The massive binary system WR 20a: light curve analysis in a colliding wind model

TL;DR

This paper develops a binary model that includes colliding winds to analyze the light curves of WR 20a, addressing limitations of the standard Roche model in wind-dominated systems. Through adaptive MCMC fitting, the authors obtain a best-fit solution with smaller component radii, lower luminosity, and a shorter distance than earlier Roche-model results, and they reproduce the observed light-curve asymmetry caused by the wind-wind collision geometry. The work demonstrates the importance of wind opacity in shaping optical light curves and shows that the derived distance can be reconciled with Westerlund 2 when recent extinction studies are considered. Cross-validation with TESS and ASAS-SN data supports the model’s portrayal of the average state of the collision zone, while highlighting non-stationary variability likely tied to wind-wind interaction processes.

Abstract

The article presents the results of the analysis of optical light curves of the massive binary system WR 20a (WN 6ha + WN 6ha). The analysis was performed with the binary system model, extending the standard Roche model for the case when both components of the system have powerful stellar winds. The model takes into account the collision of the winds and the influence of orbital motion on the collision zone. The observational light curves in the BVI filters were taken from previously published papers, in which they were analyzed using the standard Roche model. The main difference between the results of our work and the previous results is that in our model the radii of the components are about 25% smaller. As a consequence, the luminosity of the system in our model decreased by approximately 40%, and the distance to the system by 20%. In addition, the model was able to successfully describe the observed asymmetry of the light curve with respect to the phases of the conjunctions, which is impossible in the standard Roche model. The model light curves were also compared with the observational curves obtained by the TESS satellite and the ASAS-SN project. It was shown that, taking into account recent studies of interstellar extinction in the direction of the young open cluster Westerlund 2, the distance to WR 20a obtained in our calculations is consistent with the hypothesis that WR 20a is a member of the cluster.

Figures (6)

• Figure 1: An example of a light curve of WR 20a obtained by the TESS satellite for sector 36 (cycle 3, March--April 2021) using simple aperture photometry (SAP) within the standard SPOC reduction. Systematic trends are clearly visible. Due to the influence of the "third light", the depth of the minima is about $0\hbox{$\,.\!\!^{\rm m}$} 16$ (in the $BVI$ filters about $0\hbox{$\,.\!\!^{\rm m}$}4$).
• Figure 2: Posterior empirical distributions for all possible pairs of parameters $i$, $T_2$, $\mu$, $\dot{M}$. To avoid cluttering the figure, only every hundredth point is shown. Shown on the right (top) of each row are the empirical histograms of the probability distribution for a given parameter (normalized to the maximum) and the Gaussian functions approximating them (shown by solid red curves).
• Figure 3: Observational light curves in the $BVI$ filters from bonanos04, rauw07 (dots) and the optimal model light curves in our model (solid red lines). Note that in bonanos04 the uncertainties for all observational points in the $I$ filter are the same ($0\hbox{$\,.\!\!^{\rm m}$} 01$) and, obviously, are underestimated. For comparison, the model light curve for our optimal model, in which the component winds are "switched off" (see text), is shown in blue in the top panel (filter $B$).
• Figure 4: The view of the system as seen by an observer at the orbital phase 0.15. The orbit is shown by the black line, with the primary component behind and the secondary in front. The contact surface is shown in blue. To avoid clutter, only the central part of the contact surface is shown; the mesh on it is much more sparse than in real calculations.
• Figure 5: Top panel: the mean observed TESS light curve (dots) and the model light curve (solid red line) in a model with parameters found from $BVI$ filters. The uncertainties in the mean observed light curve are comparable to the size of the dots. Bottom panel: The ASAS SN light curve and the similarly computed model light curve.