On the Hunt for the Origins of the Orphan--Chenab Stream: Detailed Element Abundances with APOGEE and Gaia

TL;DR

This study combines Gaia astrometry and APOGEE spectroscopy to identify and analyze stars in the Orphan--Chenab stream, revealing its likely origin as a dwarf spheroidal galaxy through detailed chemical abundance patterns.

Contribution

It provides the first detailed chemical abundance analysis of the OC stream stars, supporting its origin from a dwarf galaxy rather than a globular cluster.

Findings

OC stream stars are not mono-metallic, with median [Fe/H] of -1.92 dex.

The chemical pattern suggests a dwarf spheroidal galaxy progenitor.

OC stream stars show alpha-element depletion compared to Milky Way populations.

Abstract

Stellar streams in the Galactic halo are useful probes of the assembly of galaxies like the Milky Way. Many tidal stellar streams that have been found in recent years are accompanied by a known progenitor globular cluster or dwarf galaxy. However, the Orphan--Chenab (OC) stream is one case where a relatively narrow stream of stars has been found without a known progenitor. In an effort to find the parent of the OC stream, we use astrometry from the early third data release of ESA's Gaia mission (Gaia EDR3) and radial velocity information from the SDSS-IV APOGEE survey to find up to 13 stars that are likely members of the OC stream. We use the APOGEE survey to study the chemical nature (for up to 13 stars) of the OC stream in the $\alpha$ (O, Mg, Ca, Si, Ti, S), odd-Z (Al, K, V), Fe-peak (Fe, Ni, Mn, Co, Cr) and neutron capture (Ce) elemental groups. We find that the stars that make up the OC stream are not consistent with a mono-metallic population and have a median metallicity of --1.92~dex with a dispersion of 0.28 dex. Our results also indicate that the $\alpha$-elements are depleted compared to the known Milky Way populations and that its [Mg/Al] abundance ratio is not consistent with second generation stars from globular clusters. The detailed chemical pattern of these stars indicates that the OC stream progenitor is very likely to be a dwarf spheroidal galaxy with a mass of ~10$^6$ M$_\odot$.