Accurate physical parameters of an orphan runaway white dwarf

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.