The properties of molecular hydrogen toward the Orion belt stars from observations by the interstellar medium absorption profile spectrograph

Absorption features from the Lyman and Werner bands of interstellar molecul ar hydrogen were recorded by the Interstellar Medium Absorption Profile Spe ctrograph (IMAPS) at lambda/Delta lambda = 80,000 in the spectra of delta O ri A and epsilon Ori. The objective was to find and study more examples of an unusual phenomenon found for one of the velocity components of H-2 in th e spectrum of zeta Ori by Jenkins & Peimbert (1997). Specifically, they det ected a gradual shift in velocity and broadening for features arising from progressively higher rotational excitations J. This effect appears to be ab sent in the spectra of both delta and epsilon Ori, which are only a few deg rees away in the sky from zeta Ori. The absence of atomic material at a lar ge negative velocity in the spectra of delta and epsilon Ori (and its prese nce in zeta Ori) supports a proposal by Jenkins & Peimbert that the line of sight to zeta intercepts a bow shock facing away from us, perhaps created by the collision of windlike material with some foreground obstruction. One edge of the molecular cloud complex Lynds 1630 is situated close to I Ori in the sky, but we present some evidence that seems to indicate that the cl oud is more distant, in which case it could not serve as the obstruction. H owever, it is possible that the outermost extension of a high-speed jet fro m a star forming within the cloud can explain the high-velocity material an d the shock front created by it. For both stars, the H-2 absorption features are separated into two velocity components. Total H-2 column densities toward delta and epsilon Ori are 5. 5 x 10(14) and 1.9 x 10(16) cm(-2), respectively. When these values are com pared to the column densities of H I, the fractions of H atoms bound in mol ecular form 2N(H-2)/[2N(H-2) + N(H I)] = 7 x 10(-6) for delta and 1.3 x 10( -4) for epsilon. The rotation temperatures of the molecules with J > 2 towa rd epsilon Ori indicate that the gas is in the general vicinity of the star s that emit UV fluxes capable of rotationally pumping the molecules. For th e strongest component of H-2 toward delta Ori, the pumping rate is lower an d consistent with a general UV flux level in the plane of the Galaxy.