18033( 2) - 29-Jul-2025 13:52:40 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18033 Version: 2 Check-in Time: 29-Jul-2025 13:52:40 STScI Edit Number: 0 Title The smoking gun uncovering the origin of the rotational spread in young Magellanic cloud clusters: NGC1866 ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Fabrizio Muratore Universita degli Studi di Padova Y CoI: Prof. Antonino Paolo Milone Universita degli Studi di Padova N CoI: Dr. Giacomo Cordoni Australian National University N CoI: Dr. Anna Fabiola Marino INAF - Osservatorio Astronomico di Padova N CoI: Dr. Alessandra Mastrobuono Universita degli Studi di Padova N Battisti CoI: Dr. Marco Tailo Universita di Bologna N CoI: Prof. E. P. Lagioia Yunnan University N CoI: Dr. Maria Vittoria Legnardi Universita degli Studi di Padova N CoI: Dr. Emanuele Dondoglio INAF - Istituto Nazionale di Astrofisica N CoI: Tuila Ziliotto Universita degli Studi di Padova N CoI: Dr. Emanuele Bortolan Universita degli Studi di Padova N ------------------------------------------------------------------------------------ Abstract Young stellar clusters in Magellanic Clouds (MCs) do not host simple stellar populations, as could be expected. Features, such as the split main sequence (MS) and the extended main sequence turn-off (eMSTO), are easily visible in all clusters younger than 2 Gyrs. Spectroscopic studies suggest that these features arise from different rotation rates among the stars. However, the occurrence of slow rotators poses a serious challenge to our understanding of stellar evolution. Indeed, pre-MS stars are expected to accelerate, so we do not expect to find slow-rotating stars. What causes some stars to slow down while others retain their spin? So far, three main mechanisms have been proposed, involving tidal interaction in binary systems, disc interaction in the pre-MS phase, and stellar mergers. Binary fraction among fast and slow rotators is the key to constrains the mechanism, indeed, each mechanism predicts different binary content. A new method has been developed to analyze a large sample of binaries, effectively distinguishing between slow and fast rotators using the F275W filter. This method has already been applied to three star clusters that are less than 100 million years old. Unfortunately, due to their young age, it is challenging to validate a single mechanism. We propose to observe NGC 1866, a 200-million-year-old star cluster in the Large Magellanic Cloud, using the F275W filter. Given that the mechanism has been active for a longer period, we expect to find more significant differences in binary content. This makes the cluster the smoking gun uncovering the braking mechanism and the origin of the split MS and the eMSTO. ------------------------------------------------------------------------------------ Observations Description ------------------------ This is a proposal aimed at obtaining high accuracy photometry of the MC cluster NGC1866 in the WFC3/UVIS/F275W band. 18033( 2) - 29-Jul-2025 13:52:40 - [ 2] ------------------------------------------------------------------------------------ 18033( 2) - 29-Jul-2025 13:52:40 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-1866 STELLAR CLUSTER, RA=05H13M41.9100S, J2000 V = 9.73 GLOBULAR CLUSTER DEC=-65D27'36.20" Reference Frame: ICRS Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18033( 2) - 29-Jul-2025 13:52:40 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 6 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 270 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18033( 2) - 29-Jul-2025 13:52:40 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: 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-1866 WFC3/UVI ACCUM UVIS-CENT F275W FLASH=20 1 1282 S PATTERN 1 1 S ER (2564 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC-1866 WFC3/UVI ACCUM UVIS-CENT F275W FLASH=20 1 1269 S PATTERN 1 2 S ER (2538 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC-1866 1 WFC3/UVI ACCUM UVIS-CEN F275W 1 none none none 1 N/A S TER NGC-1866 1 WFC3/UVI ACCUM UVIS-CEN F275W 2 none none none 1 N/A S TER NGC-1866 2 WFC3/UVI ACCUM UVIS-CEN F275W 1 none none none 2 N/A S TER NGC-1866 2 WFC3/UVI ACCUM UVIS-CEN F275W 2 none none none 2 N/A S TER 18033( 2) - 29-Jul-2025 13:52:40 - [ 6] Summary Form for Proposal 18033 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS-CENTER ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=20 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F275W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC-1866 ------------------------------------------------------------------------------------------------------------------------------------