On the Origin of Neutron-capture Elements in r-I and r-II Stars: A Differential-abundance Analysis

TL;DR

This study uses high-precision differential abundance analysis of two r-process-enhanced stars to investigate their nucleosynthesis origins, revealing commonalities in light elements and differences in neutron-capture elements, and suggesting multiple formation sites.

Contribution

It provides the first detailed differential abundance comparison between r-I and r-II stars, highlighting their elemental similarities and differences with high precision.

Findings

Light elements up to Zn are nearly equal in both stars.

r-I star shows mildly depleted light r-process elements compared to r-II.

The ratio of lighter-to-heavier r-process elements decreases from r-I to r-II stars.

Abstract

We present a strictly line-by-line differential analysis of a moderately $r$-process-enhanced star ($r$-I: HD~107752) with respect to a strongly $r$-process-enhanced star ($r$-II: CS~31082-0001) to investigate the possible common origin of their heavy-element nucleosynthesis with high-precision abundances. This study employs ESO data archive high-resolution and high signal-to-noise spectra taken with the UVES (VLT) spectrograph. Considering only the lines in common in both spectra, we estimate differential abundances of 16 light/Fe-peak elements and 15 neutron-capture elements. Abundances of O, Al, Pr, Gd, Dy, Ho, Er, and detection of Tm in HD~107752 are presented for the first time. We found three distinct features in the differential-abundance pattern. Nearly equal abundances of light elements up to Zn are present for both the stars, indicating a common origin for these elements; in addition to no noticable odd-even differential pattern. In the case of neutron-capture elements, the $r$-I star exhibits mildly depleted light $r$-process elements and more depleted heavier $r$-process elements relative to $r$-II star. We also show that among $r$-I and $r$-II stars, the ratio of lighter-to-heavier $r$-process elements (e.g. [(Sr,Y,Zr)/Eu]) exhibits a decreasing trend with respect to the overall $r$-process enhancement, forming a continuous sequence from $r$-I and $r$-II stars. Finally, we discuss the necessity of multiple sites for the formation of $r$-I stars.