Physical Conditions in Orion’s Veil

Orion's veil consists of several layers of largely neutral gas lying betweenus and the main ionizing stars of the Orion nebula. It is visible in 21cm H Iabsorption and in optical and UV absorption lines of H I and other species.Toward the Trapezium, the veil has two remarkable properties, high magneticfield (~100 microGauss) and a surprising lack of molecular hydrogen given itstotal hydrogen column density. Here we compute photoionization models of theveil to establish its gas density and its distance from the Trapezium. We use agreatly improved model of the hydrogen molecule that determines levelpopulations in ~1e5 rotational/vibrational levels and provides improvedestimates of molecular hydrogen destruction via the Lyman-Werner bands. Ourbest fit photoionization models place the veil 1-3 pc in front of the star at adensity of 1e3-1e4 cubic centimeters. Magnetic energy dominates the energy ofnon-thermal motions in at least one of the 21cm H I velocity components.Therefore, the veil is the first interstellar environment where magneticdominance appears to exist. We find that the low ratio of molecular to atomichydrogen (< 1e-4) is a consequence of high UV flux incident upon the veil dueto its proximity to the Trapezium stars and the absence of small grains in theregion.Comment: 45 pages, 20 figures, accepted for publication in Ap