Orion's Veil. IV. H 2 Excitation and Geometry

The foreground Veil of material that lies in front of the Orion Nebula is the best studied sample of the interstellar medium because we know where it is located, how it is illuminated, and the balance of thermal and magnetic energy. In this work, we present high-resolution STIS observations toward the Trapezium, with the goal of better understanding the chemistry and geometry of the two primary Veil layers, along with ionized gas along the line of sight. The most complete characterization of the rotational/vibrational column densities of H2 in the almost purely atomic components of the Veil are presented, including updates to the Cloudy model for H2 formation on grain surfaces. The observed H2 is found to correlate almost exclusively with Component B. The observed H2, observations of CI, CI*, and CI**, and theoretical calculations using Cloudy allow us to place the tightest constraints yet on the distance, density, temperature, and other physical characteristics for each cloud component. We find the H2 excitation spectrum observed in the Veil is incompatible with a recent study that argued that the Veil was quite close to the Trapezium. The nature of a layer of ionized gas lying between the Veil and the Trapezium is characterized through the emission and absorption lines it produces, which we find to be the blueshifted component observed in S iii and P iii absorption. We deduce that, within the next 30–60 thousand years, the blueshifted ionized layer and Component B will merge, which will subsequently merge with Component A in the next one million years.