[Paper Review] Binaries are the best single stars

S. E. de Mink, N. Langer+1 more|arXiv (Cornell University)|Oct 11, 2010

Stellar, planetary, and galactic studies3 citations

TL;DR

The paper argues that detached binary systems with two main-sequence stars are the best observational testbeds for single stellar evolution models, as they are the only systems where mass transfer—common in massive stars—can be definitively excluded since birth. These binaries provide direct measurements of fundamental stellar parameters, enabling stringent calibration of single-star models despite the high binary fraction among massive stars.

ABSTRACT

Stellar models of massive single stars are still plagued by major uncertainties. Testing and calibrating against observations is essential for their reliability. For this purpose one preferably uses observed stars that have never experienced strong binary interaction, i.e. "true single stars". However, the binary fraction among massive stars is high and identifying "true single stars" is not straight forward. Binary interaction affects systems in such a way that the initially less massive star becomes, or appears to be, single. For example, mass transfer results in a widening of the orbit and a decrease of the luminosity of the donor star, which makes it very hard to detect. After a merger or disruption of the system by the supernova explosion, no companion will be present. The only unambiguous identification of "true single stars" is possible in detached binaries, which contain two main-sequence stars. For these systems we can exclude the occurrence of mass transfer since their birth. A further advantage is that binaries can often provide us with direct measurements of the fundamental stellar parameters. Therefore, we argue these binaries are worth the effort needed to observe and analyze them. They may provide the most stringent test cases for single stellar models.

Motivation & Objective

• To address the challenge of calibrating massive single-star evolutionary models, which remain uncertain due to poorly constrained mixing and mass loss.
• To identify 'true single stars'—stars that have never undergone strong binary interaction—amid a high binary fraction among massive stars.
• To demonstrate that detached binaries with two main-sequence stars are the only unambiguous systems where mass transfer can be excluded since formation.
• To advocate for the use of such binaries as optimal test cases for single-star models due to their direct measurement capabilities.
• To counter the misconception that single stars without companions are inherently 'true' single stars, as many such stars are actually post-interaction remnants.

Proposed method

• Using a rapid binary evolutionary code with updated treatments of Roche-lobe overflow, mass and angular momentum transfer, and rotational effects.
• Calibrating the code against detailed binary evolution models from STARS and BEC to ensure accuracy in simulating mass transfer and orbital evolution.
• Analyzing evolutionary sequences of massive binaries with initial masses of 20 and 15 solar masses, varying initial orbital periods to explore different interaction regimes.
• Classifying systems into pre-interaction, semi-detached, and post-interaction phases based on Roche-lobe overflow and mass transfer history.
• Focusing on detached main-sequence binaries as the only systems where mass transfer can be definitively ruled out since birth.
• Leveraging direct measurements of mass, radius, and luminosity from such binaries to test single-star model predictions.

Experimental results

Research questions

• RQ1 Can detached binary systems with two main-sequence stars be used to identify stars that have never experienced strong binary interaction?
• RQ2 Why do many observed single stars appear to be single despite having undergone significant binary interaction?
• RQ3 How does binary interaction—especially mass transfer—affect the detectability and observability of companion stars?
• RQ4 What are the observational signatures of post-interaction systems where the donor star has been stripped or the system disrupted?
• RQ5 Why is excluding detected binaries from single-star model testing counter-productive, and what alternative approach ensures cleaner calibration?

Key findings

• Detached binaries with two main-sequence stars are the only systems where mass transfer can be definitively excluded since formation, making them ideal for testing single-star models.
• Stars that have experienced binary interaction often appear single due to mass transfer, which reduces the donor's luminosity and widens the orbit, making detection difficult.
• Post-interaction systems, such as those with a stripped helium star or a neutron star companion, often appear single after supernova disruption or mass ejection, leading to false single-star classifications.
• The duration of the post-interaction phase with a stripped helium star can last up to a tenth of the main-sequence lifetime or longer, yet no such massive systems have been detected, likely due to observational challenges.
• Semi-detached systems with mass transfer are detectable via eclipses and radial velocity variations, but they are not suitable for testing single-star physics due to ongoing interaction.
• Direct measurements of fundamental parameters in detached binaries—such as mass, radius, and luminosity—provide the most stringent tests for single-star evolutionary models.