Chemical tagging and the second r-process

TL;DR

This paper investigates the formation processes of elements with atomic numbers 37 to 47, providing evidence for a second r-process and exploring its characteristics and implications for galactic chemical evolution.

Contribution

It offers new insights into the second r-process, its potential formation site, and how certain element abundances can constrain astrophysical conditions.

Findings

Rb/Zr ratios constrain neutron density of formation site

Pd and Ag abundances suggest a second/weak r-process

Heavy element formation may involve multiple neutron-capture processes

Abstract

Elements in the range 37 < Z < 47 provide key information on their formation process. Several studies have shown that some of these elements are formed by an r-process, that differs from the main r-process creating europium. Through a detailed abundance study of Rb - Ag I will show, by comparing these abundances to those of Ba and Eu, that their formation processes differ. The formation process of Pd and Ag deviates from the weak/main s-process as well as from the main r-process. Hence, Pd and Ag - and to some extend Zr - are created by a second/weak r-process. However, the characteristics and formation site of this process is not well understood. The abundance ratios of Rb/Zr help constrain the neutron number density of the formation site, while comparing the Pd and Ag abundances to yield predictions can provide limitations on the entropy and electron fraction of the formation environment. This study presents clues on the second r-process. Furthermore, the formation processes of the heavy elements might not differ in a clear cut way. Several of these neutron-capture processes might yield various amounts of heavy elements (e.g. Sr and Ba) at the same time or metallicity. This could possibly help explain the large star-to-star abundance scatter for these two elements below [Fe/H]= -2.5. Knowing their origin is important in the era of large surveys (e.g Gaia-ESO). Strontium and barium will, limited by resolution and signal-to-noise ratio, be the only detectable heavy elements in the most metal-poor stars. Hence, they will, depending on metallicity, be the main tracers of the weak and main s-/r-processes. Understanding the effects of stellar parameters, synthetic spectrum codes, model atmospheres, and NLTE on the Sr abundances are crucial to describe the chemical evolution of our Galaxy. I will present these effects for Sr.