OB Stellar Associations in the Large Magellanic Cloud: Identification Method

Abstract

We describe an objective method for the identification of stellar OB associations in the Large Magellanic Cloud under the assumption that they are loose, unbound stellar systems with a young OB stellar component. The method is based on star counts and spectral classification. First we detect the areas where an enhancement of star number density occurs above 3 σ of the average field density in large regions. The boundaries at 3 σ provide the size and morphology of the detected stellar concentrations. Further examination at different magnitude ranges allows us to select the systems with a bright stellar component within the detected areas. In the second step, star counts around the peak density of each detected stellar concentration provide a typical value of the projected half-mass radius, in order to calculate the central density using the appropriate mass function slope. The central density, being a crucial parameter for the bound and unbound systems, has been used as a tentative criterion for the distinction between open clusters and associations. Finally, spectral classification from objective-prism plates provides further evidence for the existence of OB-type stars in these concentrations. The faintest magnitude at which the various systems were detected is found to be independent of the presence or absence of gas and varies by up to 4 mag. An explanation for this effect is the possible existence of pre–main-sequence stars that are not visible in the optical region.