Three-Dimensional Orbit of the Wolf-Rayet Binary Star WR 138 Determined with the CHARA Array

Core Concepts

Through a combination of long-baseline infrared interferometry and optical spectroscopy, researchers determined the three-dimensional orbit of the Wolf-Rayet binary star WR 138, revealing its evolutionary history and potential for past interactions.

Abstract

• Bibliographic Information: Holdsworth, A., Richardson, N., Schaefer, G.H., et al. (2024). Visual Orbits of Wolf-Rayet Stars II: The Orbit of the Nitrogen-Rich WR Binary WR 138 measured with the CHARA Array. The Astrophysical Journal, (submitted).
• Research Objective: This study aims to characterize the Wolf-Rayet binary system WR 138 by determining the orbital parameters and masses of both stars using a combination of spectroscopy and interferometry.
• Methodology: The researchers used long-baseline infrared interferometry with the CHARA Array to measure the binary separation and flux ratio. They combined this data with spectroscopic radial velocities from various telescopes, including Keck Observatory, to determine the orbital parameters and stellar masses.
• Key Findings: The study presents the second-ever visual orbit for a WN-type star in a binary system. The derived masses for the WR and companion O star are 13.9 ± 1.5 M⊙ and 26.3 ± 1.7 M⊙, respectively. The orbital parallax of 0.469 mas corresponds to a distance of 2.132 ± 0.054 kpc, consistent with Gaia measurements.
• Main Conclusions: The authors conclude that the system likely experienced past interactions, potentially through a common envelope phase or mass transfer during the red supergiant phase of the WN star. The study highlights the importance of combining interferometry and spectroscopy to constrain the formation mechanisms of massive binary systems.
• Significance: This research contributes valuable data to the understanding of massive binary star evolution, particularly for Wolf-Rayet stars. The precise orbital determination and mass measurements of WR 138 provide observational constraints for theoretical models of binary interactions and stellar evolution.
• Limitations and Future Research: The study acknowledges limitations in current models regarding eccentric orbits and suggests further investigation into the system's X-ray variability and polarization changes to better understand wind collisions and their impact on the binary's evolution.

Stats

The orbital period of WR 138 is 1527.99 ± 1.01 days.
The mass of the Wolf-Rayet star is 13.93 ± 1.49 M⊙.
The mass of the companion O star is 26.28 ± 1.71 M⊙.
The orbital parallax is 0.469 ± 0.012 mas.
The distance to WR 138 is 2131.97 ± 54.38 parsecs.

Quotes

Key Insights Distilled From

Visual Orbits of Wolf-Rayet Stars II: The Orbit of the Nitrogen-Rich WR Binary WR 138 measured with the CHARA Array

by Amanda Holds... at arxiv.org 11-05-2024

https://arxiv.org/pdf/2411.01062.pdf

Deeper Inquiries

How might future advancements in modeling stellar winds and binary interactions further refine our understanding of WR 138's evolution?

Answer:
Future advancements in modeling stellar winds and binary interactions hold immense potential to refine our understanding of WR 138's evolution. Here's how:

Incorporating Eccentricity: Current models, like BPASS, often struggle to accurately simulate eccentric orbits. WR 138's moderate eccentricity (e=0.191) suggests a history influenced by binary interactions. Developing models that can robustly handle eccentric orbits will be crucial to retrace the system's past interactions, including mass transfer and common envelope phases. This will provide a more accurate picture of the initial conditions of the binary and its evolutionary trajectory.

Improved Stellar Wind Physics: Sophisticated 3D hydrodynamical simulations that capture the complexities of stellar wind physics, especially in binary environments, will be transformative. These models can account for factors like radiative transfer, clumping in the winds, and the interaction of the WR star's wind with the O star's wind and its own rotation. This will lead to more precise estimates of mass-loss rates, orbital evolution, and wind properties, allowing for more accurate comparisons with observations.

Constraining Angular Momentum Transfer: The observed rapid rotation of the O star in WR 138 hints at significant angular momentum transfer, likely during a common envelope phase or through accretion. Future models that can accurately track the exchange of angular momentum during these interactions will be vital to understanding the current rotational state of the O star and its impact on the system's evolution.

