0 Assigned ID: 7218 ----- Attempting Submission 1 (Wed Apr 15 21:47:37 GMT 2026) ----- ----- Attempting Submission 2 (Thu Apr 16 05:07:17 GMT 2026) ----- HST Phase I Proposal 7218 successfully submitted. Receipt: # 7218-2 ----- Attempting Submission 3 (Thu Apr 16 15:50:31 GMT 2026) ----- HST Phase I Proposal 7218 successfully submitted. Receipt: # 7218-3 This is the initial submission of phase II for this program. The program will follow a similar triggering and observing strategy to the similar approved Cycle 33 program HST-GO-18008, with only minor changes to the filter or exposure times where necessary. We will use the strategy in this program as a fiducial triggering strategy until we identify our targets and trigger the program. -971261956 ----- Attempting Submission 1 (Mon Jul 27 00:10:46 GMT 2026) ----- 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. 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 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. The team spans supernova progenitor identification, multiwavelength follow up, precision astrometry, and time domain observations. All members contribute to vetting archival pre supernova imaging, planning and supporting TOO observations, reducing and aligning WFC3 UVIS data with MAST and JWST archives, and publishing secure progenitor associations. PI Charles D. Kilpatrick: Kilpatrick leads the program. His work focuses on identifying supernova progenitors from HST and JWST imaging, precision astrometry between post explosion and archival epochs, and massive star photometry in crowded fields. He has led or co led progenitor studies of dusty red supergiants, Type II P events, SN 2023ixf, and the JWST SN 2025pht analysis in Kilpatrick et al. ApJL. He oversees target selection and TOO triggering, WFC3 UVIS analysis, astrometric alignment with MAST and JWST, and publication. CoI Katie Auchettl: Auchettl studies multiwavelength signatures of stellar explosions and circumstellar environments, including X ray diagnostics of mass loss and interaction in core collapse supernovae. She has co authored HST and multiwavelength follow up of stripped envelope progenitors and interacting, CSM rich SNe. For this program she provides physical interpretation of progenitor associations including binaries, circumstellar dust, and interaction, and relates TOO targets to massive star evolution and explosion diversity. CoI Kyle W. Davis: Davis is a co author on the SN 2025pht study. His work includes time domain astrophysics, supernova follow up, and analysis of imaging and spectroscopy of transients. He contributes to candidate vetting, observational strategy for TOO triggers, and interpretation of SN environments in the context of progenitor association. CoI Ryan J. Foley: Foley is a co author on the SN 2025pht study. He is an expert in supernova discovery and follow up across wavelengths, progenitor studies, and transient cosmology. He supports target identification from public transient streams, prioritization of TOO triggers, and multiwavelength context for progenitor association and host environments. CoI Wynn Jacobson Galan: Jacobson Galan is a co author on the SN 2025pht study. He leads work on pre explosion activity and multiwavelength follow up of core collapse SNe, including SN 2023ixf and related events. He contributes analysis of multi epoch pre supernova variability and CSM signatures, and connects TOO targets to models of late stage massive star evolution and mass loss. CoI Ravjit Kaur: Kaur is a co author on the SN 2025pht discovery and characterization effort, including JWST progenitor analysis. She has experience with transient follow up, archival imaging, and supernova photometry. For this program she supports candidate vetting, processing of triggered HST data, archival cross checks, and crowded field photometry. CoI Aswin Suresh: Suresh is a co author on the JWST SN 2025pht progenitor study. He specializes in reducing and photometering JWST and HST imaging, PSF photometry in crowded fields, and quality assurance at SN positions. For this program he reduces WFC3 UVIS data, performs relative astrometry between TOO epochs and archival pre explosion images, and validates progenitor associations. CoI Kirsty Taggart: Taggart is a co author on the SN 2025pht study. She has contributed to progenitor environment studies, light curve and nebular analysis of SNe II P, and multiwavelength follow up of Type II and stripped envelope SNe. She contributes constraints on progenitor masses and local environments including host extinction and stellar populations, nebular phase context where applicable, and support for population level interpretation of the TOO sample. Stellar Physics and Stellar Types 0 false false false false false true false SUPPORTED First supernova at <40 Mpc with pre-explosion imaging Second supernova at <40 Mpc with pre-explosion imaging WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F555W POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false 1785110096443 /Users/ckilpatrick/Library/CloudStorage/Dropbox/Apps/Overleaf/HST Cycle 34 Progenitors TOO/apt/phaseII/OP-cache/progenitors_too-v01.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. This ToO will be executed on a supernova discovered at <40 Mpc with pre-explosion JWST, HST, or Euclid imaging. (Tic-Visit :proposal-id 18339 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "26-Jul-2026 18:54:55.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 285200 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18339 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18339 :visit-id "01" :orbit-number 1 :pcs-time 301 :si-vis-utilization 2523 :al-overhead-at-end 79 :internal-dur-at-end 348 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 On hold until trigger on a supernova discovered at <40 Mpc with pre-explosion JWST, HST, or Euclid imaging /private/var/folders/_s/tc8h43192s58wb4s6m2lbfcw0000gn/T/OPVisit4904359873249324202.xml 1785111036902 The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. This ToO will be executed on a supernova discovered at <40 Mpc with pre-explosion JWST, HST, or Euclid imaging. (Tic-Visit :proposal-id 18339 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "26-Jul-2026 19:10:36.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 285200 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18339 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18339 :visit-id "02" :orbit-number 1 :pcs-time 301 :si-vis-utilization 2491 :al-overhead-at-end 79 :internal-dur-at-end 348 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 On hold until trigger on a supernova discovered at <40 Mpc with pre-explosion JWST, HST, or Euclid imaging