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Adriano H. Cerqueira

Publications and source records attributed to Adriano H. Cerqueira.

11 recordsLinked to original sources
arXiv2016

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.

astro-ph.SR↗
arXiv2011

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.

astro-ph.SR↗
arXiv2008

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$\degr$ 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.

astro-ph↗
arXiv2004

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.

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arXiv2001

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 ($B \simeq 260 μ$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 Hα$ intensity (by factors up to 4, and 5) behind the knots of jets with helical field and $β\simeq 1-0.1$ (respectively), relative to a non magnetic jet. As a consequence, the corresponding $I_{[SII]}}/I_{H}α}$ 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 $β\simeq 0.1$ case.

astro-ph↗
arXiv2001

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

We here present the first results of fully three-dimensional (3-D) MHD simulations of radiative cooling pulsed (time-variable) jets for a set of parameters which are suitable for protostellar outflows. Considering different initial magnetic field topologies in approximate $equipartition$ with the thermal gas, i.e., (i) a longitudinal, and (ii) a helical field, both of which permeating the jet and the ambient medium; and (iii) 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 to 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 $H_α$ emissivity behind the internal knots can be about three to four 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 2-D calculations involving toroidal magnetic fields, are smoothed out or absent in the 3-D calculations.

astro-ph↗
arXiv2001

The Effects of Magnetic Fields on Radiative Cooling Jets

We here review the results of recent studies of magnetic field effects on the structure and evolution of overdense, radiative cooling, steady and pulsed jets which were carried out with the help of fully three-dimensional MHD simulations. Three initial magnetic field topologies are considered: (i) a helical and (ii) a longitudinal field, both of which permeate the jet and the ambient medium, and (iii) a pure toroidal field in the jet. Using a set of parameters which is particularly suitable for protostellar jets, we examine the emission structure of the internal knots and the leading working surface and compare with a nonmagnetic baseline and also with 2-D MHD calculations.

astro-ph↗
arXiv1998

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.

astro-ph↗
arXiv1997

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.

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