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9. A 21-cm. study of the Orion region

Published online by Cambridge University Press:  14 August 2015

T. K. Menon
T. K. Menon*
Affiliation:
Harvard College Observatory, Cambridge, Mass., U.S.A.

Extract

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An analysis of the profiles of the 21-cm. radiation from neutral hydrogen promises to be of great importance for a study of the internal motions of specific regions of the Galaxy. The two factors which influence the shape of the profiles are the velocity distribution and the density distribution of the neutral hydrogen atoms in the line of sight. The velocity distribution is essentially determined by three factors (1) galactic rotation, (2) the random motions of the gases, and (3) local peculiar motions as, for example, expansion. In the plane of the Galaxy in any specific direction the isolation of a region of particular interest is made difficult because of the superposition of the radiation along the entire line of sight. Hence regions at intermediate galactic latitudes are more suitable for study of internal motions than regions on or near the galactic equator. Also, for the study of peculiar motions, regions with small galactic rotation terms have the distinct advantage that any prevailing preferential motion will be clearly indicated by the profiles. The Orion region satisfies most of the above requirements. The galactic latitude of the section under consideration falls between − 10° and − 25°, and at the mean galactic longitude of 170° the galactic rotation term in radial velocity amounts to 7 km./sec. at a distance of 500 parsecs. Moreover the Orion region contains many features of considerable interest like the Orion Nebula, the Orion Association, the great arc of ionized hydrogen and many smaller H II regions. The great arc of Barnard (1895) [1] forms part of an almost elliptical ring of emission nebulosity with dimensions 14 × 12°. At the distance of 500 parsecs for the Orion Association these dimensions are of the order of 120 × 105 parsecs. It is of interest to note that the major axis of this ellipse is parallel to the galactic equator. This ellipticity could presumably be caused by galactic rotation, by a galactic magnetic field with lines of force along the spiral arms, or by the rotation of the whole mass itself. Further investigation is necessary to decide which of the above effects is most important.

Type
Part I: Spectral Line Investigations
Information
Symposium - International Astronomical Union , Volume 4: Radio Astronomy , 1957 , pp. 56 - 65
Copyright
Copyright © Cambridge University Press 1957 

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