Far-Ultraviolet Imaging of the Large Magellanic Cloud Populous Cluster NGC 1978 with WFPC2
Casertano, Stefano; Cole, Andrew A.; Ballester, Gilda E.; Crisp, David; Holtzman, Jon A.; Trauger, John T.; Gallagher, John S., III; Hester, J. Jeff; Westphal, James A.; Mould, Jeremy R.; Hoessel, John G.; Clarke, John T.; Stapelfeldt, Karl R.; Burrows, Christopher J.; Scowen, Paul A.; Griffiths, Richard
Abstract
We have imaged the ∼2.2 billion-year-old Large Magellanic Cloud populous cluster NGC 1978 in the far-ultraviolet and visible with the second Wide Field/Planetary Camera (WFPC2) on the Hubble Space Telescope. The far-ultraviolet images show a sparse stellar field with little apparent density enhancement in the cluster core. The visible images are dominated by the cluster' s first-ascent and second-ascent red giants, which are completely invisible to the far-ultraviolet filter. No evidence for a hot horizontal branch population of core-helium-burning stars is seen; nor is there any apparent indication of a significant blue straggler population. These results suggest that the presence of a rich, young population of field stars in the NGC 1978 region is responsible for the unusual location of the cluster in the integrated light color-color plots produced by lUE.