0 Assigned ID: 489 ----- Attempting Submission 1 (Fri Apr 08 14:19:55 GMT 2016) ----- HST Phase I Proposal 489 (n1831prop.apt) successfully submitted. Receipt: # 489-1 ----- Attempting Submission 2 (Fri Apr 08 20:51:53 GMT 2016) ----- 0 ----- Attempting Submission 1 (Thu Jul 14 16:00:16 GMT 2016) ----- Recently, deep color-magnitude diagrams (CMDs) from HST data revealed that several massive intermediate-age star clusters in the Magellanic Clouds exhibit extended main-sequence turn-offs (eMSTOs), and in some cases also dual red clumps. This poses serious questions regarding the mechanisms responsible for the formation of massive star clusters and their well-known light-element abundance variations. The nature of eMSTOs is currently a hotly debated topic of study. Several recent studies indicate that the eMSTOs are caused by an age spread of about 100-500 Myr among cluster stars, while other studies indicate that eMSTOs can be caused by a coeval population in which the relevant stars span a range of rotation velocities. Formal evidence to (dis-)prove either scenario still remains at large, mainly because the available stellar tracks that incorporate the effects of rotation are only available for masses > 1.7 Msun whereas the stars in the known eMSTOs of intermediate-age clusters are less massive. To circumvent this issue, we identified a massive star cluster in the Large Magellanic Cloud (LMC) that has the right dynamical properties to host an eMSTO along with an age at which the effects of age spreads to CMD morphology are substantially different from those of spreads of rotation rates: the ~600 Myr old cluster NGC 1831. We propose to obtain deep WFC3/UVIS imaging with filters F336W and F814W to analyze the morphologies of the MSTO and upper MS regions of NGC 1831 at high precision and compare with model predictions. This will have a lasting impact on our understanding of the eMSTO phenomenon and of star cluster formation in general. The target star cluster is observed with WFC3 using the F336W and F814W broad-band filters. We make sure we include one F814W exposure that does not saturate the brightest stars expected from its age mentioned in the literature. On the other hand, we need color uncertainties less than 0.05 mag down to well beyond the estimated magnitude of the main sequence turnoff to achieve an accurate age determination and to enable a robust identification of multiple or broad turnoff structures. Hence we also include at least two longer exposures per filter to achieve deep photometry. At least two dither positions are used for each filter to allow a robust elimination of hot pixels and cosmic ray hits during the analysis. ACS is used in parallel to the primary WFC3 exposures, using the filters F435W and F814W. This will allow a robust determination of the field population contribution to the star cluster pointing, both in terms of the star distribution in the color-magnitude diagram and the overall radial distribution of cluster stars. Used the ORIENT special requirement to avoid bright stars just outside the WFC3 and ACS fields of view, which would cause scattered light issues. Stellar Populations 12 false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F814W 1468511507054 The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. /Users/goudfroo/propo/HST/Cycle24/14688/OP-cache/n1831_phII-v01.xml (Tic-Visit :proposal-id 14688 :visit-id "01" :proposal-version -1 :trans-version "20160601" :date "14-Jul-2016 11:51:46.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 3442336 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14688 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14688 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3144 :al-overhead-at-end 121 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3478 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14688 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3250 :al-overhead-at-end 116 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3478 :pcs-gap 0 ) true 2 0