0 Assigned ID: 416 ----- Attempting Submission 1 (Fri Apr 05 12:45:18 GMT 2019) ----- 0 ----- Attempting Submission 1 (Tue Jul 16 13:37:58 GMT 2019) ----- Is the Type Ic supernova progenitor WS35 a white dwarf merger product? We propose to obtain UV spectroscopy of the newly discovered emission-line star WS35 with HST/STIS, which was found in the center of a hydrogen-free infrared nebula. Its pure C-O-Ne surface composition, high surface temperature of 200,000 K, and luminosity of 40,000 Lsun imply it is an immediate progenitor of a Type Ic supernova. Optical spectra show an unprecedented, supernova-like velocity of its wind, 16,000 km/s, whose kinetic energy exceeds the photon energy implied by WS35's luminosity. This argues for the wind acceleration being aided by a strong magnetic field and rapid rotation. It has thus been suggested that WS35 is the result of a recent white dwarf merger exceeding the Chandrasekhar mass, caught during its brief evolution from merging to collapse. We aim to test this scenario using FUV/NUV spectra of WS35, which allow to assess the velocity profile of its wind, and thereby provide a clear diagnostic of the wind acceleration mechanism. The proposed STIS UV spectroscopy will further provide unprecedented information on the surface chemical composition that is not accessible from optical spectra, in particular on the so far unconstrained neon abundance. Our results will shed new light on the formation of Type Ic supernovae, constrain the origin of Type Ia supernovae in the double-degenerate scenario, and trial the idea of generating strong magnetic fields in stellar mergers. We obtained 10 HST orbits to observe WS35 with STIS long-slit spectroscopy in low spectral resolution with gratings G140L in the FUV and G230L in the NUV. We wish to obtain data for two epochs to constrain a possible binary nature of the object. We further use time-tag mode to get information on short-term variability, which could be of the order of 5min. We split our observing time in two visits with 5 orbits each. With an integration time of 7051s per visit in the NUV and 7645s per visit in the FUV we obtain a minimum S/N of 20 per visit in the relevant wavelength ranges (~1600-3000A in the NUV, and ~1250-1600A in the FUV, cf. ETC runs STIS.sp.1368104 and STIS.sp.1368105). For time-tag mode we chose buffer times that are roughly a factor three lower than the ETC recommendations to be safe against uncertainties in the source flux and the interstellar extinction (which is included in our uploaded model spectrum). We adopt the standard 52"x0.2" slit. For target acquisition we use a standard point source acquisition. Dr. Graefener has expertise in the modelling and analysis of the spectra of Wolf-Rayet stars, the modelling of stellar winds, and the reduction of spectroscopic data, including data from HST. Dr. Gvaramadse is an expert for circumstellar nebulae and the observation and analysis of hot stars. Prof. Ignace is an expert on the theory and observation of stellar winds. Prof. Langer is an expert on the theory of stellar evolution. Prof. Sana has broad expertise in multiwavelength spectroscopy and is a former HST instrument scientist in the COS/STIS team. Dr. Shenar has expertise in the modelling of emission line stars with aspherical winds. Dr. Schwab is an expert in the modelling of late stages of stellar evolution including white dwarf mergers. Stellar Physics 6 false false true false false false false SUPPORTED STAR WOLF RAYET This object was generated by the targetselector and retrieved from the 2MASS database. J2000 J2000 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 The scheduling data was generated by CASM 27.2. CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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