17814( 5) - 08-Aug-2025 14:56:36 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17814 Version: 5 Check-in Time: 08-Aug-2025 14:56:36 STScI Edit Number: 0 Title Diversifying the UV Spectroscopic Sample of Fast Evolving Transients ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Ori Dosovitz Fox Space Telescope Science Institute Y CoI: Dr. Jennifer Andrews NOIRLab - Gemini North (HI) N CoI: Dr. Thomas G Brink University of California - Berkeley N CoI: Dr. Peter J. Brown Texas A & M University N CoI: Dr. Roger A. Chevalier The University of Virginia N CoI: Dr. Geoffrey C. Clayton Space Science Institute N CoI: Prof. Alex V. Filippenko University of California - Berkeley N CoI: Prof. Claes Fransson Stockholm University N CoI: Prof. Griffin Hosseinzadeh University of California - San Diego N CoI: Dr. Patrick Kelly University of Minnesota - Twin Cities N CoI: Dr. Nathan Smith University of Arizona N CoI: Dr. Tamas Szalai University of Szeged N CoI: Dr. Schuyler D. Van Dyk California Institute of Technology N CoI: Dr. WeiKang Zheng University of California - Berkeley N CoI: Dr. Sebastian Gomez University of Texas at Austin N CoI: Dr. Saurabh W. Jha Rutgers the State University of New Jersey N CoI: Dr. Mi Dai University of Pittsburgh N CoI: Dr. Chelsea Harris Michigan State University N CoI: Dr. Luc Dessart CNRS, Institut d'Astrophysique de Paris N CoI: Dr. Melissa Shahbandeh Space Telescope Science Institute N CoI: Dr. Yossef Zenati The Johns Hopkins University N CoI: Dr. Justin Pierel Space Telescope Science Institute N CoI: Dr. Qinan Wang Massachusetts Institute of Technology N CoI: Kyle Davis University of California - Santa Cruz N CoI: Prof. Ryan Foley University of California - Santa Cruz N CoI: Dr. Armin Rest Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract Over the past decade, higher cadence all-sky surveys have uncovered a new, extreme class of transients with rapidly evolving light curves and high peak luminosities. The characteristics of these transients challenge our conventional understanding of massive star evolution and core-collapse supernovae (SNe). Most proposed models require a compact progenitor with extreme mass-loss, but the nature of both the progenitor and the mass-loss mechanism in these models does not often conform to conventional stellar evolution scenarios. UV spectra can offer important constraints on these models. They contain a significant fraction of the total energy output, uniquely probe thermal shock radiation that is not present at optical wavelengths, and directly trace the mass-loss history through P Cygni absorption profiles in the UV resonance lines. Over the past few years, the transient community has ramped up efforts to both collect UV spectra of fast transients and update spectroscopic modeling software (i.e., CMFGEN) to handle the physical scenarios associated with this subclass. Yet, to date, only a few UV spectra exist, and only the Type Ibn SN 17814( 5) - 08-Aug-2025 14:56:36 - [ 2] 2020nxt was successfully modeled by CMFGEN at UV wavelengths. Furthermore, within this SN Ibn subclass, there is known spectroscopic diversity amongst the events. Here we propose a disruptive ToO with HST/STIS to obtain 2 epochs of UV spectra of a fast transient that is within 250 Mpc. We will implement detailed CMFGEN modeling to constrain the progenitor system and pre-explosion mass-loss history. This program is not appropriate for ``Flex Thursday'' as the rates are too low. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose to obtain 2 epochs (3 orbits each) of STIS-MAMA spectra of a single Fast Transient supernova. The brightness is really the most important factor, but fast transients can vary in brightness substantially (magnitudes). We offer rough guidelines of a target within roughly 250 Mpc and as close to light-curve maximum as possible. The goal is to detect UV signatures that can distinguish between progenitor scenarios. These observations will also trace circumstellar interaction, constrain the progenitor mass loss, and identify late-time heating mechanisms for potential warm dust. The initial spectrum will be triggered as a *disruptive* ToO, and observations should be scheduled as soon as possible. The second epoch should be obtained again within 10 days (the goal is to maximize the time between the two epochs, so there may be some wiggle room depending on the brightness and evolution timeline of the SN). We will use the data to trace circumstellar interaction, constrain the progenitor mass loss, and measure the CNO processing. Comparisons to other SN and FBOT subclasses will also help distinguish between