Two massive stars possibly ejected from NGC 3603 via a three-body encounter

TL;DR

This paper reports the discovery of a star ejected from NGC 3603, likely due to a three-body dynamical encounter, and estimates the properties of the ejected stars based on observational data and conservation laws.

Contribution

It proposes a novel scenario where two massive stars were ejected from NGC 3603 via a three-body encounter, and estimates their masses and merger history.

Findings

The O6 V star was ejected from NGC 3603.

The O2 If*/WN6 star was also ejected and likely resulted from a binary merger.

The ejected stars' properties support a three-body dynamical ejection scenario.

Abstract

We report the discovery of a bow-shock-producing star in the vicinity of the young massive star cluster NGC 3603 using archival data of the Spitzer Space Telescope. Follow-up optical spectroscopy of this star with Gemini-South led to its classification as O6 V. The orientation of the bow shock and the distance to the star (based on its spectral type) suggest that the star was expelled from the cluster, while the young age of the cluster (~2 Myr) implies that the ejection was caused by a dynamical few-body encounter in the cluster's core. The relative position on the sky of the O6 V star and a recently discovered O2 If*/WN6 star (located on the opposite side of NGC 3603) allows us to propose that both objects were ejected from the cluster via the same dynamical event -- a three-body encounter between a single (O6 V) star and a massive binary (now the O2 If*/WN6 star). If our proposal is correct, then one can "weigh" the O2 If*/WN6 star using the conservation of the linear momentum. Given a mass of the O6 V star of 30 Msun, we found that at the moment of ejection the mass of the O2 If*/WN6 star was 175 Msun. Moreover, the observed X-ray luminosity of the O2 If*/WN6 star (typical of a single star) suggests that the components of this originally binary system have merged (e.g., because of encounter hardening).