0 Assigned ID: 4430 ----- Attempting Submission 1 (Wed May 24 17:14:27 GMT 2023) ----- Type Ia supernovae (SNe Ia) are foundational objects in modern astronomy. The energy they release drives galactic evolution, the elements synthesized in their explosions enrich their surroundings, and their high intrinsic luminosities make them the most accurate cosmological distance indicators. Yet almost a century of in-depth study has failed to reveal the progenitor systems or explosion mechanisms of SNe Ia. Ultraviolet (UV) spectra are an under-utilized, but valuable resource to answer these lingering questions. UV spectra probe the outermost layers of the SN, where the differences produced by various explosion scenarios are largest, but are inaccessible at other wavelengths. HST's extensive archive of SNe Ia UV spectra, totaling more than 500 orbits and 281 individual spectra, has been severely under-used by studies analyzing the physics of SNe Ia. Here, we propose to use this archive to study the complete sample of HST UV spectral observations in order to catalog and characterize the observed diversity of SNe Ia in the UV, and connect spectral measurements back to the fundamental physics that drive UV spectral formation and SN Ia explosions. Previous UV spectroscopic population studies have only used ~10 objects and are too small to draw robust conclusions about the UV in SNe Ia. This study will serve as the foundation upon which rest-frame UV studies using Rubin/LSST, Roman, and JWST data will be built, and will ensure that the last years of Hubble's UV spectral capacity are maximized by the SN community in the pursuit of new discoveries with SNe Ia. The team members have a long history of collaboration on supernovae (SNe) and transients representing a well tuned team. Group members have led various projects with HST, Swift, and JWST, spanning both Type Ia and core collapse SNe. The group consists of a healthy combination of students, postdocs, and members with many decades of experience in the field. The team has both theorists and observers who have a history of working on SNe in the UV all the way through to the MIR. PI: James DerKacy is an expert in SN spectroscopy. He has extensive experience with multi-wavelength (UV to MIR) spectral observations, reductions, and NLTE spectral modeling. As PI, he will lead the analysis of the HST data and coordinate with STScI. He will coordinate with the other Co-I's and share scientific responsibility of the combined results with the Co-I's and other team members. Chris Ashall is an expert in SN observations and physics. He has experience working from the UV through to the MIR with spectra and photometry. He has written both observational and theory papers, and has extensive experience with abundance tomography modeling in the UV. Eddie Baron is an expert in numerical radiative transfer, supernova spectroscopy, and supernova phenomenology. Peter Brown is an expert in UV properties of supernovae, observational techniques and leads multi-year programs with HST and Swift. Anthony Burrow is a grad student at the University of Oklahoma working on SNe Ia classification and spectroscopic modeling. Peter Hoeflich is the main developer of HYDRA, which has been widely applied to calculate SN explosions, light curves, flux, and polarization spectra using detailed nuclear, atomic and molecular networks. Lifan Wang is an expert in supernova and observational techniques. Stellar Physics and Stellar Types 0 Regular false false true false false false SUPPORTED