15945( 3) - 23-Jul-2019 16:25:29 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 15945 Version: 3 Check-in Time: 23-Jul-2019 16:25:29 STScI Edit Number: 0 Title A two orbits proposal to solve the age spread dilemma in young Magellanic Clouds clusters ------------------------------------------------------------------------------------ Type Cycle GO 27 ------------------------------------------------------------------------------------ Investigators Contact? PI: Mr. Giacomo Cordoni Universita degli Studi di Padova Y CoI: Prof. Antonino Paolo Milone Universita degli Studi di Padova N CoI: Dr. Michele Cignoni Universita di Pisa N CoI: Dr. Edoardo Prospero Lagioia Universita degli Studi di Padova N CoI: Dr. Tailo Marco Universita degli Studi di Padova N CoI: Dr. Anna Fabiola Marino University of Padova N ------------------------------------------------------------------------------------ Abstract This is a two-orbits proposal to explore pre-main sequence (pre-MS) stars in Large Magellanic Cloud young clusters to assess, once and for all, if multiple bursts of star formation can be supported by these systems. The discovery that nearly all star clusters younger than 2Gyr in both Magellanic Clouds exhibit double/extended main-sequence turn offs (eMSTOs) has been one of the major advances made by HST in stellar astrophysics. Despite a huge effort has been undertaken to understand these observations, the mechanism responsible for the eMSTO is still controversial. It has been suggested that the eMSTO is due to coeval stellar populations with different rotation, but it seems that rotation alone is not able to reproduce the eMSTOs. On the other hand, the eMSTO could be due to a spread in age, so that these clusters would be the younger counterparts of old globular clusters with multiple populations. We propose a novel approach to disentangle between age and rotation by using the Turn-On (TOn), where the pre-MS reaches the main sequence in the color-magnitude diagram (CMD). The most remarkable feature introduced by different ages on this region of the CMD would be the appearance of multiple or broaden TOn peaks in the luminosity function. Specifically a single narrow peak is expected in the case of a single generation, whereas multiple/broaden TOn peaks would imply multiple stellar generations. The proposed observations will allow us, to understand, whether the age spread contributes to eMSTO in young clusters or not, and will be surely seminal for future investigation of the still-eluding multiple stellar populations phenomenon. ------------------------------------------------------------------------------------ Observations Description ------------------------ We plan deep photometry to reach a S/N of about 50, at a magnitude of 24.0 in the F814W band and study the luminosity function (LF). Since deep observations are needed to well sample the LF and the CMD at the faintest magnitudes, we need filters that provide a high S/N in a limited exposure time, and the F606W and F814W filters fulfill exactly these requirements. According to the WFC3 Exposure Time Calculator (ETC), to reach a S/N sim 50, needed for our goal, at a magnitude of 24.0 in the F814W band we need an exposure time of at least 3000 seconds. On the other hand, the F606W 15945( 3) - 23-Jul-2019 16:25:29 - [ 2] requires a shorter exposure time to reach the same S/N. To achieve the desired S/N we, therefore, need a total of two orbits. We plan to observe NGC 1818 for 2268 seconds (724s+772s+772s) and 2400 seconds (795s+795s+810s) in the F606W and F814W respectively. All exposures will be optimally dithered to ensure the best possible photometric accuracy. We request therefore a total of only two orbits. Parallel fields will be taken through the WFC/ACS F606W for a total of 1820 seconds (560s+630s+630s) and F814W, for 1990 seconds (650s+690s+650s). They will serve the purpose of field decontamination, allowing us to subtract non-cluster members in our cluster field. Specifically, the statistical subtraction technique exploits the comparison of the color-magnitude diagram in the cluster field with that of a reference field. Data reduction Photometry and astrometry will be derived with the software developed by Jay Anderson for ACS/WFC and WFC3/UVIS cameras. Reduced Gyro Mode (RGM) operations According to the report ''Science Implications of Operating HST Reduced