The photospheres of the hottest fastest stars in the Galaxy

We perform nonlocal thermodynamic equilibrium (NLTE) model atmosphere analyses of the three hottest hypervelocity stars (space velocities between approximate to 1500-2800 km s(-1)) known to date, which were recently discovered spectroscopically and identified as runaways from Type Ia supernovae. The hottest of the three (J0546+0836, effective temperature T-eff = 95 000 +/- 15 000 K, surface gravity log g = 5.5 +/- 0.5) has an oxygen-dominated atmosphere with a significant amount of carbon (C = 0.10 +/- 0.05, O = 0.90 +/- 0.05, mass fractions). Its mixed absorption+emission line spectrum exhibits photospheric absorption lines from O V and O VI as well as O III and O IV emission lines that are formed in a radiation-driven wind with a mass-loss rate of the order of (M) over dot = 10(-8) M-circle dot yr(-1). Spectroscopically, J0546+0836 is a [WC]-PG1159 transition-type pre-white dwarf. The second object (J0927-6335) is a PG1159-type white dwarf with a pure absorption-line spectrum dominated by C III/C IV and O III/O IV. We find T-eff = 60 000 +/- 5000 K, log g = 7.0 +/- 0.5, and a carbon-and oxygen-dominated atmosphere with C = 0.47 +/- 0.25, O = 0.48 +/- 0.25, and possibly a minute amount of helium (He = 0.05 +/- 0.05). Comparison with post-AGB evolutionary tracks suggests a mass of M approximate to 0.5 M-circle dot for both objects, if such tracks can safely be applied to these stars. We find the third object (J1332-3541) to be a relatively massive (M = 0.89 M-circle dot) hydrogen-rich (DAO) white dwarf with T-eff = 65 657 +/- 2390 K, log g = 8.38 +/- 0.08, and abundances H = 0.65 +/- 0.04 and He = 0.35 +/- 0.04. We discuss our results in the context of the "dynamically driven double-degenerate double-detonation" (D-6) scenario proposed for the origin of these stars.