scholarly journals Neutron star mergers and rare core-collapse supernovae as sources of r-process enrichment in simulated galaxies

2020 ◽  
Vol 494 (4) ◽  
pp. 4867-4883 ◽  
Author(s):  
Freeke van de Voort ◽  
Rüdiger Pakmor ◽  
Robert J J Grand ◽  
Volker Springel ◽  
Facundo A Gómez ◽  
...  
Keyword(s):  
Neutron Star ◽  
Milky Way ◽  
Small Variation ◽  
Abundance Ratio ◽  
Core Collapse ◽  
Binary Neutron Star ◽  
Low Metallicity ◽  
Process Event ◽  
R Process ◽  
Process Elements

ABSTRACT We use cosmological, magnetohydrodynamical simulations of Milky Way-mass galaxies from the Auriga project to study their enrichment with rapid neutron capture (r-process) elements. We implement a variety of enrichment models from both binary neutron star mergers and rare core-collapse supernovae. We focus on the abundances of (extremely) metal-poor stars, most of which were formed during the first ∼Gyr of the Universe in external galaxies and later accreted on to the main galaxy. We find that the majority of metal-poor stars are r-process enriched in all our enrichment models. Neutron star merger models result in a median r-process abundance ratio, which increases with metallicity, whereas the median trend in rare core-collapse supernova models is approximately flat. The scatter in r-process abundance increases for models with longer delay times or lower rates of r-process-producing events. Our results are nearly perfectly converged, in part due to the mixing of gas between mesh cells in the simulations. Additionally, different Milky Way-mass galaxies show only small variation in their respective r-process abundance ratios. Current (sparse and potentially biased) observations of metal-poor stars in the Milky Way seem to prefer rare core-collapse supernovae over neutron star mergers as the dominant source of r-process elements at low metallicity, but we discuss possible caveats to our models. Dwarf galaxies that experience a single r-process event early in their history show highly enhanced r-process abundances at low metallicity, which is seen both in observations and in our simulations. We also find that the elements produced in a single event are mixed with ≈108 M⊙ of gas relatively quickly, distributing the r-process elements over a large region.

scholarly journals The Dynamics of Binary Neutron Star Mergers and GW170817

2020 ◽  
Vol 70 (1) ◽  
pp. 95-119 ◽  
Author(s):  
David Radice ◽  
Sebastiano Bernuzzi ◽  
Albino Perego
Keyword(s):  
Neutron Star ◽  
Nuclear Astrophysics ◽  
Theoretical Models ◽  
Physical Processes ◽  
Binary Neutron Star ◽  
Compact Binary ◽  
Open Questions ◽  
Binary Mergers ◽  
R Process ◽  
Process Elements

With the first observation of a binary neutron star merger through gravitational waves and light, GW170817, compact binary mergers have now taken the center stage in nuclear astrophysics. They are thought to be one of the main astrophysical sites of production of r-process elements, and merger observations have become a fundamental tool to constrain the properties of matter. Here, we review our current understanding of the dynamics of neutron star mergers in general and of GW170817 in particular. We discuss the physical processes governing the inspiral, merger, and postmerger evolution, and we highlight the connections between these processes, the dynamics, and the multimessenger observables. Finally, we discuss open questions and issues in the field and the need to address them through a combination of better theoretical models and new observations.

scholarly journals Chemical Evolution of R-process Elements in the Hierarchical Galaxy Formation

2015 ◽  
Vol 11 (S317) ◽  
pp. 318-319
Author(s):  
Yutaka Komiya ◽  
Toshikazu Shigeyama
Keyword(s):  
Neutron Star ◽  
Chemical Evolution ◽  
Galaxy Formation ◽  
Milky Way ◽  
Galactic Chemical Evolution ◽  
Halo Stars ◽  
R Process ◽  
Process Elements ◽  
Milky Way Halo

