18398( 2) - 13-Aug-2026 14:58:15 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18398 Version: 2 Check-in Time: 13-Aug-2026 14:58:15 STScI Edit Number: 0 Title Hidden in Plain Sight? Unraveling the Potential Merger Histories of Ultra-Massive White Dwarfs in the Initial-Final Mass Relation ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Steffani Grondin University of California - San Diego Y CoI: Mr. David R Miller University of British Columbia Y CoI: Maria Drout University of Toronto N CoI: Dr. Kyle Kremer University of California - San Diego N CoI: Prof. Pier-Emmanuel Tremblay University of Warwick N CoI: Dr. Kyle Rocha University of California - San Diego N CoI: Dr. Ilaria Caiazzo Institute of Science and Technology N Austria ------------------------------------------------------------------------------------ Abstract The initial-final mass relation (IFMR) is fundamental to stellar astrophysics, linking a star's birth mass to its white dwarf (WD) remnant with broad implications for Galactic chemical enrichment, star formation histories, and the boundary between WD to neutron star formation. While the IFMR has been extensively studied, its high-mass end remains sparsely populated and poorly constrained, largely owing to the rarity of ultra-massive WDs (M > 1.1 Msun). This uncertainty is further compounded by binary evolution, since mergers are expected to be a dominant channel for producing ultra-massive WDs. Distinguishing between WDs formed through single-star evolution versus those via a binary merger is challenging, where optical spectroscopy alone cannot always rule out a merger origin. Here, we request 7 orbits of HST/COS far-UV spectroscopy to search for carbon features in four ultra-massive WDs that anchor the high-mass end of the IFMR. If carbon features are detected, they will provide strong evidence that some of the WDs currently anchoring this regime are merger remnants rather than single-star products, motivating a recalibration of the IFMR. If carbon is not detected, these observations will strengthen the case that these systems are consistent with single-star evolution and therefore remain robust calibrators of the high-mass end. In either outcome, this program delivers a direct empirical constraint on the least well constrained end of a fundamental relation in stellar astrophysics, where HST is the only instrument capable of obtaining the far-UV spectroscopy required. ------------------------------------------------------------------------------------ Observations Description ------------------------ This proposal will obtain UV spectra for four ultramassive white dwarfs in star clusteds that currently anchor the high-mass end of the white dwarf initial-final mass relation. These measurements will allow us to search for carbon features (e.g., C III), which would indicate potential evidence for a merger history. Observations will be obtained in the G130M grism and a central wavelength of 1291 AA, using the FP-POS 3 and 4 positions. 18398( 2) - 13-Aug-2026 14:58:15 - [ 2] The impacts of reduced gyro operations on this program would primarily be reduced schedulability due to the smaller field of regard. ------------------------------------------------------------------------------------ 18398( 2) - 13-Aug-2026 14:58:15 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HYADES-WD1 STAR, DA RA=02H38M36.4186S +/- J2000 V = 19.17 0.00016479", DEC=+76D42'18.76" +/- 0.000169126" Reference Frame: ICRS Extended: NO Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2016 0.013440816694407574 -0.026491 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 2 ALPHAPER-WD1 STAR, DA RA=02H58M43.3094S +/- J2000 V = 17.04 0.000062610485", DEC=+50D20'52.02" +/- 0.0000620509" Reference Frame: ICRS Extended: NO Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2016 0.0028027905420715754 -0.024163 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 3 ALPHAPER-WD2 STAR, DA RA=03H56M58.0087S +/- J2000 V = 17.41 0.0000931751", DEC=+45D01'11.15" +/- 0.000068625435" Reference Frame: ICRS Extended: NO Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2016 0.0021084713252891565 -0.030504999999999997 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 4 ALPHAPER-WD5 STAR, DA RA=22H28M14.7816S +/- J2000 V = 17.41 0.000050933402", DEC=+45D34'34.33" +/- 0.000050106153" Reference Frame: ICRS Extended: NO Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2016 0.0021292644407163923 -0.030504999999999997 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18398( 2) - 13-Aug-2026 14:58:15 - [ 4] Visit: 01 (HYADES-WD1-2orbits-set1) 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 HYADES-WD1 COS/FUV ACQ/PEA PSA G130M 1291 LIFETIME-POS=LP7 1 23.5 S KXD (23.5 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 HYADES-WD1 COS/FUV ACQ/PEA PSA G130M 1291 NUM-POS=5, STEP-SIZE=0.9, 1 23.5 S KD CENTER=FLUX-WT-FLR, (23.5 S) LIFETIME-POS=LP7 ------------------------------------------------------------------------------------------------------------------------------------ 3 HYADES-WD1 COS/FUV TIME-TA PSA G130M 1291 FP-POS=3, BUFFER-TIME=4445, 1 1905 S G LIFETIME-POS=LP7 see below (2088 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 HYADES-WD1 COS/FUV TIME-TA PSA G130M 1291 FP-POS=4, BUFFER-TIME=4445, 1 1905 S G LIFETIME-POS=LP7 