Interstellar Structure of the Orion Region. I.

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Abstract
This paper investigates the distribution and motions of neutral hydrogen gas inside the giant H ii ring in the Orion region. The observational material consists of the profiles of the 21-cm line of neutral hydrogen obtained with the Harvard 24-foot radio telescope. It is shown that the H ii ring is unlik]ey to be the edge of a sphere. The total mass of hydrogen, both ionized and neutral, inside the H ii ring is found to be about 110000 solar masses. The shapes of the 21-cm profiles are interpreted on the following mode]. The H ii ring is assumed to be the edge of an almost spheroidal mass of hydrogen, the central portion of which is at rest and the outer shell expanding with a velocity of the order of to km/sec. On the assumption that the initial expansion began from the center of the H ii ring, it is found that the expansion interval is about 2.4 X 108 years. The center of expansion agrees closely with that of the three stars in Columbae, AE Aurigae, and 53 Arietis found by Blaauw and Morgan. The variation in radial velocity of the peaks of the 21-cm profiles is interpreted to be due to non-spherically symmetrical mass distribution inside the H ii ring. The present theories of stellar associations are reviewed and shown to be inadequate to interpret all the observations in the case of the Orion region.

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