The Fundamental Parameters of Four Massive Eclipsing Binaries in Westerlund 1

TL;DR

This study measures fundamental parameters of four massive eclipsing binaries in Westerlund 1, providing new data to constrain massive star evolution, and estimates the cluster's distance and the progenitor mass of a magnetar.

Contribution

It presents new high-precision measurements of masses and radii for eight stars in Westerlund 1, including the first direct distance estimate to the cluster using eclipsing binaries.

Findings

Masses range from 10 to 40 solar masses.

Confirmed high mass of the magnetar progenitor.

Estimated cluster distance at 3.7 +/- 0.6 kpc.

Abstract

Only a small number of high mass stars (> 30 Mo) have fundamental parameters (i.e. masses and radii) measured with high enough accuracy from eclipsing binaries to constrain formation and evolutionary models of massive stars. This work aims to increase this limited sample, by studying the 4 massive eclipsing binary candidates discovered by Bonanos in the young massive cluster Westerlund 1. We present new follow-up echelle spectroscopy of these binaries and models of their light and radial velocity curves. We obtain fundamental parameters (i.e. masses, radii) for the 8 component stars, finding masses that span a range of 10-40 Mo, and contributing accurate fundamental parameters for 1 additional very massive star, the 33 Mo component of W13. WR77o is found to have a ~40 Mo companion, which provides a second dynamical constraint on the mass of the progenitor of the magnetar known in the cluster. We also use W13 to estimate the first, direct, eclipsing binary distance to Westerlund 1 and therefore the magnetar, and find it to be at 3.7 +/- 0.6 kpc. Our results confirm previous evidence for a high mass for the progenitor of the magnetar. In addition, the availability of eclipsing binaries with accurate parameters opens the way for direct, independent, high precision eclipsing binary distance measurements to Westerlund 1.