Optical Spectroscopy Reveals Hidden Neutron-capture Elemental Abundance Differences among APOGEE-identified Chemical Doppelg\"angers

TL;DR

This study demonstrates that optical spectroscopy can uncover hidden differences in neutron-capture element abundances among stars that appear chemically identical in APOGEE data, revealing deeper chemical diversity in the Milky Way.

Contribution

The paper shows that optical spectroscopy detects neutron-capture element differences in APOGEE-identified stellar pairs, highlighting limitations of APOGEE's chemical tagging.

Findings

Neutron-capture element differences of 0.02 to 0.38 dex detected

Pairs sharing APOGEE abundances can differ significantly in neutron-capture elements

Optical spectroscopy reveals critical chemical differences not seen in APOGEE data

Abstract

Grouping stars by chemical similarity has the potential to reveal the Milky Way's evolutionary history. The APOGEE stellar spectroscopic survey has the resolution and sensitivity for this task. However, APOGEE lacks access to strong lines of neutron-capture elements ($Z > 28$) which have nucleosynthetic origins that are distinct from those of the lighter elements. We assess whether APOGEE abundances are sufficient for selecting chemically similar disk stars by identifying 25 pairs of chemical ``doppelgangers'' in APOGEE DR17 and following them up with the Tull spectrograph, an optical, $R \sim 60{,}000$ echelle on the McDonald Observatory 2.7-m telescope. Line-by-line differential analyses of pairs' optical spectra reveals neutron-capture (Y, Zr, Ba, La, Ce, Nd, and Eu) elemental abundance differences of $\Delta$[X/Fe] $\rm \sim 0.020 \pm 0.015$ to $0.380 \pm 0.15$ dex (4--140%), and up to 0.05 dex (12%) on average, a factor of 1--2 times higher than intra-cluster pairs. This is despite the pairs sharing nearly identical APOGEE-reported abundances and [C/N] ratios, a tracer of giant-star age. This work illustrates that even when APOGEE abundances derived from SNR $> 300$ spectra are available, optically-measured neutron-capture element abundances contain critical information about composition similarity. These results hold implications for the chemical dimensionality of the disk, mixing within the interstellar medium, and chemical tagging with the neutron-capture elements.