On the origin of the Hercules group: I. chemical signatures indicating the outer bar origin

TL;DR

This study analyzes the chemical signatures of the Hercules group, revealing its likely origin from the outer thin bar of the Galaxy, based on elemental abundance patterns indicating a dynamical transport mechanism.

Contribution

It provides new chemical evidence linking the Hercules group to the outer Galactic bar, expanding understanding of its origin beyond previous Fe-focused studies.

Findings

Hercules stars are enriched in iron-peak and Na, Al elements.

They are deficient in alpha elements compared to local stars.

Chemical signatures suggest an origin from the outer Galactic bar.

Abstract

The Hercules kinematic group is a kinematic anomaly of stars observed in the solar neighbourhood (SNd). In this series of papers, we present a comprehensive study of this structure. This paper focuses on its chemical signatures over several groups of elements. The next paper discusses its kinematical properties. While studies suggested a non-native origin of Hercules stars due to the distinct chemical and kinematic features, previous studies focussed mainly on the Fe abundances. We adopt chemical data with abundances of elements from GALAH and APOGEE to seek further chemical implications on the origin. Our analysis reveals that the low alpha population of the low angular momentum Hercules group is significantly enhanced in iron-peak (Fe, Ni, Mn) and Odd-Z (Na, Al) elements, and slightly deficient in alpha elements (O, Ca, Ti) compared to kinematically local stars. The super enhancement in iron-peak elements and deficiency in alpha elements support their origin from the outer thin bar in the inner Galaxy. Moreover, the enhancement in Na and Al indicates these stars as the youngest stars in the old sequence from the inner thick disc. Hence, the origin of these stars can be related to the outer bar region. These chemical signatures require the underlying dynamical mechanism that forms the Hercules group to be capable of transporting stars in the inner Galaxy out to the SNd. The next paper will consider the Trojan orbits as the favoured mechanism.