The r-process signature found in the ultra-faint dwarf galaxy Reticulum II

We present the non-local thermodynamic equilibrium (NLTE) abundances of up to 10 chemical species in a sample of 59 very metal-poor (VMP, −4 ≤ [Fe/H] ≾−2) stars in seven dwarf spheroidal galaxies (dSphs) and in the Milky Way (MW) halo. Our results are based on high-resolution spectroscopic datasets and homogeneous and accurate atmospheric parameters determined in Paper I. We show that once the NLTE effects are properly taken into account, all massive galaxies in our sample, that is, the MW halo and the classical dSphs Sculptor, Ursa Minor, Sextans, and Fornax, reveal a similar plateau at [α/Fe] ≃ 0.3 for each of the α-process elements: Mg, Ca, and Ti. We put on a firm ground the evidence for a decline in α/Fe with increasing metallicity in the Boötes I ultra-faint dwarf galaxy (UFD), that is most probably due to the ejecta of type Ia supernovae. For Na/Fe, Na/Mg, and Al/Mg, the MW halo and all dSphs reveal indistinguishable trends with metallicity, suggesting that the processes of Na and Al synthesis are identical in all systems, independent of their mass. The dichotomy in the [Sr/Ba] versus [Ba/H] diagram is observed in the classical dSphs, similarly to the MW halo, calling for two different nucleosynthesis channels for Sr. We show that Sr in the massive galaxies is well correlated with Mg suggesting a strong link to massive stars and that its origin is essentially independent of Ba, for most of the [Ba/H] range. Our three UFDs, that is Boötes I, UMa II, and Leo IV, are depleted in Sr and Ba relative to Fe and Mg, with very similar ratios of [Sr/Mg] ≃−1.3 and [Ba/Mg] ≃−1 on the entire range of their Mg abundances. The subsolar Sr/Ba ratios of Boötes I and UMa II indicate a common r-process origin of their neutron-capture elements. Sculptor remains the classical dSph, in which the evidence for inhomogeneous mixing in the early evolution stage, at [Fe/H] <−2, is the strongest.

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Neutron star binary orbits in their host potential: effect on early r-process enrichment

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2554 ◽

2019 ◽

Vol 490 (1) ◽

pp. 296-311 ◽

Cited By ~ 7

Author(s):

Matteo Bonetti ◽

Albino Perego ◽

Massimo Dotti ◽

Gabriele Cescutti

Keyword(s):

Neutron Star ◽

Dwarf Galaxy ◽

Supernova Explosion ◽

Potential Effect ◽

Host Galaxy ◽

Host Galaxies ◽

Gas Reservoir ◽

Star Binary ◽

R Process ◽

Process Elements

ABSTRACT Coalescing neutron star binary (NSB) systems are primary candidates for r-process enrichment of galaxies. The recent detection of r-process elements in ultrafaint dwarf (UFD) galaxies and the abundances measured in classical dwarfs challenges the NSB merger scenario both in terms of coalescence time-scales and merger locations. In this paper, we focus on the dynamics of NSBs in the gravitational potentials of different types of host galaxies and on its impact on the subsequent galactic enrichment. We find that, for a ∼t−1 delay time distribution, even when receiving a low kick (∼10 km s−1) from the second supernova explosion, in shallow dwarf galaxy potentials NSBs tend to merge with a large off-set from the host galaxy. This results in a significant geometrical dilution of the amount of produced r-process elements that fall back and pollute the host galaxy gas reservoir. The combination of dilution and small number statistics produces a large scatter in the expected r-process enrichment within a single UFD or classical dwarf galaxy. Comparison between our results and observed europium abundances reveals a tension that even a systematic choice of optimistic parameters in our models cannot release. Such a discrepancy could point to the need of additional r-process production sites that suffer less severe dilution or to a population of extremely fast merging binaries.

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The S2 Stream: the shreds of a primitive dwarf galaxy.*

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3250 ◽

2020 ◽

Author(s):

David S Aguado ◽

G C Myeong ◽

Vasily Belokurov ◽

N Wyn Evans ◽

Sergey E Koposov ◽

...

Keyword(s):

Dwarf Galaxy ◽

Total Sample ◽

Galactic Disc ◽

Capture Process ◽

Abundance Patterns ◽

Cold Stream ◽

Low Metallicity ◽

R Process ◽

Process Elements ◽

Formation Efficiency

Abstract The S2 stream is a kinematically cold stream that is plunging downwards through the Galactic disc. It may be part of a hotter and more diffuse structure called the Helmi stream. We present a multi-instrument chemical analysis of the stars in the metal-poor S2 stream using both high- and low-resolution spectroscopy, complemented with a re-analysis of the archival data to give a total sample of 62 S2 members. Our high-resolution program provides α-elements (C, Mg, Si, Ca and Ti), iron-peak elements (V, Cr, Mn, Fe, Ni), n-capture process elements (Sr, Ba) and other elements such as Li, Na, Al, and Sc for a subsample of S2 objects. We report coherent abundance patterns over a large metallicity spread (∼1 dex) confirming that the S2 stream was produced by a disrupted dwarf galaxy. The combination of S2’s α-elements displays a mildly decreasing trend with increasing metallicity which can be tentatively interpreted as a “knee” at [Fe/H]<−2. At the low metallicity end, the n-capture elements in S2 may be dominated by r-process production however several stars are Ba-enhanced, but unusually poor in Sr. Moreover, some of the low-[Fe/H] stars appear to be carbon-enhanced. We interpret the observed abundance patterns with the help of chemical evolution models that demonstrate the need for modest star-formation efficiency and low wind efficiency confirming that the progenitor of S2 was a primitive dwarf galaxy.

