The Properties of Molecular Hydrogen toward the Orion Belt Stars from Observations by the Interstellar Medium Absorption Profile Spectrograph

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Abstract
Absorption features from the Lyman and Werner bands of interstellar molecular hydrogen were recorded by the Interstellar Medium Absorption Profile Spectrograph (IMAPS) at a wavelength resolving power R=80,000 in the spectra of delta Ori and epsilon Ori. The objective was to find and study more examples of an unusual phenomenon found for one of the velocity components of H2 in the spectrum of zeta Ori by Jenkins & Peimbert (1997). Specifically, they detected a gradual shift in velocity and broadening for features arising from progressively higher rotational excitations J. This effect appears to be absent in the spectra of both delta and epsilon Ori, which are only a few degrees away in the sky from zeta Ori. The absence of atomic material at a large negative velocity in the spectra of delta and epsilon Ori (and its presence 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 wind-like material with some foreground obstruction. For both stars, the H2 absorption features are separated into two velocity components. Total H2 column densities toward delta and epsilon Ori are 5.5e14 and 1.9e16 cm^-2, respectively. The rotation temperatures of the molecules with J > 2 toward epsilon Ori indicate that the gas is in the general vicinity of the stars that emit UV fluxes capable of rotationally pumping the molecules. For the strongest component of H2 toward delta Ori, the pumping rate is lower and consistent with a general UV flux level in the plane of the Galaxy.

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