Multimessenger observations of the neutron star merger GW170817 and its kilonova proved that neutron star mergers can synthesize large quantities of r-process elements. If neutron star mergers in ...fact dominate all r-process element production, then the distribution of kilonova ejecta compositions should match the distribution of r-process abundance patterns observed in stars. The lanthanide fraction (XLa) is a measurable quantity in both kilonovae and metal-poor stars, but it has not previously been explicitly calculated for stars. Here we compute the lanthanide fraction distribution of metal-poor stars (Fe/H < − 2.5) to enable comparison to current and future kilonovae. The full distribution peaks at log XLa ∼ −1.8, but r-process-enhanced stars (Eu/Fe > 0.7) have distinctly higher lanthanide fractions: . We review observations of GW170817 and find general consensus that the total , somewhat lower than the typical metal-poor star and inconsistent with the most highly r-enhanced stars. For neutron star mergers to remain viable as the dominant r-process site, future kilonova observations should be preferentially lanthanide-rich (including a population of ∼10% with ). These high-XLa kilonovae may be fainter and more rapidly evolving than GW170817, posing a challenge for discovery and follow-up observations. Both optical and (mid-)infrared observations will be required to robustly constrain kilonova lanthanide fractions. If such high-XLa kilonovae are not found in the next few years, that likely implies that the stars with the highest r-process enhancements have a different origin for their r-process elements.
Abstract
We present high-resolution spectroscopy of four stars in two candidate ultra-faint dwarf galaxies (UFDs), Grus I (Gru I) and Triangulum II (Tri II). Neither object currently has a clearly ...determined velocity dispersion, placing them in an ambiguous region of parameter space between dwarf galaxies and globular clusters (GCs). No significant metallicity difference is found for the two Gru I stars, but both stars are deficient in neutron-capture elements. We verify previous results that Tri II displays significant spreads in metallicity and
α
/Fe. Neutron-capture elements are not detected in our Tri II data, but we place upper limits at the lower envelope of Galactic halo stars, consistent with previous very low detections. Stars with similarly low neutron-capture element abundances are common in UFDs but rare in other environments. This signature of low neutron-capture element abundances traces chemical enrichment in the least massive star-forming dark matter halos and further shows that the dominant sources of neutron-capture elements in metal-poor stars are rare. In contrast, all known GCs have similar ratios of neutron-capture elements to those of halo stars, suggesting that GCs do not form at the centers of their own dark matter halos. The low neutron-capture element abundances may be the strongest evidence that Gru I and Tri II are (or once were) galaxies rather than GCs, and we expect future observations of these systems to robustly find nonzero velocity dispersions or signs of tidal disruption. However, the nucleosynthetic origin of this low neutron-capture element floor remains unknown.
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.
ABSTRACT We investigate anew the distribution of absolute carbon abundance, A(C) = log ϵ(C), for carbon-enhanced metal-poor (CEMP) stars in the halo of the Milky Way, based on high-resolution ...spectroscopic data for a total sample of 305 CEMP stars. The sample includes 147 CEMP-s (and CEMP-r/s) stars, 127 CEMP-no stars, and 31 CEMP stars that are unclassified, based on the currently employed Ba/Fe criterion. We confirm previous claims that the distribution of A(C) for CEMP stars is (at least) bimodal, with newly determined peaks centered on A(C) = 7.96 (the high-C region) and A(C) = 6.28 (the low-C region). A very high fraction of CEMP-s (and CEMP-r/s) stars belongs to the high-C region, while the great majority of CEMP-no stars resides in the low-C region. However, there exists complexity in the morphology of the A(C)-Fe/H space for the CEMP-no stars, a first indication that more than one class of first-generation stellar progenitors may be required to account for their observed abundances. The two groups of CEMP-no stars we identify exhibit clearly different locations in the A(Na)-A(C) and A(Mg)-A(C) spaces, also suggesting multiple progenitors. The clear distinction in A(C) between the CEMP-s (and CEMP-r/s) stars and the CEMP-no stars appears to be as successful, and likely more astrophysically fundamental, for the separation of these sub-classes as the previously recommended criterion based on Ba/Fe (and Ba/Eu) abundance ratios. This result opens the window for its application to present and future large-scale low- and medium-resolution spectroscopic surveys.
