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 theta super(1) Ori C, the veil has two remarkable properties, a high magnetic field (~100 mu G) and a surprising lack of H sub(2), given its total column density. Here we compute photoionization models of the veil to establish its gas density and its distance from theta super(1) Ori C. We use a greatly improved model of the H sub(2) molecule that determines level populations in 10 super(5) rotational/vibrational levels and provides improved estimates of H sub(2) 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 super(3)-10 super(4) cm super(-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 sub(2)/H super(0) (