The photospheres of the hottest fastest stars in the Galaxy

Abstract

We perform nonlocal thermodynamic equilibrium (NLTE) model atmosphere analyses of the three hottest hypervelocity stars (space velocities between $\approx$ 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 $T_{\mathrm{eff}}$ = 95,000 $\pm$ 15,000 K, surface gravity log g = $5.5\pm 0.5$) has an oxygen-dominated atmosphere with a significant amount of carbon (C = $0.10\pm 0.05$, O = $0.90\pm 0.05$, 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 $10^{-8}$ $M_{\odot }$ 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 $T_{\mathrm{eff}}$ = 60,000 $\pm$ 5000 K, log g = $7.0\pm 0.5$, and a carbon- and oxygen-dominated atmosphere with C = $0.47\pm 0.25$, O = $0.48\pm 0.25$, and possibly a minute amount of helium (He = $0.05\pm 0.05$). Comparison with post-AGB evolutionary tracks suggests a mass of $M\approx 0.5$ $M_{\odot }$ 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=0.89M_{\odot }$) hydrogen-rich (DAO) white dwarf with $T_{\mathrm{eff}}$ = 65,657 $\pm$ 2390 K, log g = $8.38\pm 0.08$, and abundances H = $0.65\pm 0.04$ and He = $0.35\pm 0.04$. 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.

Accepted for publication in A&A