How an overweight and rapidly rotating PG 1159 star in the Galactic halo challenges evolutionary models

Abstract

PG 1159 stars are thought to be progenitors of the majority of H-deficient white dwarfs. Their unusual He-, C-, and O-dominated surface composition is typically believed to result from a late thermal pulse experienced by a single (pre-)white dwarf. Yet, other formation channels - involving close binary evolution - have recently been proposed and could lead to similar surface compositions. Here we present a non-local thermodynamic equilibrium spectral analysis based on new UV and archival optical spectra of one of the hottest PG 1159 stars, $\text{RX J}0122.9\text{ -}7521$. We find $T_{\text{eff}}=175$ kK and a surface gravity of log $g=7.7$, and an astonishingly low O/C ratio of $7.3\times 10^{-3}$ by mass. By combining the spectroscopic surface gravity and Gaia parallax with a spectral energy distribution fit, we derive a mass of $M_{\text{spec}}=1.8_{-0.7}^{+1.1}$ $M_{\odot }$. Although this spectroscopic mass is higher than predicted by evolutionary models, it is subject to substantial uncertainty. Furthermore, we find that $\text{RX J}0122.9\text{ -}7521$ shows strongly rotationally broadened lines, suggesting that the previously reported photometric period of $41$ min indeed corresponds to the rotational period of this star. Our kinematic analysis shows that $\text{RX J}0122.9\text{ -}7521$ belongs to the Galactic halo, which - assuming single-star evolution - is in stark contrast to its relatively high mass. The rapid rotation, high mass, and halo kinematics, as well as the lack of evidence for a close companion, lead us to believe that $\text{RX J}0122.9\text{ -}7521$ formed through the merger of two white dwarfs. Yet, none of the current models can explain the surface abundances of $\text{RX J}0122.9\text{ -}7521$.

Accepted for publication in A&A

References in corpus (29)

• Array Programming with NumPy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• galpy: A Python Library for Galactic Dynamics
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• How to Constrain Your M Dwarf: measuring effective temperature, bolometric luminosity, mass, and radius
• The unWISE Catalog: Two Billion Infrared Sources from Five Years of WISE Imaging
• On the Spectral Evolution of Hot White Dwarf Stars. I. A Detailed Model-Atmosphere Analysis of Hot White Dwarfs from SDSS DR12
• An Analysis of the Shapes of Interstellar Extinction Curves. VII. Milky Way Spectrophotometric Optical-through-Ultraviolet Extinction and Its R-Dependence
• Magnetic fields in isolated and interacting white dwarfs
• New nonadiabatic pulsation computations on full PG1159 evolutionary models: the theoretical GW Vir instability strip revisited
• Evolutionary Models for the Remnant of the Merger of Two Carbon-Oxygen Core White Dwarfs
• The DECam Local Volume Exploration Survey Data Release 2
• Fe emission and ionized excess absorption in the luminous quasar 3C109 with XMM-Newton
• Spectral classification of the 100 pc white dwarf population from Gaia-DR3 and the Virtual Observatory
• An Isolated White Dwarf with a 70 second Spin Period
• A deep test of radial differential rotation in a helium-atmosphere white dwarf: I. Discovery of pulsations in PG 0112+104
• Discovery of a hot ultramassive rapidly rotating DBA White Dwarf
• A proto-helium white dwarf stripped by a substellar companion via common-envelope ejection -- Uncovering the true nature of a candidate hypervelocity B-type star
• Formation of ultra-massive carbon-oxygen white dwarfs from the merger of carbon-oxygen and helium white dwarf pairs
• An evolutionary channel for CO-rich and pulsating He-rich subdwarfs
• First discovery of trans-iron elements in a DAO-type white dwarf (BD$-22{}^{\circ }3467$)
• The bright blue side of the night sky: Spectroscopic survey of bright and hot (pre-) white dwarfs
• Discovery of hot subdwarfs with extremely high carbon and oxygen abundances
• Mysterious, Variable, and Extremely Hot: White Dwarfs Showing Ultra-High Excitation Lines I. Photometric Variability
• Post-merger evolution of double helium white dwarfs and distribution of helium-rich hot subdwarfs
• The far-ultraviolet spectra of two hot PG1159 stars
• Hard X-ray Emission Associated with White Dwarfs. IV. Signs of Accretion from Sub-stellar Companions
• Spectroscopic survey of faint planetary-nebula nuclei III. A [WC] central star and two new PG1159 nuclei
• Detection of forbidden line components of lithium-like carbon in stellar spectra

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