A massive white-dwarf merger product before final collapse

Abstract: Gravitational wave emission can lead to the coalescence of close pairs of compact objects orbiting each other. For the case of neutron stars such mergers may yield masses above the Tolman-Oppenheimer-Volkoff limit, leading to the formation of black holes. For the case of white dwarfs the merger product may exceed the Chandrasekhar limit, leading either to a thermonuclear explosion as Type Ia supernova, or to a collapse forming a neutron star. If a Type Ia supernova explosion is avoided, the merger of two massive white dwarfs is expected to form a hydrogen- and helium-free circumstellar nebula with a hot and luminous, rapidly rotating and highly magnetized central star for several 10,000 yr before its final collapse. Here we report the discovery of a hot star with an emission line dominated spectrum in the centre of a circular mid-infrared nebula. Both the star and the nebula appear to be free of hydrogen and helium. Our tailored stellar atmosphere and wind models indicate a stellar surface temperature of about 200,000 K, and a record outflow velocity of 16,000 km/s. This extreme velocity, together with the derived mass outflow rate, imply rapid stellar rotation and a strong magnetic field aiding the wind acceleration. The Gaia distance of the star leads to a luminosity of 10^{4.5} Lsun, which matches models of the post-merger evolution of super-Chandrasekhar mass white dwarfs. The high stellar temperature and the nebular size argue for a short remaining lifetime of the star, which will produce a bright optical and high-energy transient upon collapse. Our observations indicate that super-Chandrasekhar mass white dwarf mergers can indeed avoid a thermonuclear explosion as Type Ia supernova, and provide empirical evidence for magnetic field generation in stellar mergers.

• The paper demonstrates WS35 as a massive white-dwarf merger product before collapse, evidenced by an extreme emission-line spectrum and a record outflow velocity of 16,000 km/s.
• Observations with WISE, the Russian 6-m telescope, and Gaia data confirm its high surface temperature (~200,000 K) and an oxygen-rich, helium-free composition.
• The findings imply that post-merger evolution can lead directly to neutron star formation via accretion-induced collapse, challenging conventional Type Ia supernova scenarios.

Overview of "A Massive White-Dwarf Merger Product Prior to Collapse"

The paper "A Massive White-Dwarf Merger Product Prior to Collapse" by Gvaramadze et al. presents the discovery of an intriguing astrophysical phenomenon: a massive white-dwarf merger product that is on the brink of collapse. This research provides empirical evidence for the generation of magnetic fields in stellar mergers and outlines the unique characteristics of an extraordinary object identified in the constellation Cassiopeia, designated as WS35. Observations made with cutting-edge instruments, including data from the Wide-field Infrared Survey Explorer ({\it WISE}) and the Russian 6-m telescope, are combined with theoretical models to provide insights into the nature and evolution of this object.

Key Findings

The central star of WS35 is characterized by an extreme emission-line spectrum, consistent with an oxygen-rich Wolf-Rayet (WO) star, and it is part of a hydrogen- and helium-free mid-infrared nebula. Uniquely, this star exhibits a stellar surface temperature of approximately 200,000 K, and its outflow velocity of 16,000 km/s is the fastest recorded for a star of this type. Spectral analysis suggests a chemical composition predominantly of oxygen and carbon, with mass fractions of 0.8 and 0.2, respectively. The absence of helium lines implies a composition nearly devoid of helium.

The derived mass-loss rate is significant, suggesting that WS35 is rapidly spinning and highly magnetized, features that align with theoretical models of post-merger super-Chandrasekhar white dwarf evolution. The {\it Gaia} data provides a distance measurement to WS35, further supporting its luminosity and matching theoretical predictions for post-merger white dwarf evolution.

Implications and Theoretical Context

The study's implications reach both practical and theoretical aspects of astrophysics. Practically, WS35 serves as a prospective precursor for a future bright optical and high-energy transient event, likely leading to the formation of a low-mass neutron star. Theoretically, this research adds to our understanding of stellar mergers and accretion-induced collapse (AIC) scenarios, as WS35 represents a case where a merger product avoids a thermonuclear explosion as a Type Ia supernova. This supports the theory that such mergers can lead directly to neutron star formation.

The findings offer substantial evidence that the merger of two carbon-oxygen white dwarfs results in significant magneto-hydrodynamical interactions, supporting theoretical models like those proposed by Ji et al., which predict strong magnetic fields in the resultant object. Moreover, WS35 challenges previous assumptions regarding the evolutionary pathways of super-Chandrasekhar mass white dwarf mergers, highlighting that not all such events culminate in Type Ia supernovae. Instead, this object provides an empirical counterexample and portends the eventual production of a high-energy transient event followed by a potentially unique supernova.

Future Directions

Future research should focus on monitoring WS35 to capture its behavior leading up to and including its anticipated collapse. Additionally, its peculiar emission lines and impressive magnetic field strength merit further investigation into the underlying physics governing such stellar phenomena. Spectroscopic studies across different wavelengths, as well as simulations embracing magnetohydrodynamic processes, could unify empirical data with evolving theoretical frameworks. Broader surveys may identify similar objects, aiding in the statistical understanding of white dwarf merger products across the galaxy.

In conclusion, the discovery and analysis of WS35 reflect significant progress in astrophysical research, particularly in understanding the dynamics of post-merger evolution in compact stars. The study's results have bridged observational data with theoretical predictions and opened new avenues for exploring stellar evolution pathways that diverge from traditional narratives.