How Neutron Star Mergers make Heavy Elements

Introduction

The incredible density, gravity, and electromagnetic field strengths of neutron stars (NS) make them laboratories for physics under extreme conditions. But probing these exotic objects is difficult.

The Impact of Gravitational Waves

With the 2017 gravitational wave detection of a NS-NS merger, the landscape changed, and we can now get high-quality spectra of the decompressed neutron-rich matter emerging from the collision. This is a new transient astrophysical phenomenon called a 'kilonova'.

Significance of Kilonovae

Kilonovae are a potential treasure trove of information on some of the biggest open questions in physics:

1. Understanding the nuclear and astrophysical pathways that created half of all the heavy elements (Z > 30) in the universe.
2. The physics of very hot and extremely dense matter.

For this reason, they are considered a scientific priority, and kilonova science is the target of several large new and upgraded facilities.

Challenges in Kilonova Research

Kilonovae are challenging due to several factors:

• The phenomenon is short-lived, requiring rapid follow-up with large telescopes.
• The outflow is heavy element-dominated, making it extremely demanding to model.
• The merger itself covers a huge dynamic range and involves complex nuclear physics.

To interpret the spectra, we require new atomic data, which does not yet exist for most of the heavy elements.

The HEAVYMETAL Initiative

To tackle these challenges, HEAVYMETAL assembles experts in various fields:

• Astrophysical observations
• Hydrodynamical merger simulation
• Numerical radiative transfer
• Laboratory heavy element spectroscopy and atomic structure calculation

With this team, we will be able to determine the structures and overall geometries of the merger outflow, the elemental abundances, and their stratification within the ejecta.

Goals and Insights

By the full exploitation of kilonovae, we will trace the nucleosynthesis pathways in NS mergers and provide important insights on:

• Heavy nuclei
• Neutrino interactions
• The nature of high-density matter

We will also chart the role of compact object mergers as the cosmic forge of the heaviest elements.