Gemini/GRACES Spectroscopy of Stars in Triangulum II

TL;DR

This study uses high-resolution Gemini-GRACES spectroscopy to analyze two stars in Triangulum II, revealing their chemical compositions, binary nature, and supporting the galaxy's metallicity spread indicative of chemical evolution.

Contribution

First detailed high-resolution spectroscopic analysis of stars in Triangulum II, confirming metallicity spread and chemical signatures consistent with dwarf galaxy evolution.

Findings

Stars show typical calcium and barium but low magnesium and sodium.

One star exhibits potassium enhancement similar to NGC 2419.

Detected binary star with velocity variation, consistent with high binary fraction in dwarf galaxies.

Abstract

The chemical abundance ratios and radial velocities for two stars in the recently discovered Triangulum II faint dwarf galaxy have been determined from high resolution, medium signal-to-noise ratio spectra from the Gemini-GRACES facility. These stars have stellar parameters and metallicities similar to those derived from their photometry and medium-resolution Ca II triplet spectra, and supports that Triangulum II has a metallicity spread consistent with chemical evolution in a dwarf galaxy. The elemental abundances show that both stars have typical calcium abundances and barium upper limits for their metallicities, but low magnesium and sodium. This chemical composition resembles some stars in dwarf galaxies, attributed to inhomogeneous mixing in a low star formation environment, and/or yields from only a few supernova events. One of our targets (Star40) has an enhancement in potassium, and resembles some stars in the unusual outer halo star cluster, NGC 2419. Our other target (Star46) appears to be a binary based on a change in its radial velocity (Delta v(rad) = 24.5 +/- 2.1 km/s). This is consistent with variations found in binary stars in other dwarf galaxies. While this serves as a reminder of the high binary fraction in these ultra faint dwarf galaxies, this particular object has had little impact on the previous determination of the velocity dispersion in Triangulum II.