18240( 3) - 15-May-2026 01:35:27 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18240 Version: 3 Check-in Time: 15-May-2026 01:35:27 STScI Edit Number: 0 Title Feedback in Low-metallicity Environments from Emerging sTar clusters (FLEET) ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Angela Adamo Stockholm University Y CoI: Prof. Daniela Calzetti University of Massachusetts - Amherst N CoI: Dr. Linda J. Smith Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract JWST has opened a unique window into the earliest phases of star formation where dust, ionized gas, and massive stars interact while consuming the gas reservoir. The duration of the emerging phase of star formation is among the most challenging to constrain. Analyses in metal-rich spirals find that the 3.3 micron PAH feature, a tracer of photo-dissociation regions (PDRs), remains associated with emerging young star clusters (eYSCs) for about 4 Myr, too long for large fractions of ionizing radiation to escape. At low metallicity, however, harder radiation fields and altered dust composition may change this picture. FLEET will target nine dwarf galaxies (3-8 Mpc) with vigorous star formation, spanning metallicities of 10-40% Zsun, to sample eYSCs (10^3-10^5 Msun) and their associated H II regions and PDRs at 1-5 pc resolution. From multiband NIRCam and MIRI imaging (complemented by HST observations), we will determine the timescales for clusters to emerge from their natal clouds as a function of their mass. By controlling for proximity and cluster properties (emerging stage, mass, age), we will trace PAH grain composition and its evolution, testing model predictions (inhibited growth vs. destruction) that diverge at the probed metallicity range. We will calibrate the PAH and 21 micron emission as unobscured tracers of star formation. The results of the FLEET program will establish whether cluster emergence is sufficiently short (<<4 Myr) to allow ionizing photon escape, and provide a benchmark for studies of distant dwarf galaxies and simulations of emission and dust content in low-metallicity molecular clouds across a broad range of eYSC properties. ------------------------------------------------------------------------------------ Observations Description ------------------------ The program will acquire 3 orbits with WFC3/UVIS to observe NGC625 in the F336W, F438W, F547M, F657N, F814W filters standard for optical studies. For Visit 1 (NGC625) we use as pattern WFC3-UVIS-GAP-LINE with 3 dithers which cover the gap while sampling the PSF(half px). We cover 1 dither step in each orbit, acquiring 5 exposures (one per filter). For the target ESO245 we use the WFC3-UVIS-MOS-DITH-LINE with 4 positions to cover the gap while appling small dither to better sample the gap. Single exposures include POSTFLASH to reduce CTE effects. ------------------------------------------------------------------------------------ 18240( 3) - 15-May-2026 01:35:27 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC-625 GALAXY, DWARF COMPACT, RA=01H35M5.4108S, J2000 V = 11.25 STAR FORMING REGION DEC=-41D26'4.55" 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 ------------------------------------------------------------------------------------------------------------------------------------ 3 ESO245 GALAXY, DWARF COMPACT, RA=01H45M4.1019S, J2000 V = 12.63 STAR FORMING REGION DEC=-43D35'30.23" 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 ------------------------------------------------------------------------------------------------------------------------------------ 18240( 3) - 15-May-2026 01:35:27 - [ 3] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-GAP-LINE Pattern_Purpose MOSAIC Number_Of_Points 3 Point_Spacing 2.414 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 85.759 