiatively cooling, supermagnetosonic jets, with the help of fully three-dimensional SPMHD simulations. Evaluated for a set of parameters which are mainly suitable for protostellar jets (with density ratios between the jet and the ambient medium 3-10, and ambient Mach number ~ 24), these simulations are also compared with baseline non-magnetic and adiabatic calculations. We find that, after amplification by compression and re-orientation in nonparallel shocks at the working surface, the magnetic field that is carried backward with the shocked gas into the cocoon improves the jet collimation relative to the purely hydrodynamic (HD) systems. Low-amplitude, approximately equally spaced internal shocks (which are absent in the HD systems) are produced by MHD K-H reflection pinch modes. The longitudinal field geometry also excites non-axisymmetric helical modes which cause some beam wiggling. The strength and amount of these modes are, however, reduced (by ~ twice) in the presence of radiative cooling relative to the adiabatic cases. Besides, a large density ratio between the jet and the ambient medium also reduces, in general, the number of the internal shocks. As a consequence, the weakness of the induced internal shocks makes it doubtful that the magnetic pinches could produce by themselves the bright knots observed in the overdense, radiatively cooling protostellar jets.Comment: To appear in ApJ; 36 pages + 16 (gif) figures. PostScript files of figures are available at http://www.iagusp.usp.br/preprints/preprint.htm

Three-dimensional MHD simulations of Radiatively cooling, Pulsed Jets

(Abridged) We here investigate, by means of fully 3-D Smoothed Particle Magnetohydrodynamic numerical simulations, the effects of magnetic fields on overdense, radiatively cooling, pulsed jets, using different initial magnetic field topologies and strengths ($B \simeq 260 \mu$G-0). The relative differences that have been previously detected in 2-D simulations involving distinct magnetic field configurations are diminished in the 3-D flows. While the presence of toroidal magnetic components can modify the morphology close to the jet head inhibiting its fragmentation in the early jet evolution, as previously reported in the literature, the impact of the pulsed-induced internal knots causes the appearance of a complex morphology at the jet head (as required by the observations of H-H jets) even in the MHD jet models with toroidal components. The detailed structure and emission properties of the internal working surfaces can be also significantly altered by the presence of magnetic fields. The increase of the magnetic field strength improves the jet collimation, and amplifies the density (by factors up to 1.4, and 4) and the H\alpha$intensity (by factors up to 4, and 5) behind the knots of jets with helical field and$\beta \simeq 1-0.1$(respectively), relative to a non magnetic jet. As a consequence, the corresponding$I_{[SII]}}/I_{H}\alpha}$ratio (which is frequently used to determine the excitation level of HH objects) can be decreased in the MHD models with toroidal components relative to non-magnetic calculations. We also find that the helical mode of the K-H instability can be triggered in MHD models with helical magnetic fields, causing some jet wiggling. No evidence for the formation of the nose cones is found in the 3-D flows, nor even in the$\beta \simeq 0.1$ case.Comment: 31 pages, 10 figures (see higher resolution figures in: http://www.iagusp.usp.br/~dalpino/mhd-jets/apj0301.tar.gz), ApJ in pres

Three-dimensional hydrodynamical simulations of the large scale structure of W50-SS433

We present 3D hydrodynamical simulations of a precessing jet propagating inside a supernova remnant (SNR) shell, particularly applied to the W50-SS433 system in a search for the origin of its peculiar elongated morphology. Several runs were carried out with different values for the mass loss rate of the jet, the initial radius of the SNR, and the opening angle of the precession cone. We found that our models successfully reproduce the scale and morphology of W50 when the opening angle of the jets is set to 10$degr$ or if this angle linearly varies with time. For these models, more realistic runs were made considering that the remnant is expanding into an interstellar medium (ISM) with an exponential density profile (as HI observations suggest). Taking into account all these ingredients, the large scale morphology of the W50-SS 433 system, including the asymmetry between the lobes (formed by the jet-SNR interaction), is well reproduced.Fil: Zavala, Jesús. Universidad Nacional Autónoma de México; MéxicoFil: Velázquez, Pablo F.. Universidad Nacional Autónoma de México; MéxicoFil: Cerqueira, Adriano H. Universidade Estadual de Santa Cruz; BrasilFil: Dubner, Gloria Mabel. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentin