0 Assigned ID: 7813 ----- Attempting Submission 1 (Thu Apr 16 23:49:10 GMT 2026) ----- HST Phase I Proposal 7813 successfully submitted. Receipt: # 7813-1 ----- Attempting Submission 2 (Thu Apr 16 23:56:43 GMT 2026) ----- HST Phase I Proposal 7813 successfully submitted. Receipt: # 7813-2 0 ----- Attempting Submission 1 (Tue Aug 18 21:59:50 GMT 2026) ----- Massive stars (above 8 solar masses) end their lives in core-collapse supernovae (CCSNe), but key aspects of this process remain poorly understood with the observed spectral and light-curve diversities, reflecting a range of progenitor properties. Type II SNe progenitors - red supergiants - have been directly observed, and the progenitors of stripped-envelope SNe (Types Ib/c) are thought to be even more massive stars (e.g., Wolf-Rayet stars) and/or equally massive stars in a binary system with sufficient mass loss. Because massive stars have short lifetimes, they are expected to explode near their star-forming birthplaces. However, some CCSNe appear "hostless" with no obvious host galaxy detected, offering unique insight into progenitor environments. Possible explanations include undetected faint dwarf hosts or runaway massive stars ejected from their galaxies. We propose deep, high-resolution UV (F300X) imaging with WFC3/UVIS to search for faint dwarf galaxies with recent star formation at the explosion sites of nine nearby (z < 0.05), spectroscopically confirmed hostless CCSNe (6 Type II, 3 Type Ic-BL). These targets lack clear host associations with high probabilities of chance alignment with unrelated field galaxies. Only HST provides the required UV sensitivity and spatial resolution. We request 2 orbits per target to reach 26.1 AB mag with sufficient signal-to-noise for robust host detection or stringent non-detection limits. These observations will probe the faint end of the galaxy UV luminosity function and inform models of massive star evolution, galaxy formation, and stellar kinematics. To maximize the signal while focusing on UV light to track recent star formation, we place the supernova location at the center of UVIS chip 2 and choose the most transmissive filter whose red edge is less than 300nm - F300X. We calculate the on-source exposure times required by WFC3/UVIS F300X to obtain signal-to-noise ratios greater than 10. With four dithers (for optimal cosmic-ray rejection and point-spread function sampling) for a 2'' source with a 1'' extraction radius, we find 80 minutes of on-source time (or 2 orbits including the overhead) to reach m_F300X = 26.1 (AB) for all the hostless core-collapse supernova sample. The proposers have extensive experience with both HST and supernova/transients science. PI Hiramatsu analyzed the HST WFC3 observations of a very peculiar supernova and host galaxies of tidal disruption events from Cycle 24 and 25 GO programs (published by Sollerman et al. 2019 and French et al. 2020, respectively), and is currently leading the Cycle 32-33 SNAP program (222 orbits) to study the host galaxies of interacting supernovae in their rest-frame UV. Co-I Gomez led GO programs in Cycles 27 and 28 to study the first robust pair-instability supernova, and in Cycle 32 program to study the host galaxies of luminous supernovae in their rest-frame UV. Co-I Berger led a Cycle 20 GO program, and was the thesis advisor of the PI of the Cycle 21 program to observe the host galaxies of the earliest sample of superluminous supernovae (published by Lunnan et al. 2015) in their rest-frame UV; he also led three GO programs in Cycles 14 and 15 to study gamma-ray burst host galaxies, and 3 ToO programs in Cycles 25, 27, and 28 to study kilonovae associated with gamma-ray bursts. Co-I Blanchard led four GO programs in Cycles 24-26 to study superluminous supernovae and a tidal disruption event, and is currently leading the Cycle 32-33 GO program to study superluminous supernovae. Co-I Tsuna is an expert in theoretical modeling of supernovae and their progenitor evolution. Stellar Physics and Stellar Types Galaxies 6 false true false false false false false SUPPORTED GALAXY UNDESIGNATED J2000 J2000 g=24.7 GALAXY UNDESIGNATED J2000 J2000 g=24.7 GALAXY UNDESIGNATED J2000 J2000 g=24.7 GALAXY UNDESIGNATED J2000 J2000 g=23.3 GALAXY UNDESIGNATED J2000 J2000 g=24.7 GALAXY UNDESIGNATED J2000 J2000 g=24.7 GALAXY UNDESIGNATED J2000 J2000 g=24.7 GALAXY UNDESIGNATED J2000 J2000 g=24.7 GALAXY UNDESIGNATED J2000 J2000 g=23.3 WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false 1787089392318 /Users/daichihiramatsu/UF Dropbox/Daichi Hiramatsu/Astrophysics/Proposal/HST/HostlessSNe_Cycle34/OP-cache/HostlessSNe_Cycle34-v01.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18390 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 17:43:12.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18390 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18390 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2565 :al-overhead-at-end 105 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18390 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2545 :al-overhead-at-end 125 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) true 2 0 1787089392788 /Users/daichihiramatsu/UF Dropbox/Daichi Hiramatsu/Astrophysics/Proposal/HST/HostlessSNe_Cycle34/OP-cache/HostlessSNe_Cycle34-v02.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18390 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 17:43:12.