THE HELIUM TO HYDROGEN RATIO IN THE SMALL MAGELLANIC CLOUD

Abstract

The Small Magellanic Cloud contains a number of emission nebulosities of which the brightest is NGC 346, illustrated in Plate I. The brighter part of the nebula has dimensions of about V X 3', while the fainter outer portion has an over-all size of about 5' X 10'. The spatial relationship of NGC 346 with respect to other objects in the Small Cloud is shown in Plate II of a recent paper by Hugh Johnson.1 He notes that NGC 346 is comparable in size with Gum's nebula in Vela-Puppis, the largest known H il region in the galaxy. However, NGC 346 is much brighter. In view of Schmidt's suggestion that the Small Magellanic Cloud might be deficient in helium,2 a determination of the He/H ratio in this object appeared to be of interest. Our study of NGC 346 was part of a general spectrophotometric investigation of gaseous nebulae in the Magellanic Clouds. We succeeded in observing most of the brighter objects listed in Henize's catalog.3 Most of the LMC nebulae were observed at Mount Bingar ; those in the SMC were observed with the 50-inch and 74-inch telescopes at Mount Stromlo. From these measurements it is possible to obtain not only the relative intensities of the principal lines, but also the absolute integrated monochromatic brightnesses of the smaller nebulae and the surface brightness of larger ones. In order to make these spectrophotometric measurements of emission nebulosities, we used a photoelectric spectrum scanner