# Clues on the Origin and Evolution of Massive Contact Binaries: Atmosphere Analysis of VFTS 352

## TL;DR

This study analyzes the atmosphere and wind properties of the massive contact binary VFTS 352 to understand its evolutionary status and potential to form black hole mergers, using phase-resolved spectroscopy and advanced modeling.

## Contribution

It provides detailed atmospheric and wind parameters of VFTS 352, revealing insights into its mixing processes and evolutionary stage through spectral disentangling and NLTE modeling.

## Key findings

- Both stars are hotter than single-star models predict.
- Chemical abundances show no significant mixing signatures.
- Results help constrain binary evolution pathways leading to black hole mergers.

## Abstract

The massive O4.5 V + O5.5 V binary VFTS 352 in the Tarantula nebula is one of the shortest-period and most massive overcontact binaries known. Recent theoretical studies indicate that some of these systems could ultimately lead to the formation of gravitational waves via black hole binary mergers through the chemically homogeneous evolution pathway. By analyzing ultraviolet-optical phase-resolved spectroscopic data, we aim to constrain atmospheric and wind properties that could be later used to confront theoretical predictions from binary evolution. In particular, surface abundances are powerful diagnostics of the evolutionary status, mass transfer and the internal mixing processes. From a set of 32 VLT/FLAMES visual and 8 HST/COS ultraviolet spectra, we used spectral disentangling to separate the primary and secondary components. Using a genetic algorithm wrapped around the NLTE model atmosphere and spectral synthesis code FASTWIND, we perform an 11-parameter optimization to derive the atmospheric and wind parameters of both components, including the surface abundances of He, C, N, O and Si. We find that both components are hotter than expected compared to single-star evolutionary models indicating that additional mixing processes may be at play. However the derived chemical abundances do not show significant indications of mixing when adopting baseline values typical for the system environment.