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

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.