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 molecular hydrogen were recorded by the Interstellar Medium Absorption Profile Spectrograph (IMAPS) at λ/∆λ = 80, 000 in the spectra of δ Ori A and ǫ 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 (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 δ and ǫ 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 ǫ 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 wind-like 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 ǫ Ori are 5.5 × 10 14 and 1.9 × 10cm, respectively. When these values are compared to the Princeton University Observatory, Princeton, NJ 08544 Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085 Present address: Dept. of Astronomy, Whitman College, 345 Boyer Ave., Walla Walla, WA 99362. Code 681, NASA Goddard Space Flight Center, Greenbelt, MD 20771