Adriano H. Cerqueira

Three-dimensional Magnetohydrodynamic Simulations of Radiatively Cooling, Pulsed Jets

We here investigate, by means of fully three-dimensional smoothed particle magnetohydrodynamic numerical simulations, the effects of different initial magnetic field configurations on the evolution of overdense, radiatively cooling, pulsed jets using the following different initial magnetic field topologies: (1) longitudinal, (2) helical geometry permeating both the jet and the ambient medium, and (3) purely toroidal geometry permeating the jet only. We explore the effects of different pulsational periods, as well as different values of the magnetic field strength (? 0.1-? or B 260-0 ?G). The presence of a helical or toroidal field tends to affect the global characteristics of the fluid more than a longitudinal field. However, the relative differences that have been previously detected in two-dimensional simulations involving distinct magnetic field configurations are diminished in the three-dimensional flows. While the presence of toroidal magnetic components can modify the morphology close to the jet head, inhibiting its fragmentation in the early evolution of the jet, as previously reported in the literature, the impact of the pulse-induced internal knots causes the appearance of a clumpy, complex morphology at the jet head (as required by the observations of Herbig-Haro [HH] jets) even in the MHD jet models with helical or toroidal configurations. The detailed structure and emission properties of the internal working surfaces can also be significantly altered by the presence of magnetic fields. The increase of the magnetic field strength (decrease of ?) improves the jet collimation and amplifies the density (by factors up to 1.4 and 4) and the H? intensity (by factors up to 4 and 5) behind the knots of jets with a helical field and ? 1-0.1 relative to a nonmagnetic jet. As a consequence, the corresponding I[S II]/IH? 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 nonmagnetic calculations by about the same amounts, although the intensity estimates above are very approximate. We also find that the helical mode of the Kelvin-Helmholtz instability can be triggered in MHD models with helical magnetic fields, causing some wiggling of the jet axis. No evidence for the formation of the nose cones that are commonly detected in two-dimensional jet simulations with initial toroidal magnetic fields is found in the three-dimensional flows or even in the ? 0.1 case. The implications of our results for HH jets are briefly discussed.

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 the morphology of overdense, radiatively cooling, supermagnetosonic jets, with the help of fully three-dimensional smoothed particle magnetohydrodynamic simulations. Evaluated for a set of parameters that are mainly suitable for protostellar jets (with density ratios between those of the jet and the ambient medium η≈3-10, and ambient Mach number Ma≈24), these simulations are also compared with baseline nonmagnetic and adiabatic calculations. Two initial magnetic field topologies (in approximate equipartition with the gas, β=pth/pB1) are considered: (1) a helical field and (2) a longitudinal field, both of which permeate both the jet and the ambient medium. We find that, after amplification by compression and reorientation 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, but this effect is larger in the presence of the helical field. In both magnetic configurations, low-amplitude, approximately equally spaced (λ≈2-4Rj) internal shocks (which are absent in the HD systems) are produced by magnetohydrodynamic (MHD) Kelvin-Helmholtz reflection pinch modes. The longitudinal field geometry also excites nonaxisymmetric helical modes that cause some beam wiggling. The strength and amount of these modes are, however, reduced (by about 2 times) in the presence of radiative cooling relative to the adiabatic cases. Besides, a large density ratio, η, 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 by themselves produce the bright knots observed in the overdense, radiatively cooling protostellar jets. Magnetic fields may leave also important signatures on the head morphologies of the radiative cooling jets. The amplification of the nonparallel components of the magnetic fields, particularly in the helical field geometry, reduces the postshock compressibility and increases the postshock cooling length. This may lead to stabilization of the cold shell of shocked material that develops at the head against both the Rayleigh-Taylor and global thermal instabilities. As a consequence, the clumps that develop by fragmentation of the shell in the HD jets tend to be depleted in the helical field geometry. The jet immersed in the longitudinal field, on the other hand, still retains the clumps, although they have their densities decreased relative to the HD counterparts. As stressed in our previous work, since the fragmented shell structure resembles the knotty pattern commonly observed in HH objects behind the bow shocks of protostellar jets, this result suggests that, as long as (equipartition) magnetic fields are present, they should probably be predominantly longitudinal at the heads of these jets.

Magnetic Field Effects on the Head Structure of Protostellar Jets

We present the results of three-dimensional smooth particle magnetohydrodynamics numerical simulations of supermagnetosonic, overdense, radiatively cooling jets. Together with a baseline nonmagnetic calculation, two initial magnetic configurations (in approximate equipartition with the gas) are considered: (1) a helical field and (2) a longitudinal field, both of which permeate both the jet and the ambient medium. 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 that is found to develop at the head of purely hydrodynamical jets by fragmentation of the cold shell of shocked material. 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 protostellar 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 Kelvin-Helmholtz pinch modes that drive low-amplitude internal shocks along the beam. These shocks are not strong however, and it is likely that they could only play a secondary role in the formation of the bright knots observed in protostellar jets.

