0 Assigned ID: 1558 ----- Attempting Submission 1 (Sat Sep 30 15:07:44 GMT 2017) ----- 0 ----- Attempting Submission 1 (Tue Nov 21 11:29:16 GMT 2017) ----- GD492: a nearby hypervelocity emissary from a thermonuclear supernova event GD 492 is a nearby hypervelocity white dwarf with a unique chemical composition displaying the nucleosynthesis fingerprints of C-, O-, and Si- burning. From our ground-based optical spectroscopy, we determine a He-dominated atmosphere, but loaded with a large amount of Ne, Mn, O (30, 5, 1%, by mass, respectively) and traces of ten other elements. The large space velocity, exceeding the Galactic escape velocity, and the considerable alpha enhancements strongly suggest that GD 492 is an exotic white dwarf remnant of a failed Type Ia supernova. The super-Solar Mn/Fe ratio provides evidence that this partial thermonuclear explosion occurred in a single-degenerate binary. Through kinematic analysis, we constrained the supernova event to have occurred most likely < 10 Myr ago, at a nearby Galactic disc location towards the Carina arm. Here we apply for five HST orbits to obtain high-quality STIS near-ultraviolet spectroscopy of GD492 which we will use to: (1) accurately measure the effective temperature, which is significantly affected by the currently unconstrained amount of ultraviolet line blanketing. Together with the forthcoming Gaia DR2 parallax, this measurement will also tightly constrain the radius and mass of GD 492. (2) unambiguously measure the bulk atmospheric composition, i.e. the Ne/He ratio; determine the degree of C-burning through the detection of (or placing a firm upper limit on) the C-abundance; and derive significantly improved constraints on O- and Si- burning via more accurate abundance measurements of Mn, S, and Ni. Both scientific goals critically require space-based ultraviolet spectroscopy. The goal of these observations is to obtain a high S/N spectrum of the target star, GD492. The star has a moderate, but well known proper motion, and is in a sparse field, so acquisition with the STIS CCD should be straightforward. We were awarded five orbits, but as requested, we split the observations into a two and a three-orbit visit to facilitate the scheduling of the observations. We would be equally happy should it be possible to schedule the observations as a single five-orbit visit, as that would give us an extra ~650sec on-target, but the difference in the total S/N is too small to request a single five-orbit visit. Please attempt to schedule the observations in one of the Dec/Jan/Mar to ensure the spectroscopy is available in time for the Gaia Data Release 2, which will provide an accurate parallax. 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