Evidence against anomalous compositions for giants in the Galactic Nuclear Star Cluster

TL;DR

This study shows that strong Sc I lines in cool M giants are not unique to the Galactic Center and are likely due to line formation effects, cautioning against using these lines for abundance analysis in such stars.

Contribution

It demonstrates that strong Sc I lines are common in cool M giants and are not indicative of anomalous chemical abundances, highlighting the need for improved line formation models.

Findings

Strong Sc I lines are present in both Galactic Center and local M giants.

The lines are likely due to line formation effects, not abundance anomalies.

Similar effects are observed in lines of vanadium, titanium, and yttrium.

Abstract

Very strong Sc I lines have been found recently in cool M giants in the Nuclear Star Cluster in the Galactic Center. Interpreting these as anomalously high scandium abundances in the Galactic Center would imply a unique enhancement signature and chemical evolution history for nuclear star clusters, and a potential test for models of chemical enrichment in these objects. We present high resolution K-band spectra (NIRSPEC/Keck II) of cool M giants situated in the solar neighborhood and compare them with spectra of M giants in the Nuclear Star Cluster. We clearly identify strong Sc I lines in our solar neighborhood sample as well as in the Nuclear Star Cluster sample. The strong Sc I lines in M giants are therefore not unique to stars in the Nuclear Star Cluster and we argue that the strong lines are a property of the line formation process that currently escapes accurate theoretical modeling. We further conclude that for giant stars with effective temperatures below approximately 3800 K these Sc I lines should not be used for deriving the scandium abundances in any astrophysical environment until we better understand how these lines are formed. We also discuss the lines of vanadium, titanium, and yttrium identified in the spectra, which demonstrate a similar striking increase in strength below 3500 K effective temperature.