TriAnd | Yuri Abuchaim

Overview

The Triangulum-Andromeda (TriAnd) overdensity is a stellar structure located in the outer disk of the Milky Way, approximately 18 kpc from the Sun, in the direction of galactic coordinates l ~ -125° and b ~ -25°.

This study presents the most comprehensive chemodynamical analysis of TriAnd to date, using a sample of 31 candidate stars observed with the high-resolution GRACES spectrograph (R = 40,000) at the Gemini North telescope (8.1m).

The origin of TriAnd has been controversial since its discovery, with previous studies suggesting scenarios ranging from in situ formation (born in the galactic disk) to an accreted population (from satellite galaxies). Our work aims to resolve this debate through a combined analysis of dynamical parameters and chemical abundances.

Methodology

Sample Selection

M giant stars selected from the 2MASS catalog
Photometric criteria to separate giants from dwarfs
Selection in the region -90° < l < -160° and -10° > b > -45°
Proper motion filtering using Gaia DR2 to reduce contamination

Spectroscopic Analysis

Observations with GRACES/Gemini (R ~ 40,000, 4000-10000 Å)
Determination of atmospheric parameters (T_eff, log g, [Fe/H]) via MOOG
Abundance analysis for 11 elements: Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Ba, Eu
NLTE corrections applied for Fe, Mg, Ca, Cr and Si

Galactic Dynamics

Calculation of stellar orbits using AGAMA (integration for 5 Gyr)
Galactic potential model from McMillan (2017)
1000 Monte Carlo realizations for each star
Comparison with outer disk and halo populations

Key Results

Stellar Dynamics

Most stars show low eccentricity (e < 0.4) and inclination, compatible with disk populations
6 stars display high eccentricity (e > 0.4), suggesting accreted origin
Orbital energy (E) and angular momentum (L_z) consistent with outer disk for the main population

Chemical Abundances

Mean metallicity [Fe/H] ~ -0.8 dex, more metal-poor than the outer disk
α-element patterns (Mg, Si, Ca, Ti) similar to the outer thin disk
High-eccentricity stars show chemical patterns similar to Gaia-Sausage/Enceladus (GSE)
High [Ba/Fe] and [Ba/Eu] suggest predominant enrichment by s-process
Identification of 3 possible highly r-process enriched r-II stars

Our results indicate that TriAnd is predominantly an in situ population of the outer disk, possibly formed by disk dynamical heating, with a small contamination of accreted stars that spatially coincide with the structure.

Parâmetros orbitais das estrelas TriAnd — Orbital parameters of TriAnd stars: (a) Eccentricity vs. inclination; (b) Orbital energy vs. angular momentum; (c-d) Spatial projections with velocity vectors.

Abundâncias de elementos alfa — Relation [α/Fe] vs [Fe/H] for Mg, Si, Ca and Ti, comparing TriAnd stars (blue) with outer disk populations (gray) and GSE (yellow).

Implications and Conclusions

This study resolves the controversy about TriAnd's nature, demonstrating that:

The main population has an in situ origin, formed in the galactic disk
The apparent "chemical complexity" reported in previous studies is due to contamination by accreted stars
The outer galactic disk presents substructures formed by dynamical perturbations
Future studies should consider the possibility of mixed populations in stellar overdensities

How to Cite

Abuchaim, Y., Perottoni, H. D., Rossi, S., et al. 2023, ApJ, 949, 48
Title: The Chemodynamical Nature of the Triangulum-Andromeda Overdensity
DOI: 10.3847/1538-4357/acc9bc

Collaborators: University of São Paulo, Universitat de Barcelona, University of Chicago, UNAM, Federal University of Goiás, NOIRLab, National Observatory, Federal University of Rio de Janeiro