AbstractThe main astronomical source of r-process elements has not yet been identified. One plausible site is neutron star mergers (NSMs). From the perspective of Galactic chemical evolution, however, it has been pointed out that the NSM scenario is incompatible with observations. Recently, Tsujimoto & Shigeyama (2014) pointed out that NSM ejecta can spread into much larger volume than ejecta from a supernova. We re-examine the chemical evolution of r-process elements under the NSM scenario considering this difference in propagation of the ejecta. We find that the NSM scenario can be compatible with the observed abundances of the Milky Way halo stars.

scholarly journals The GALAH survey: temporal chemical enrichment of the galactic disc

2019 ◽  
Vol 491 (2) ◽  
pp. 2043-2056 ◽  
Author(s):  
Jane Lin ◽  
Martin Asplund ◽  
Yuan-Sen Ting ◽  
Luca Casagrande ◽  
Sven Buder ◽  
...  
Keyword(s):  
Main Sequence ◽  
Abundance Ratio ◽  
Core Collapse ◽  
Chemical Enrichment ◽  
R Process ◽  
Short Time ◽  
Process Elements ◽  
Process Element ◽  
Larger Sample ◽  
Short Time Delay

ABSTRACT We present isochrone ages and initial bulk metallicities ($\rm [Fe/H]_{bulk}$, by accounting for diffusion) of 163 722 stars from the GALAH Data Release 2, mainly composed of main-sequence turn-off stars and subgiants ($7000\, \mathrm{ K}> T_{\mathrm{ eff}}> 4000\, \mathrm{ K}$ and $\log g>3$ dex). The local age–metallicity relationship (AMR) is nearly flat but with significant scatter at all ages; the scatter is even higher when considering the observed surface abundances. After correcting for selection effects, the AMR appears to have intrinsic structures indicative of two star formation events, which we speculate are connected to the thin and thick discs in the solar neighbourhood. We also present abundance ratio trends for 16 elements as a function of age, across different $\rm [Fe/H]_{bulk}$ bins. In general, we find the trends in terms of [X/Fe] versus age from our far larger sample to be compatible with studies based on small (∼100 stars) samples of solar twins, but we now extend them to both sub- and supersolar metallicities. The α-elements show differing behaviour: the hydrostatic α-elements O and Mg show a steady decline with time for all metallicities, while the explosive α-elements Si, Ca, and Ti are nearly constant during the thin-disc epoch (ages $\lesssim \! 12$ Gyr). The s-process elements Y and Ba show increasing [X/Fe] with time while the r-process element Eu has the opposite trend, thus favouring a primary production from sources with a short time delay such as core-collapse supernovae over long-delay events such as neutron star mergers.

scholarly journals Evidence for ≳4 Gyr timescales of neutron star mergers from Galactic archaeology

2020 ◽  
Vol 634 ◽  
pp. L2 ◽  
Author(s):  
Á. Skúladóttir ◽  
S. Salvadori
Keyword(s):  
Neutron Star ◽  
Milky Way ◽  
Dwarf Galaxy ◽  
Direct Detection ◽  
Compact Object ◽  
Core Collapse ◽  
Chemical Abundances ◽  
Capture Process ◽  
Process Elements ◽  
First Time

The nucleosynthetic site of the rapid (r) neutron-capture process is currently being debated. The direct detection of the neutron star merger GW170817, through gravitational waves and electromagnetic radiation, has confirmed such events as important sources of the r-process elements. However, chemical evolution models are not able to reproduce the observed chemical abundances in the Milky Way when neutron star mergers are assumed to be the only r-process site and realistic time distributions of such events are taken into account. Now for the first time, we combine all the available observational evidence of the Milky Way and its dwarf galaxy satellites to show that the data can only be explained if there are (at least) two distinct r-process sites: a quick source with timescales comparable to core-collapse supernovae, tquick ≲ 108 yr, and a delayed source with characteristic timescales tdelayed ≳ 4 Gyr. The delayed r-process source most probably originates in neutron star mergers, as the timescale fits well with that estimated for GW170817. Given the short timescales of the quick source, it is likely associated with massive stars, though a specific fast-track channel for compact object mergers cannot be excluded at this point. Our approach demonstrates that only by looking at all the available data will we be able to solve the puzzle that is the r-process.

scholarly journals HERBS II: Detailed chemical compositions of Galactic bulge stars

