SIMBAD references

Absorption features from the Lyman and Werner bands of interstellar molecular hydrogen were recorded by the Interstellar Medium Absorption Profile Spectrograph (IMAPS) at λ/Δλ=80,000 in the spectra of δ Ori A and ε 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 ζ Ori by Jenkins & Peimbert (1997ApJ...477..265J). 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 δ and ε Ori, which are only a few degrees away in the sky from ζ Ori. The absence of atomic material at a large negative velocity in the spectra of δ and ε Ori (and its presence in ζ Ori) supports a proposal by Jenkins & Peimbert that the line of sight to ζ 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 ζ Ori in the sky, but we present some evidence that seems to indicate that the cloud is more distant, in which case it could not serve as the obstruction. However, it is possible that the outermost extension of a high-speed jet from a star forming within the cloud can explain the high-velocity material and the shock front created by it.

For both stars, the H₂ absorption features are separated into two velocity components. Total H₂ column densities toward δ and ε Ori are 5.5x10¹⁴ and 1.9x10¹⁶ cm^–2, respectively. When these values are compared to the column densities of H I, the fractions of H atoms bound in molecular form 2N(H₂)/[2N(H₂)+N(H I)]=7x10^–6 for δ and 1.3x10^–4 for ε. The rotation temperatures of the molecules with J>2 toward ε 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 δ Ori, the pumping rate is lower and consistent with a general UV flux level in the plane of the Galaxy.