Deflection of stellar jets by ambient magnetic fields

We deal with the question whether the deflection of stellar jets/outflows t hat is observed for example in L 1221 can be caused by strong ambient magne tic fields. We present results of fully three-dimensional (3D) numerical ma gnetohydrodynamics (MHD) simulations of adiabatic jets propagating into an initially homogeneous medium containing an oblique magnetic field. In our m odel, the jet is longitudinally magnetized by a field of the strength of th e x-component of the ambient magnetic held. We varied the jet velocity v(j) (in the range 2 to 40 km s(-1)), the ambient magnetic field strength B-alp ha (50, 100 mu G), and the angle alpha(10 degrees to 70 degrees) between th e initial jet velocity and the ambient field to find the dependence of the critical velocity v(cr) (up to which a jet is deflected by the magnetic hel d) on B-alpha and alpha. Our numerical results indicate that v(cr) is a dec reasing function of the field angle alpha. We find a nearly linear dependen ce of v(cr) on the ambient field strength B-alpha. Our numerical results ar e in good qualitative and rough quantitative agreement with an analytical m odel derived from a two-dimensional consideration of the ram pressure balan ce governing the motion of the jet head. This model predicts an unrealistic ally high ambient field strength of similar to 1000 mu G to deflect a jet o f 100 km s(-1) (appropriate to stellar jets). Applying our results to L 122 1 and HH 110/HH 270 we thus suggest that in these sources the jet/outflow m ust interact with a relatively strong magnetic field (greater than or equal to 100 mu G) contained in a dense clump of ambient gas (n greater than or equal to 10(4) cm(-3)) if deflection is to be caused by magnetic forces. In the case of L 1221 two other possible explanations of the observational da ta can be given by assuming (i) that only a low speed wind surrounding the driving jet is bent by an ambient field while the outflow-driving jet itsel f remains straight and invisible due to high obscuration by ambient materia l, or (ii) that the outflow contains no high speed driving jet at all and c an therefore be deflected by a moderate ambient field (less than or similar to 200 mu G).