Physical conditions in Orion's veil

Orion's veil consists of several layers of largely neutral gas lying between us and the main ionizing stars of the Orion Nebula. It is visible in 21 cm H I absorption and in optical and UV absorption lines of H I and other species. Toward θ¹ Ori C, the veil has two remarkable properties, a high magnetic field (≈100 ,μG) and a surprising lack of H₂, given its total column density. Here we compute photoionization models of the veil to establish its gas density and its distance from θ¹ Ori C. We use a greatly improved model of the H₂ molecule that determines level populations in 10⁵ rotational/vibrational levels and provides improved estimates of H₂ destruction via the Lyman-Werner bands. Our best-fit photoionization models place the veil 1-3 pc in front of the star at a density of 10 ³-10⁴ cm^-3. Magnetic energy dominates the energy of nonthermal motions in at least one of the 21 cm H I velocity components. Therefore, the veil is the first interstellar environment in which magnetic dominance appears to exist. We find that the low ratio of H ₂/H⁰ (<10^-4) is a consequence of high UV flux incident on the veil due to its proximity to the Trapezium stars and the absence of small grains in the region.