We present the kinematics of 35 highly r-process-enhanced (Eu/Fe ≥ +0.7) metal-poor (−3.8 < Fe/H < −1.4) field stars. We calculate six-dimensional positions and velocities, evaluate energies and ...integrals of motion, and compute orbits for each of these stars using parallaxes and proper motions from the second Gaia data release and published radial velocities. All of these stars have halo kinematics. Most stars (66%) remain in the inner regions of the halo (<13 kpc), and many (51%) have orbits that pass within 2.6 kpc of the Galactic center. Several stars (20%) have orbits that extend beyond 20 kpc, including one with an orbital apocenter larger than the Milky Way virial radius. We apply three clustering methods to search for structure in phase space, and we identify eight groups. No abundances are considered in the clustering process, but the Fe/H dispersions of the groups are smaller than would be expected by random chance. The orbital properties, clustering in phase space and metallicity, and the lack of highly r-process-enhanced stars on disk-like orbits, indicate that such stars likely were accreted from disrupted satellites. Comparison with the galaxy luminosity-metallicity relation suggests MV −9 for most of the progenitor satellites, characteristic of ultra-faint or low-luminosity classical dwarf spheroidal galaxies. Environments with low rates of star formation and Fe production, rather than the nature of the r-process site, may be key to obtaining the Eu/Fe ratios found in highly r-process-enhanced stars.
Abstract
R
-process enhanced stars with Eu/Fe ≥ +0.7 (so-called
r
-II stars) are believed to have formed in an extremely neutron-rich environment in which a rare astrophysical event (e.g., a ...neutron-star merger) occurred. This scenario is supported by the existence of an ultra-faint dwarf galaxy, Reticulum II, where most of the stars are highly enhanced in
r
-process elements. In this scenario, some small fraction of dwarf galaxies around the Milky Way were
r
enhanced. When each r-enhanced dwarf galaxy accreted to the Milky Way, it deposited many
r
-II stars in the Galactic halo with similar orbital actions. To search for the remnants of the
r
-enhanced systems, we analyzed the distribution of the orbital actions of
N
= 161
r
-II stars in the solar neighborhood by using Gaia EDR3 data. Since the observational uncertainty is not negligible, we applied a newly developed greedy optimistic clustering method to the orbital actions of our sample stars. We found six clusters of
r
-II stars that have similar orbits and chemistry, one of which is a new discovery. Given the apparent phase-mixed orbits of the member stars, we interpret that these clusters are good candidates for remnants of completely disrupted
r
-enhanced dwarf galaxies that merged with the ancient Milky Way.
The cosmic dispersion in the abundances of the heavy elements strontium and barium in halo stars is well known. Strontium and barium are detected in most cool, metal-poor giants, but are these ...elements always detectable? To identify stars that could be considered probable candidates for lacking these elements, I examine the stellar abundance data available in the literature for 1148 field stars and 226 stars in dwarf galaxies, 776 of which have metallicities lower than Fe/H < -2.0. Strontium or barium have been detected in all field, globular cluster, and dwarf galaxy environments studied. All upper limits are consistent with the lowest detected ratios of Sr/H and Ba/H, The frequent appearance of these elements raises the intriguing prospect that at least one kind of neutron-capture reaction operates as often as the nucleosynthesis mechanisms that produce lighter elements, such as magnesium, calcium, or iron, although the yields of heavy elements may be more variable.
ABSTRACT The heaviest metals found in stars in most ultra-faint dwarf (UFD) galaxies in the Milky Way halo are generally underabundant by an order of magnitude or more when compared with stars in the ...halo field. Among the heavy elements produced by n-capture reactions, only Sr and Ba can be detected in red giant stars in most UFD galaxies. This limited chemical information is unable to identify the nucleosynthesis process(es) responsible for producing the heavy elements in UFD galaxies. Similar Sr/Ba and Ba/Fe ratios are found in three bright halo field stars, BD−18°5550, CS 22185-007, and CS 22891-200. Previous studies of high-quality spectra of these stars report detections of additional n-capture elements, including Eu. The Eu/Ba ratios in these stars span +0.41 to +0.86. These ratios and others among elements in the rare Earth domain indicate an r-process origin. These stars have some of the lowest levels of r-process enhancement known, with Eu/H spanning −3.95 to −3.32, and they may be considered nearby proxies for faint stars in UFD galaxies. Direct confirmation, however, must await future observations of additional heavy elements in stars in the UFD galaxies themselves.
Abstract
The recent detection of a binary neutron star merger and the clear evidence of the decay of radioactive material observed in this event have, after 60 years of effort, provided an ...astrophysical site for the rapid neutron-capture (
r
-) process which is responsible for the production of the heaviest elements in our universe. However, observations of metal-poor stars with highly enhanced
r
-process elements have revealed abundance patterns suggesting that multiple sites may be involved. To address this issue, and to advance our understanding of the
r
-process, we have initiated an extensive search for bright (
V
< 13.5), very metal-poor (Fe/H < −2) stars in the Milky Way halo exhibiting strongly enhanced
r
-process signatures. This paper presents the first sample collected in the southern hemisphere using the echelle spectrograph on du Pont 2.5 m telescope at Las Campanas Observatory. We have observed and analyzed 107 stars with −3.13 < Fe/H < −0.79. Of those, 12 stars are strongly enhanced in heavy
r
-process elements (
r
-II), 42 stars show moderate enhancements of heavy
r
-process material (
r
-I), and 20 stars exhibit low abundances of the heavy
r
-process elements and higher abundances of the light
r
-process elements relative to the heavy ones (limited-
r
). This search is more successful at finding
r
-process-enhanced stars compared to previous searches, primarily due to a refined target selection procedure that focuses on red giants.