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18390 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18390 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2565 :al-overhead-at-end 105 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18390 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2545 :al-overhead-at-end 125 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) true 2 0 1787089393240 /Users/daichihiramatsu/UF Dropbox/Daichi Hiramatsu/Astrophysics/Proposal/HST/HostlessSNe_Cycle34/OP-cache/HostlessSNe_Cycle34-v03.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18390 :visit-id "03" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 17:43:13.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18390 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18390 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2565 :al-overhead-at-end 105 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18390 :visit-id "03" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2545 :al-overhead-at-end 125 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) true 2 0 1787089393688 /Users/daichihiramatsu/UF Dropbox/Daichi Hiramatsu/Astrophysics/Proposal/HST/HostlessSNe_Cycle34/OP-cache/HostlessSNe_Cycle34-v04.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18390 :visit-id "04" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 17:43:13.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18390 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18390 :visit-id "04" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2565 :al-overhead-at-end 105 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3001 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18390 :visit-id "04" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2545 :al-overhead-at-end 125 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3001 :pcs-gap 0 ) true 2 0 1787089394142 /Users/daichihiramatsu/UF Dropbox/Daichi Hiramatsu/Astrophysics/Proposal/HST/HostlessSNe_Cycle34/OP-cache/HostlessSNe_Cycle34-v05.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18390 :visit-id "05" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 17:43:14.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18390 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18390 :visit-id "05" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2565 :al-overhead-at-end 105 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2974 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18390 :visit-id "05" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2545 :al-overhead-at-end 125 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2974 :pcs-gap 0 ) true 2 0 1787089394586 /Users/daichihiramatsu/UF Dropbox/Daichi Hiramatsu/Astrophysics/Proposal/HST/HostlessSNe_Cycle34/OP-cache/HostlessSNe_Cycle34-v06.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18390 :visit-id "06" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 17:43:14.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18390 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18390 :visit-id "06" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2565 :al-overhead-at-end 105 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3044 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18390 :visit-id "06" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2545 :al-overhead-at-end 125 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3044 :pcs-gap 0 ) true 2 0 1787089395054 /Users/daichihiramatsu/UF Dropbox/Daichi Hiramatsu/Astrophysics/Proposal/HST/HostlessSNe_Cycle34/OP-cache/HostlessSNe_Cycle34-v07.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18390 :visit-id "07" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 17:43:14.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18390 :visit-id "07" :obset-id "07" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18390 :visit-id "07" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2565 :al-overhead-at-end 105 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3181 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18390 :visit-id "07" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2545 :al-overhead-at-end 125 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3181 :pcs-gap 0 ) true 2 0 1787089395506 /Users/daichihiramatsu/UF Dropbox/Daichi Hiramatsu/Astrophysics/Proposal/HST/HostlessSNe_Cycle34/OP-cache/HostlessSNe_Cycle34-v08.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18390 :visit-id "08" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 17:43:15.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18390 :visit-id "08" :obset-id "08" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18390 :visit-id "08" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2565 :al-overhead-at-end 105 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2992 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18390 :visit-id "08" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2545 :al-overhead-at-end 125 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2992 :pcs-gap 0 ) true 2 0 1787089396236 /Users/daichihiramatsu/UF Dropbox/Daichi Hiramatsu/Astrophysics/Proposal/HST/HostlessSNe_Cycle34/OP-cache/HostlessSNe_Cycle34-v09.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18390 :visit-id "09" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 17:43:15.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18390 :visit-id "09" :obset-id "09" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18390 :visit-id "09" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2565 :al-overhead-at-end 105 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3011 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18390 :visit-id "09" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2545 :al-overhead-at-end 125 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3011 :pcs-gap 0 ) true 2 0