Star clusters, particularly those objects in the disk-bulge-halo interface are as yet poorly charted, despite the fact that they carry important information about the formation and the structure of ...the Milky Way. Here, we present a detailed chemical abundance study of the recently discovered object Gaia 1. Photometry has previously suggested it as an intermediate-age, moderately metal-rich system, although the exact values for its age and metallicity remained ambiguous in the literature. We measured detailed chemical abundances of 14 elements in four red giant members, from high-resolution (R = 25 000) spectra that firmly establish Gaia 1 as an object associated with the thick disk. The resulting mean Fe abundance is −0.62 ± 0.03(stat.)± 0.10(sys.) dex, which is more metal-poor than indicated by previous spectroscopy from the literature, but it is fully in line with values from isochrone fitting. We find that Gaia 1 is moderately enhanced in the α-elements, which allowed us to consolidate its membership with the thick disk via chemical tagging. The cluster’s Fe-peak and neutron-capture elements are similar to those found across the metal-rich disks, where the latter indicate some level of s-process activity. No significant spread in iron nor in other heavy elements was detected, whereas we find evidence of light-element variations in Na, Mg, and Al. Nonetheless, the traditional Na-O and Mg-Al (anti-)correlations, typically seen in old globular clusters, are not seen in our data. This confirms that Gaia 1 is rather a massive and luminous open cluster than a low-mass globular cluster. Finally, orbital computations of the target stars bolster our chemical findings of Gaia 1’s present-day membership with the thick disk, even though it remains unclear which mechanisms put it in that place.
Abstract
Extensive progress has recently been made in our understanding of heavy-element production via the
r
-process in the universe, specifically with the first observed neutron star binary merger ...(NSBM) event associated with the gravitational-wave signal detected by LIGO, GW170817. The chemical abundance patterns of metal-poor
r
-process-enhanced stars provide key evidence for the dominant site(s) of the
r
-process and whether NSBMs are sufficiently frequent or prolific
r
-process sources to be responsible for the majority of
r
-process material in the universe. We present atmospheric stellar parameters (using a nonlocal thermodynamic equilibrium analysis) and abundances from a detailed analysis of 141 metal-poor stars carried out as part of the
R
-Process Alliance (RPA) effort. We obtained high-resolution “snapshot” spectroscopy of the stars using the MIKE spectrograph on the 6.5 m Magellan Clay telescope at Las Campanas Observatory in Chile. We find 10 new highly enhanced
r
-II (with Eu/Fe > +1.0), 62 new moderately enhanced
r
-I (+0.3 < Eu/Fe ≤ +1.0), and 17 new limited-
r
(Eu/Fe < +0.3) stars. Among those, we find 17 new carbon-enhanced metal-poor (CEMP) stars, of which five are CEMP-no. We also identify one new
s
-process-enhanced (Ba/Eu > +0.5) and five new
r
/
s
(0.0 < Ba/Eu < +0.5) stars. In the process, we discover a new ultra-metal-poor (UMP) star at Fe/H = −4.02. One of the
r
-II stars shows a deficit in
α
and Fe-peak elements, typical of dwarf galaxy stars. Our search for
r
-process-enhanced stars by RPA efforts has already roughly doubled the known
r
-process sample.
We present results from a medium-resolution (R ∼ 2000) spectroscopic follow-up campaign of 1694 bright (V < 13.5), very metal-poor star candidates from the RAdial Velocity Experiment (RAVE). Initial ...selection of the low-metallicity targets was based on the stellar parameters published in RAVE Data Releases 4 and 5. Follow up was accomplished with the Gemini-N and Gemini-S, the ESO/NTT, the KPNO/Mayall, and the SOAR telescopes. The wavelength coverage for most of the observed spectra allows for the determination of carbon and -element abundances, which are crucial for considering the nature and frequency of the carbon-enhanced metal-poor (CEMP) stars in this sample. We find that 88% of the observed stars have ≤ −1.0, 61% have ≤ −2.0, and 3% have ≤ −3.0 (with four stars at ≤ −3.5). There are 306 CEMP star candidates in this sample, and we identify 169 CEMP Group I, 131 CEMP Group II, and 6 CEMP Group III stars from the A(C) versus Fe/H diagram. Inspection of the abundance ratios reveals that five of the CEMP Group II stars can be classified as "mono-enriched second-generation" stars. Gaia DR1 matches were found for 734 stars, and we show that transverse velocities can be used as a confirmatory selection criteria for low-metallicity candidates. Selected stars from our validated list are being followed-up with high-resolution spectroscopy to reveal their full chemical-abundance patterns for further studies.
