2000MNRAS.315...11L

We report the first infrared proper motion measurements of the Herbig-Haro objects in OMC-1 using a 4-yr time baseline. The [Feii]-emitting bullets are moving of the order of 0.08arcsec per year, or at about 170km.s^–1. The direction of motion is similar to that inferred from their morphology. The proper motions of H₂-emitting wakes behind the [Feii] bullets, and of newly found H₂ bullets, are also measured. H₂ bullets have smaller proper motion than [Feii] bullets, while H₂ wakes with leading [Feii] bullets appear to move at similar speeds to their associated bullets. A few instances of variability in the emission can be attributed to dense, stationary clumps in the ambient cloud being overrun, setting up a reverse-oriented bullet. Differential motion between [Feii] bullets and their trailing H₂ wakes is not observed, suggesting that these are not separating, and also that they have reached a steady-state configuration over at least 100yr. The most distant bullets have, on average, larger proper motions, but are not consistent with free expansion. Nevertheless, an impulsive, or short-lived (≪1000yr), duration for their origin seems likely.