3-D numerical simulations of rotating jets - The case of the DG Tau microjet

We here present results of three-dimensional Smoothed Particle hydro and magnetohydrodynamics simulations of rotating jets, also including the effects of radiative cooling, precession and velocity variability. Using initial conditions and parameters which are particularly suitable for the DG Tau microjet, we have been able to approximately reproduce its complex knotty morphology and kinematics. We have also obtained radial velocity maps which are in good agreement with the data obtained by Bacciotti et al., thus indicating that their interpretation that the DG Tau microjet is rotating is correct. Finally, we have found that a magnetic field of the order of ≈0.5 mG is sufficient to collimate the jet against the lateral expansion that is caused by the centrifugal forces.

On the Influence of Magnetic Fields on the Structure of Protostellar Jets

We here present the first results of fully three-dimensional MHD simulations of radiative cooling pulsed (time-variable) jets for a set of parameters that are suitable for protostellar outflows. Considering different initial magnetic field topologies in approximate equipartition with the thermal gas, i.e., (1) a longitudinal and (2) a helical field, both of which permeate the jet and the ambient medium, and (3) a purely toroidal field permeating only the jet, we find that the overall morphology of the pulsed jet is not very much affected by the presence of the different magnetic field geometries in comparison with a nonmagnetic calculation. Instead, the magnetic fields tend to affect essentially the detailed structure and emission properties behind the shocks at the head and at the pulse-induced internal knots, particularly for the helical and toroidal geometries. In these cases, we find, for example, that the Hα emissivity behind the internal knots can be about 3-4 times larger than that of the purely hydrodynamical jet. We also find that some features, like the nose cones that often develop at the jet head in two-dimensional calculations involving toroidal magnetic fields, are smoothed out or absent in the three-dimensional calculations.

MHD NUMERICAL SIMULATIONS OF PROTO-STELLAR JETS

We will summarize in this paper the effects that the presence of the magnetic field can cause to proto-stellar jet dynamics, structure and emission line properties, and the differences between two- and three-dimensional numerical simulations will be emphasized.

TRANSVERSE VELOCITY SHIFTS IN PROTOSTELLAR JETS: ROTATION OR VELOCITY ASYMMETRIES?

Observations of several protostellar jets show systematic differences in radial velocity transverse to the jet propagation direction, which have been interpreted as evidence of rotation in the jets. In this paper we discuss the origin of these velocity shifts, and show that they could be originated by rotation in the flow, or by side to side asymmetries in the shock velocity, which could be due to asymmetries in the jet ejection velocity/density or in the ambient medium. For typical poloidal jet velocities (~ 100-200 km/s), an asymmetry >~ 10% can produce velocity shifts comparable to those observed. We also present three dimensional numerical simulations of rotating, precessing and asymmetric jets, and show that, even though for a given jet there is a clear degeneracy between these effects, a statistical analysis of jets with different inclination angles can help to distinguish between the alternative origins of transverse velocity shifts. Our analysis indicate that side to side velocities asymmetries could represent an important contribution to transverse velocity shifts, being the most important contributor for large jet inclination angles (with respect the the plane of the sky), and can not be neglected when interpreting the observations.

3D 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 ◦ 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.

3-D SPH Simulations of Radiatively Cooling Magnetized Jets

Considerable amount of numerical work on magnetized, adiabatic, light jets has been done to study extragalactic jets (see, e.g., [1] for a review). Only recently, these MHD studies have been extended to heavy, radiatively cooling jets [e.g., 2, 3, 8, 9, 10].

Three‐dimensional numerical simulations of rotating jets

In this paper, we briefly review the role played by the magnetic field in protostellar (or Herbig‐Haro) jets. Among its most important effects on the jet dynamics at the large (sub‐parsec and parsec) scales, we may distinguish: i) deflection of the jet in the interstellar medium; ii) change of the rate at which the Rayleigh‐Taylor and Kelvin‐Helmholtz instabilities develop in the jet head and along the beam; and iii) change of the emission line structure behind the working surfaces. Magnetic fields also seem to play a key role in the jet launching mechanism from the protostar and accretion disk that surrounds it. We present here recent results from three‐dimensional Smoothed Particle Hydrodynamical numerical simulations of rotating jets which are in good agreement with high resolution observations of the DG Tau microjet that have shown the presence of a pattern in the radial velocity that is consistent with a rotating jet launched from a Keplerian accretion disk by magneto‐centrifugal forces. We have also...