----- Attempting Submission 1 (Fri Jul 09 15:18:07 GMT 2010) ----- The recent discovery of multiple stellar populations in massive Galactic globular clusters poses a serious challenge for models of star cluster formation and evolution. The finding of multiple main sequences in the massive clusters NGC 2808 and omega Centauri, and multiple sub-giant-branch in NGC 1851 and many other globulars have demonstrated that star clusters are not as simple as we have imagined for decades. Surprisingly the only way to explain the main sequence splitting appears to be Helium enrichment, up to an astonishingly high Y~0.40.An unique angle on this problem can be provided by intermediate-age clusters in the Magellanic Clouds with peculiar main-sequence turn-off morphologies. Recent discoveries, based on ACS data of unparalleled photometric accuracy, have demonstrated that the CMDs of a large fraction of these clusters (~70 %) are not consistent with the simple, single stellar population hypothesis. Explanations for what conditions could give rise to multiple populations in Galactic Globular Clusters remain controversial; this is even more the case for LMC clusters To properly constraint the multipopulation phenomenon in Magellanic Cloud star clusters, we propose deep UV/IR imaging of NGC 1846, a star cluster where multiple populations have already been identified. The proposed observation will allow us to accurately measure the age difference between the stellar populations providing fundamental clues on the formation mechanism. Our simulations of WFC3 performance suggest that we will be able to detect even the main sequence splitting caused by small He differences (Delta Y ~0.02).

UVIS and NIR WFC3, and ACS WFC observations of the Large Magellanic Cloud stellar cluster NGC1846 RESOLVED STELLAR POPULATIONS 12 false 0.0 0.0 false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER J2000 00 00 0.0000 +00 00 0.00 J2000 1278688601346 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. /tmp/OPVisit5639200323210927342.xml (Tic-Visit :proposal-id 12219 :visit-id "01" :proposal-version -1 :trans-version "20100505" :date "09-Jul-2010 17:16:38.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 7837328 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12219 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12219 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3264 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3599 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12219 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3364 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3599 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12219 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3364 :al-overhead-at-end 116 :internal-dur-at-end 701 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3599 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12219 :visit-id "01" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3355 :al-overhead-at-end 0 :internal-dur-at-end 1079 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3599 :pcs-gap 0 ) true 4 0