18441( 3) - 19-Aug-2026 14:02:26 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18441 Version: 3 Check-in Time: 19-Aug-2026 14:02:26 STScI Edit Number: 0 Title A UV Search for a Surviving He-Star Companion in SN 1181: A Critical Test of the SN Iax Progenitor Channel ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Kohki Uno Columbia University in the City of New Y York CoI: Prof. Daichi Hiramatsu University of Florida N CoI: Daichi Tsuna Harvard University N CoI: Dr. Tomoya Kinugawa Shinshu University N CoI: Dr. Takatoshi Ko University of Tokyo, Institute of N Astronomy ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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 18441( 3) - 19-Aug-2026 14:02:26 - [ 2] 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. ------------------------------------------------------------------------------------ 18441( 3) - 19-Aug-2026 14:02:26 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SN1181 STAR, SUPERNOVA, RA=00H53M11.2100S, J2000 V = 15.4 SUPERNOVA TYPE IA DEC=+67D30'2.40" Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 18441( 3) - 19-Aug-2026 14:02:26 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18441( 3) - 19-Aug-2026 14:02:26 - [ 5] Visit: 01 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 SN1181 WFC3/UVI ACCUM UVIS2-C1K F225W FLASH=17 2 250 S PATTERN 1 1-4 S 1C-SUB (2000 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SN1181 WFC3/UVI ACCUM UVIS2-C1K F225W FLASH=17 2 250 S S 1C-SUB (2000 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 SN1181 WFC3/UVI ACCUM UVIS2-C1K F275W FLASH=17 1 250 S S 1C-SUB (1000 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 SN1181 WFC3/UVI ACCUM UVIS2-C1K F336W FLASH=17 1 250 S S 1C-SUB (1000 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN1181 1 WFC3/UVI ACCUM UVIS2-C1 F225W 1 none 1 none 1 N/A S K1C-SUB SN1181 1 WFC3/UVI ACCUM UVIS2-C1 F225W 1 none 2 none 1 N/A S K1C-SUB SN1181 1 WFC3/UVI ACCUM UVIS2-C1 F225W 2 none 1 none 1 N/A S K1C-SUB SN1181 1 WFC3/UVI ACCUM UVIS2-C1 F225W 2 none 2 none 1 N/A S K1C-SUB SN1181 1 WFC3/UVI ACCUM UVIS2-C1 F225W 3 none 1 none 2 N/A S K1C-SUB SN1181 1 WFC3/UVI ACCUM UVIS2-C1 F225W 3 none 2 none 2 N/A S K1C-SUB SN1181 1 WFC3/UVI ACCUM UVIS2-C1 F225W 4 none 1 none 3 N/A S K1C-SUB SN1181 1 WFC3/UVI ACCUM UVIS2-C1 F225W 4 none 2 none 3 N/A S K1C-SUB SN1181 2 WFC3/UVI ACCUM UVIS2-C1 F225W 1 none 1 none 1 N/A S K1C-SUB SN1181 2 WFC3/UVI ACCUM UVIS2-C1 F225W 1 none 2 none 1 N/A S K1C-SUB SN1181 2 WFC3/UVI ACCUM UVIS2-C1 F225W 2 none 1 none 2 N/A S K1C-SUB SN1181 2 WFC3/UVI ACCUM UVIS2-C1 F225W 2 none 2 none 2 N/A S K1C-SUB 18441( 3) - 19-Aug-2026 14:02:26 - [ 6] 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN1181 2 WFC3/UVI ACCUM UVIS2-C1 F225W 3 none 1 none 2 N/A S K1C-SUB SN1181 2 WFC3/UVI ACCUM UVIS2-C1 F225W 3 none 2 none 2 N/A S K1C-SUB SN1181 2 WFC3/UVI ACCUM UVIS2-C1 F225W 4 none 1 none 3 N/A S K1C-SUB SN1181 2 WFC3/UVI ACCUM UVIS2-C1 F225W 4 none 2 none 3 N/A S K1C-SUB SN1181 3 WFC3/UVI ACCUM UVIS2-C1 F275W 1 none none none 1 N/A S K1C-SUB SN1181 3 WFC3/UVI ACCUM UVIS2-C1 F275W 2 none none none 2 N/A S K1C-SUB SN1181 3 WFC3/UVI ACCUM UVIS2-C1 F275W 3 none none none 3 N/A S K1C-SUB SN1181 3 WFC3/UVI ACCUM UVIS2-C1 F275W 4 none none none 3 N/A S K1C-SUB SN1181 4 WFC3/UVI ACCUM UVIS2-C1 F336W 1 none none none 1 N/A S K1C-SUB SN1181 4 WFC3/UVI ACCUM UVIS2-C1 F336W 2 none none none 2 N/A S K1C-SUB SN1181 4 WFC3/UVI ACCUM UVIS2-C1 F336W 3 none none none 3 N/A S K1C-SUB SN1181 4 WFC3/UVI ACCUM UVIS2-C1 F336W 4 none none none 3 N/A S K1C-SUB 18441( 3) - 19-Aug-2026 14:02:26 - [ 7] Summary Form for Proposal 18441 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-C1K1C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=17 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1-4 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F225W, F275W, F336W ------------------------------------------------------------------------------------------------------------------------------------ Target Names SN1181 ------------------------------------------------------------------------------------------------------------------------------------