18394( 2) - 19-Aug-2026 15:09:35 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18394 Version: 2 Check-in Time: 19-Aug-2026 15:09:35 STScI Edit Number: 0 Title Bridging the Gap Between Strong and Weak Mass Loss with Nebular UV spectra of SN 2025tis and SN 2026ayt ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Kyra Azalee Bostroem California Institute of Technology Y CoI: Prof. Stefano Valenti University of California - Davis N CoI: Prof. David J. Sand University of Arizona N CoI: Prof. Griffin Hosseinzadeh University of California - San Diego N CoI: Dr. Saurabh W. Jha Rutgers the State University of New Jersey N CoI: Dr. Manisha Shrestha Monash University N CoI: Dr. Conor Ransome University of Arizona N CoI: Dr. Bhagya Subrayan University of Arizona N CoI: Jeniveve Pearson University of Arizona N CoI: Dr. Aravind Pazhayath Ravi University of California - Davis N CoI: Mr. Nicolas Eduardo Meza Retamal University of California - Davis N CoI: Ms. Darshana Mehta University of California - Davis N CoI: Dr. Lindsey Kwok Northwestern University N CoI: Dr. Sergiy Sergiyovich Vasylyev University of California - San Diego N CoI: Dr. Jennifer Andrews NOIRLab - Gemini North (HI) N CoI: Daryl Janzen University of Saskatchewan N CoI: Dr. Yize Dong Harvard University N CoI: Dr. Jacob Jencson California Institute of Technology N CoI: Dr. Michael Jon Lundquist California Association for Research in N Astronomy (CARA) ------------------------------------------------------------------------------------ Abstract Winds from massive stars and their supernova explosions are responsible for the majority of elements in the universe and the regulation of star formation on a galactic scale. However, empirical mass-loss rates disagree and there is mounting evidence of periods of enhanced mass loss as the star nears explosion. While strongly interaction between supernova ejecta and circumstellar material can be observed in the optical at early times, this disappears on the time scale of weeks and is not sensitive at any time to weaker mass loss. Fortunately, signatures of weaker mass loss can be observed in the UV throughout the supernova's evolution. We propose to observe SN2025tis and SN2026ayt, two supernovae are spectroscopically different in early FUV and NUV observations, in the FUV and NUV 200-800\, d after explosion to explore the diversity of UV spectra in supernovae with different amounts of circumstellar material. Additionally, both of these supernovae occurred in dwarf galaxies, giving us the possibility of probing supernovae and CSM interaction in low metallicity environments. This work will connect early and late-time observations, a spectroscopic and photometric sample, and strong and weak interaction in the UV, laying the groundwork for interpreting high redshift supernovae from the Roman Space Telescope, Rubin Observatory, and JWST and preparing for a larger sample in the 2040s with the Habitable Worlds Observatory. ------------------------------------------------------------------------------------ 18394( 2) - 19-Aug-2026 15:09:35 - [ 2] Observations Description ------------------------ This program seeks to observe SN 2025tis and SN 2026ayt in the FUV and NUV with STIS. Each supernova will be observed over two epochs separated by 200-400d. Visits over 4 orbits were split in to NUV and FUV visits with a requirement that they be taken with 14 days of each other (although closer is preferable) ------------------------------------------------------------------------------------ 18394( 2) - 19-Aug-2026 15:09:35 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SN2025TIS EXT-STAR, SUPERNOVA RA=03H55M22.7940S, J2000 V = 19.4 TYPE II DEC=-28D09'32.69" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 2 SN2026AYT EXT-STAR, SUPERNOVA RA=09H27M23.3900S, J2000 V = 18.1 TYPE II DEC=-32D00'30.88" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 18394( 2) - 19-Aug-2026 15:09:35 - [ 4] Visit: 01 (SN2026ayt epoch 1 FUV) Visit Requirements: 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 SN2026AYT STIS/CCD ACQ F28X50LP MIRROR 1 5 S (5 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SN2026AYT STIS/FUV ACCUM 52X0.2 G140L 1425 1 2100 S -MAMA (2100 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 SN2026AYT STIS/FUV ACCUM 52X0.2 G140L 1425 1 2550 S -MAMA (2550 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 SN2026AYT STIS/FUV ACCUM 52X0.2 G140L 1425 1 2550 S -MAMA (2550 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 SN2026AYT STIS/FUV ACCUM 52X0.2 G140L 1425 1 2550 S -MAMA (2550 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2026AYT 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN2026AYT 2 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 1 N/A -MAMA SN2026AYT 3 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 2 N/A -MAMA SN2026AYT 4 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 3 N/A -MAMA SN2026AYT 5 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 