Two Wolf-Rayet stars at the heart of colliding-wind binary Apep

Abstract: Infrared imaging of the colliding-wind binary Apep has revealed a spectacular dust plume with complicated internal dynamics that challenges standard colliding-wind binary physics. Such challenges can be potentially resolved if a rapidly-rotating Wolf-Rayet star is located at the heart of the system, implicating Apep as a Galactic progenitor system to long-duration gamma-ray bursts. One of the difficulties in interpreting the dynamics of Apep is that the spectral composition of the stars in the system was unclear. Here we present visual to near-infrared spectra that demonstrate that the central component of Apep is composed of two classical Wolf-Rayet stars of carbon- (WC8) and nitrogen-sequence (WN4-6b) subtypes. We argue that such an assignment represents the strongest case of a classical WR+WR binary system in the Milky Way. The terminal line-of-sight wind velocities of the WC8 and WN4-6b stars are measured to be $2100 \pm 200$ and $3500 \pm 100$ km s$^{-1}$, respectively. If the mass-loss rate of the two stars are typical for their spectral class, the momentum ratio of the colliding winds is expected to be $\approx$ 0.4. Since the expansion velocity of the dust plume is significantly smaller than either of the measured terminal velocities, we explore the suggestion that one of the Wolf-Rayet winds is anisotropic. We can recover a shock-compressed wind velocity consistent with the observed dust expansion velocity if the WC8 star produces a significantly slow equatorial wind with a velocity of $\approx$530 km s$^{-1}$. Such slow wind speeds can be driven by near-critical rotation of a Wolf-Rayet star.

• The paper classifies the Apep system as a binary containing two Wolf-Rayet stars (WC8 and WN4-6b) using spectroscopy.
• Spectroscopic analysis revealed wind velocities of 2100 km/s and 3500 km/s for the two stars, indicating a relatively balanced colliding-wind momentum ratio of approximately 0.4.
• The disparity between observed dust plume velocity (~600 km/s) and predictions supports an anisotropic wind driven by a rapidly rotating Wolf-Rayet star.

Analysis of "Two Wolf-Rayet Stars at the Heart of Colliding-Wind Binary Apep"

The paper "Two Wolf-Rayet stars at the heart of colliding-wind binary Apep," authored by J. R. Callingham et al., offers a meticulous examination of the enigmatic Apep system, emphasizing its classification as a binary consisting of two Wolf-Rayet (WR) stars. This study provides significant insights into the dynamics and composition of Apep, establishing it as a prime candidate for understanding colliding-wind binary (CWB) systems, particularly those involving Wolf-Rayet stars.

Key Findings

The primary contribution of this study is the identification and classification of the central components of Apep as two classical Wolf-Rayet stars, namely of the carbon sequence (WC8) and nitrogen sequence (WN4-6b) subtypes. This conclusion is drawn from visual to near-infrared spectroscopic data that precisely delineates the emission line characteristics for the WR stars. Notably, the study argues that this configuration represents one of the most robust cases for a WR-WR binary in our Galaxy.

Spectroscopic Insights

The authors employ visual-red spectroscopy to resolve the previous ambiguities surrounding the stellar composition of Apep. The spectra analysis reveals two primary wind velocities: 2100 km/s for the WC8 star and 3500 km/s for the WN4-6b star. Such detailed velocity measurements are pivotal for delineating the dynamics within the CWB system and align with the authors' assertion of a momentum ratio of approximately 0.4 for the colliding winds—a far more balanced scenario than traditionally observed in WR+O star binaries.

Implications for CWB Dynamics

Curiously, the study acknowledges a disparity between the expected and observed expansion velocities of the Apep dust plume. While classical models predict a much higher dust expansion velocity, the observational data indicates a mere ~600 km/s. The authors explore the hypothesis of an anisotropic wind driven by rapid rotation of the Wolf-Rayet star, conceivably producing a slower, equatorial wind that reconciles the velocity discrepancy. This proposition offers valuable evidence supporting models that suggest rapidly rotating WR stars could lie at the heart of asymmetric wind structures.

Broader Implications

The Apep system, with its potential association as a progenitor of long-duration gamma-ray bursts (GRBs), provides an invaluable testbed for examining astrophysical conditions conducive to GRB formation. By establishing a clearer picture of the Apep system's structure and dynamics, this research lays critical groundwork for understanding the formative stages of such catastrophic cosmic events.

Future Directions

The findings advocate for high-resolution observations such as VLBI to further investigate the proposed structure and dynamics of the wind-collision region in Apep. Such efforts could validate current theoretical models of WR star evolution, especially those positing rapid rotation effects leading to anisotropic wind distributions.

Conclusion

Through comprehensive spectral analysis, this study advances our understanding of WR binaries in the Galactic context, offering new avenues to explore both stellar wind dynamics and massive stellar evolution. The rigorous classification of Apep as a WC8+WN4-6b system not only marks a significant observation in stellar astrophysics but also prompts reconsideration of existing models to adequately describe these complex and rare systems.