Molecular Hydrogen in the Direction of ζ Orionis A

Abstract

A spectrum of ζ Ori A over the wavelength interval 950-1150 Å recorded by the Interstellar Medium Absorption Profile Spectrograph (IMAPS) on the ORFEUS-SPAS I mission shows Lyman and Werner band absorption features from molecular hydrogen in rotational levels J = 0, 1, 2, 3, and 5. Most of the molecules are found in two distinct velocity components. One is at a heliocentric radial velocity of about -1 km s^-1 with log N(H₂) = 14.5 and a rotational temperature T_rot = 950 K, while the other is at +25 km s^-1 with log N(H₂) = 15.9 and T_rot = 320 K. Some extra H₂ exists in a much weaker component [log N(H₂) = 14.0] between the two main peaks.

The H₂ component at -1 km s^-1 exhibits profile shapes that become broader and show small displacements toward more negative velocities as J increases. These changes are inconsistent with a simple interpretation that UV optical pumping in an optically thin, uniform medium creates the H₂ in excited rotational levels. Differential shielding of the UV radiation at certain velocities does not appear to be a satisfactory explanation for the effect.

Evidence from atomic features at other velocities may offer some insight into the origin of this unusual behavior exhibited by the H₂ profiles. Absorption features from moderately ionized atoms at -94 km s^-1 and more highly ionized species at about -36 km s^-1 suggest that along the line of sight to ζ Ori A, there may be a standing bow shock with an initial compression ratio of 2.6. This shock is probably created when a negative-velocity gas flow collides with an obstruction, in this case a neutral cloud at 0 km s^-1. If this interpretation is correct, the H₂ with the changing profiles may represent molecules forming in the postshock gas that is undergoing further compression as it recombines and cools. We suggest that molecules can form initially by associative detachment of H^- in a moving, warm, partly ionized medium behind the front. The H₂ in this area is most conspicuous in the higher J levels. Later, when the gas becomes very cool, neutral, and more compressed as it comes nearly to a halt, it is more easily seen in the lowest J levels. In this part of the medium, the principal way of producing H₂ should be from reactions on the surfaces of dust grains, as one expects for quiescent interstellar clouds.