A study of diffuse southern Ha nebulae

Abstract

The result of a photographic survey for diffuse Ha emission nebulosities (HII regions) in the Southern Milky Way from galactic longitude 1+190° to 350° are given. A catalogue, which includes a number of new objects, lists positions and dimensions of 85 physically separate H II regions together with details of the exciting stars and their distance moduli when available. The faintest objects detected have n3=1, where n3 is the number of third level hydrogen atoms in a column 1 cm2 in cross-section passing through the nebula in the direction of the line of sight. A scheme of classification describing the large scale structural features of H II regions is suggested. The main criterion is the radial variation of surface brightness within the nebula. Most H II regions may then be arranged in an almost continuous sequence from those having a very high central surface brightness, (n3 up to 200), which decreased with increasing distance from the centre, (class I), to fainter objects (n3 ˷5) in which most of the emission is concentrated in a ring or in an incomplete ring (class IV). One of the intermediate classes (class III) has a practically uniform surface brightness over a circular disk with the exciting star at the geometrical centre. This class corresponds most closely to Stromgren’s idealised H II regions, which when appear as a special case in a more general sequence of structural forms. The classification is also applicable to emission nebulosities in other stellar systems where only large-scale features may be observed. The statistical relations between the apparent photographic magnitude of the exciting star and the apparent diameter of the nebulosity is discussed and compared with Hubble’s original relation derived from blue-sensitive plate material. The spatial distribution of the Southern HII regions can be studied in only a tentative fashion at present. The result suggest certain details spiral structure in the galaxy.