Modeling X-ray Emission and Polarization Variability: As highlighted in the paper, dedicated X-ray and polarimetric observations of WR 138 across its orbit can provide valuable data on the colliding winds. Coupling these observations with advanced models that can predict the X-ray and polarization signatures of colliding winds will offer crucial insights into the geometry, temperature, and density of the wind collision region, further constraining the system's properties.

By incorporating these advancements, future models will be able to reconstruct a more detailed and accurate evolutionary history of WR 138, shedding light on the interplay between stellar winds and binary interactions in shaping the lives of massive stars.

Could alternative explanations, such as interactions with a now-escaped third star, account for the observed properties of WR 138?

Answer:
While the paper rules out a current close third companion based on the interferometric data, interactions with a now-escaped third star could potentially explain some of the observed properties of WR 138. Here's a breakdown:
Arguments in Favor:

Eccentricity: The moderate eccentricity of WR 138's orbit could be a relic of a past interaction with a third star. The Kozai-Lidov mechanism, for instance, can induce eccentricity oscillations in a triple system, potentially leaving the inner binary with a non-zero eccentricity even after the third star is ejected.
Rapid Rotation of the O Star: A close encounter with a third star could have imparted angular momentum to the system, contributing to the O star's rapid rotation.
Arguments Against:

Lack of Observational Evidence: Currently, there's no direct observational evidence, such as stellar streams or a detected runaway star, to support a past triple system scenario for WR 138.
Fine-Tuning Problem: Reproducing the observed properties of WR 138 through a past triple interaction would require fine-tuning of the initial conditions and parameters of the triple system. This makes it a less probable explanation compared to binary evolution scenarios.
Conclusion:
While a past interaction with a now-escaped third star cannot be definitively ruled out, it remains a less likely explanation for the observed properties of WR 138 compared to binary evolution scenarios. Future observations, particularly deep imaging or astrometric searches for a potential escaped companion, could help to further explore this possibility.

What broader implications might the study of massive binary systems like WR 138 have for our understanding of galaxy evolution and the chemical enrichment of the universe?

Answer:
The study of massive binary systems like WR 138 has profound implications for our understanding of galaxy evolution and the chemical enrichment of the universe. Here's why:

Understanding Massive Star Evolution: Massive stars are the cosmic engines that drive the evolution of galaxies. They are the primary sources of heavy elements (elements heavier than helium) through stellar nucleosynthesis and supernova explosions. By studying systems like WR 138, we gain insights into the mass-loss processes, binary interactions, and ultimate fates of these stars, refining our understanding of their lifecycles and their impact on galactic ecosystems.

Constraining Supernova Progenitors: WR stars are thought to be progenitors of Type Ib/c supernovae, some of the most energetic events in the universe. Precisely measuring the masses of WR stars in binaries like WR 138 helps constrain the mass range of stars that end their lives as these supernovae, providing crucial information for supernova models and our understanding of these explosive events.

Chemical Enrichment of Galaxies: The heavy elements synthesized in massive stars and dispersed through their winds and supernova explosions are incorporated into subsequent generations of stars and planets. By studying the mass loss and wind properties of WR stars in binaries, we can better understand how these elements are released into the interstellar medium, enriching galaxies and shaping their chemical evolution over cosmic time.

Gravitational Wave Sources: Some massive binary systems, including those containing WR stars, are predicted to merge and produce detectable gravitational waves. Characterizing the orbital parameters and masses of these systems, as done for WR 138, is essential for predicting the rates and properties of these mergers, contributing to the growing field of gravitational wave astronomy.

Formation of Exotic Objects: The interactions in massive binary systems can lead to the formation of exotic objects like X-ray binaries, gamma-ray bursts, and even potentially massive black hole binaries. Studying systems like WR 138 provides valuable clues about the formation pathways of these objects, deepening our understanding of the diverse stellar populations in the universe.

In conclusion, the study of massive binary systems like WR 138 is not just about understanding individual stars, but about unraveling the intricate processes that govern the evolution of galaxies, the dispersal of heavy elements, and the formation of some of the most energetic and enigmatic objects in the cosmos.