A spectrum of \zeta Ori A over the wavelength interval 950 - 1150Å recorded by 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 ( { H } _ { 2 } ) = 14.5 and a rotational temperature T _ { rot } = 950 K , while the other is at +25 km s ^ { -1 } with \log N ( { H } _ { 2 } ) = 15.9 and T _ { rot } = 320 K. Some extra H _ { 2 } exists in a much weaker component ( \log N ( { H } _ { 2 } ) = 14.0 ) between the two main peaks .

The H _ { 2 } 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 H _ { 2 } 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 on the origin of this unusual behavior exhibited by the H _ { 2 } 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 \zeta 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 H _ { 2 } with the changing profiles may represent molecules forming in the postshock gas flow 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 H _ { 2 } 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 H _ { 2 } should be from reactions on the surfaces of dust grains , as one expects for quiescent interstellar clouds .