0 Assigned ID: 523 ----- Attempting Submission 1 (Fri Mar 06 16:12:09 GMT 2020) ----- HST Phase I Proposal 523 successfully submitted. Receipt: # 523-1 ----- Attempting Submission 2 (Fri Mar 06 21:00:01 GMT 2020) ----- 0 ----- Attempting Submission 1 (Mon Jun 29 19:13:53 GMT 2020) ----- HST Phase II Proposal 16282 successfully submitted. Receipt: # 16282-1 ----- Attempting Submission 2 (Mon Jun 29 19:49:00 GMT 2020) ----- HST Phase II Proposal 16282 successfully submitted. Receipt: # 16282-2 ----- Attempting Submission 3 (Mon Jun 29 20:54:32 GMT 2020) ----- HST Phase II Proposal 16282 successfully submitted. Receipt: # 16282-3 ----- Attempting Submission 4 (Mon Jun 29 21:29:55 GMT 2020) ----- Globular Clusters (GCs) in the Milky Way are the primary laboratories for establishing the ages of the oldest stellar populations and for measuring the color-magnitude relation of stars. The gold standard for these studies has involved high-precision visible light investigations with the Hubble Space Telescope (HST). However, the shape of the color-magnitude relation in the visible bandpasses offers little leverage to disentangle the effects of distance, reddening, and metallicity, and these uncertainties impact our derived age measurements for GCs. Recently, a new feature has been observed in several HST WFC3-IR color-magnitude diagrams (CMDs) of nearby GCs. At low stellar masses, the stellar main sequence in an infrared (IR) CMD exhibits a sharp "kink" (due to opacity effects in M dwarfs), such that lower mass and cooler dwarfs become bluer in the F110W - F160W color baseline and not redder. As demonstrated by Correnti et al. (2016, 2018), this inversion of the color- magnitude relation offers a new opportunity to fit GC properties in the IR baseline, and to reduce their uncertainties. Here, we propose a 1-orbit HST WFC3-IR program to measure the color-magnitude relation of stars (down to 2.5 mag below the MS "kink") for the metal-rich bulge cluster NGC6553. We will establish the most accurate age for the cluster to date, with sub-Gyr precision. Combining these observations with the sample analyzed by Correnti et al. (2016, 2018), will establish an independent and sensitive test to the age-metallicity relation of clusters. The target cluster NGC6553 is observed with WFC3/IR using the F110W and F160W broad-band filters. In the target cluster, we expect the magnitude of the kink to occur at M_F110W ~ 5.9 mag and M_F160W ~ 5.1 mag which translates to an apparent magnitude of F110W ~ 20.6 mag at the distance of the cluster. To accurately map the shape of the main sequence and the color-magnitude relation below the main sequence kink we need high signal-to-noise photometry ~ 4 magnitudes below this limit. To resample the PSF, mitigate hot pixels, and minimize errors from flat fielding we apply a 4-point dither pattern. We use the STEP100 sample sequence with 9 samples for each filters. Observations are arranged in 1 visit that covers the available orbit. We sligthly offset our pointing from the target center in order to have a complete overlap with the field of view of the existing WFC3/UVIS observations (GO-15232) and to avoid a sampling of only the innermost region of the cluster. We are not imposing any constraints on the observations orient nor any special requirements. All the members of our Team have strong expertise in imaging data reduction for deep and accurate PSF-fitting stellar photometry in crowded fields. They have led, or participated to, several HST/WFC2, ACS, and WFC3 projects devoted to the understanding of the formation and evolution of globular clusters (GCs) and their parent populations. We have developed tools to achieve high-precision photometric characterization from ACS, WFC3-UVIS, and WFC3-IR observations of GCs (e.g., Kalirai et al. 2012, Cohen et al. 2018). Moreover, Drs Brown, Cohen, and Gennaro are recognized experts in Bulge studies, focused on the Bulge GC systems and Bulge star formation history and initial mass function. For example, Dr. Brown is the PI of the Galactic Bulge Treasury Program (Brown et al. 2009, 2010). In the last decade, the members of our Team have published hundreds of refereed papers based on their proprietary HST data. The feasibility of this project and the ability of this Team to achieve the proposed goals has been demonstrated from the analysis of archival and new (GO-14124, PI: Correnti) WFC3-IR observations of five globular clusters which led to the publication of two refereed papers (Correnti et al. 2016, 2018). Stellar Populations and the Interstellar Medium Stellar Physics and Stellar Types 6 false false false false false false false SUPPORTED STELLAR CLUSTER BULGE GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/IR Imaging No No F110W WFC3/IR Imaging No No F160W 1593465842872 /private/var/folders/dm/dc_d1hss43g3x89tg6hc_23c00019q/T/OPVisit9165827123647349982.xml The scheduling data was generated by CASM 28.2. CASM was run in Phase II mode. 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