The spatial distributions of blue main-sequence stars in Magellanic Cloud star clusters

The color-magnitude diagrams (CMDs) of young star clusters show that, particularly at ultraviolet wavelengths, their upper main sequences (MSs) bifurcate into a sequence comprising the bulk population and a blue periphery. The spatial distribution of stars is crucial to understand the reasons for these distinct stellar populations. This study uses high-resolution photometric data obtained with the Hubble Space Telescope to study the spatial distributions of the stellar populations in seven Magellanic Cloud star clusters. The cumulative radial number fractions of blue stars within four clusters are strongly anti-correlated with those of the high-mass-ratio binaries in the bifurcated region, with negative Pearson coefficients < -0.7. Those clusters generally are young or in an early dynamical evolutionary stage. In addition, a supporting N-body simulation suggests the increasing percentage of blue-MS stars from the cluster centers to their outskirts may be associated with the dissolution of soft binaries. This study provides a different perspective to explore the MS bimodalities in young clusters and adds extra puzzles. A more comprehensive study combined with detailed simulations is needed in the future.
Comment: 19 pages, 9 figures, 3 tables. Accepted for publication in ApJ