see below (2712 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 | ------------------------------------------------------------------------------------------------------------------------------------ HYADES-WD1 1 COS/FUV ACQ/PEA PSA G130M 1291 none none none none 1 N/A KXD HYADES-WD1 2 COS/FUV ACQ/PEA PSA G130M 1291 none none none none 1 N/A KD HYADES-WD1 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 2088.0 S G HYADES-WD1 4 COS/FUV TIME-TA PSA G130M 1291 none none none none 2 2712.0 S G 18398( 2) - 13-Aug-2026 14:58:15 - [ 5] Visit: 02 (HYADES-WD1-2orbits-set2) 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 HYADES-WD1 COS/FUV ACQ/PEA PSA G130M 1291 LIFETIME-POS=LP7 1 23.5 S KXD (23.5 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 HYADES-WD1 COS/FUV ACQ/PEA PSA G130M 1291 NUM-POS=5, STEP-SIZE=0.9, 1 23.5 S KD CENTER=FLUX-WT-FLR, (23.5 S) LIFETIME-POS=LP7 ------------------------------------------------------------------------------------------------------------------------------------ 3 HYADES-WD1 COS/FUV TIME-TA PSA G130M 1291 FP-POS=3, BUFFER-TIME=4445, 1 1905 S G LIFETIME-POS=LP7 see below (2088 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 HYADES-WD1 COS/FUV TIME-TA PSA G130M 1291 FP-POS=4, BUFFER-TIME=4445, 1 1905 S G LIFETIME-POS=LP7 see below (2712 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 | ------------------------------------------------------------------------------------------------------------------------------------ HYADES-WD1 1 COS/FUV ACQ/PEA PSA G130M 1291 none none none none 1 N/A KXD HYADES-WD1 2 COS/FUV ACQ/PEA PSA G130M 1291 none none none none 1 N/A KD HYADES-WD1 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 2088.0 S G HYADES-WD1 4 COS/FUV TIME-TA PSA G130M 1291 none none none none 2 2712.0 S G 18398( 2) - 13-Aug-2026 14:58:15 - [ 6] Visit: 03 (ALPHAPER-WD1) 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 ALPHAPER-WD COS/NUV ACQ/IMA PSA MIRRORB 1 31.2 S 1 GE (31.2 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ALPHAPER-WD COS/FUV TIME-TA PSA G130M 1291 FP-POS=3, BUFFER-TIME=1198, 1 735 S 1 G LIFETIME-POS=LP7 see below (875 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ALPHAPER-WD COS/FUV TIME-TA PSA G130M 1291 FP-POS=4, BUFFER-TIME=1198, 1 735 S 1 G LIFETIME-POS=LP7 see below (875 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 | ------------------------------------------------------------------------------------------------------------------------------------ ALPHAPER-WD1 1 COS/NUV ACQ/IMA PSA MIRRORB none none none none 1 N/A GE ALPHAPER-WD1 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 875.0 S G ALPHAPER-WD1 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 875.0 S G 18398( 2) - 13-Aug-2026 14:58:15 - [ 7] Visit: 04 (ALPHAPER-WD2) 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 ALPHAPER-WD COS/NUV ACQ/IMA PSA MIRRORB 1 39.4 S 2 GE (39.4 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ALPHAPER-WD COS/FUV TIME-TA PSA G130M 1291 FP-POS=3, BUFFER-TIME=1415, 1 705 S 2 G LIFETIME-POS=LP7 see below (864 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ALPHAPER-WD COS/FUV TIME-TA PSA G130M 1291 FP-POS=4, BUFFER-TIME=1415, 1 705 S 2 G LIFETIME-POS=LP7 see below (864 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 | ------------------------------------------------------------------------------------------------------------------------------------ ALPHAPER-WD2 1 COS/NUV ACQ/IMA PSA MIRRORB none none none none 1 N/A GE ALPHAPER-WD2 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 864.0 S G ALPHAPER-WD2 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 864.0 S G 18398( 2) - 13-Aug-2026 14:58:15 - [ 8] Visit: 05 (ALPHAPER-WD5) 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 ALPHAPER-WD COS/NUV ACQ/IMA PSA MIRRORB 1 39.2 S 5 GE (39.2 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ALPHAPER-WD COS/FUV TIME-TA PSA G130M 1291 FP-POS=3, BUFFER-TIME=1401, 1 705 S 5 G LIFETIME-POS=LP7 see below (864 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ALPHAPER-WD COS/FUV TIME-TA PSA G130M 1291 FP-POS=4, BUFFER-TIME=1401, 1 705 S 5 G LIFETIME-POS=LP7 see below (864 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 | ------------------------------------------------------------------------------------------------------------------------------------ ALPHAPER-WD5 1 COS/NUV ACQ/IMA PSA MIRRORB none none none none 1 N/A GE ALPHAPER-WD5 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 864.0 S G ALPHAPER-WD5 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 864.0 S G 18398( 2) - 13-Aug-2026 14:58:15 - [ 9] Summary Form for Proposal 18398 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, ACQ/PEAKD, ACQ/PEAKXD, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=1198, BUFFER-TIME=1401, BUFFER-TIME=1415, BUFFER-TIME=4445, CENTER=FLUX-WT-FLR, FP-POS=3, FP-POS=4, LIFETIME-POS=LP7, NUM-POS=5, STEP-SIZE=0.9 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G130M, MIRRORB ------------------------------------------------------------------------------------------------------------------------------------ Target Names ALPHAPER-WD1, ALPHAPER-WD2, ALPHAPER-WD5, HYADES-WD1 ------------------------------------------------------------------------------------------------------------------------------------