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

A spatially unresolved velocity feature, with an approaching radial velocity of ≈100 km s−1 with respect to the systemic radial velocity, in a position–velocity array of [O iii] 5007-A 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 Hubble Space Telescope (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−1 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 surface of the bullet would be indistinguishable from a continuously emitted jet in current observations.