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Register a new userA Study of Radio Polarization in Protostellar Jets
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Added by
Mariana Andrea C\\'ecere
Publication date
2015
fields
Physics
and research's language is
English
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Synchrotron radiation is commonly observed in connection with shocks of different velocities, ranging from relativistic shocks associated with active galactic nuclei, gamma-ray bursts or microquasars to weakly- or non-relativistic flows as those observed in supernova remnants. Recent observations of synchrotron emission in protostellar jets are important not only because they extend the range over which the acceleration process works, but also because they allow to determine the jet and/or interstellar magnetic field structure, thus giving insights on the jet ejection and collimation mechanisms. In this paper, we compute for the first time polarized (synchrotron) and non polarized (thermal X-ray) synthetic emission maps from axisymmetrical simulations of magnetized protostellar jets. We consider models with different jet velocities and variability, as well as toroidal or helical magnetic field. Our simulations show that variable, low-density jets with velocities $sim$ 1000 km s$^{-1}$ and $sim$ 10 times lighter than the environment can produce internal knots with significant synchrotron emission, and thermal X-rays in the shocked region of the leading bow shock moving in a dense medium. While models with a purely toroidal magnetic field show a very large degree of polarization, models with helical magnetic field show lower values and a decrease of the degree of polarization, in agreement with observations of protostellar jets.
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We aim at studying the causal link between the knotty jet structure in CARMA 7, a young Class 0 protostar in the Serpens South cluster, and episodic accretion in young protostellar disks. We used numerical hydrodynamics simulations to derive the protostellar accretion history in gravitationally unstable disks around solar-mass protostars. We compared the time spacing between luminosity bursts Deltatau_mod, caused by dense clumps spiralling on the protostar, with the differences of dynamical timescales between the knots Deltatau_obs in CARMA 7. We found that the time spacing between the bursts have a bi-modal distribution caused by isolated and clustered luminosity bursts. The former are characterized by long quiescent periods between the bursts with Deltatau_mod = a few * (10^3-10^4) yr, whereas the latter occur in small groups with time spacing between the bursts Deltatau_mod= a few * (10-10^2) yr. For the clustered bursts, the distribution of Deltatau_mod in our models can be fit reasonably well to the distribution of Deltatau_obs in the protostellar jet of CARMA 7, if a certain correction for the (yet unknown) inclination angle with respect to the line of sight is applied. The K-S test on the model and observational data sets suggests the best-fit values for the inclination angles of 55-80 deg., which become narrower (75-80 deg.) if only strong luminosity bursts are considered. The dynamical timescales of the knots in the jet of CARMA 7 are too short for a meaningful comparison with the long time spacings between isolated bursts in our models. The exact sequences of time spacings between the luminosity bursts in our models and knots in the jet of CARMA 7 were found difficult to match. (abridged)
X-ray observations of protostellar jets show evidence of strong shocks heating the plasma up to temperatures of a few million degrees. In some cases, the shocked features appear to be stationary. They are interpreted as shock diamonds. We aim at investigating the physics that guides the formation of X-ray emitting stationary shocks in protostellar jets, the role of the magnetic field in determining the location, stability, and detectability in X-rays of these shocks, and the physical properties of the shocked plasma. We performed a set of 2.5-dimensional magnetohydrodynamic numerical simulations modelling supersonic jets ramming into a magnetized medium and explored different configurations of the magnetic field. The model takes into account the most relevant physical effects, namely thermal conduction and radiative losses. We compared the model results with observations, via the emission measure and the X-ray luminosity synthesized from the simulations. Our model explains the formation of X-ray emitting stationary shocks in a natural way. The magnetic field collimates the plasma at the base of the jet and forms there a magnetic nozzle. After an initial transient, the nozzle leads to the formation of a shock diamond at its exit which is stationary over the time covered by the simulations (~ 40 - 60 yr; comparable with time scales of the observations). The shock generates a point-like X-ray source located close to the base of the jet with luminosity comparable with that inferred from X-ray observations of protostellar jets. For the range of parameters explored, the evolution of the post-shock plasma is dominated by the radiative cooling, whereas the thermal conduction slightly affects the structure of the shock.
A number of works reported on the existence of a large scale alignment of the polarization plane of extragalactic sources as well as the alignment of radio-sources structural axes. However, both claims and their interpretation remain controversial. For the first time we explore the parsec-scale jets alignments. Additionally, we use archival polarimetric data at different wavelengths in order to compare relative orientations of the jets and the polarization planes of their emission. Using the flux density distribution in very long baseline interferometry (VLBI) radio maps from the Astrogeo database, we determine the parsec-scale jet orientation for the largest sample of active galactic nuclei (AGN) to date. Employing the method of parallel transport and a sample statistics characterizing the jet orientation dispersion among neighbors, we test whether the identified jets are significantly aligned. We show that the parsec-scale jets in our sample do not demonstrate any significant global alignments. Moreover, the jet direction is found to be weakly correlated with the polarization plane direction at different frequencies.
The formation of brown dwarfs (BDs) due to the fragmentation of proto-stellar disks undergoing pairwise encounters was investigated. High resolution allowed the use of realistic initial disk models where both the vertical structure and the local Jeans mass were resolved. The results show that objects with masses ranging from giant planets to low mass stars can form during such encounters from initially stable disks. The parameter space of initial spin-orbit orientations and the azimuthal angles for each disk was explored. An upper limit on the initial Toomre Q value of ~2 was found for fragmentation to occur. Depending on the initial configuration, shocks, tidal-tail structures and mass inflows were responsible for the condensation of disk gas. Retrograde disks were generally more likely to fragment. When the interaction timescale was significantly shorter than the disks dynamical timescales, the proto-stellar disks tended to be truncated without forming objects. The newly-formed objects had masses ranging from 0.9 to 127 Jupiter masses, with the majority in the BD regime. They often resided in star-BD multiples and in some cases also formed hierarchical orbiting systems. Most of them had large angular momenta and highly flattened, disk-like shapes. The objects had radii ranging from 0.1 to 10 AU. The disk gas was assumed to be locally isothermal, appropriate for the short cooling times in extended proto-stellar disks, but not for condensed objects. An additional case with explicit cooling that reduced to zero for optically thick gas was simulated to test the extremes of cooling effectiveness and it was still possible to form objects in this case. Detailed radiative transfer is expected to lengthen the internal evolution timescale for these objects, but not to alter our basic results.
We present a multi-epoch (20 years baseline) kinematical investigation of HH52, 53, and 54 at optical and near-IR wavelengths, along with medium and high- resolution spectroscopic analyses, probing the kinematical and physical time variability conditions of the gas along the flows. By means of multi-epoch and multi-wavelength narrow-band images, we derived proper motions, tangential velocities, velocity and flux variability of the knots. Radial velocities and physical parameters of the gas were derived from spectroscopy. Finally, spatial velocities and inclination of the flows were obtained by combining both imaging and spectroscopy. The P.M. analysis reveals three distinct, partially overlapping outflows. In 20 years, about 60% of the knots show some degree of flux variability. Our set of observations apparently indicates acceleration and deceleration in a variety of knots along the jets. For about 20% of the knots, mostly coincident with working surfaces or interacting knots along the flows, a relevant variability in both flux and velocity is observed. We argue that both variabilities are related and that all or part of the kinetic energy lost by the interacting knots is successively radiated. The analysis indicates the presence of very light, ionised, and hot flows, impacting a denser medium. Several knots are deflected. At least for a couple of them (HH54 G and G0), the deflection originates from the collision of the two. For the more massive parts of the flow, the deflection is likely the result of the flow collision with a dense cloud or with clumps.
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