The photospheres of the hottest fastest stars in the Galaxy

¹ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany
e-mail: werner@astro.uni-tuebingen.de
² Landessternwarte Heidelberg, Zentrum für Astronomie, Ruprecht-Karls-Universität, Königstuhl 12, 69117 Heidelberg, Germany
³ Department of Astronomy, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁴ Department of Physics, University of Warwick, Coventry CV4 7AL, UK

Received: 16 October 2023
Accepted: 16 November 2023

Abstract

We perform nonlocal thermodynamic equilibrium (NLTE) model atmosphere analyses of the three hottest hypervelocity stars (space velocities between ≈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 Ṁ= 10⁻⁸ M_⊙ 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 ≈ 0.5 M_⊙ 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_⊙) 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” (D⁶) scenario proposed for the origin of these stars.