Assigned ID: 389 ----- Attempting Submission 1 (Fri Feb 24 13:26:40 GMT 2012) ----- HST Phase I Proposal 389 (Halpha_N1856.pdf.apt) successfully submitted. ----- Attempting Submission 2 (Fri Feb 24 16:17:43 GMT 2012) ----- ----- Attempting Submission 1 (Mon Jul 09 23:45:24 GMT 2012) ----- Recently, deep HST images provided conclusive evidence that several massive intermediate-age star clusters (SC) in the LMC and SMC present extended main-sequence turn-offs (eMSTOs), and in some cases also dual red clumps. This poses serious questions regarding the mechanisms responsible for SC formation. While most recent studies indicate that the eMSTOs are caused by a range in stellar age of about 150-500 Myr among cluster stars, the obvious and important question is: Why has this not been observed yet in SCs in that age range? Our calculations show that prolonged star formation in SCs requires a cluster escape velocity in excess of about 10 km/s, and there really is only one SC in the Local Group that has the right mass and size to be able to retain mass loss in slow winds of the first stellar generation and be young enough to have some of its second stellar generation being still on the pre-main sequence (PMS) and showing emission-line signatures: the 300 Myr old cluster NGC 1856 in the LMC. Since this SC has no adequate HST imaging in the archive to detect an eMSTO, we propose to obtain deep broad-band imaging in F438W, F555W, F814W with WFC3/UVIS to analyze the MSTO and use F656N to search for Balmer-line emission signatures from PMS stars of the secondary stellar generation. The detection of the PMS phase in this cluster (or lack thereof) will have a lasting impact on our understanding of the eMSTO phenomenon. This SC represents a unique opportunity to render insights on the relative importance of various types of polluter stars of the first generation that have been proposed to be responsible for the chemical enrichment of second-generation stars in star clusters. The target star cluster is observed with WFC3 using the F438W, F555W, and F814W broad-band filters and the F656N narrow-band Halpha filter. For the broad-band filters, we make sure we include one 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 if the target cluster in question is indeed found to host multiple stellar populations. Hence we also include at least two longer exposures per filter to achieve deep photometry. At least two dither positions are used for each target star cluster 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 F555W, F658N, and F814W. This will allow a robust determination of the field population contribution to the star cluster pointing. RESOLVED STELLAR POPULATIONS 12 false 0.0 0.0 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 F438W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F656N WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F658N ACS/WFC Imaging No No F814W 1341876593551 /Users/tpuzia/Science/proposals/hst/cycle20/Halpha_N1856/phase2/13011_ph2_subm.apt/OP-cache/13011_ph2_subm-v01.xml The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. The visit orientation requirements are set to avoid a bright star in the WFC3 field of view. (Tic-Visit :proposal-id 13011 :visit-id "01" :proposal-version -1 :trans-version "20120604" :date "09-Jul-2012 19:29:52.0" :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 4881584 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13011 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13011 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3249 :al-overhead-at-end 80 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3582 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13011 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3360 :al-overhead-at-end 80 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3582 :pcs-gap 0 ) true 2 0