2019 ◽  
Vol 486 (4) ◽  
pp. 5349-5361 ◽  
Author(s):  
L Duong ◽  
M Asplund ◽  
D M Nataf ◽  
K C Freeman ◽  
M Ness
Keyword(s):  
Globular Clusters ◽  
Milky Way ◽  
Abundance Ratio ◽  
Chemical Compositions ◽  
Core Collapse ◽  
Detailed Chemistry ◽  
Red Clump ◽  
Red Giant ◽  
R Process ◽  
Detailed Chemical

ABSTRACT This work explores the detailed chemistry of the Milky Way bulge using the HERMES spectrograph on the Anglo-Australian Telescope. Here, we present the abundance ratios of 13 elements for 832 red giant branch and clump stars along the minor bulge axis at latitudes b = −10○, − 7.5○, and −5○. Our results show that none of the abundance ratios vary significantly with latitude. We also observe disc-like [Na/Fe] abundance ratios, which indicate that the bulge does not contain helium-enhanced populations as observed in some globular clusters. Helium enhancement is therefore not the likely explanation for the double red-clump observed in the bulge. We confirm that bulge stars mostly follow abundance trends observed in the disc. However, this similarity is not confirmed across all elements and metallicity regimes. The more metal-poor bulge population at [Fe/H] ≲ − 0.8 is enhanced in the elements associated with core collapse supernovae (SNeII). In addition, the [La/Eu] abundance ratio suggests higher r-process contribution, and likely higher star formation in the bulge compared to the disc. This highlights the complex evolution in the bulge, which should be investigated further, both in terms of modelling; and with additional observations of the inner Galaxy.

scholarly journals Turbulent mixing of r-process elements in the Milky Way

2020 ◽  
Vol 496 (2) ◽  
pp. 1891-1901 ◽  
Author(s):  
Paz Beniamini ◽  
Kenta Hotokezaka
Keyword(s):  
Solar System ◽  
Milky Way ◽  
Gas Diffusion ◽  
Star Formation History ◽  
Model Parameters ◽  
Early Solar System ◽  
Element Abundances ◽  
Process Event ◽  
R Process ◽  
Process Elements

ABSTRACT We study turbulent gas diffusion affects on r-process abundances in Milky Way stars, by a combination of an analytical approach and a Monte Carlo simulation. Higher r-process event rates and faster diffusion, lead to more efficient mixing corresponding to a reduced scatter of r-process abundances and causing r-process enriched stars to start appearing at lower metallicities. We use three independent observations to constrain the model parameters: (i) the scatter of radioactively stable r-process element abundances, (ii) the largest r-process enrichment values observed in any solar neighborhood stars, and (iii) the isotope abundance ratios of different radioactive r-process elements (244Pu/238U and 247Cm/238U) at the early Solar system as compared to their formation. Our results indicate that the Galactic r-process rate and the diffusion coefficient are respectively r < 4 × 10−5 yr−1, D > 0.1 kpc2 Gyr−1 (r < 4 × 10−6 yr−1, D > 0.5 kpc2 Gyr−1 for collapsars or similarly prolific r-process sources) with allowed values satisfying an approximate anticorrelation such that D ≈ r−2/3, implying that the time between two r-process events that enrich the same location in the Galaxy, is τmix ≈ 100−200 Myr. This suggests that a fraction of ∼0.8 (∼0.5) of the observed 247Cm (244Pu) abundance is dominated by one r-process event in the early Solar system. Radioactively stable element abundances are dominated by contributions from ∼10 different events in the early Solar system. For metal poor stars (with [Fe/H] ≲ −2), their r-process abundances are dominated by either a single or several events, depending on the star formation history.

scholarly journals Neutron Star Mergers Might Not Be the Only Source of r-process Elements in the Milky Way

2019 ◽  
Vol 875 (2) ◽  
pp. 106 ◽  
Author(s):  
Benoit Côté ◽  
Marius Eichler ◽  
Almudena Arcones ◽  
Camilla J. Hansen ◽  
Paolo Simonetti ◽  
...  
Keyword(s):  
Neutron Star ◽  
Milky Way ◽  
R Process ◽  
Process Elements

scholarly journals Chemical evolution of r-process elements in the Draco dwarf spheroidal galaxy

