0 Assigned ID: 620 ----- Attempting Submission 1 (Fri Apr 05 18:41:25 GMT 2019) ----- ----- Attempting Submission 2 (Sun Apr 07 15:30:39 GMT 2019) ----- 0 ----- Attempting Submission 1 (Mon Jul 15 14:42:09 GMT 2019) ----- HST Phase II Proposal 15918 (15918.apt) successfully submitted. Receipt: # 15918-1 ----- Attempting Submission 2 (Mon Jul 15 14:52:01 GMT 2019) ----- Recent discoveries have been revolutionising the field of thermonuclear supernovae with the identification of a new family of remarkable stars that, like their prototype LP 40-365, are suggested to be the former accreting white dwarfs that likely survived to peculiar (possibly Type Iax) supernova explosions. Their element abundance pattern shows ONeMg-dominated atmospheres, presenting large traces of partial Si-burning, iron-peak elements, which are heavily produced in near Chandrasekhar-mass thermonuclear explosions. Thanks to the accurate parallaxes of the Second Data Release of Gaia, these stars are confirmed as inflated low-mass white dwarfs that have been ejected with velocities in the range of 600 km/s. The observational evidence collected so far strongly suggests these stars were the former white dwarf accretors in single-degenerate binaries, which were partly burned by the explosions that unbound the binaries they belonged to. Near- and far-ultraviolet spectroscopy is crucial to precisely measure key properties of these stars, as this wavelength range enables to accurately measure their effective temperature leading to improved determinations of their atmospheric bulk composition and, in combination with Gaia parallaxes, their radii and masses. The proposed target is the hottest and the brightest star of this family, enabling a favorable perspective on the likely earliest phases of their formation. Hence, the HST observations will allow us to gain deeper insight in the evolutionary status and formation mechanisms of these stars via comparison with theoretical models. The primary goal of this proposal is to obtain the far- and near-ultraviolet spectral energy distribution of this star. We choose the STIS G140L/G230L gratings and the widest slit, 52x2, to ensure that we optimise the throughput and minimise possible slit flux losses. The observation sequence is straight-foward: acquisition with the optical CCD and the FP28x50LP broadband filter, one orbit of STIS/G230L spectroscopy, and two orbits of STIS/G140L spectroscopy. Boris 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 accreting metal-rich debris from disrupted planetesimals (e.g. Gaensicke et al. 2012, MNRAS, 424, 333). Ulrich Heber has long-standing experience in the spectral characterisation of hot subluminous stars (Heber et al. 2016, PASP, 128, 2001), and binary systems, as well as on the study of hypervelocity and runaway stars (Heber et al. 2008, A&A, 483, 21). JJ 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, leading to the identification of the new LP 40-365-like objects. Mark Hollands and Roberto Raddi have worked on the spectroscopic and kinematic analysis of LP 40-365 (Raddi, Hollands, Gaensicke, et al.2018, ApJ, 858, 3; Raddi, Hollands, Koester et al. 2018, MNRAS, 479, 96), and have led the identification of the new LP40-365 stars, including the target for the proposed HST/STIS observations, J1825-3757. The optical spectroscopic follow-up of this newly identified class of runaway white dwarfs, and the analysis of the STIS/G230L spectrum of LP40-365 (GO 15431) are summarised in the manuscript (Raddi, Hollands, Koester, et al. 2019, arXiv:1902.05061, currently submitted to MNRAS). Detlev 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, A34). He has written a stellar atmosphere and spectral sysnthesis code, which is also used for the analysis of the new class runaway stars. Ken Shen is a theoretical astrophysicist working on thermonuclear supernovae. He has worked on the modelling of the early evolution of inflated white dwarf soon after the Type Iax supernova explosion (Shen & Schwab 2017, ApJ, 834, 180). He has expertise in binary star population synthesis modelling and has contributed to shaping the theoretical interpretation of LP40-365 and the other similar stars we found. Odette Toloza is an expert in the analysis of time-tagged ultraviolet spectroscopy of white dwarfs (e.g. Toloza et al. 2016, MNRAS, 459, 3929). Her work focuses on the study of accreting white dwarfs in cataclysmic variables, hence, possible thermonuclear supernova progenitors. Stellar Physics 6 false false true false false false false SUPPORTED STAR SDB NO J2000 J2000 GALEX NUV = 14.6 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 1587136567610 /private/var/folders/46/4jpf0wpx40jbz16h87362w240001d2/T/OPVisit8436578232979265330.xml The scheduling data was generated by CASM 27.2. 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