554 research outputs found
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
Optimal integrated abundances for chemical tagging of extragalactic globular clusters
High-resolution integrated light (IL) spectroscopy provides detailed abundances of distant globular clusters whose stars cannot be resolved. Abundance comparisons with other systems (e.g. for chemical tagging) require understanding the systematic offset
The proper motion and changing jet morphology of Cygnus X-3
We present analysis of 25 years' worth of archival VLA, VLBA and EVN
observations of the X-ray binary Cygnus X-3. From this, we deduce the source
proper motion, allowing us to predict the location of the central binary system
at any given time. However, the line of sight is too scatter-broadened for us
to measure a parallactic distance to the source. The measured proper motion
allows us to constrain the three-dimensional space velocity of the system,
implying a minimum peculiar velocity of 9 km/s. Reinterpreting VLBI images from
the literature using accurate core positions shows the jet orientation to vary
with time, implying that the jets are oriented close to the line of sight and
are likely to be precessing.Comment: 4 pages, 1 figure. To appear in proceedings of "Science and
Technology of Long Baseline Real-Time Interferometry: The 8th International
e-VLBI Workshop, EXPReS09", 22-26 June 2009, Madrid, Spai
The R-Process Alliance: Chemical Abundances for a Trio of R-Process-Enhanced Stars -- One Strong, One Moderate, One Mild
We present detailed chemical abundances of three new bright (V ~ 11),
extremely metal-poor ([Fe/H] ~ -3.0), r-process-enhanced halo red giants based
on high-resolution, high-S/N Magellan/MIKE spectra. We measured abundances for
20-25 neutron-capture elements in each of our stars. J1432-4125 is among the
most r-process rich r-II stars, with [Eu/Fe]= +1.44+-0.11. J2005-3057 is an r-I
star with [Eu/Fe] = +0.94+-0.07. J0858-0809 has [Eu/Fe] = +0.23+-0.05 and
exhibits a carbon abundance corrected for evolutionary status of [C/Fe]_corr =
+0.76, thus adding to the small number of known carbon-enhanced r-process
stars. All three stars show remarkable agreement with the scaled solar
r-process pattern for elements above Ba, consistent with enrichment of the
birth gas cloud by a neutron star merger. The abundances for Sr, Y, and Zr,
however, deviate from the scaled solar pattern. This indicates that more than
one distinct r-process site might be responsible for the observed
neutron-capture element abundance pattern. Thorium was detected in J1432-4125
and J2005-3057. Age estimates for J1432-4125 and J2005-3057 were adopted from
one of two sets of initial production ratios each by assuming the stars are
old. This yielded individual ages of 12+-6 Gyr and 10+-6 Gyr, respectively.Comment: 30 pages, includes a long table, 5 figure
The r-Process Pattern of a Bright, Highly r-Process-Enhanced, Metal-Poor Halo Star at [Fe/H] ~ -2
A high-resolution spectroscopic analysis is presented for a new highly
r-process-enhanced ([Eu/Fe] = 1.27, [Ba/Eu] = -0.65), very metal-poor ([Fe/H] =
-2.09), retrograde halo star, RAVE J153830.9-180424, discovered as part of the
R-Process Alliance survey. At V = 10.86, this is the brightest and most
metal-rich r-II star known in the Milky Way halo. Its brightness enables
high-S/N detections of a wide variety of chemical species that are mostly
created by the r-process, including some infrequently detected lines from
elements like Ru, Pd, Ag, Tm, Yb, Lu, Hf, and Th, with upper limits on Pb and
U. This is the most complete r-process census in a very metal-poor r-II star.
J1538-1804 shows no signs of s-process contamination, based on its low [Ba/Eu]
and [Pb/Fe]. As with many other r-process-enhanced stars, J1538-1804's
r-process pattern matches that of the Sun for elements between the first,
second, and third peaks, and does not exhibit an actinide boost.
Cosmo-chronometric age-dating reveals the r-process material to be quite old.
This robust main r-process pattern is a necessary constraint for r-process
formation scenarios (of particular interest in light of the recent neutron star
merger, GW 170817), and has important consequences for the origins of r-II
stars. Additional r-I and r-II stars will be reported by the R-Process Alliance
in the near future.Comment: Accepted for publication in ApJ letter
Correlated radio/X-ray behaviour of Cyg X-3
In order to test the recently proposed classification of the radio/X-ray states of the X-ray binary Cyg X-3, we present an analysis of the radio data available for the system at much higher spatial resolutions than used for defining the states. The radio data set consists of archival VLBA data at 5 and 15 GHz and new e-EVN data at 5 GHz. In the X-ray regime we use data that are quasi-simultaneous with radio, monitoring and pointed RXTE observations. We find that when the radio emission from both jet and core is globally considered, the behaviour of Cyg X-3 at milliarcsecond scales is consistent with that described at arcsecond scales. However, when the radio emission is disentangled in a core component and a jet component the situation changes. It becomes clear that in active states the radio emission from the jet is dominating that from the core. This shows that in these states the overall radio flux cannot be used as a direct tracer of the accretion state
Element abundance patterns in stars indicate fission of nuclei heavier than uranium
The heaviest chemical elements are naturally produced by the rapid
neutron-capture process (r-process) during neutron star mergers or supernovae.
The r-process production of elements heavier than uranium (transuranic nuclei)
is poorly understood and inaccessible to experiments, so must be extrapolated
using nucleosynthesis models. We examine element abundances in a sample of
stars that are enhanced in r-process elements. The abundances of elements Ru,
Rh, Pd, and Ag (atomic numbers Z = 44 to 47, mass numbers A = 99 to 110)
correlate with those of heavier elements (63 150). There is no
correlation for neighboring elements (34 <= Z <= 42 and 48 <= Z <= 62). We
interpret this as evidence that fission fragments of transuranic nuclei
contribute to the abundances. Our results indicate that neutron-rich nuclei
with mass numbers >260 are produced in r-process events.Comment: Authors' version of manuscript published in Science on December 07,
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