Resolving stellar pulsational shock waves in the extreme helium star V652 Her

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V652 Her is pulsating supersonically with a Mach-8 shock wave. We propose to use line profile variations in strong spectral lines to see directly for the first time the outwardly propagating shock waves in this star. This will advance our understanding of the physics of pulsational shock waves, in general, with direct applications to our recent model of pulsational shocks to explain newly discovered behaviour in the strongly magnetic rapidly oscillating Ap (roAp) stars. It will allow us specifically to estimate energy losses in the upper atmosphere of V652 Her, with important implications for modelling the Fe-opacity driving in this rare extreme helium star. At present no models include energy losses; with Subaru observations to guide us, we will introduce for the first time the physics of such energy losses into our models. We will also measure the compression at minimum radius. We will have the first temporal resolution of the rapid outward acceleration of the atmosphere -- acceleration 10 times that of the surface gravity of the star for a stunningly brief 15 minutes before the atmosphere goes into free-fall for the rest of the 2.5-hr pulsation cycle. The high resolution and wide wavelength range of HDS will also give an unprecedented high S/N spectrum from which we will derive detailed abundances of the CNO-cycle produced elements in a star in a rare, and not fully understood, stage of stellar evolution -- a probable merged white dwarf binary.