18021( 3) - 12-Mar-2026 18:52:22 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18021 Version: 3 Check-in Time: 12-Mar-2026 18:52:22 STScI Edit Number: 0 Title FUV imaging of massive Magellanic Clouds clusters ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Prof. Hugues Sana Institute of Astronomy, KU Leuven Y CoI: Dr. Julia Bodensteiner Universiteit van Amsterdam N CoI: Dr. Hannah Penn Earnshaw California Institute of Technology N CoI: Dr. Bethany Ann Ludwig Institute of Astronomy, KU Leuven N CoI: Pablo Ignacio Marchant Campos Ghent University N CoI: Dr. Reinhold Willcox Institute of Astronomy, KU Leuven N ------------------------------------------------------------------------------------ Abstract Stripped stars are one of the most exciting types of massive binary interaction products. They are the hot, helium-rich cores of intermediate and high-mass stars that have lost their envelope through mass transfer or common-envelope evolution. Stripped stars dramatically enhance the ionization radiation budget of massive stars and are excellent candidate progenitors of hydrogen-poor core-collapse supernovae. Given their temperature and small sizes, stripped stars are hard to detect in the optical. Using NUV SWIFT photometry, a newly proposed method, based on NUV-optical color-indexes, has recently been successful in identifying a dozen such stripped stars in the Magellanic Clouds (MCs). However, the quality of the PSF and its photometric precision have limited these studies to low-density field regions, and to more massive stripped stars with no or optically-faint companions. Here we propose to take the detection of stripped stars to the next level, by testing and validating the use of HST FUV photometry in two 25-40 Myr massive MC clusters. As FUV photometry is challenging given Brightness Object Protection constraints, we used archival WFC3/F225W photometry and VLT/MUSE integral field spectroscopy to avoid problematic objects in the clusters targeted here. Characterizing the full stripped-star population is crucial to constrain binary evolution models and can only be achieved with FUV imaging. Our pilot project will open the possibility to detect stripped stars in their native clusters (hence assign them an age!), particularly stripped stars with optically bright companions (unidentifiable in the NUV or optical) and lower-mass stripped stars than possible so far. ------------------------------------------------------------------------------------ Observations Description ------------------------ We aim to obtain deep STIS.MAMA FUV F25QTZ imaging at 4 positions in 2 different clusters in the magellanic clouds to: - identified stripped stars in dense environments - characterise the FUV crowding of sources in the MCs to prepare the upcoming UVEX mission 18021( 3) - 12-Mar-2026 18:52:22 - [ 2] Using archival mF225W data (published in Milone+2018, 2023), we identified these positions so that no stars is brighter than mF225w=13.5 Using ground-based integra-field spectroscopy (Bodensteiner+2022 for NGC330; unpublished for NGC1818), we check that there are no O-type stars in the field of view. The combination of these two requirements (Teff<30kK and mF225w) allow us to guarantee to safety of the instrument. We also check the total count rate by integrating the source fluxes over the FOV and found out that the global count rate of the detector stays <10 000 cnt/s We plan 1 orbit per position. Given we don't need particular acquisition, we have a ~2400sec integration duration that we split into 2 sub-exposure dither by 2arcsec to correct for cosmic and detector defects. We aim to reach sources with (Vega)MagFUV = 23 (ABmag~24.5) at 5sigma. ------------------------------------------------------------------------------------ 18021( 3) - 12-Mar-2026 18:52:22 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1818-FIEL STELLAR CLUSTER, OPEN RA=05H04M16.1500S, J2000 V = 15.5 mF225>13.5 VEGAMAG D1 CLUSTER DEC=-66D26'15.40" 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 ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1818-FIEL STELLAR CLUSTER, OPEN RA=05H04M11.4500S, J2000 V = 15.062 mF225>13.5 VEGAMAG D2 CLUSTER DEC=-66D26'23.80" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC1818-FIEL STELLAR CLUSTER, OPEN RA=05H04M10.9200S, J2000 V = 16.1 mF225>13.5 VEGAMAG D3 CLUSTER DEC=-66D25'54.30" 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 ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC1818-FIEL STELLAR CLUSTER, OPEN RA=05H04M16.4100S, J2000 V = 15.785 mF225>13.5 VEGAMAG D4 CLUSTER DEC=-66D25'47.20" 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 ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC330-FIELD STELLAR CLUSTER, OPEN RA=00H56M23.2400S, J2000 V = 12.6 mF225>13.5 VEGAMAG 1 CLUSTER DEC=-72D28'7.00" 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 ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC330-FIELD STELLAR CLUSTER, OPEN RA=00H56M17.3600S, J2000 V = 13.52 mF225>13.5 VEGAMAG 2 CLUSTER DEC=-72D27'59.50" 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 ------------------------------------------------------------------------------------------------------------------------------------ 18021( 3) - 12-Mar-2026 18:52:22 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC330-FIELD STELLAR CLUSTER, OPEN RA=00H56M15.0100S, J2000 V = 13.81 mF225>13.5 VEGAMAG 3 CLUSTER DEC=-72D27'30.50" 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 ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC330-FIELD STELLAR CLUSTER, OPEN RA=00H56M21.4500S, J2000 V = 13.14 mF225>13.5 VEGAMAG 4 CLUSTER DEC=-72D27'27.80" 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 ------------------------------------------------------------------------------------------------------------------------------------ 18021( 3) - 12-Mar-2026 18:52:22 - [ 5] Visit: 01 Visit Requirements: ORIENT 35D TO 55 D; ORIENT 125D