We present high-resolution Magellan/MIKE spectroscopy of 42 red giant stars in seven stellar streams confirmed by the Southern Stellar Stream Spectroscopic Survey (S5): ATLAS, Aliqa Uma, Chenab, ...Elqui, Indus, Jhelum, and Phoenix. Abundances of 30 elements have been derived from over 10,000 individual line measurements or upper limits using photometric stellar parameters and a standard LTE analysis. This is currently the most extensive set of element abundances for stars in stellar streams. Three streams (ATLAS, Aliqa Uma, and Phoenix) are disrupted metal-poor globular clusters, although only weak evidence is seen for the light-element anticorrelations commonly observed in globular clusters. Four streams (Chenab, Elqui, Indus, and Jhelum) are disrupted dwarf galaxies, and their stars display abundance signatures that suggest progenitors with stellar masses ranging from 106 to 107 M . Extensive description is provided for the analysis methods, including the derivation of a new method for including the effect of stellar parameter correlations on each star's abundance and uncertainty. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.
Abstract
We present a nearly complete rapid neutron-capture process (
r
-process) chemical inventory of the metal-poor (Fe/H = −1.46 ± 0.10)
r
-process-enhanced (Eu/Fe = +1.32 ± 0.08) halo star HD ...222925. This abundance set is the most complete for any object beyond the solar system, with a total of 63 metals detected and seven with upper limits. It comprises 42 elements from 31 ≤
Z
≤ 90, including elements rarely detected in
r
-process-enhanced stars, such as Ga, Ge, As, Se, Cd, In, Sn, Sb, Te, W, Re, Os, Ir, Pt, and Au. We derive these abundances from an analysis of 404 absorption lines in ultraviolet spectra collected using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and previously analyzed optical spectra. A series of appendices discusses the atomic data and quality of fits for these lines. The
r
-process elements from Ba to Pb, including all elements at the third
r
-process peak, exhibit remarkable agreement with the solar
r
-process residuals, with a standard deviation of the differences of only 0.08 dex (17%). In contrast, deviations among the lighter elements from Ga to Te span nearly 1.4 dex, and they show distinct trends from Ga to Se, Nb through Cd, and In through Te. The
r
-process contribution to Ga, Ge, and As is small, and Se is the lightest element whose production is dominated by the
r
-process. The lanthanide fraction, log
X
La
= −1.39 ± 0.09, is typical for
r
-process-enhanced stars and higher than that of the kilonova from the GW170817 neutron-star merger event. We advocate adopting this pattern as an alternative to the solar
r
-process-element residuals when confronting future theoretical models of heavy-element nucleosynthesis with observations.
We report the discovery of a new actinide-boost star, 2MASS J09544277+5246414, originally identified as a very bright (V = 10.1), extremely metal-poor (Fe/H = −2.99) K giant in the LAMOST survey, and ...found to be highly r-process-enhanced (r-II; Eu/Fe = +1.28), during the snapshot phase of the R-Process Alliance (RPA). Based on a high signal-to-noise ratio (S/N), high-resolution spectrum obtained with the Harlan J. Smith 2.7 m telescope, this star is the first confirmed actinide-boost star found by RPA efforts. With an enhancement of Th/Eu = +0.37, 2MASS J09544277+5246414 is also the most actinide-enhanced r-II star yet discovered, and only the sixth metal-poor star with a measured uranium abundance (U/Fe = +1.40). Using the Th/U chronometer, we estimate an age of 13.0 4.7 Gyr for this star. The unambiguous actinide-boost signature of this extremely metal-poor star, combined with additional r-process-enhanced and actinide-boost stars identified by the RPA, will provide strong constraints on the nature and origin of the r-process at early times.
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