Massive-binary EvoluTion Across the metallicity Ladder

Introduction

Do massive stars undergo supernova explosions when collapsing into black holes? What mechanisms drove the Cosmos into an epoch of reionization? How was dust produced in the Early Universe? Scarcely any field of astronomy remains unaffected by massive stars: stars born with more than eight solar masses.

Current Understanding

Yet, studies in the Local Universe reveal substantial gaps in our understanding of massive stars related to:

• Mass loss
• Internal mixing
• Core-collapse
• Stellar interactions

Uncertainties worsen at the low-metallicity conditions of the Early Universe. The primary reason for this is a severe lack of empirical constraints on massive binaries across the metallicity axis, driven by the rarity of massive stars and the shortage of adequate monitoring campaigns to study them.

Project Overview

METAL leverages hundreds of hours worth of novel spectroscopic and interferometric data collected as PI using observatories such as the Very Large Telescope (VLT) and the Hubble Space Telescope, including a VLT Large Programme (116hr; 2023 - 2025).

Monitoring Campaigns

These campaigns monitor thousands of massive stars in our Galaxy and the Magellanic Clouds. Targeting:

• Unevolved OB-type stars
• Evolved Wolf-Rayet and Oe/Be stars
• Elusive black holes

Objectives and Outcomes

METAL will elucidate the initial conditions, evolution, and ultimate fates of massive stars at three metallicity anchors. Groundbreaking outcomes include:

1. Unprecedented statistics on the multiplicity, initial mass function, and structure of massive stars at low metallicity
2. A tenfold increase in the sample of dormant black-hole binaries and the first such sample at low metallicity
3. A revised mass-loss prescription and a comprehensive binary characterization of evolved massive stars at two metallicity anchors

Impact

The deliverables will be the defining calibrators for next-generation evolution models of massive stars, ushering in advancements in models of not only stellar evolution but also galaxy evolution, unresolved stellar populations, and core-collapse supernovae.