Bridging the Gap Between Strong and Weak Mass Loss with Nebular UV spectra of SN 2025tis and SN 2026ayt

0 Assigned ID: 7583 ----- Attempting Submission 1 (Thu Apr 16 19:47:24 GMT 2026) ----- HST Phase I Proposal 7583 successfully submitted. Receipt: # 7583-1 ----- Attempting Submission 2 (Thu Apr 16 19:48:56 GMT 2026) ----- 0 ----- Attempting Submission 1 (Wed Aug 19 06:08:56 GMT 2026) ----- Bridging the Gap Between Strong and Weak Mass Loss with Nebular UV spectra of SN 2025tis and SN 2026ayt 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. 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) K. A. Bostroem is a former member of the STIS team at STScI and has extensive experience with the STIS, Swift, Gemini, and Keck data reduction. Her research focuses on understanding the progenitors of hydrogen-rich CC SNe. She is the PI of programs to obtain HST/STIS UV data of SN 2022acko, SN2023ixf, SN2024ggi, SN2025tis, and SN2026ayt and is the author of papers on early interaction in SN 2023ixf optical spectra, interaction in the early UV spectra of SN 2023ixf, and an analysis of the late time UV spectra of SN 2023ixf and SN 2024ggi. S. Valenti is the PI of a Cycle 31 program and a DD program to obtain HST/STIS FUV and NUV spectra of SN 2023ixf and PI of a program to obtain UIV photometry of 10 supernoave within 100 Mpc to search for CSM interaction. He has worked comprehensively on core collapse supernovae (CC SNe). M. Lundquist is an active member of the DLT40 team and specializes in the reduction and analysis of Gemini and Keck observations. He is currently a staff astronomer at the W. M. Keck Observatory. Prior to joining the DLT40 team, he worked as a Gemini Science Fellow at the Gemini Observatories. J. Andrews is an active member of the DLT40 team, has expertise observing with HST/STIS, and is an assistant astronomer at the Gemini Observatory. Her research focuses on characterizing the progenitors of core collapse (CC) SNe, specifically focusing on their mass-loss history via circumstellar interaction. D. Sand is the co-PI of the DLT40 survey and an expert in the progenitors of Type Ia SNe and SN environments. He has extensive experience with Swift, and Gemini observations of SNe and their environments. M. Shrestha is a Lecturer at Monash University. Her research focuses on the transients such as long gamma-ray bursts, core collapse (CC) SNe, and kilonova. She is part of DLT40 and SAGUARO collaborations. J. Jencson is an active member of the DLT40 team and has extensive experience in reducing and analyzing Swift observations of transients. His research focuses on multi-wavelength studies of hydrogen-rich transients and searches for dust-obscured transients and faint/failed SNe. G. Hosseinzadeh is an expert in early SN~Ia light curves and has experience analyzing early UV data. J. Pearson is focusing her thesis research on understanding the progenitors and environments of CC SNe and is an active member of the DLT40 team. Y. Dong is an post doc at the Center for Astrophysics and has experience analyzing both CC~SNe and SN~I. N. Meza is a graduate student in the DLT40 team focusing his thesis work on CC~SNe. D. Janzen is a member of the SkyNet team which is responsible for maintaining the telescopes that the DLT40 team uses for their search. S. Jha is an expert in Type Ia supernovae and the PI of a program on SALT to obtain spectra of supernova transients. SALT is ideally located for obtaining rapid classification spectra of supernovae discovered by our Australian telescope. A. P. Ravi is a postdoc in the DLT40 team and has expertise in high energy emission from supernovae. He leads the UC Davis NIR observing program. D. Mehta is a graduate student in the DLT40 team focusing on analyzing UV photometry of Type II supernovae to search for CSM interaction. L. Kwok is the PI of a Cycle 3 JWST proposal to observe SNe~Ia from infancy until nebular phase. She is an expert in the mid-infrared properties of both normal and peculiar SNe~Ia. B. Subrayan specializes in multi-wavelength observations of supernovae, supernova remnants, tidal disruption events, and extreme nuclear transients. She has extensive experience in optical and infrared long-slit spectroscopy of core-collapse supernovae. She runs a rapid target of opportunity program at MMT Observatory, giving her longslit experience at big telescopes. S. S. Vasylyev is a postdoctoral scholar at UC San Diego, specializing in core collapse SNe (CC-SNe). He has extensive experience with UV-optical spectroscopy and polarimetry of young supernovae, particularly those with CSM interaction signatures. He is one of the leads of the PASSTA program. He is a member of LSST-DA, helping build infrastructure to maximize science from LSST together with other facilities available through other approved programs. C. Ransome has extensive experience in the photometric and spectroscopic follow-up of supernovae. His research focus is on supernovae that interact with surrounding material. He has experience in HST and JWST data and will aid in analysis. Stellar Physics and Stellar Types 0 false true false false false true true SUPPORTED EXT-STAR SUPERNOVA TYPE II J2000 J2000 EXT-STAR SUPERNOVA TYPE II J2000 J2000 Timing link suitability includes constraints ([002 After 001 by 200.0 to 500.0 days]) The scheduling data was generated by CASM 34.0. CASM was run in Phase I mode. STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/CCD Imaging No No 50CCD STIS/NUV-MAMA Spectroscopic No No G230L 2376 Timing link suitability includes constraints ([002 After 001 by 200.0 to 500.0 days] [003 After 002 by at most 14.0 days]) The scheduling data was generated by CASM 34.0. CASM was run in Phase I mode. STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/CCD Imaging No No 50CCD Timing link suitability includes constraints ([003 After 002 by at most 14.0 days]) The scheduling data was generated by CASM 34.0. CASM was run in Phase I mode. STIS/CCD Imaging No No 50CCD STIS/NUV-MAMA Spectroscopic No No G230L 2376 Timing link suitability includes constraints ([005 After 004 by at most 14.0 days] [006 After 004 by 200.0 to 500.0 days]) The scheduling data was generated by CASM 34.0. CASM was run in Phase I mode. STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/CCD Imaging No No 50CCD Timing link suitability includes constraints ([005 After 004 by at most 14.0 days]) The scheduling data was generated by CASM 34.0. CASM was run in Phase I mode. STIS/CCD Imaging No No 50CCD STIS/NUV-MAMA Spectroscopic No No G230L 2376 Timing link suitability includes constraints ([006 After 004 by 200.0 to 500.0 days]) The scheduling data was generated by CASM 34.0. CASM was run in Phase I mode. STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/CCD Imaging No No 50CCD STIS/NUV-MAMA Spectroscopic No No G230L 2376 1787118425776 /Users/bostroem/Desktop/research/proposals/HST_Cy34_nebular_UV_26ayt_25tis/APT/OP-cache/apt_v1-v01.xml Timing link suitability includes constraints ([02 After 01 by 200.0 to 400.0 days] [05 After 01 by at most 14.0 days]) The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18394 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 22:47:05.