951 research outputs found
Characterizing the Heavy Elements in Globular Cluster M22 and an Empirical s-process Abundance Distribution Derived from the Two Stellar Groups
We present an empirical s-process abundance distribution derived with
explicit knowledge of the r-process component in the low-metallicity globular
cluster M22. We have obtained high-resolution, high signal-to-noise spectra for
6 red giants in M22 using the MIKE spectrograph on the Magellan-Clay Telescope
at Las Campanas Observatory. In each star we derive abundances for 44 species
of 40 elements, including 24 elements heavier than zinc (Z=30) produced by
neutron-capture reactions. Previous studies determined that 3 of these stars
(the "r+s group") have an enhancement of s-process material relative to the
other 3 stars (the "r-only group"). We confirm that the r+s group is moderately
enriched in Pb relative to the r-only group. Both groups of stars were born
with the same amount of r-process material, but s-process material was also
present in the gas from which the r+s group formed. The s-process abundances
are inconsistent with predictions for AGB stars with M =< 3 Msun and suggest an
origin in more massive AGB stars capable of activating the Ne-22(alpha,n)Mg-25
reaction. We calculate the s-process "residual" by subtracting the r-process
pattern in the r-only group from the abundances in the r+s group. In contrast
to previous r- and s-process decompositions, this approach makes no assumptions
about the r- and s-process distributions in the solar system and provides a
unique opportunity to explore s-process yields in a metal-poor environment.Comment: Accepted for publication in the Astrophysical Journal. 18 pages, 8
figure
Observational Constraints on First-Star Nucleosynthesis. I. Evidence for Multiple Progenitors of CEMP-no Stars
We investigate anew the distribution of absolute carbon abundance, (C) (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- (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
(C) for CEMP stars is (at least) bimodal, with newly determined peaks
centered on (C) (the high-C region) and (C) (the low-C
region). A very high fraction of CEMP- (and CEMP-r/s) stars belong to the
high-C region, while the great majority of CEMP-no stars reside in the low-C
region. However, there exists complexity in the morphology of the (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 (Na)-(C) and (Mg)-(C) spaces,
also suggesting multiple progenitors. The clear distinction in (C) between
the CEMP- (and CEMP-) stars and the CEMP-no stars appears to be $as\
successfullikely\ more\ astrophysically\ fundamental$, 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.Comment: 26pages, 7 figures, and 3 Tables ; Accepted for publication in ApJ;
added more data and corrected minor inconsistencies existed in the compiled
data of the previous studie
The R-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-Poor Star with an Extreme R-Process Enhancement of [Eu/H] = -0.14
We present a detailed abundance analysis of the bright (V = 9.02), metal-poor
([Fe/H] = -1.47 +/- 0.08) field red horizontal-branch star HD 222925, which was
observed as part of an ongoing survey by the R-Process Alliance. We calculate
stellar parameters and derive abundances for 46 elements based on 901 lines
examined in a high-resolution optical spectrum obtained using the Magellan
Inamori Kyocera Echelle spectrograph. We detect 28 elements with 38 <= Z <= 90;
their abundance pattern is a close match to the Solar r-process component. The
distinguishing characteristic of HD 222925 is an extreme enhancement of
r-process elements ([Eu/Fe] = +1.33 +/- 0.08, [Ba/Eu] = -0.78 +/- 0.10) in a
moderately metal-poor star, so the abundance of r-process elements is the
highest ([Eu/H] = -0.14 +/- 0.09) in any known r-process-enhanced star. The
abundance ratios among lighter (Z <= 30) elements are typical for metal-poor
stars, indicating that production of these elements was dominated by normal
Type II supernovae, with no discernible contributions from Type Ia supernovae
or asymptotic giant branch stars. The chemical and kinematic properties of HD
222925 suggest it formed in a low-mass dwarf galaxy, which was enriched by a
high-yield r-process event before being disrupted by interaction with the Milky
Way.Comment: Accepted for publication in the Astrophysical Journal (17 pages, 4
figures, 3 tables
The Ubiquity of the Rapid Neutron-Capture Process
To better characterize the abundance patterns produced by the r-process, we
have derived new abundances or upper limits for the heavy elements zinc (Zn),
yttrium (Y), lanthanum (La), europium (Eu), and lead (Pb). Our sample of 161
metal-poor stars includes new measurements from 88 high resolution and high
signal-to-noise spectra obtained with the Tull Spectrograph on the 2.7m Smith
Telescope at McDonald Observatory, and other abundances are adopted from the
literature. We use models of the s-process in AGB stars to characterize the
high Pb/Eu ratios produced in the s-process at low metallicity, and our new
observations then allow us to identify a sample of stars with no detectable
s-process material. In these stars, we find no significant increase in the
Pb/Eu ratios with increasing metallicity. This suggests that s-process material
was not widely dispersed until the overall Galactic metallicity grew
considerably, perhaps even as high as [Fe/H]=-1.4. We identify a dispersion of
at least 0.5 dex in [La/Eu] in metal-poor stars with [Eu/Fe]<+0.6 attributable
to the r-process, suggesting that there is no unique "pure" r-process elemental
ratio among pairs of rare earth elements. We confirm earlier detections of an
anti-correlation between Y/Eu and Eu/Fe bookended by stars strongly enriched in
the r-process (e.g., CS 22892-052) and those with deficiencies of the heavy
