0 Assigned ID: 808 ----- Attempting Submission 1 (Fri Apr 05 21:44:25 GMT 2019) ----- HST Phase I Proposal 808 (d6.apt) successfully submitted. Receipt: # 808-1 ----- Attempting Submission 2 (Fri Apr 05 23:38:41 GMT 2019) ----- 0 ----- Attempting Submission 1 (Mon Jul 15 15:20:05 GMT 2019) ----- The study of Type Ia supernovae (SNe Ia) has undergone a revolution within the last year following the discovery of three extremely unique stars in Gaia's Data Release 2. Their hypervelocities (1000-2500 km/s), atmospheric compositions (predominantly C/O with contributions from Mg, Si, S, and Fe-group elements), inflated radii, and the kinematic association of one of the stars with a known SN remnant are all strong evidence that they are the former white dwarf (WD) secondaries in merging double WD systems in which the primary WD exploded as a SN Ia. While analysis of these paradigm-shifting stars in the optical is ongoing, a complete understanding is currently limited by degeneracies in the effective temperatures, surface gravities, and metal abundances. Near-ultraviolet spectroscopy is crucial to break these degeneracies. Our proposed ultraviolet observations of the brightest of these stars will allow for the derivation of its radius, mass, and bulk atmospheric composition, enabling constraints on the precise nucleosynthetic yields of the nearby explosion that polluted the surviving companion's surface as well as characterization of its current stellar structure. This will enable modeling of the future evolution of these stars, which will facilitate searches for more SN Ia survivors that will answer the decades-old mystery of the nature of SN Ia progenitors. The primary goal of this proposal is to obtain the near-ultraviolet spectral energy distribution of this star. We choose the STIS G230L grating and the widest slit, 52x2, to ensure that we optimise the throughput and minimise possible slit flux losses. The observation sequence is straight-foward: two identical three-orbit visits, composed of an acquisition with the optical CCD and the FP28x50LP broadband filter, followed by G230L spectroscopy. Gaensicke has been using HST for ultraviolet spectroscopy of white dwarfs since Cycle 6, addressing science questions that range from accreting white dwarfs as possible SN Ia progenitors to white dwarfs with evolved planetary systems (e.g. Gaensicke et al. 2012, MNRAS 424, 333G ). Heber has long experience in the spectral characterisation of hot subluminous stars and binary systems, as well as on the study of hypervelocity and runaway stars (Heber et al. 2016, PASP 128, h2001 ). Hermes is an expert of photometric and spectroscopic characterisation of pulsating white dwarfs and binary systems, also involving the use of HST UV data (Hermes et al. 2014, MNRAS 444, 1674). He took part in the follow-up campaign of runaway supernova remnants. Hollands and Raddi have worked on the spectroscopic and kinematic analysis of LP 40-365 (Raddi et al. 2018MNRAS 479L, 96, and Raddi et al. 2018, ApJ 858, 3), and have analyzed the HST/STIS data on LP 40-365 (Raddi et al. arXiv:1902.05061) Koester has been working on many aspects related to white dwarf physics for over forty years, and he has been leading the atmospheric analysis of many projects involving HST ultraviolet spectroscopy of white dwarfs (e.g. Koester et al. 2014, A&A 566, 34). Shen is a theoretical astrophysicist working on thermonuclear supernovae. He has been working on modelling the dynamically driven double-degenerate double-detonation (D6) scenario for sub-Chandrasekhar mass Type Ia supernovae (Shen et al. 2018, ApJ 854, 52), predicting the properties of surviving white dwarf companions and accretors (Shen & Schwab 2017, ApJ 834, 180) and identifying the three D6 stars (Shen et al. 2018, ApJ 865, 15) with Gaia DR2. Toloza is an expert in the analysis of time-tagged HST ultraviolet spectroscopy of white dwarfs (e.g. Toloza et al. 2016, MNRAS 459, 3929). Stellar Physics 6 false false true false false false false SUPPORTED STAR SDB NO J2000 J2000 GALEX NUV=19.57 STIS/NUV-MAMA Spectroscopic No No G230L 2376 1563203685121 /tmp/OPVisit7557735552912733243.xml The scheduling data was generated by CASM 27.2. CASM was run in Phase II mode. 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