4 N/A -MAMA 18394( 2) - 19-Aug-2026 15:09:35 - [ 5] Visit: 02 (SN2026ayt epoch 2) Visit Requirements: AFTER 01 BY 200 D TO 400 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 SN2026AYT STIS/CCD ACQ F28X50LP MIRROR 1 5 S (5 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SN2026AYT STIS/FUV ACCUM 52X0.2 G140L 1425 1 2100 S -MAMA (2100 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 SN2026AYT STIS/FUV ACCUM 52X0.2 G140L 1425 1 2550 S -MAMA (2550 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 SN2026AYT STIS/NUV ACCUM 52X0.2 G230L 2376 1 2575 S -MAMA (2575 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2026AYT 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN2026AYT 2 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 1 N/A -MAMA SN2026AYT 3 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 2 N/A -MAMA SN2026AYT 4 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 3 N/A -MAMA 18394( 2) - 19-Aug-2026 15:09:35 - [ 6] Visit: 03 (SN2025tis epoch 1) Visit Requirements: 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 SN2025TIS STIS/CCD ACQ F28X50LP MIRROR 1 5 S (5 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SN2025TIS STIS/FUV ACCUM 52X0.2 G140L 1425 1 870 S (870 -MAMA S) ------------------------------------------------------------------------------------------------------------------------------------ 3 SN2025TIS STIS/NUV ACCUM 52X0.2 G230L 2376 1 870 S (870 -MAMA 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2025TIS 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN2025TIS 2 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 1 N/A -MAMA SN2025TIS 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A -MAMA 18394( 2) - 19-Aug-2026 15:09:35 - [ 7] Visit: 04 (SN2025tis epoch 2 FUV) Visit Requirements: AFTER 03 BY 200 D TO 400 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 SN2025TIS STIS/CCD ACQ F28X50LP MIRROR 1 5 S (5 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SN2025TIS STIS/FUV ACCUM 52X0.2 G140L 1425 1 2100 S -MAMA (2100 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 SN2025TIS STIS/FUV ACCUM 52X0.2 G140L 1425 1 2550 S -MAMA (2550 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 SN2025TIS STIS/FUV ACCUM 52X0.2 G140L 1425 1 2550 S -MAMA (2550 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2025TIS 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN2025TIS 2 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 1 N/A -MAMA SN2025TIS 3 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 2 N/A -MAMA SN2025TIS 4 STIS/FUV ACCUM 52X0.2 G140L 1425 none none none none 3 N/A -MAMA 18394( 2) - 19-Aug-2026 15:09:35 - [ 8] Visit: 05 (SN2026ayt epoch 1 NUV) Visit Requirements: AFTER 01 BY 0 D TO 14 D On Hold Comments: Additional Comments: If possible, please schedule with Visit 1 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 SN2026AYT STIS/CCD ACQ F28X50LP MIRROR 1 5 S (5 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SN2026AYT STIS/NUV ACCUM 52X0.2 G230L 2376 1 2100 S -MAMA (2100 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2026AYT 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN2026AYT 2 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A -MAMA 18394( 2) - 19-Aug-2026 15:09:35 - [ 9] Visit: 06 (SN2025tis epoch 2 NUV) Visit Requirements: AFTER 04 BY 0 D TO 14 D On Hold Comments: Additional Comments: If possible, please schedule with Visit 4 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 SN2025TIS STIS/CCD ACQ F28X50LP MIRROR 1 5 S (5 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SN2025TIS STIS/NUV ACCUM 52X0.2 G230L 2376 1 2100 S -MAMA (2100 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 SN2025TIS STIS/NUV ACCUM 52X0.2 G230L 2376 1 2575 S -MAMA (2575 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 SN2025TIS STIS/NUV ACCUM 52X0.2 G230L 2376 1 2575 S -MAMA (2575 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 SN2025TIS STIS/NUV ACCUM 52X0.2 G230L 2376 1 2575 S -MAMA (2575 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2025TIS 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A SN2025TIS 2 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 1 N/A -MAMA SN2025TIS 3 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 2 N/A -MAMA SN2025TIS 4 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 3 N/A -MAMA SN2025TIS 5 STIS/NUV ACCUM 52X0.2 G230L 2376 none none none none 4 N/A -MAMA 18394( 2) - 19-Aug-2026 15:09:35 - [ 10] Summary Form for Proposal 18394 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.2, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 0 D TO 14 D, AFTER 01 BY 200 D TO 400 D, AFTER 03 BY 200 D TO 400 D, AFTER 04 BY 0 D TO 14 D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, G230L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names SN2025TIS, SN2026AYT ------------------------------------------------------------------------------------------------------------------------------------