H Emission From Point Sources In The Magellanic Clouds

Methods

Many, if not most, emission-line objects in the Magellanic Clouds have been identified through the detection of H on photographic plates. Although a single direct exposure in H provides a ready means of identifying extended objects, it is insufficient for the identification of point sources with emission lines, for these appear indistinguishable from normal stars. What is needed is information on the H line itself and on the neighbouring continuum as well, so that emission-line objects can stand out by appearing different in these two observations. The continuum data can be provided by either obtaining a separate direct plate through an adjacent waveband (or a broader waveband which includes but dilutes the H emission) or making a single exposure through an objective prism.

The latter technique has been the more commonly used, especially in the days before fast plate measuring machines and powerful computers. However, in crowded fields such as those of the Magellanic Clouds, it suffers somewhat from the overlapping of neighbouring images. Consequently, most work has been carried out through restricted wavebands sufficiently narrow to minimise this problem. On the other hand, the large numbers of faint stars cause more of a problem when comparing two direct plates because, whereas they are so blended on objective-prism plates that they merely merge into the sky background, they remain as individual objects on direct plates, with the consequence that complex software is required for separating the blended images and for matching the plates. Also, the lists of objects found by comparing two plates taken at different times will include large numbers of variable stars that must be eliminated in another way, usually through comparison with a third plate.

Similar work has been carried out using other emission lines such as [OIII]5007 for planetary nebulae and HeII4686 for WR and symbiotic stars.

© Copyright Astronomical Society of Australia 1997