3-D SPH Simulations of Radiatively Cooling Magnetized Jets

a r X i v :a s t r o -p h /9808198v 1 19 A u g 19983-D SPH SIMULATIONS OF RADIATIVELY COOLING MAGNETIZED JETS

E.M.DE GOUVEIA DAL PINO AND A.H.CERQUEIRA University of S?a o Paulo,Instituto Astron?o mico e Geof ′isico Av.Miguel St′e fano,4200,04301-904S?a o Paulo,SP,Brazil 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].In this work,we summarize the results of three-dimensional smooth par-ticle magnetohydrodynamics numerical simulations of supermagnetosonic,radiatively cooling jets [3].Two initial magnetic con?gurations (in ～equipar-tition with the gas)are considered:(i)a helical and (ii)a longitudinal ?eld which permeate both the jet and the ambient medium [2].These MHD simulations have been compared with a baseline of previous non-magnetic radiative cooling and adiabatic calculation [e.g.,4,5,6,7].We ?nd that magnetic ?elds have important e?ects on the dynamics and structure of ra-diative cooling jets,especially at the head.Both magnetic ?eld geometries are able to improve jet collimation,although this e?ect is more pronounced in the helical case.In both magnetic con?gurations,the con?ning pressure of the cocoon is able to excite MHD Kelvin-Helmholtz pinch and kink modes that drive low-amplitude internal shocks and wiggling along the beam (see Fig.1).These modes are however,inhibited by the presence of radiatively cooling and by increasing the density contrast,η,between the jet and the

ambient medium [4,5,6,7].As a consequence,the weakness of the induced shocks makes it doubtful that they could produce by themselves the bright knots observed in the overdense,radiatively cooling protostellar jets.Also we ?nd that the presence of a helical ?eld (Fig.1,bottom)suppresses the formation of the clumpy structure which is found to develop at the head of purely hydrodynamical [3]jets by fragmentation of the cold shell of shocked material.On the other hand,a cooling jet embedded in a longi-tudinal magnetic ?eld (Fig.1,top)retains the 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 sug-

2 E.M.DE GOUVEIA DAL PINO AND A.H.CERQUEIRA

gests that a predominantly helical magnetic ?eld con?guration is unlikely at the jet head in those cases

[2].

Figure 1.Figure 1:Left ?gray-scale head of radiative cooling jets with η=3and ambient Mach number M a =24:a hydrodynam-ical jet (HD3r,top),an MHD jet with initial longitudinal B-?eld (ML3r,middle),and an MHD jet with initial helical B-?eld (MH3r,bottom)after they have propagated ≈30R j .Right ?mid-plane velocity ?eld distribution [2].

References

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