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The Faintest Dwarf Galaxies

Annual Review of Astronomy and Astrophysics ◽

10.1146/annurev-astro-091918-104453 ◽

2019 ◽

Vol 57 (1) ◽

pp. 375-415 ◽

Cited By ~ 63

Author(s):

Joshua D. Simon

Keyword(s):

Dark Matter ◽

Binary Stars ◽

Luminosity Function ◽

Milky Way ◽

Dwarf Galaxy ◽

Sloan Digital Sky Survey ◽

Stellar Kinematics ◽

Chemical Abundance ◽

Abundance Patterns ◽

R Process

The lowest luminosity ([Formula: see text] L[Formula: see text]) Milky Way satellite galaxies represent the extreme lower limit of the galaxy luminosity function. These ultra-faint dwarfs are the oldest, most dark matter–dominated, most metal-poor, and least chemically evolved stellar systems known. They therefore provide unique windows into the formation of the first galaxies and the behavior of dark matter on small scales. In this review, we summarize the discovery of ultra-faint dwarfs in the Sloan Digital Sky Survey in 2005 and the subsequent observational and theoretical progress in understanding their nature and origin. We describe their stellar kinematics, chemical abundance patterns, structural properties, stellar populations, orbits, and luminosity function, as well as what can be learned from each type of measurement. We conclude the following: ▪ In most cases, the stellar velocity dispersions of ultra-faint dwarfs are robust against systematic uncertainties such as binary stars and foreground contamination. ▪ The chemical abundance patterns of stars in ultra-faint dwarfs require two sources of r-process elements, one of which can likely be attributed to neutron star mergers. ▪ Even under conservative assumptions, only a small fraction of ultra-faint dwarfs may have suffered significant tidal stripping of their stellar components. ▪ Determining the properties of the faintest dwarfs out to the virial radius of the Milky Way will require very large investments of observing time with future telescopes. Finally, we offer a look forward at the observations that will be possible with future facilities as the push toward a complete census of the Local Group dwarf galaxy population continues.

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Distribution of R- and S-Process Elements in the Galactic Halo

Symposium - International Astronomical Union ◽

10.1017/s0074180900035555 ◽

1988 ◽

Vol 132 ◽

pp. 501-506

Author(s):

C. Sneden ◽

C. A. Pilachowski ◽

K. K. Gilroy ◽

J. J. Cowan

Keyword(s):

Heavy Element ◽

Galactic Halo ◽

Low Noise ◽

Heavy Elements ◽

Process Synthesis ◽

Element Abundances ◽

Halo Stars ◽

Abundance Patterns ◽

R Process ◽

Process Elements

Current observational results for the abundances of the very heavy elements (Z>30) in Population II halo stars are reviewed. New high resolution, low noise spectra of many of these extremely metal-poor stars reveal general consistency in their overall abundance patterns. Below Galactic metallicities of [Fe/H] Ã −2, all of the very heavy elements were manufactured almost exclusively in r-process synthesis events. However, there is considerable star-to-star scatter in the overall level of very heavy element abundances, indicating the influence of local supernovas on element production in the very early, unmixed Galactic halo. The s-process appears to contribute substantially to stellar abundances only in stars more metal-rich than [Fe/H] Ã −2.

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Nuclear Processes Related to the Ap Stars and the Heaviest Chemical Elements

International Astronomical Union Colloquium ◽

10.1017/s0252921100080520 ◽

1976 ◽

Vol 32 ◽

pp. 169-182

Author(s):

B. Kuchowicz

Keyword(s):

Nuclear Physics ◽

Nuclear Reactions ◽

Chemical Elements ◽

Fission Products ◽

Abundance Pattern ◽

Isotopic Shifts ◽

Processed Material ◽

R Process ◽

Ap Stars ◽

Nuclear Processes

SummaryIsotopic shifts in the lines of the heavy elements in Ap stars, and the characteristic abundance pattern of these elements point to the fact that we are observing mainly the products of rapid neutron capture. The peculiar A stars may be treated as the show windows for the products of a recent r-process in their neighbourhood. This process can be located either in Supernovae exploding in a binary system in which the present Ap stars were secondaries, or in Supernovae exploding in young clusters. Secondary processes, e.g. spontaneous fission or nuclear reactions with highly abundant fission products, may occur further with the r-processed material in the surface of the Ap stars. The role of these stars to the theory of nucleosynthesis and to nuclear physics is emphasized.

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