Highly r-process enhanced stars in ultra-faint dwarf galaxies

Abstract

Highly r-process enhanced metal-poor stars (MP r-II, $[Eu/Fe]>1$ and $[Fe/H]\lesssim -1.5$) have been observed in ultra-faint dwarf (UFD) galaxy, specifically in Reticulum~II (Ret~II). The fact that only a few UFDs contain such stars implies that the r-process site may reflect very rare, but individually prolific events, such as neutron star mergers (NSMs). Considering the relatively short star formation history (SFH) of UFDs, it is puzzling how they could experience such a rare phenomenon. In this work, we show the results of cosmological hydrodynamic zoom-in simulations of isolated UFDs ($M_{vir}\approx 10^7-10^8$ solar mass and $M_{\ast }\approx 10^3-10^4$ solar mass at $z=0$) to explain the formation of MP r-II stars in UFDs. We employ a simple toy model for NSM events, adopting parameters consistent with observations, such as the NSM rate (1 per $M_{\ast }\approx 10^5$ solar mass) and europium (Eu) mass ($M_{Eu}\approx 10^{-5}$ solar mass). We identify only one simulated galaxy ($M_{vir}\approx 4.6\times 10^7$, $M_{\ast }\approx 3.4\times 10^3$ solar mass at $z=0$) with abundances similar to Ret~II in a simulation volume that hosts $\sim 30$ UFD analogs, indicating that such abundances are possible but rare. By exploring a range of key parameters, we demonstrate that the most important factor in determining the formation of MP r-II stars in UFDs is how quickly subsequent stars can be formed out of r-process enriched gas. We find that it takes between 10 to 100~Myr to form the first and second burst of MP r-II stars. Over this period, Eu-polluted gas maintains the required high abundance ratios of $[Eu/Fe]>1$.

12 pages, 8 figures, Accepted for publication in MNRAS

References in corpus (33)

• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• An Ordinary Short Gamma-Ray Burst with Extraordinary Implications: Fermi-GBM Detection of GRB 170817A
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. II. UV, Optical, and Near-IR Light Curves and Comparison to Kilonova Models
• Spectroscopic identification of r-process nucleosynthesis in a double neutron star merger
• INTEGRAL Detection of the First Prompt Gamma-Ray Signal Coincident with the Gravitational Wave Event GW170817
• Formation of Double Neutron Star Systems
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. IV. Detection of Near-infrared Signatures of r-process Nucleosynthesis with Gemini-South
• Model-independent evidence in favor of an end to reionization by z~6
• Evidence of patchy hydrogen reionization from an extreme Ly$\alpha$ trough below redshift six
• The Tidal Evolution of Local Group Dwarf Spheroidals
• The Quenching of the Ultra-Faint Dwarf Galaxies in the Reionization Era
• Segue 1: An Unevolved Fossil Galaxy from the Early Universe
• Complete element abundances of nine stars in the r-process galaxy Reticulum II
• Production of the entire range of r-process nuclides by black hole accretion disc outflows from neutron star mergers
• The Star Formation Histories of Local Group Dwarf Galaxies II. Searching For Signatures of Reionization
• Galactic r-process enrichment by neutron star mergers in cosmological simulations of a Milky Way-mass galaxy
• The History of R-Process Enrichment in the Milky Way
• An R-process enhanced star in the dwarf galaxy Tucana III
• Lyman-alpha emitters gone missing: evidence for late reionization?
• Recovery from population III supernova explosions and the onset of second generation star formation
• The effect of feedback and reionization on star formation in low-mass dwarf galaxy haloes
• r-process Production Sites as inferred from Eu Abundances in Dwarf Galaxies
• Natal Kicks and Time Delays in Merging Neutron Star Binaries - Implications for r-process nucleosynthesis in Ultra Faint Dwarfs and in the Milky Way
• Enrichment history of r-process elements shaped by a merger of neutron star pairs
• The Old Host-Galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational Wave Source
• Chemical Diversity in the Ultra-faint Dwarf Galaxy Tucana II
• Simulating Neutron Star Mergers as r-process Sources in Ultra Faint Dwarf Galaxies
• Evidence for $\gtrsim 4$ Gyr timescales of neutron star mergers from Galactic archaeology
• The role of faint population III supernovae in forming CEMP stars in ultra-faint dwarf galaxies
• Origin of the CEMP-no Group Morphology in the Milky Way
• Chemical Feature of Eu abundance in the Draco dwarf spheroidal galaxy
• Probing the assembly of dwarf galaxies through cosmic time with damped Lyman-$\alpha$ absorption spectroscopy
• New Population Synthesis Models of the Cosmic UV Background

Cited by in corpus (14)

• Signatures of R-process Enrichment in Supernovae from Collapsars
• 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
• Dwarf galaxy formation with and without dark matter-baryon streaming velocities
• Hydrodynamic mixing of accretion disk outflows in collapsars: implications for r-process signatures
• Discovery of an Extremely r-process-enhanced Thin-disk Star with [Eu/H] = +0.78
• The emergence of globular clusters and globular-cluster-like dwarfs
• Triggered Population III star formation: the effect of H${}_2$ self-shielding
• Understanding Stellar Mass-Metallicity and Size Relations in Simulated Ultra-Faint Dwarf Galaxies
• The first r-process enhanced star confirmed to be a member of the Galactic bulge
• Exploring effects of IMF sampling and SN feedback injection on star formation and metallicity in ultra-faint dwarf galaxies
• Segue 2 Recently Collided with the Cetus-Palca Stream: New Opportunities to Constrain Dark Matter in an Ultra-Faint Dwarf
• Structural parameters and possible association of the Ultra-Faint Dwarfs Pegasus III and Pisces II from deep Hubble Space Telescope photometry
• Forward-modelling the Luminosity, Distance, and Size distributions of the Milky Way Satellites