Solving the mystery of extreme light variability in the massive eccentric system MACHO 80.7443.1718

TL;DR

This paper investigates the extreme light variability of the massive eccentric binary system MACHO 80.7443.1718, proposing a wind-wind collision model and analyzing its stellar parameters to understand its unique evolutionary state.

Contribution

The study provides a detailed analysis of ExtEV's parameters and demonstrates that wind-wind collision and atmospheric effects explain its light variability, revealing an unusual mass-loss rate.

Findings

Wind-wind collision explains light variability.

Primary star has ~30 R_sun radius and high mass-loss rate.

System is in a rare evolutionary phase with enhanced mass loss.

Abstract

The evolution of massive stars is heavily influenced by their binarity, and the massive eccentric binary system MACHO 80.7443.1718 (ExtEV) serves as a prime example. This study explores whether the light variability of ExtEV, observed near the periastron during its 32.8-day orbit, can be explained by a wind-wind collision (WWC) model and reviews other potential explanations. Using broadband photometry, TESS data, ground-based $UBV$ time-series photometry, and high-resolution spectroscopy, we analysed the system's parameters. We ruled out the presence of a Keplerian disk and periodic Roche-lobe overflow. Our analysis suggests the primary component has a radius of about $30\,{\rm R}_\odot$, luminosity of $\sim6.6\times10^5\,{\rm L}_\odot$, and mass between $25$ and $45\,{\rm M}_\odot$, with a high wind mass-loss rate of $4.5\times10^{-5}\,{\rm M}_\odot\,{\rm yr}^{-1}$, likely enhanced by tidal interactions, rotation, and tidally excited oscillations. We successfully modelled ExtEV's light curve, identifying atmospheric eclipse and light scattering in the WWC cone as key contributors. The system's mass-loss rate exceeds theoretical predictions, indicating that ExtEV is in a rare evolutionary phase, offering insights into enhanced mass loss in massive binary systems.