progenitor systems and central engine powering mechanisms. The first epoch will include observations in both the G140 and G230L bandpasses. The second epoch will include observations in only the G230L bandpass. We choose STIS over COS for several reasons. Most importantly, STIS offers spatially resolved spectra that exploit the intrinsically high resolution of HST over a large spectral range (accommodating both the C III] 1909 Ang and Mg II 2800 Ang lines). The long slit length will allow us to subtract contamination from zodiacal light, earthshine, the host galaxy, and H II regions. STIS also has superior sensitivity to COS in the NUV. While COS is optimized for isolated, faint point sources in the FUV with medium resolution, we would suffer from less wavelength coverage and require more integrations. We would also lose the advantages of the slit described above. Important Note: COS, however, will be sufficient in the case that STIS is unavailable in the aging spacecraft. ------------------------------------------------------------------------------------ 17814( 5) - 08-Aug-2025 14:56:36 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 2 SN-2025SHI EXT-STAR, SUPERNOVA RA=22H48M42.2790S, J2000 V = 17.5 DEC=+27D34'57.99" Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 17814( 5) - 08-Aug-2025 14:56:36 - [ 4] Visit: 01 (STIS-SN2025shi-Epoch1 (NUV)) Visit Requirements: ON HOLD ; TOO RESPONSE TIME 2.0D On Hold Comments: Target-of-Opportunity. The initial spectrum will be triggered as a disruptive ToO, and observations should be scheduled as soon as possible (2-5 days as stated in the CfP). Less than 2 days could be possible since PI Fox works at STScI and has worked on ultra-rapid responses with HST planners in the past. The main goal, however, is to make sure the observations are obtained around peak, which typically happens about 3 days post-discovery. Additional Comments: Likely to acquire on bright SN. 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 SN-2025SHI STIS/CCD ACQ F28X50LP MIRROR 1 30 S (30 S) Comments: Used 10, 000K Blackbody normalized to V=16. SN should be bright during initial acquisition. ------------------------------------------------------------------------------------------------------------------------------------ 2 SN-2025SHI STIS/NUV ACCUM 52X0.2 G230L 2376 1 2000 S -MAMA see below (2014 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN-2025SHI 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN-2025SHI 2 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 2014.0 S -MAMA 17814( 5) - 08-Aug-2025 14:56:36 - [ 5] Visit: 02 (STIS-SN2025shi-Epoch1 (FUV)) Visit Requirements: AFTER 01 BY 0 D TO 1.1 D; ON HOLD ; TOO RESPONSE TIME 2.0D On Hold Comments: Target-of-Opportunity. The initial spectrum will be triggered as a disruptive ToO, and observations should be scheduled as soon as possible (2-5 days as stated in the CfP). Less than 2 days could be possible since PI Fox works at STScI and has worked on ultra-rapid responses with HST planners in the past. The main goal, however, is to make sure the observations are obtained around peak, which typically happens about 3 days post-discovery. Additional Comments: Likely to acquire on bright SN. 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 SN-2025SHI STIS/CCD ACQ F28X50LP MIRROR 1 30 S (30 S) Comments: Used 10, 000K Blackbody normalized to V=16. SN should be bright during initial acquisition. ------------------------------------------------------------------------------------------------------------------------------------ 2 SN-2025SHI STIS/FUV ACCUM 52X0.2D1 G140L 1425 1 2000 S -MAMA see below (2014 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN-2025SHI 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN-2025SHI 2 STIS/FUV ACCUM 52X0.2D1 G140L 1425 none none none none 1 2014.0 S -MAMA 17814( 5) - 08-Aug-2025 14:56:36 - [ 6] Visit: 03 (STIS-SN2025shi-Epoch1 (FUV)) Visit Requirements: AFTER 02 BY 0 D TO 1.1 D; ON HOLD ; TOO RESPONSE TIME 2.0D On Hold Comments: Target-of-Opportunity. The initial spectrum will be triggered as a disruptive ToO, and observations should be scheduled as soon as possible (2-5 days as stated in the CfP). Less than 2 days could be possible since PI Fox works at STScI and has worked on ultra-rapid responses with HST planners in the past. The main goal, however, is to make sure the observations are obtained around peak, which typically happens about 3 days post-discovery. Additional Comments: Likely to acquire on bright SN. 