2015 ◽  
Vol 11 (S317) ◽  
pp. 310-311
Author(s):  
M. N. Ishigaki ◽  
T. Tsujimoto ◽  
T. Shigeyama ◽  
W. Aoki
Keyword(s):  
Neutron Star ◽  
High Resolution ◽  
Neutron Capture ◽  
Milky Way ◽  
Origin And Evolution ◽  
Dwarf Spheroidal Galaxies ◽  
Giant Stars ◽  
Upper Limits ◽  
R Process ◽  
Process Elements

AbstractA dominant astrophysical site for r-process, which is responsible for producing heavy neutron-capture elements, is unknown. Dwarf spheroidal galaxies around the Milky Way halo provide ideal laboratories to investigate the origin and evolution of r-process elements. We carried out high-resolution spectroscopic observations of three giant stars in the Draco dwarf spheroidal galaxy to estimate their europium abundances. We found that the upper-limits of [Eu/H] are very low in the range [Fe/H] < −2, while this ratio is nearly constant at higher metallicities. This trend is not well reproduced with models which assume that Eu is produced together with Fe by SNe, and may suggest the contribution from other objects such as neutron-star mergers.

scholarly journals The AMBRE Project: r-process elements in the Milky Way thin and thick discs

2018 ◽  
Vol 619 ◽  
pp. A143 ◽  
Author(s):  
G. Guiglion ◽  
P. de Laverny ◽  
A. Recio-Blanco ◽  
N. Prantzos
Keyword(s):  
Chemical Evolution ◽  
Milky Way ◽  
Element Abundances ◽  
Thick Disc ◽  
Thin Disc ◽  
Solar Metallicity ◽  
R Process ◽  
Process Elements ◽  
Process Element ◽  
The Eu

Context. The chemical evolution of neutron capture elements in the Milky Way disc is still a matter of debate. There is a lack of statistically significant catalogues of such element abundances, especially those of the r-process. Aims. We aim to understand the chemical evolution of r-process elements in Milky Way disc. We focus on three pure r-process elements Eu, Gd, and Dy. We also consider a pure s-process element, Ba, in order to disentangle the different nucleosynthesis processes. Methods. We take advantage of high-resolution FEROS, HARPS, and UVES spectra from the ESO archive in order to perform a homogeneous analysis on 6500 FGK Milky Way stars. The chemical analysis is performed thanks to the automatic optimization pipeline GAUGUIN. We present abundances of Ba (5057 stars), Eu (6268 stars), Gd (5431 stars), and Dy (5479 stars). Based on the [α/Fe] ratio determined previously by the AMBRE Project, we chemically characterize the thin and the thick discs, and a metal-rich α-rich population. Results. First, we find that the [Eu/Fe] ratio follows a continuous sequence from the thin disc to the thick disc as a function of the metallicity. Second, in thick disc stars, the [Eu/Ba] ratio is found to be constant, while the [Gd/Ba] and [Dy/Ba] ratios decrease as a function of the metallicity. These observations clearly indicate a different nucleosynthesis history in the thick disc between Eu and Gd–Dy. The [r/Fe] ratio in the thin disc is roughly around +0.1 dex at solar metallicity, which is not the case for Ba. We also find that the α-rich metal-rich stars are also enriched in r-process elements (like thick disc stars), but their [Ba/Fe] is very different from thick disc stars. Finally, we find that the [r/α] ratio tends to decrease with metallicity, indicating that supernovae of different properties probably contribute differently to the synthesis of r-process elements and α-elements. Conclusions. We provide average abundance trends for [Ba/Fe] and [Eu/Fe] with rather small dispersions, and for the first time for [Gd/Fe] and [Dy/Fe]. This data may help to constrain chemical evolution models of Milky Way r- and s-process elements and the yields of massive stars. We emphasize that including yields of neutron-star or black hole mergers is now crucial if we want to quantitatively compare observations to Galactic chemical evolution models.

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