Angle_Between_Sides Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-MOS-DITH-LINE WFC3-UVIS-MOS-DITH-LINE Pattern_Purpose MOSAIC DITHER Number_Of_Points 2 2 Point_Spacing 2.4 0.119 Line_Spacing Coordinate_Frame POS-TARG POS-TARG Pattern_Orient 85.754 33.606 Angle_Between_Sides Center_Pattern YES NO ------------------------------------------------------------------------------------------------------------------------------------ 18240( 3) - 15-May-2026 01:35:27 - [ 4] Visit: 01 Visit Requirements: ORIENT 0D TO 45 D; ORIENT 90D TO 130 D; ORIENT 189D TO 225 D; ORIENT 270D TO 314 D 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-625 WFC3/UVI ACCUM UVIS-CENT F336W FLASH=20 1 400 S PATTERN 1 1-5 S ER (1200 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC-625 WFC3/UVI ACCUM UVIS-CENT F438W FLASH=20 1 350 S S ER (1050 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC-625 WFC3/UVI ACCUM UVIS-CENT F547M FLASH=15 1 400 S S ER (1200 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC-625 WFC3/UVI ACCUM UVIS-CENT F657N FLASH=20 1 500 S S ER (1500 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC-625 WFC3/UVI ACCUM UVIS-CENT F814W FLASH=12 1 320 S (960 S ER 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-625 1 WFC3/UVI ACCUM UVIS-CEN F336W 1 none none none 1 N/A S TER NGC-625 1 WFC3/UVI ACCUM UVIS-CEN F336W 2 none none none 2 N/A S TER NGC-625 1 WFC3/UVI ACCUM UVIS-CEN F336W 3 none none none 3 N/A S TER NGC-625 2 WFC3/UVI ACCUM UVIS-CEN F438W 1 none none none 1 N/A S TER NGC-625 2 WFC3/UVI ACCUM UVIS-CEN F438W 2 none none none 2 N/A S TER NGC-625 2 WFC3/UVI ACCUM UVIS-CEN F438W 3 none none none 3 N/A S TER NGC-625 3 WFC3/UVI ACCUM UVIS-CEN F547M 1 none none none 1 N/A S TER NGC-625 3 WFC3/UVI ACCUM UVIS-CEN F547M 2 none none none 2 N/A S TER NGC-625 3 WFC3/UVI ACCUM UVIS-CEN F547M 3 none none none 3 N/A S TER NGC-625 4 WFC3/UVI ACCUM UVIS-CEN F657N 1 none none none 1 N/A S TER 18240( 3) - 15-May-2026 01:35:27 - [ 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-625 4 WFC3/UVI ACCUM UVIS-CEN F657N 2 none none none 2 N/A S TER NGC-625 4 WFC3/UVI ACCUM UVIS-CEN F657N 3 none none none 3 N/A S TER NGC-625 5 WFC3/UVI ACCUM UVIS-CEN F814W 1 none none none 1 N/A S TER NGC-625 5 WFC3/UVI ACCUM UVIS-CEN F814W 2 none none none 2 N/A S TER NGC-625 5 WFC3/UVI ACCUM UVIS-CEN F814W 3 none none none 3 N/A S TER 18240( 3) - 15-May-2026 01:35:27 - [ 6] Visit: 02 Visit Requirements: ORIENT 270D TO 290 D; ORIENT 175D TO 195 D; ORIENT 0D TO 25 D; ORIENT 84D TO 99 D 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 ESO245 WFC3/UVI ACCUM UVIS-CENT F657N FLASH=20 1 550 S PATTERN 2 1 S ER (2200 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 | ------------------------------------------------------------------------------------------------------------------------------------ ESO245 1 WFC3/UVI ACCUM UVIS-CEN F657N 1 1 none none 1 N/A S TER ESO245 1 WFC3/UVI ACCUM UVIS-CEN F657N 1 2 none none 1 N/A S TER ESO245 1 WFC3/UVI ACCUM UVIS-CEN F657N 2 1 none none 1 N/A S TER ESO245 1 WFC3/UVI ACCUM UVIS-CEN F657N 2 2 none none 1 N/A S TER 18240( 3) - 15-May-2026 01:35:27 - [ 7] Summary Form for Proposal 18240 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS-CENTER ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=12, FLASH=15, FLASH=20 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 0D TO 25 D, ORIENT 0D TO 45 D, ORIENT 175D TO 195 D, ORIENT 189D TO 225 D, ORIENT 270D TO 290 D, ORIENT 270D TO 314 D, ORIENT 84D TO 99 D, ORIENT 90D TO 130 D, PATTERN 1 1-5, PATTERN 2 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F336W, F438W, F547M, F657N, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ESO245, NGC-625 ------------------------------------------------------------------------------------------------------------------------------------