The structure and kinematics of the HH 444 photoionized jet in the sigma Orionis region

HH 444 is a stellar jet which is immersed in the H II region photoionized b y sigma Orionis. Spatially resolved, high spectral resolution observations of the H alpha and [N II] lambda lambda 6548, 6584 lines have been obtained for this object. These observations reveal for the first time the detailed velocity structure of the stellar jet and its associated bow-shock. The li ne profiles are modeled with gasdynamic numerical simulations, using a 3D c ode that also includes the transfer of the ionizing radiation from an exter nal source (i.e., sigma Orionis for the case of HH 444). In order to reprod uce the observed characteristics of the line profiles, we find that an axia lly peaked initial velocity profile for the jet and a non-zero initial open ing angle for the ow with a time-variable ejection velocity is required. Wi th these three ingredients we can reproduce the observed "acceleration" of the material along the jet beam, the rapid H alpha intensity drop for incre asing distances from the source, and the knotty structure of the jet, respe ctively.