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 348400 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18394 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18394 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2589 :al-overhead-at-end 0 :internal-dur-at-end 113 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18394 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2578 :al-overhead-at-end 85 :internal-dur-at-end 132 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18394 :visit-id "01" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2578 :al-overhead-at-end 85 :internal-dur-at-end 132 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18394 :visit-id "01" :orbit-number 4 :pcs-time 380 :si-vis-utilization 2578 :al-overhead-at-end 85 :internal-dur-at-end 394 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) true 4 0 1787117887121 /Users/bostroem/Desktop/research/proposals/HST_Cy34_nebular_UV_26ayt_25tis/APT/OP-cache/apt_v1-v05.xml Timing link suitability includes constraints ([05 After 01 by at most 14.0 days]) The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. If possible, please schedule with Visit 1 (Tic-Visit :proposal-id 18394 :visit-id "05" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 22:38:07.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 87408 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18394 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18394 :visit-id "05" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2589 :al-overhead-at-end 0 :internal-dur-at-end 394 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) true 1 0 1787117852138 /Users/bostroem/Desktop/research/proposals/HST_Cy34_nebular_UV_26ayt_25tis/APT/OP-cache/apt_v1-v02.xml Timing link suitability includes constraints ([02 After 01 by 200.0 to 400.0 days]) The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18394 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 22:37:32.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 278848 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18394 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18394 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2589 :al-overhead-at-end 0 :internal-dur-at-end 113 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18394 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2578 :al-overhead-at-end 85 :internal-dur-at-end 132 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18394 :visit-id "02" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2603 :al-overhead-at-end 198 :internal-dur-at-end 319 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) true 3 0 1787117852490 /Users/bostroem/Desktop/research/proposals/HST_Cy34_nebular_UV_26ayt_25tis/APT/OP-cache/apt_v1-v03.xml Timing link suitability includes constraints ([04 After 03 by 200.0 to 400.0 days]) The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18394 :visit-id "03" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 22:37:32.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 174416 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18394 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18394 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 394 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) true 1 0 1787117853089 /Users/bostroem/Desktop/research/proposals/HST_Cy34_nebular_UV_26ayt_25tis/APT/OP-cache/apt_v1-v04.xml Timing link suitability includes constraints ([04 After 03 by 200.0 to 400.0 days] [06 After 04 by at most 14.0 days]) The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18394 :visit-id "04" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 22:37:32.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 261408 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18394 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18394 :visit-id "04" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2589 :al-overhead-at-end 0 :internal-dur-at-end 113 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18394 :visit-id "04" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2578 :al-overhead-at-end 85 :internal-dur-at-end 132 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18394 :visit-id "04" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2578 :al-overhead-at-end 85 :internal-dur-at-end 394 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) true 3 0 1787117912879 /Users/bostroem/Desktop/research/proposals/HST_Cy34_nebular_UV_26ayt_25tis/APT/OP-cache/apt_v1-v06.xml Timing link suitability includes constraints ([06 After 04 by at most 14.0 days]) The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. If possible, please schedule with Visit 4 (Tic-Visit :proposal-id 18394 :visit-id "06" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 22:38:32.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 348400 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18394 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18394 :visit-id "06" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2589 :al-overhead-at-end 0 :internal-dur-at-end 113 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18394 :visit-id "06" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2603 :al-overhead-at-end 85 :internal-dur-at-end 132 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18394 :visit-id "06" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2603 :al-overhead-at-end 85 :internal-dur-at-end 132 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18394 :visit-id "06" :orbit-number 4 :pcs-time 380 :si-vis-utilization 2603 :al-overhead-at-end 85 :internal-dur-at-end 394 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2983 :pcs-gap 0 ) true 4 0