We revisit the observed frequencies of carbon-enhanced metal-poor (CEMP) stars as a function of the metallicity in the Galaxy, using data from the literature with available high-resolution ...spectroscopy. Our analysis excludes stars exhibiting clear overabundances of neutron-capture elements and takes into account the expected depletion of surface carbon abundance that occurs due to CN processing on the upper red giant branch. This allows for the recovery of the initial carbon abundance of these stars, and thus for an accurate assessment of the frequencies of carbon-enhanced stars. The correction procedure we develop is based on stellar-evolution models and depends on the surface gravity, log g, of a given star. Our analysis indicates that for stars with Fe/H < or =, slant -2.0, 20% exhibit C/Fe > or =, slanted +0.7. This fraction increases to 43% for Fe/H < or =, slant -3.0 and 81% for Fe/H < or =, slant -4.0, which is higher than have been previously inferred without taking the carbon abundance correction into account. These CEMP star frequencies provide important inputs for Galactic and stellar chemical evolution models, as they constrain the evolution of carbon at early times and the possible formation channels for the CEMP-no stars. We also have developed a public online tool with which carbon corrections using our procedure can be easily obtained.
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 search for dynamical substructures in the LAMOST DR3 very metal-poor (VMP) star catalog. After cross-matching with Gaia DR2, there are ∼3300 VMP stars with available high-quality astrometric ...information that have halo-like kinematics. We apply a method based on the self-organizing map StarGO to find groups clustered in the 4D space of orbital energy and angular momentum. We identify 57 dynamically tagged groups (DTGs), which we label DTG-1 to DTG-57. Most of them belong to existing massive substructures in the nearby halo, such as the Gaia Sausage or Sequoia. The stream identified by Helmi et al. is recovered, but the two disjointed portions of the substructure appear to have distinct dynamical properties. The very retrograde substructure Rg5 found previously by Myeong et al. is also retrieved. We report six new DTGs with highly retrograde orbits, two with very prograde orbits, and 12 with polar orbits. By mapping other data sets (APOGEE halo stars, and catalogs of r-process-enhanced and carbon-enhanced metal-poor CEMP stars) onto the trained neuron map, we can associate stars with detailed chemical abundances with the DTGs and look for associations with chemically peculiar stars. The highly eccentric Gaia Sausage groups contain representatives of both debris from the satellite itself (which is -poor) and the Splashed Disk, sent up into eccentric halo orbits from the encounter (and which is -rich). The new prograde substructures also appear to be associated with the Splashed Disk. The DTGs belonging to the Gaia Sausage host two relatively metal-rich r-II stars and six CEMP stars in different subclasses, consistent with the idea that the Gaia Sausage progenitor is a massive dwarf galaxy. Rg5 is dynamically associated with two highly r-process-enhanced stars with Fe/H ∼ −3. This finding indicates that its progenitor might be an ultrafaint dwarf galaxy that has experienced r-process enrichment from neutron star mergers.
Abstract
We present observational evidence that an aspherical supernova explosion could have occurred in the first stars in the early universe. Our results are based on the first determination of a ...Zn abundance in a
Hubble Space Telescope
/Cosmic Origins Spectrograph high-resolution UV spectrum of a hyper-metal-poor (HMP) star, HE 1327−2326, with
. We determine Zn/Fe = 0.80 ± 0.25 from a UV Zn
i
line at 2138 Å, detected at 3.4
σ
. Yields of a 25
M
⊙
aspherical supernova model with artificially modified densities exploding with
E
= 5 × 10
51
erg best match the entire abundance pattern of HE 1327−2326. Such high-entropy hypernova explosions are expected to produce bipolar outflows, which could facilitate the external enrichment of small neighboring galaxies. This has already been predicted by theoretical studies of the earliest star-forming minihalos. Such a scenario would have significant implications for the chemical enrichment across the early universe, as HMP carbon-enhanced metal-poor (CEMP) stars such as HE 1327−2326 might have formed in such externally enriched environments.
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.
This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were ...identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed up at high spectral resolution (R ∼ 31,500) with the 3.5 m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe i lines, taking into account non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have Fe/H < −1.5, 105 have Fe/H < −2.0, and 4 have Fe/H < −3.0. Nine new carbon-enhanced metal-poor stars have been discovered, three of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3 ≤ Eu/Fe ≤ +1.0 and Ba/Eu < 0) and 4 new r-II stars (with Eu/Fe > +1.0). Nineteen stars are found to exhibit a "limited-r" signature (Sr/Ba > +0.5, Ba/Eu < 0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, , and Fe-peak elements match those of typical Milky Way (MW) halo stars, except for one r-I star that has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars that are consistent with membership in the MW halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.
Abstract
Orbital characteristics based on Gaia Early Data Release 3 astrometric parameters are analyzed for ∼4000 metal-poor stars (Fe/H ≤ −0.8) compiled from the Best and Brightest survey. Selected ...as metal-poor candidates based on broadband near- and far-IR photometry, 43% of these stars had medium-resolution (1200 ≲
R
≲ 2000) validation spectra obtained over a 7 yr campaign from 2014 to 2020 with a variety of telescopes. The remaining stars were chosen based on photometric metallicity determinations from the Huang et al. recalibration of the Sky Mapper Southern Survey. Dynamical clusters of these stars are obtained from the orbital energy and cylindrical actions using the
HDBSCAN
unsupervised learning algorithm. We identify 52 dynamically tagged groups (DTGs) with between five and 21 members; 18 DTGs have at least 10 member stars. Milky Way (MW) substructures such as Gaia-Sausage-Enceladus, the Metal-Weak Thick-Disk, Thamnos, the Splashed Disk, and the Helmi Stream are identified. Associations with MW globular clusters are determined for eight DTGs; no recognized MW dwarf galaxies were associated with any of our DTGs. Previously identified dynamical groups are also associated with our DTGs, with emphasis placed on their structural determination and possible new identifications. Chemically peculiar stars are identified as members of several DTGs, with six DTGs that are associated with
r
-process-enhanced stars. We demonstrate that the mean carbon and
α
-element abundances of our DTGs are correlated with their mean metallicity in an understandable manner. Similarly, we find that the mean metallicity, carbon, and
α
-element abundances are separable into different regions of the mean rotational-velocity space.
We describe a method for the determination of stellar C/Fe abundance ratios using low-resolution (R = 2000) stellar spectra from the Sloan Digital Sky Survey (SDSS) and its Galactic sub-survey, the ...Sloan Extension for Galactic Understanding and Exploration (SEGUE). By means of a star-by-star comparison with a set of SDSS/SEGUE spectra with available estimates of C/Fe based on published high-resolution analyses, we demonstrate that we can measure C/Fe from SDSS/SEGUE spectra with S/N > or =, slanted 15 A super(-1) to a precision better than 0.35 dex for stars with atmospheric parameters in the range T sub(eff) = 4400, 6700 K, log g = 1.0, 5.0, Fe/H = -4.0, +0.5, and C/Fe = -0.25, +3.5. We find that the differential frequency slowly rises from almost zero to about 14% at Fe/H ~ -2.4, followed by a sudden increase, by about a factor of three, to 39% from Fe/H ~ -2.4 to Fe/H ~ -3.7.
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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