Two jets from the Orion (M42) `proplyds' - kinematics, morphologies and origins

A spatially unresolved velocity feature, with an approaching radial velocity of ~100 km/s with respect to the systemic radial velocity, in a position-velocity array of [O III] 5007 line profiles is identified as the kinematical counterpart of a jet from the proplyd LV 5 (158-323) in the core of the Orion Nebula. The only candidate in HST imagery for this jet appears to be a displaced, ionized knot. Also an elongated jet projects from the proplyd GMR 15 (161-307). Its receding radial velocity difference appears at ~80 km/s in the same position-velocity array. A standard' model for jets from young, low mass stars invokes an accelerating, continuous flow outwards with an opening angle of a few degrees. Here an alternative explanation is suggested which may apply to some, if not all, of the proplyd jets. In this, a bullet’ of dense material is ejected which ploughs through dense circumstellar ambient gas. The decelerating tail of material ablated from the bullet’s surface would be indistinguishable from a continuously emitted jet in current observations.