TO 145 D; ORIENT 215D TO 235 D; ORIENT 305D TO 325 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 NGC1818-FIE STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S LD1 -MAMA G (1173 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1818-FIE STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S POS TARG 2, 2 LD1 -MAMA G (1173 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC1818-FIELD1 1 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 N/A -MAMA G NGC1818-FIELD1 2 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 N/A -MAMA G 18021( 3) - 12-Mar-2026 18:52:22 - [ 6] Visit: 02 Visit Requirements: ORIENT 35D TO 55 D; ORIENT 125D TO 145 D; ORIENT 215D TO 235 D; ORIENT 305D TO 325 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 NGC1818-FIE STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S LD2 -MAMA G (1173 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1818-FIE STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S POS TARG 2, 2 LD2 -MAMA G (1173 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC1818-FIELD2 1 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 N/A -MAMA G NGC1818-FIELD2 2 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 N/A -MAMA G 18021( 3) - 12-Mar-2026 18:52:22 - [ 7] Visit: 03 Visit Requirements: ORIENT 35D TO 55 D; ORIENT 125D TO 145 D; ORIENT 215D TO 235 D; ORIENT 305D TO 325 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 NGC1818-FIE STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S LD3 -MAMA G (1173 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1818-FIE STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S POS TARG 2, 2 LD3 -MAMA G (1173 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC1818-FIELD3 1 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 N/A -MAMA G NGC1818-FIELD3 2 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 N/A -MAMA G 18021( 3) - 12-Mar-2026 18:52:22 - [ 8] Visit: 04 Visit Requirements: ORIENT 35D TO 55 D; ORIENT 125D TO 145 D; ORIENT 215D TO 235 D; ORIENT 305D TO 325 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 NGC1818-FIE STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S LD4 -MAMA G (1173 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1818-FIE STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S POS TARG 2, 2 LD4 -MAMA G (1173 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC1818-FIELD4 1 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 N/A -MAMA G NGC1818-FIELD4 2 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 N/A -MAMA G 18021( 3) - 12-Mar-2026 18:52:22 - [ 9] Visit: 05 Visit Requirements: ORIENT 35D TO 55 D; ORIENT 125D TO 145 D; ORIENT 215D TO 235 D; ORIENT 305D TO 325 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 NGC330-FIEL STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S D1 -MAMA G see below (1188 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC330-FIEL STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S POS TARG 2, 2 D1 -MAMA G see below (1188 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC330-FIELD1 1 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 1188.0 S -MAMA G NGC330-FIELD1 2 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 1188.0 S -MAMA G 18021( 3) - 12-Mar-2026 18:52:22 - [ 10] Visit: 06 Visit Requirements: ORIENT 35D TO 55 D; ORIENT 125D TO 145 D; ORIENT 215D TO 235 D; ORIENT 305D TO 325 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 NGC330-FIEL STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S D2 -MAMA G see below (1188 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC330-FIEL STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S POS TARG 2, 2 D2 -MAMA G see below (1188 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC330-FIELD2 1 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 1188.0 S -MAMA G NGC330-FIELD2 2 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 1188.0 S -MAMA G 18021( 3) - 12-Mar-2026 18:52:22 - [ 11] Visit: 07 Visit Requirements: ORIENT 35D TO 55 D; ORIENT 125D TO 145 D; ORIENT 215D TO 235 D; ORIENT 305D TO 325 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 NGC330-FIEL STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S D3 -MAMA G see below (1188 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC330-FIEL STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S POS TARG 2, 2 D3 -MAMA G see below (1188 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC330-FIELD3 1 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 1188.0 S -MAMA G NGC330-FIELD3 2 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 1188.0 S -MAMA G 18021( 3) - 12-Mar-2026 18:52:22 - [ 12] Visit: 08 Visit Requirements: ORIENT 35D TO 55 D; ORIENT 125D TO 145 D; ORIENT 215D TO 235 D; ORIENT 305D TO 325 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 NGC330-FIEL STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S D4 -MAMA G see below (1188 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC330-FIEL STIS/FUV TIME-TA F25QTZ MIRROR BUFFER-TIME=950 1 1173 S POS TARG 2, 2 D4 -MAMA G see below (1188 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC330-FIELD4 1 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 1188.0 S -MAMA G NGC330-FIELD4 2 STIS/FUV TIME-TA F25QTZ MIRROR none none none none 1 1188.0 S -MAMA G 18021( 3) - 12-Mar-2026 18:52:22 - [ 13] Summary Form for Proposal 18021 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F25QTZ ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=950 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 125D TO 145 D, ORIENT 215D TO 235 D, ORIENT 305D TO 325 D, ORIENT 35D TO 55 D, POS TARG 2, 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1818-FIELD1, NGC1818-FIELD2, NGC1818-FIELD3, NGC1818-FIELD4, NGC330-FIELD1, NGC330-FIELD2, NGC330-FIELD3, NGC330-FIELD4 ------------------------------------------------------------------------------------------------------------------------------------