We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor (Fe/H = -1.62 + or - 0.09) star HD 94028. Previous studies revealed that this ...star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., Pb/Fe = +0.79 + or - 0.32) and rapid neutron-capture process (r process; e.g., Eu/Fe = +0.22 + or - 0.12), including unusually large molybdenum (Mo/Fe = +0.97 + or - 0.16) and ruthenium (Ru/Fe = +0.69 + or - 0.17) enhancements. However, this star is not enhanced in carbon (C/Fe = -0.06 + or - 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar As/Ge ratio (+0.99 + or - 0.23) and the enhanced Mo/Fe and Ru/Fe ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar As/Ge ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced As/Ge ratios, hinting that operation of the i process may have been common in the early Galaxy.
Abstract
We observe two metal-poor main-sequence stars that are members of the recently discovered Sylgr stellar stream. We present radial velocities, stellar parameters, and abundances for 13 ...elements derived from high-resolution optical spectra collected using the Magellan Inamori Kyocera Echelle spectrograph. The two stars have identical compositions (within 0.13 dex or 1.2
σ
) among all elements detected. Both stars are very metal-poor (Fe/H = −2.92 ± 0.06). Neither star is highly enhanced in C (C/Fe < +1.0). Both stars are enhanced in the
α
elements Mg, Si, and Ca (
α
/Fe = +0.32 ± 0.06), and the ratios among Na, Al, and all Fe-group elements are typical for other stars in the halo and ultra-faint and dwarf spheroidal galaxies at this metallicity. Sr is mildly enhanced (Sr/Fe = +0.22 ± 0.11), but Ba is not enhanced (Ba/Fe < −0.4), indicating that these stars do not contain high levels of neutron-capture elements. The Li abundances match those found in metal-poor unevolved field stars and globular clusters (GCs) (log
ϵ
(Li) = 2.05 ± 0.07), which implies that environment is not a dominant factor in determining the Li content of metal-poor stars. The chemical compositions of these two stars cannot distinguish whether the progenitor of the Sylgr stream was a dwarf galaxy or a GC. If the progenitor was a dwarf galaxy, the stream may originate from a dense region such as a nuclear star cluster. If the progenitor was a GC, it would be the most metal-poor GC known.
Abstract
We present new abundances derived from Cu
i
, Cu
ii
, Zn
i
, and Zn
ii
lines in six warm (5766 ≤
≤ 6427 K), metal-poor (−2.50 ≤ Fe/H ≤ −0.95) dwarf and subgiant (3.64 ≤ log
g
≤ 4.44) stars. ...These abundances are derived from archival high-resolution ultraviolet spectra from the Space Telescope Imaging Spectrograph on board the
Hubble Space Telescope
and ground-based optical spectra from several observatories. Ionized Cu and Zn are the majority species, and abundances derived from Cu
ii
and Zn
ii
lines should be largely insensitive to departures from local thermodynamic equilibrium (LTE). We find good agreement between the Zn/H ratios derived separately from Zn
i
and Zn
ii
lines, suggesting that departures from LTE are, at most, minimal (≲0.1 dex). We find that the Cu/H ratios derived from Cu
ii
lines are 0.36 ± 0.06 dex larger than those derived from Cu
i
lines in the most metal-poor stars (Fe/H < −1.8), suggesting that LTE underestimates the Cu abundance derived from Cu
i
lines. The deviations decrease in more metal-rich stars. Our results validate previous theoretical non-LTE calculations for both Cu and Zn, supporting earlier conclusions that the enhancement of Zn/Fe in metal-poor stars is legitimate, and the deficiency of Cu/Fe in metal-poor stars may not be as large as previously thought.
Abstract
We report the discovery of 30 stars with extreme space velocities (≳480
) in the Gaia-DR2 archive. These stars are a subset of 1743 stars with high-precision parallax, large tangential ...velocity (
v
tan
> 300
), and measured line-of-sight velocity in DR2. By tracing the orbits of the stars back in time, we find at least one of them is consistent with having been ejected by the supermassive black hole at the Galactic Center. Another star has an orbit that passed near the Large Magellanic Cloud about 200 Myr ago. Unlike previously discovered blue hypervelocity stars, our sample is metal-poor (−1.5 < Fe/H < −1.0) and quite old (>1
). We discuss possible mechanisms for accelerating old stars to such extreme velocities. The high observed space density of this population, relative to potential acceleration mechanisms, implies that these stars are probably bound to the Milky Way (MW). If they are bound, the discovery of this population would require a local escape speed of around ∼600
and consequently imply a virial mass of
M
200
∼ 1.4 × 10
12
M
⊙
for the MW.
The elements germanium (Ge, Z = 32), arsenic (As, Z = 33), and selenium (Se, Z = 34) span the transition from charged-particle or explosive synthesis of the iron-group elements to neutron-capture ...synthesis of heavier elements. Among these three elements, only the chemical evolution of germanium has been studied previously. Here we use archive observations made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope and observations from several ground-based facilities to study the chemical enrichment histories of seven stars with metallicities -2.6 < or =, slant Fe/H < or =, slant -0.4. We perform a standard abundance analysis of germanium, arsenic, selenium, and several other elements produced by neutron-capture reactions. When combined with previous derivations of germanium abundances in metal-poor stars, our sample reveals an increase in the Ge/Fe ratios at higher metallicities. This could mark the onset of the weak s-process contribution to germanium. In contrast, the As/Fe and Se/Fe ratios remain roughly constant. These data do not directly indicate the origin of germanium, arsenic, and selenium at low metallicity, but they suggest that the weak and main components of the s-process are not likely sources.
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