Chemodynamical Analysis of Metal-rich High-eccentricity Stars in the Milky Way's Disk

TL;DR

This study analyzes metal-rich, high-eccentricity stars in the Milky Way's disk, revealing their diverse origins including accretion, disk heating, and merger-induced starburst, based on chemodynamical data from SDSS and LAMOST.

Contribution

It provides a detailed chemodynamical characterization of the Splash stars, identifying their multiple formation mechanisms and origins in the Milky Way's evolutionary history.

Findings

High-eccentricity stars split into two populations with distinct properties.

Most high-eccentricity stars are heated disk stars or from merger-induced starbursts.

Half of the stars are accreted from Gaia Sausage/Enceladus.

Abstract

We present a chemodynamical analysis of 11,562 metal-rich, high-eccentricity halo-like main-sequence (MS) stars, which has been referred to as the Splash or Splashed Disk, selected from Sloan Digital Sky Survey (SDSS) and Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). When divided into two groups, a low-[$\alpha$/Fe] population (LAP) and a high-[$\alpha$/Fe] population (HAP), based on kinematics and chemistry, we find that they exhibit very distinct properties, indicative of different origins. From a detailed analysis of their orbital inclinations, we suggest that the HAP arises from a large fraction (~ 90%) of heated disk stars and a small fraction (~ 10%) of in situ stars from a starburst population, likely induced by interaction of the Milky Way with Gaia Sausage/Enceladus (GSE) or other early merger. The LAP comprises about half accreted stars from the GSE and half formed by the GSE-induced starburst. Our findings further imply that the Splash stars in our sample originated from at least three different mechanisms - accretion, disk heating, and a merger induced starburst.