Gyro Mode'' of May 6th, the largest science imparts of operating in RGM is the reduction in the number of orbits per week (from ~84/week to ~73/week), and a longer search time for the ''Target acquisition'', with a consequent reduction of the science observing time during visibility periods by ~ 120 seconds. Given these premises, operating in RGM will have a non-negligible impact on our observations, nonetheless, they will provide the required accuracy to reach our scientific goal. ------------------------------------------------------------------------------------ 15945( 3) - 23-Jul-2019 16:25:29 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-1818 EXT-CLUSTER, GLOBULAR RA=05H04M18.3900S, J2000 V = 9.7 CLUSTER DEC=-66D25'46.70" Reference Frame: Other: SIMBAD Comments: This object was generated by the target selector and retrieved from the SIMBAD database. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2015.5 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 15945( 3) - 23-Jul-2019 16:25:29 - [ 4] Visit: 01 (NGC1818) Visit Requirements: SCHED 100% On Hold Comments: Additional Comments: NGC-1818 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-1818 WFC3/UVI ACCUM UVIS-CENT F814W 1 795 S (795 POS TARG 0, 0; PAR 2 S ER S) WITH 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 ANY ACS/WFC ACCUM WFCENTER F814W 1 650 S (650 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC-1818 WFC3/UVI ACCUM UVIS-CENT F814W 1 795 S (795 POS TARG 3, 3; PAR 4 S ER S) WITH 3 ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY ACS/WFC ACCUM WFCENTER F814W 1 690 S (690 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC-1818 WFC3/UVI ACCUM UVIS-CENT F606W 1 724 S (724 POS TARG 0, 0; PAR 6 S ER S) WITH 5 ------------------------------------------------------------------------------------------------------------------------------------ 6 ANY ACS/WFC ACCUM WFCENTER F606W 1 560 S (560 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC-1818 WFC3/UVI ACCUM UVIS-CENT F814W 1 810 S (810 POS TARG -3, -3; PAR 8 S ER S) WITH 7 ------------------------------------------------------------------------------------------------------------------------------------ 8 ANY ACS/WFC ACCUM WFCENTER F814W 1 650 S (650 S) ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC-1818 WFC3/UVI ACCUM UVIS-CENT F606W 1 772 S (772 POS TARG 3, 3; PAR 10 S ER S) WITH 9 ------------------------------------------------------------------------------------------------------------------------------------ 10 ANY ACS/WFC ACCUM WFCENTER F606W 1 630 S (630 S) ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC-1818 WFC3/UVI ACCUM UVIS-CENT F606W 1 772 S (772 POS TARG -3, -3; PAR 12 S ER S) WITH 11 ------------------------------------------------------------------------------------------------------------------------------------ 12 ANY ACS/WFC ACCUM WFCENTER F606W 1 630 S (630 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ 15945( 3) - 23-Jul-2019 16:25:29 - [ 5] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ NGC-1818 1 WFC3/UVI ACCUM UVIS-CEN F814W none none none none 1 N/A S TER ANY 2 ACS/WFC ACCUM WFCENTER F814W none none none none 1 N/A NGC-1818 3 WFC3/UVI ACCUM UVIS-CEN F814W none none none none 1 N/A S TER ANY 4 ACS/WFC ACCUM WFCENTER F814W none none none none 1 N/A NGC-1818 5 WFC3/UVI ACCUM UVIS-CEN F606W none none none none 1 N/A S TER ANY 6 ACS/WFC ACCUM WFCENTER F606W none none none none 1 N/A NGC-1818 7 WFC3/UVI ACCUM UVIS-CEN F814W none none none none 2 N/A S TER ANY 8 ACS/WFC ACCUM WFCENTER F814W none none none none 2 N/A NGC-1818 9 WFC3/UVI ACCUM UVIS-CEN F606W none none none none 2 N/A S TER ANY 10 ACS/WFC ACCUM WFCENTER F606W none none none none 2 N/A NGC-1818 11 WFC3/UVI ACCUM UVIS-CEN F606W none none none none 2 N/A S TER ANY 12 ACS/WFC ACCUM WFCENTER F606W none none none none 2 N/A 15945( 3) - 23-Jul-2019 16:25:29 - [ 6] Summary Form for Proposal 15945 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS-CENTER, WFCENTER ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PAR 10 WITH 9, PAR 12 WITH 11, PAR 2 WITH 1, PAR 4 WITH 3, PAR 6 WITH 5, PAR 8 WITH 7, POS TARG -3, -3, POS TARG 0, 0, POS TARG 3, 3, SCHED 100% ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANY, NGC-1818 ------------------------------------------------------------------------------------------------------------------------------------