The very massive binary NGC3603-A1

TL;DR

This study uses advanced spectroscopy to analyze the massive binary star system NGC3603-A1, revealing it likely hosts the most massive star measured so far, with detailed mass estimates for its components.

Contribution

First direct measurement of the absolute masses of both stars in the NGC3603-A1 binary system using radial velocities and inclination data.

Findings

Primary star mass: 116 ± 31 solar masses

Secondary star mass: 89 ± 16 solar masses

A1 is likely the most massive star measured to date

Abstract

Using VLT/SINFONI, we have obtained repeated AO-assisted, NIR spectroscopy of the three central WN6ha stars in the core of the very young (~1 Myr), massive and dense Galactic cluster NGC3603. One of these stars, NGC3603-A1, is a known 3.77-day, double-eclipsing binary, while another one, NGC3603-C, is one of the brightest X-ray sources among all known Galactic WR stars, which usually is a strong indication for binarity. Our study reveals that star C is indeed an 8.9-day binary, although only the WN6ha component is visible in our spectra; therefore we temporarily classify star C as an SB1 system. A1, on the other hand, is found to consist of two emission-line stars of similar, but not necessarily of identical spectral type, which can be followed over most the orbit. Using radial velocities for both components and the previously known inclination angle of the system, we are able to derive absolute masses for both stars in A1. We find M_1 = (116 \pm 31) Mo for the primary and M_2 = (89 \pm 16) Mo for the secondary component of A1. While uncertainties are large, A1 is intrinsically half a magnitude brighter than WR20a, the current record holder with 83 and 82 Mo, respectively; therefore, it is likely that the primary in A1 is indeed the most massive star weighed so far.