Tearing the Veil: interaction of the Orion Nebula with its neutral environment

TL;DR

This study uses high-resolution HI 21cm observations to explore the interaction between the Orion Nebula and its surrounding neutral environment, revealing details about the gas phases, feedback mechanisms, and structural models of the region.

Contribution

It provides new high-resolution observational evidence of HI absorption and emission in the Orion Nebula, clarifies the interaction of ionized flows with the Veil, and refines the nebula's structural model.

Findings

HI absorption observed towards the HII region's radio continuum.

HI emission from the Orion Bar PDR and Orion-KL outflow.

Interaction of ionized flows with the Veil causes velocity shifts and revises the nebula's distance estimates.

Abstract

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.