Final submission of SALT3 UV AR proposal. -706834198 Assigned ID: 3579 ----- Attempting Submission 1 (Fri Mar 25 21:28:21 GMT 2022) ----- HST Phase I Proposal 3579 successfully submitted. Receipt: # 3579-1 ----- Attempting Submission 2 (Fri Mar 25 22:29:02 GMT 2022) ----- Virtually all upcoming cosmological analyses leveraging the several hundred thousand Type Ia supernovae (SNIa) expected from the Vera C. Rubin Observatory and Roman Space Telescope (Zhan+2018; Hounsell+2018) will include the widely used SALT3 light curve fitter. Unfortunately, SALT3 is very poorly trained in the ultraviolet (UV; <3,500A) due to a lack of well-calibrated spectra used to constrain UV spectral features. The atmosphere absorbs ~100% of UV light <3,000A, which means the (primarily ground-based) SALT3 training sample at these wavelengths is comprised of only 85 SNIa spectra that cover just half of the rest-frame model phase range with a median S/N of ~1.4. Over 50% of these spectra are at z>0.4, also leaving SALT3 susceptible to a redshift-evolving bias. The extremely unreliable result has not been a major barrier due to a relative lack of new rest-frame UV observations, but reliably fitting ~35% of all Roman SNIa observations will require SALT3 to be robust below 3,500A. The HST archive now contains 179 STIS G230L and G430L spectra not in the SALT3 training sample from 26 bright, nearby (median z ~ 0.005) SNIa. These unique spectra provide continuous, high-precision, well-calibrated rest-frame UV information from 1,700-5,500A for ~85% of the SALT3 phase range, enabling a 300A extension to the model and drastic improvements in the 2,000-3,500A wavelength range. This proposal triples the UV spectral SALT3 training sample (with double the S/N), quantifies or excludes any redshift-evolution of SNIa in the UV impacting distance measurements at >3sigma, and increases the percentage of reliably fit Roman SNIa data from the current ~66% to ~99%. This team includes experts in SNIa cosmology, who have been leaders in SNIa analysis with HST, the development of SN light curve models, and precision SNIa cosmology for over two decades. * Light Curve Model Development: W. D. Kenworthy, D. Jones, M. Dai, R. Kessler, and J. Pierel were primary develop- ers of the SALT3 model and training framework, including the SALTShaker training code (Kenworth et al., 2021). R. Kessler wrote the SuperNova Analysis Software (SNANA), one of the most widely-used tools for SN cosmology over the last decade, which is used for all simulated training sample tests in this program (Kessler et al., 2009). J. Pierel developed an early extrapolation of the SALT2 model to UV wavelengths, including similar extensions of CC SN models (Pierel et al., 2018), based on the extensive work of R. Foley (Foley et al., 2016.) * SN spectral/photometric analysis: All members of the team are experts in core-collapse and/or Type Ia SN analysis. O. Fox, S. Gezari, J. Pierel, A.Rest, C. Rojas-Bravo, L. Strolger, and Q.Wang, in particular have extensive experience dealing with UV photometry for a variety of SN types, while R. Foley has led HST -GO programs that produced a majority of the spectra included in this program (12592, 13286, 13646, 14925, 15876, 16238). S. Gomez, P.McGill, D.Jones, M. Siebert, and S. Shahbandeh are experts in reducing and analyzing SN spectra of various types, including methods for mitigating host contamination, measuring reddening, etc. * SNIa STIS Spectra R. Foley, D. Jones, and M. Siebert have written several papers relying on the analysis of STIS spectra of SNIa (Foley et al., 2012,2013,2016; Pan et al., 2015,2020; Siebert et al., 2020), and R. Foley and M. Siebert in particular have previously reduced a large number of the spectra that are in this program. Stellar Physics and Stellar Types Large Scale Structure of the Universe 0 Regular false false false false false false SUPPORTED