Internal Velocities in the Orion Nebula: Large Proper-Motion Features

The proper motions of Herbig-Haro (HH) objects in the Orion Nebula were measured with a ±10 km s -1 accuracy using the Hubble Space Telescope Wide Field Planetary Camera 2 images in [S II], [N II], Hα, and [O III], taken 4–6 yr apart. Seven HH flows in the outer region as well as seven HH objects in the inner region of the BN-KL complex were identified. The H 2 finger system was confirmed to be created by an explosive event that took place approximately 1000 yr ago. We found a new HH flow toward the northwest originating from the vicinity of OMC-1S, which may be a part of the low-velocity bipolar flows centered at FIR 4. The proper motion of HH 202 was also measured with high accuracy for the first time. The proper-motion vectors of HH 202 and HH 203/204 are aligned well with their projected symmetric axis, which may indicate that they emanated from the same source. The proper-motion measurements in various emission lines provide generally the same results in the wide range of velocity from 20 to 400 km s -1 , as expected for shocks in a steady state.