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 SN-2025SHI STIS/CCD ACQ F28X50LP MIRROR 1 30 S (30 S) Comments: Used 10, 000K Blackbody normalized to V=16. SN should be bright during initial acquisition. ------------------------------------------------------------------------------------------------------------------------------------ 2 SN-2025SHI STIS/FUV ACCUM 52X0.2D1 G140L 1425 1 2000 S -MAMA see below (2014 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN-2025SHI 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN-2025SHI 2 STIS/FUV ACCUM 52X0.2D1 G140L 1425 none none none none 1 2014.0 S -MAMA 17814( 5) - 08-Aug-2025 14:56:36 - [ 7] Visit: 04 (STIS-SN2025shi-Epoch2 (NUV)) Visit Requirements: AFTER 01 BY 6 D TO 12 D; ON HOLD On Hold Comments: Target-of-Opportunity. This visit will be scheduled for 6-12 days following the first visit (as specified in the Timing Requirements), ideally aiming for 10 days. But we'd like to have some flexibility with our final submission to account for the brightness and light-curve evolution. Additional Comments: Likely to acquire on bright SN. 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 SN-2025SHI STIS/CCD ACQ F28X50LP MIRROR 1 30 S (30 S) Comments: Used 10, 000K Blackbody normalized to V=16. SN should be bright during initial acquisition. ------------------------------------------------------------------------------------------------------------------------------------ 2 SN-2025SHI STIS/NUV ACCUM 52X0.2 G230L 2376 1 2000 S -MAMA see below (2010 S) Comments: Assuming a 7, 000K Blackbody around 10 days post-max. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN-2025SHI 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN-2025SHI 2 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 2010.0 S -MAMA 17814( 5) - 08-Aug-2025 14:56:36 - [ 8] Visit: 05 (STIS-SN2025shi-Epoch2 (NUV)) Visit Requirements: AFTER 04 BY 0 Orbits TO 1.1 Orbits; ON HOLD On Hold Comments: Target-of-Opportunity. This visit will be scheduled for 6-12 days following the first visit (as specified in the Timing Requirements), ideally aiming for 10 days. But we'd like to have some flexibility with our final submission to account for the brightness and light-curve evolution. Additional Comments: Likely to acquire on bright SN. 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 SN-2025SHI STIS/CCD ACQ F28X50LP MIRROR 1 30 S (30 S) Comments: Used 10, 000K Blackbody normalized to V=16. SN should be bright during initial acquisition. ------------------------------------------------------------------------------------------------------------------------------------ 2 SN-2025SHI STIS/NUV ACCUM 52X0.2 G230L 2376 1 2000 S -MAMA see below (2010 S) Comments: Assuming a 7, 000K Blackbody around 10 days post-max. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN-2025SHI 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN-2025SHI 2 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 2010.0 S -MAMA 17814( 5) - 08-Aug-2025 14:56:36 - [ 9] Visit: 06 (STIS-SN2025shi-Epoch2 (NUV)) Visit Requirements: AFTER 05 BY 0 Orbits TO 1.1 Orbits; ON HOLD On Hold Comments: Target-of-Opportunity. This visit will be scheduled for 6-12 days following the first visit (as specified in the Timing Requirements), ideally aiming for 10 days. But we'd like to have some flexibility with our final submission to account for the brightness and light-curve evolution. Additional Comments: Likely to acquire on bright SN. 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 SN-2025SHI STIS/CCD ACQ F28X50LP MIRROR 1 30 S (30 S) Comments: Used 10, 000K Blackbody normalized to V=16. SN should be bright during initial acquisition. ------------------------------------------------------------------------------------------------------------------------------------ 2 SN-2025SHI STIS/NUV ACCUM 52X0.2 G230L 2376 1 2000 S -MAMA see below (2010 S) Comments: Assuming a 7, 000K Blackbody around 10 days post-max. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN-2025SHI 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN-2025SHI 2 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 2010.0 S -MAMA 17814( 5) - 08-Aug-2025 14:56:36 - [ 10] Summary Form for Proposal 17814 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.2, 52X0.2D1, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 0 D TO 1.1 D, AFTER 01 BY 6 D TO 12 D, AFTER 02 BY 0 D TO 1.1 D, AFTER 04 BY 0 Orbits TO 1.1 Orbits, AFTER 05 BY 0 Orbits TO 1.1 Orbits, ON HOLD , TOO RESPONSE TIME 2.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, G230L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names SN-2025SHI ------------------------------------------------------------------------------------------------------------------------------------