18339( 2) - 03-Aug-2026 16:23:47 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18339 Version: 2 Check-in Time: 03-Aug-2026 16:23:47 STScI Edit Number: 0 Title Identification of Supernova Progenitor Stars in High-Resolution Space-based Imaging ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Charles Kilpatrick Northwestern University Y CoI: Prof. Ryan Foley University of California - Santa Cruz N CoI: Dr. Kirsty Taggart Universities Space Research Association N CoI: Dr. Wynn Vicente Jacobson-Galan California Institute of Technology N CoI: Kyle Davis University of California - Santa Cruz N CoI: Dr. Katie Auchettl University of Melbourne N CoI: Aswin Suresh Northwestern University N CoI: Ravjit Kaur University of California - Santa Cruz N ------------------------------------------------------------------------------------ Abstract Directly establishing the observational properties of exploding stars is the only direct way to connect nearby core-collapse supernovae (SNe) to their initial conditions. The growing archive of Hubble Space Telescope (HST), James Webb Space Telescope (JWST), Euclid, and soon Roman imaging of galaxies within ~40 Mpc increasingly yields SN sites with candidate pre-explosion counterparts, including heavily dust-enshrouded red supergiants and stripped-envelope systems. Ground-based imaging at natural seeing often leaves multiple possible associations within the SN localization, and chance alignments must be excluded even when a progenitor candidate later disappears. We therefore propose WFC3/UVIS Target-of-Opportunity imaging to obtain a diffraction-limited image of each new SN, aligned to archival pre-explosion HST and JWST data with sufficient astrometric precision to confirm or reject progenitor identifications. The program supports science on the red supergiant problem, the diversity of Type Ib/c progenitors, LBV-related SNe IIn, and binary-driven stripping. Cycle 34 overlaps the start of Nancy Grace Roman Space Telescope Cycle 1 science operations; Roman wide area surveys will provide complementary archival data at resolutions comparable to HST imaging. At the estimated event rate, we expect of order two TOO triggers during Cycle 34. ------------------------------------------------------------------------------------ Observations Description ------------------------ This is a target-of-opportunity proposal to observe up to 2 supernovae at <40 Mpc with pre-explosion JWST, HST, and/or Euclid imaging. The goal of the proposal is to obtain imaging at the site of each supernova in order to precisely align to the pre-explosion imaging and identify or rule out the presence of a counterpart in the pre-explosion data. We are requesting a 3 week minimum response time from the time of trigger in order to guarantee that we can calibrate our exposures to the expected brightness of the supernova at the time of observation and avoid either saturating or failing to detect the primary supernova target. Our fiducial observation setup assumes that we will observe the supernova 18339( 2) - 03-Aug-2026 16:23:47 - [ 2] around or slightly after maximum light, with a required brightness of >16.5 mag to avoid saturating this source, which would limit the astrometric precision in centroiding on the supernova. We therefore use 68s exposures in F555W and use the UVIS2-C1K1C-SUB readout mode to reduce overheads between individual exposures. As we are targeting sources in galaxies at <40 Mpc, we expect the field around the supernova to be crowded with astrometric calibrators, and so the smaller readout region will not limit our science case. We have also adjusted FLASH=15 to avoid CTE losses. To improve PSF sampling, we use 4 iterations of a WFC3-UVIS-DITHER-BOX pattern for a total of 16 individual exposures. ------------------------------------------------------------------------------------ 18339( 2) - 03-Aug-2026 16:23:47 - [ 3] TARGET LIST Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ 1 TOO1 FIRST SUPERNOVA AT <40 MPC WITH V = 16.5 PRE-EXPLOSION IMAGING ------------------------------------------------------------------------------------------------------------------------------------ 2 TOO2 SECOND SUPERNOVA AT <40 MPC WITH V = 16.5 PRE-EXPLOSION IMAGING ------------------------------------------------------------------------------------------------------------------------------------ 18339( 2) - 03-Aug-2026 16:23:47 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18339( 2) - 03-Aug-2026 16:23:47 - [ 5] Visit: 01 (TOO-1) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 21.0D; TOO MAX RESPONSE TIME 100.0D On Hold Comments: On hold until trigger on a supernova discovered at <40 Mpc with pre-explosion JWST, HST, or Euclid imaging Additional Comments: This ToO will be executed on a supernova discovered at <40 Mpc with pre-explosion JWST, HST, or Euclid imaging. 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 TOO1 WFC3/UVI ACCUM UVIS2-C1K F555W FLASH=21 4 68 S PATTERN 1 1 S 1C-SUB see below (1120 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 | ------------------------------------------------------------------------------------------------------------------------------------ TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 1 none 1 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 1 none 2 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 1 none 3 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 1 none 4 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 2 none 1 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 2 none 2 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 2 none 3 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 2 none 4 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 3 none 1 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 3 none 2 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 3 none 3 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 3 none 4 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 4 none 1 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 4 none 2 none 1 70.0 S S K1C-SUB TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 4 none 3 none 1 70.0 S S K1C-SUB 18339( 2) - 03-Aug-2026 16:23:47 - [ 6] 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 | ------------------------------------------------------------------------------------------------------------------------------------ TOO1 1 WFC3/UVI ACCUM UVIS2-C1 F555W 4 none 4 none 1 70.0 S S K1C-SUB 18339( 2) - 03-Aug-2026 16:23:47 - [ 7] Visit: 02 (TOO-2) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 21.0D; TOO MAX RESPONSE TIME 100.0D On Hold Comments: On hold until trigger on a supernova discovered at <40 Mpc with pre-explosion JWST, HST, or Euclid imaging Additional Comments: This ToO will be executed on a supernova discovered at <40 Mpc with pre-explosion JWST, HST, or Euclid imaging. 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 TOO2 WFC3/UVI ACCUM UVIS2-C1K F555W FLASH=21 4 68 S (1088 PATTERN 1 1 S 1C-SUB 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 | ------------------------------------------------------------------------------------------------------------------------------------ TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 1 none 1 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 1 none 2 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 1 none 3 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 1 none 4 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 2 none 1 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 2 none 2 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 2 none 3 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 2 none 4 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 3 none 1 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 3 none 2 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 3 none 3 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 3 none 4 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 4 none 1 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 4 none 2 none 1 N/A S K1C-SUB TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 4 none 3 none 1 N/A S K1C-SUB 18339( 2) - 03-Aug-2026 16:23:47 - [ 8] 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 | ------------------------------------------------------------------------------------------------------------------------------------ TOO2 1 WFC3/UVI ACCUM UVIS2-C1 F555W 4 none 4 none 1 N/A S K1C-SUB 18339( 2) - 03-Aug-2026 16:23:47 - [ 9] Summary Form for Proposal 18339 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-C1K1C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=21 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ON HOLD , PATTERN 1 1, TOO MAX RESPONSE TIME 100.0D, TOO MIN RESPONSE TIME 21.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W ------------------------------------------------------------------------------------------------------------------------------------ Target Names TOO1, TOO2 ------------------------------------------------------------------------------------------------------------------------------------