elements (e.g., HD 122563). We can reproduce the range of Y/Eu ratios using
simulations of high-entropy neutrino winds of core-collapse supernovae that
include charged-particle and neutron-capture components of r-process
nucleosynthesis. The heavy element abundance patterns in most metal-poor stars
do not resemble that of CS 22892-052, but the presence of heavy elements such
as Ba in nearly all metal-poor stars without s-process enrichment suggests that
the r-process is a common phenomenon.Comment: Accepted for publication in the Astrophysical Journal. 25 pages, 13
figure
The Chemical Abundances Of Stars In The Halo (CASH) Project. II. A Sample Of 14 Extremely Metal-Poor Stars
We present a comprehensive abundance analysis of 20 elements for 16 new low-metallicity stars from the Chemical Abundances of Stars in the Halo (CASH) project. The abundances have been derived from both Hobby-Eberly Telescope High Resolution Spectrograph snapshot spectra (R similar to 15,000) and corresponding high-resolution (R similar to 35,000) Magellan Inamori Kyocera Echelle spectra. The stars span a metallicity range from [Fe/H] from -2.9 to -3.9, including four new stars with [Fe/H] < -3.7. We find four stars to be carbon-enhanced metal-poor (CEMP) stars, confirming the trend of increasing [C/Fe] abundance ratios with decreasing metallicity. Two of these objects can be classified as CEMP-no stars, adding to the growing number of these objects at [Fe/H]< -3. We also find four neutron-capture-enhanced stars in the sample, one of which has [Eu/Fe] of 0.8 with clear r-process signatures. These pilot sample stars are the most metal-poor ([Fe/H] less than or similar to -3.0) of the brightest stars included in CASH and are used to calibrate a newly developed, automated stellar parameter and abundance determination pipeline. This code will be used for the entire similar to 500 star CASH snapshot sample. We find that the pipeline results are statistically identical for snapshot spectra when compared to a traditional, manual analysis from a high-resolution spectrum.Physics Frontier Center/Joint Institute for Nuclear Astrophysics (JINA) PHY 02-16783, PHY 0822648Carnegie Institution of WashingtonNSF AST-0908978Astronom
The Extreme Overabundance of Molybdenum in Two Metal-Poor Stars
We report determinations of the molybdenum abundances in five mildly to
extremely metal-poor turnoff stars using five Mo II lines near 2000A. In two of
the stars, the abundance of molybdenum is found to be extremely enhanced, as
high or higher than the neighboring even-Z elements ruthenium and zirconium. Of
the several nucleosynthesis scenarios envisioned for the production of nuclei
in this mass range in the oldest stars, a high-entropy wind acting in a
core-collapse supernova seems uniquely capable of the twin aspects of a high
molybdenum overproduction confined to a narrow mass range. Whatever the details
of the nucleosynthesis mechanism, however, this unusual excess suggests that
very few individual nucleosynthesis events were responsible for the synthesis
of the light trans-Fe heavy elements in these cases, an unexpected result given
that both are only moderately metal-poor.Comment: updated in v2, including text missing from the third-to-last
paragraph in the published versio
Detection of the Second r-process Peak Element Tellurium in Metal-Poor Stars
Using near-ultraviolet spectra obtained with the Space Telescope Imaging
Spectrograph onboard the Hubble Space Telescope, we detect neutral tellurium in
three metal-poor stars enriched by products of r-process nucleosynthesis, BD+17
3248, HD 108317, and HD 128279. Tellurium (Te, Z=52) is found at the second
r-process peak (A=130) associated with the N=82 neutron shell closure, and it
has not been detected previously in Galactic halo stars. The derived tellurium
abundances match the scaled solar system r-process distribution within the
uncertainties, confirming the predicted second peak r-process residuals. These
results suggest that tellurium is predominantly produced in the main component
of the r-process, along with the rare earth elements.Comment: Accepted for publication in the Astrophysical Journal Letters (5
pages, 2 figures
New Detections of Arsenic, Selenium, and Other Heavy Elements in Two Metal-Poor Stars
We use the Space Telescope Imaging Spectrograph on board the Hubble Space
Telescope to obtain new high-quality spectra covering the 1900 to 2360 Angstrom
wavelength range for two metal-poor stars, HD 108317 and HD 128279. We derive
abundances of Cu II, Zn II, As I, Se I, Mo II, and Cd II, which have not been
detected previously in either star. Abundances derived for Ge I, Te I, Os II,
and Pt I confirm those derived from lines at longer wavelengths. We also derive
upper limits from the non-detection of W II, Hg II, Pb II, and Bi I. The mean
[As/Fe] ratio derived from these two stars and five others in the literature is
unchanged over the metallicity range -2.8 = +0.28
+/- 0.14 (std. dev. = 0.36 dex). The mean [Se/Fe] ratio derived from these two
stars and six others in the literature is also constant, = +0.16 +/-
0.09 (std. dev. = 0.26 dex). The As and Se abundances are enhanced relative to
a simple extrapolation of the iron-peak abundances to higher masses, suggesting
that this mass region (75 < A < 82) may be the point at which a different
nucleosynthetic mechanism begins to dominate the quasi-equilibrium alpha-rich
freezeout of the iron peak. = +0.56 +/- 0.23 in HD 108317 and HD
128279, and we infer that lines of Cu I may not be formed in local
thermodynamic equilibrium in these stars. The [Zn/Fe], [Mo/Fe], [Cd/Fe], and
[Os/Fe] ratios are also derived from neutral and ionized species, and each
ratio pair agrees within the mutual uncertainties, which range from 0.15 to
0.52 dex.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 10
figure
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