Molecular Hydrogen in the Direction of zeta Ori A

Abstract

Lyman and Werner band absorption features from interstellar molecular hydrogen in rotational levels J = 0, 1, 2, 3 and 5 were observed in the spectrum of zeta Ori A by the Interstellar Medium Absorption Profile Spectrograph (IMAPS). Most of the molecules are found in two distinct velocity components, with one of them showing profile shapes that become broader and show small displacements toward more negative velocities as J increases. This change in profile shapes, along with evidence from atomic lines at high negative velocities, supports an interpretation that the molecules are initially created by associative detachment of H- in a partially ionized, warm gas that is cooling and recombining behind a stationary shock front. The front probably arises from an interaction of high speed gas from the Orion stars and the ambient medium. These molecules are most conspicuous in the high J levels. Later, when the gas becomes much cooler, mostly neutral, and more compressed as it comes nearly to a halt, the H2 is primarily created by reactions on the surfaces of dust grains and is more easily seen in the lowest J levels.