0 Assigned ID: 7492 ----- Attempting Submission 1 (Thu Apr 16 17:11:54 GMT 2026) ----- Could not connect to APT Status Server COS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS COS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS 0 ----- Attempting Submission 1 (Tue Aug 11 23:41:55 GMT 2026) ----- There are three warnings in this proposal which we believe do not impact the program in any way (rounding of exposure times and connection to APT status server), but please let us know if this is not the case. Hidden in Plain Sight? Unraveling the Potential Merger Histories of Ultra-Massive White Dwarfs in the Initial-Final Mass Relation 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. 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. The impacts of reduced gyro operations on this program would primarily be reduced schedulability due to the smaller field of regard. Dr. Steffani Grondin (Postdoctoral Fellow at UCSD) and Mr. David Miller (PhD Candidate at UBC) will act as team leads of this project. Dr. Grondin is an expert in white dwarf binaries, using multi-wavelength observations (HST, Swift, Gemini, Keck, Shane) to study these systems and constrain the common envelope phase of binary evolution. Mr. Miller is an expert on the white dwarf initial-final mass relation and ultra-massive white dwarfs, having discovered the systems in this proposal during his PhD. Prof. Maria Drout is an expert in identification and characterization of evolved stellar systems in the Milky Way and Local Group Galaxies, with an emphasis on supernova and gravitational wave progenitor systems. Prof. Kyle Kremer is an expert in compact object binaries and mergers in stellar clusters, with a focus on double white dwarf binaries as LISA progenitors, and has experience analyzing HST spectra of these systems. Prof Pier-Emmanuel Tremblay is an expert in modelling white dwarf atmospheres, in particular, carbon features. Dr. Kyle Rocha is an expert in binary population synthesis, with a focus on compact object binary pathways as a core developer of the POSYDON population synthesis code. Prof. Ilaria Caiazzo is an expert in both theoretical and observational white dwarf astrophysics, with specific expertise in using white dwarf magnetic fields to identify systems that are merger products. 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