Exploring the Galactic Anticenter substructure with LAMOST & Gaia DR2

TL;DR

This study analyzes the kinematic and chemical properties of stars in the Galactic Anticenter Substructure using LAMOST and Gaia DR2 data, revealing their likely origin as outer, metal-poor disk stars with nearly circular orbits.

Contribution

It provides new insights into the nature and origin of GASS, suggesting it is part of the outer metal-poor disk extending to 30 kpc, formed after the thick disk.

Findings

GASS stars have nearly circular orbits on both sides of the Galactic plane.

GASS stars are more metal-poor and have lower alpha-abundance than typical thin disk stars.

GASS likely represents the outer, metal-poor disk extending to 30 kpc.

Abstract

We characterize the kinematic and chemical properties of 589 Galactic Anticenter Substructure Stars (GASS) with K-/M- giants in Integrals-of-Motion space. These stars likely include members of previously identified substructures such as Monoceros, A13, and the Triangulum-Andromeda cloud (TriAnd). We show that these stars are on nearly circular orbits on both sides of the Galactic plane. We can see velocity($V_{Z}$) gradient along Y-axis especially for the south GASS members. Our GASS members have similar energy and angular momentum distributions to thin disk stars. Their location in [$\alpha$/M] vs. [M/H] space is more metal poor than typical thin disk stars, with [$\alpha$/M] \textbf{lower} than the thick disk. We infer that our GASS members are part of the outer metal-poor disk stars, and the outer-disk extends to 30 kpc. Considering the distance range and $\alpha$-abundance features, GASS could be formed after the thick disk was formed due to the molecular cloud density decreased in the outer disk where the SFR might be less efficient than the inner disk.