Orion's Cloak - A rapidly expanding shell of gas centered on the Orion OB1 association
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Abstract
The structure of the interstellar gas surrounding the Orion OB1 association and the neighboring lambda Orionis association is detailed. UV absorption lime spectra of various ionization stages of C, N, Si and S in the directions of 12 stars were obtained by means of the spectrometer on board the Copernicus satellite. The presence of a shell of material surrounding the two associations and expanding at 100 to 120 km/sec, designated Orion's Cloak, was revealed, together with sporadically occurring higher column density matter at lower velocities. Results are interpreted to indicate the presence of a rapidly moving radiative shock outside the H II region of the association stars and inside this feature, a lower velocity, higher column density cloud which appears to be directly ionized by association stars. It is suggested that the gas features are caused by the effects of a recent supernova and of multiple supernovae, stellar winds and rocket-accelerated clouds in addition to stellar ionization.