0 Assigned ID: 7739 ----- Attempting Submission 1 (Thu Apr 16 22:39:06 GMT 2026) ----- update the post-flash -1278902327 ----- Attempting Submission 1 (Mon Aug 17 15:41:22 GMT 2026) ----- HST Phase II Proposal 18441 successfully submitted. Receipt: # 18441-1 ----- Attempting Submission 2 (Wed Aug 19 02:40:07 GMT 2026) ----- SN 1181, the only historical supernova identified as a faint Type Iax supernova (SN Iax), provides an important laboratory for testing the progenitor channel of this class. Its remnant hosts a surviving white dwarf (WD), pointing to an incomplete thermonuclear explosion and favoring a pure-deflagration scenario. A central prediction of this model is that the He-star companion that donated material onto the WD before explosion should also survive. Detecting or ruling out such a companion would provide direct evidence for the progenitor channel of a faint SN Iax and would test whether the faint and bright SN Iax subclasses share a common single-degenerate origin. More broadly, it would inform debate over the explosion channels of Type Ia supernovae. We propose three orbits of HST WFC3/UVIS imaging in F225W, F275W, and F336W to carry out the UV search for a surviving He-star companion in the SN 1181 remnant. Our binary stellar evolution calculations and existing constraints by GALEX indicate that the surviving companion is most likely a low-mass He star (0.3-0.5 Msun) with an effective temperature of 20,000-40,000 K, making UV observations essential. HST WFC3/UVIS provides the sensitivity and spatial resolution required to detect such a source or place stringent limits on its presence. This program is designed to deliver a decisive result: either the detection of the long-sought surviving companion or strong constraints that exclude it. In either case, the proposed observations will provide the most direct test yet of the progenitor system of a faint SN Iax and an important step toward clarifying the physical diversity of thermonuclear supernovae. We will obtain WFC3/UVIS imaging of the SN 1181 field in a single 3-orbit visit using the UVIS2-C1K1C-SUB aperture centered on the central white dwarf. The approximately 40 arcsec x 40 arcsec subarray fully covers the expected companion search region of approximately 10 arcsec radius. Two orbits are devoted to sixteen 250 s F225W exposures, for a total integration time of 4000 s. The third orbit contains four 250 s F275W exposures and four 250 s F336W exposures, for total integration times of 1000 s in each filter. All exposures use the four-point WFC3-UVIS-DITHER-BOX pattern for hot-pixel and cosmic-ray mitigation and improved PSF sampling. The central white dwarf is expected to saturate in the individual exposures. The science targets are the surrounding stars rather than the central source. Dividing the observations into 250 s integrations reduces blooming relative to the originally proposed 500 s integrations while preserving the Phase I total exposure times. No orientation or timing constraints are required. Kohki Uno (PI, Columbia University) is a postdoctoral fellow specializing in astronomical transients. He has extensive experience in HST analysis of supernovae as a Co-I on GO-18016. He has also served as PI on multiple observing programs using large ground-based telescopes, including Keck and Subaru. He brings broad expertise relevant to this program and will lead the data analysis. Daichi Hiramatsu (CoPI and administrative PI, University of Florida) is an expert in observations and modeling of supernovae and their progenitors, with extensive experience in HST UV imaging through his PI program SNAP 17770. Daichi Tsuna (CoPI, Harvard University) is a postdoctoral fellow with expertise on theoretical modeling of supernovae and stellar evolution. He will work on applying models of stellar/binary evolution to explain and interpret the HST observations. Tomoya Kinugawa (Co-I, Shinshu University) is an associate professor with expertise on theoretical calculation of binary evolution. He will work on applying models of binary evolution path to explain and interpret the HST observations. Takatoshi Ko (Co-I, University of Tokyo) is a researcher specializing in supernovae and supernova remnants. His work focuses in particular on the historical supernova SN 1181 and its remnant. He has extensive experience in the analysis and interpretation of transient and remnant data, and has led multiple observing programs as PI using major facilities including Subaru and the VLA. Stellar Physics and Stellar Types Stellar Populations and the Interstellar Medium 6 false false false false false false false SUPPORTED STAR SUPERNOVA SUPERNOVA TYPE IA NO PA30 IRAS00500+6713 J2000 J2000 WFC3/UVIS Imaging No No F225W WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F336W POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false 1787106847232 /private/var/folders/xb/h80dxm6d1lz5qhhpwr5y8j1m0000gn/T/OPVisit5972318771445700153.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. 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