The Binary Frequency of r-Process-Element Enhanced Metal-Poor Stars and its Implications: Chemical Tagging in the Primitive Halo of the Milky Way

TL;DR

This study investigates the binary nature of r-process-enhanced metal-poor stars to understand their chemical origins, finding most are single and suggesting their elemental enhancements reflect the primordial composition of their birth clouds.

Contribution

It provides the first radial velocity monitoring of such stars, revealing most are single and challenging binary mass transfer scenarios for their r-process enrichment.

Findings

Most r-process-enhanced stars are single, not binaries.

Binary stars in the sample have orbital properties similar to typical Population I binaries.

The r-process and carbon enhancements are likely inherited from natal molecular clouds.

Abstract

A few rare halo giants in the range [Fe/H] $\simeq -2.9\pm0.3$ exhibit {\it r}-process element abundances that vary as a group by factors up to [$r$/Fe] $\sim80$, relative to those of the iron peak and below. Yet, the astrophysical production site of these {\it r}-process elements remains unclear. We report initial results from four years of monitoring the radial velocities of 17 {\it r}-process-enhanced metal-poor giants to detect and characterise binaries in this sample. We find three (possibly four) spectroscopic binaries with orbital periods and eccentricities that are indistinguishable from those of Population I binaries with giant primaries, and which exhibit no signs that the secondary components have passed through the AGB stage of evolution or exploded as supernovae. The other 14 stars in our sample appear to be single -- including the prototypical $r$-process-element enhanced star CS 22892-052, which is also enhanced in carbon, but not in {\it s}-process elements. We conclude that the {\it r}-process (and potentially carbon) enhancement of these stars was not a local event due to mass transfer or winds from a binary companion, but was imprinted on the natal molecular clouds of these (single and binary) stars by an external source. These stars are thus spectacular chemical tracers of the inhomogeneous nature of the early Galactic halo system.