0 Assigned ID: 652 ----- Attempting Submission 1 (Fri Apr 07 17:12:56 GMT 2017) ----- 0 ----- Attempting Submission 1 (Thu Jul 20 10:04:41 GMT 2017) ----- Searching for fossil fragments of the Galactic bulge formation process We have discovered that the stellar system Terzan5 (Ter5) in the Galactic bulge harbors stellar populations with very different IRON content (delta[Fe/H]~1 dex, Ferraro+09, Nature 462, 483) and AGES (12 Gyr and 4.5 Gyr for the sub-solar and super-solar metallicity populations, respectively, Ferraro+16, ApJ,828,75). This evidence demonstrates that Ter5 is not a globular cluster, and identifies it as (1) a site in the Galactic bulge where recent star formation occurred, and (2) the remnant of a massive system able to retain the iron-enriched gas ejected by violent supernova explosions. The striking chemical similarity between Ter5 and the bulge opens the fascinating possibility that we discovered the fossil remnant of a pristine massive structure that could have contributed to the Galactic bulge assembly. Prompted by this finding, here we propose to secure deep HST optical observations for the bulge stellar system Liller1, that shows a similar complexity as Ter5, with evidence of two stellar populations with different iron content. The immediate goal is to properly explore the main sequence turnoff region of the system for unveiling possible splits due to stellar populations of different ages. As demonstrated by our experience with Ter5, the requested HST observations, in combination with the K-band diffraction limited images that we already secured with GeMS-Gemini, are essential to achieve this goal. The project will allow us to establish if other fossil remnants of the bulge formation epoch do exist, thus probing that the merging of pre-evolved massive structures has been an important channel for the formation of the Galactic bulge. We plan to secure optical images in two classical ACS passbands (F814W and F606W) of the stellar system Liller1 in the galactic bulge. The observations are organized in just one visit: 3 orbits are needed to secure 6 exposures (each 1300 sec -long) in the F606W filter and 2 orbits to secure 6 exposures (each ~850 sec-long) in the F814W filter. All the exposures have been dithered by using appropriate offsets. Parallel observation with the WFC3 in the F606W,F814W,F110W and F160W filters are also planned in order to sample the bulge field population in the region surrounding the cluster. Stellar Populations 0 false false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER J2000 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W The scheduling data was generated by CASM 25.2. 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