Tearing the Veil: interaction of the Orion Nebula with its neutral
  environment
  We present HI 21cm observations of the Orion Nebula, obtained with the Karl
G. Jansky Very Large Array, at an angular resolution of 7.2"x5.7" and a
velocity resolution of 0.77 km/s. Our data reveal HI absorption towards the
radio continuum of the HII region, and HI emission arising from the Orion Bar
photon-dominated region (PDR) and from the Orion-KL outflow. In the Orion Bar
PDR, the HI signal peaks in the same layer as the H2 near-infrared vibrational
line emission, in agreement with models of the photodissociation of H2. The gas
temperature in this region is approximately 540K, and the HI abundance in the
interclump gas in the PDR is 5-10% of the available hydrogen nuclei. Most of
the gas in this region therefore remains molecular. Mechanical feedback on the
Veil manifests itself through the interaction of ionized flow systems in the
Orion Nebula, in particular the Herbig-Haro object HH202, with the Veil. These
interactions give rise to prominent blueward velocity shifts of the gas in the
Veil. The unambiguous evidence for interaction of this flow system with the
Veil shows that the distance between the Veil and the Trapezium stars needs to
be revised downwards to about 0.4pc. The depth of the ionized cavity is about
0.7pc, which is much smaller than the depth and the lateral extent of the Veil.
Our results reaffirm the blister model for the M42 HII region, while also
revealing its relation to the neutral environment on a larger scale.
