The Most Metal-poor Stars in the Magellanic Clouds are $r$-process Enhanced

Abstract

The chemical abundances of a galaxy's metal-poor stellar population can be used to investigate the earliest stages of its formation and chemical evolution. The Magellanic Clouds are the most massive of the Milky Way's satellite galaxies and are thought to have evolved in isolation until their recent accretion by the Milky Way. Unlike the Milky Way's less massive satellites, little is know about the Magellanic Clouds' metal-poor stars. We have used the mid-infrared metal-poor star selection of Schlaufman & Casey (2014) and archival data to target nine LMC and four SMC giants for high-resolution Magellan/MIKE spectroscopy. These nine LMC giants with $-2.4\lesssim [\text{Fe/H}]\lesssim -1.5$ and four SMC giants with $-2.6\lesssim [\text{Fe/H}]\lesssim -2.0$ are the most metal-poor stars in the Magellanic Clouds yet subject to a comprehensive abundance analysis. While we find that at constant metallicity these stars are similar to Milky Way stars in their $\alpha$, light, and iron-peak elemental abundances, both the LMC and SMC are enhanced relative to the Milky Way in the $r$-process element europium. These abundance offsets are highly significant, equivalent to $3.9\sigma$ for the LMC, $2.7\sigma$ for the SMC, and $5.0\sigma$ for the complete Magellanic Cloud sample. We propose that the $r$-process enhancement of the Magellanic Clouds' metal-poor stellar population is a result of the Magellanic Clouds' isolated chemical evolution and long history of accretion from the cosmic web combined with $r$-process nucleosynthesis on a timescale longer than the core-collapse supernova timescale but shorter than or comparable to the thermonuclear (i.e., Type Ia) supernova timescale.

Accepted for publication in The Astronomical Journal. 30 pages, 7 figures, and 5 tables

References in corpus (60)

• The NumPy array: a structure for efficient numerical computation
• The Gaia mission
• Modules for Experiments in Stellar Astrophysics (MESA)
• Mass and environment as drivers of galaxy evolution in SDSS and zCOSMOS and the origin of the Schechter function
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• 3D-HST+CANDELS: The Evolution of the Galaxy Size-Mass Distribution since $z=3$
• Galactic chemical evolution: Carbon through Zinc
• A distance to the Large Magellanic Cloud that is precise to one per cent
• Are the Magellanic Clouds on their First Passage about the Milky Way?
• Magnetorotationally driven Supernovae as the origin of early galaxy $r$-process elements?
• Determining stellar atmospheric parameters and chemical abundances of FGK stars with iSpec
• The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble Space Telescope / Wide Field Planetary Camera 2 Observations
• A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-Poor Stars
• The Importance of Satellite Quenching for the Build-Up of the Red Sequence of Present Day Galaxies
• The Quenching of the Ultra-Faint Dwarf Galaxies in the Reionization Era
• Modern stellar spectroscopy caveats
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE) Spectrographs
• Target Selection for the SDSS-IV APOGEE-2 Survey
• SkyMapper Southern Survey: Second Data Release (DR2)
• New Rare Earth Element Abundance Distributions for the Sun and Five r-Process-Rich Very Metal-Poor Stars
• The Solar Twin Planet Search. I. Fundamental parameters of the stellar sample
• Non-LTE line formation of Fe in late-type stars - III. 3D non-LTE analysis of metal-poor stars
• Complete element abundances of nine stars in the r-process galaxy Reticulum II
• Galactic r-process enrichment by neutron star mergers in cosmological simulations of a Milky Way-mass galaxy
• Systematic non-LTE study of the $-2.6\le$ [Fe/H] $\le 0.2$ F and G dwarfs in the solar neighbourhood. II. Abundance patterns from Li to Eu
• The History of R-Process Enrichment in the Milky Way
• Multi-Element Abundance Measurements from Medium-Resolution Spectra. IV. Alpha Element Distributions in Milky Way Dwarf Satellite Galaxies
• The Gas Content and Stripping of Local Group Dwarf Galaxies
• The Chemical Enrichment History of the Large Magellanic Cloud
• An R-process enhanced star in the dwarf galaxy Tucana III
• High-Resolution Spectroscopic Study of Extremely Metal-Poor Star Candidates from the SkyMapper Survey
• The $R$-Process Alliance: Fourth Data Release from the Search for $r$-Process-Enhanced Stars in the Galactic Halo
• The Best and Brightest Metal-Poor Stars
• The GALAH Survey: Non-LTE departure coefficients for large spectroscopic surveys
• linemake: An Atomic and Molecular Line List Generator
• The solar silicon abundance based on 3D non-LTE calculations
• The R-process Alliance: First Magellan/MIKE Release from the Southern Search for R-Process-enhanced Stars
• The Southern Stellar Stream Spectroscopic Survey (S5): Chemical Abundances of Seven Stellar Streams
• Supernova Fallback: A Possible Site for the r-Process
• Ultrafaint Dwarf Galaxies - the lowest mass relics from before reionization
• $R$-process enhancements of Gaia-Enceladus in GALAH DR3
• The Need for Speed: Escape velocity and dynamical mass measurements of the Andromeda galaxy
• The total mass and dark halo properties of the Small Magellanic Cloud
• Non-LTE analysis of K I in late-type stars
• Red Giants in the Small Magellanic Cloud. II. Metallicity Gradient and Age-Metallicity Relation
• Constraining cosmic scatter in the Galactic halo through a differential analysis of metal-poor stars
• Zirconium, Barium, Lanthanum and Europium Abundances in Open Clusters
• Evidence for $\gtrsim 4$ Gyr timescales of neutron star mergers from Galactic archaeology
• Chemical evolution of classical and ultra-faint dwarf spheroidal galaxies
• Magellanic satellites in $\Lambda$CDM cosmological hydrodynamical simulations of the Local Group
• Age-Dating Red Giant Stars Associated with Galactic Disk and Halo Substructures
• The ESA Gaia Archive: Data Release 1
• Star Formation in Ultrafaint Dwarfs: Continuous or Single-age Bursts?
• Ultrafaint Dwarfs - Star Formation and Chemical Evolution in the Smallest Galaxies
• Elemental abundances in M31: [Fe/H] and [α/Fe] in M31 Dwarf Galaxies Using Coadded Spectra
• The Most Metal-poor Stars in the Inner Bulge
• Elemental Abundances in M31: Properties of the Inner Stellar Halo
• Elemental Abundances in M31: Iron and Alpha Element Abundances in M31's Outer Halo
• The universality of the rapid neutron-capture process revealed by a possible disrupted dwarf galaxy star
• Binary neutron star merger in common envelope jet supernovae

Cited by in corpus (5)

• The chemical composition of globular clusters in the Local Group
• Timing the r-Process Enrichment of the Ultra-Faint Dwarf Galaxy Reticulum II
• Origin of highly $r$-process-enhanced stars in a cosmological zoom-in simulation of a Milky Way-like galaxy
• Galactic chemical evolution of exoplanet host stars: Are high-mass planetary systems young?
• Metal-Poor Stars Observed with the Southern African Large Telescope II. An Extended Sample