523 research outputs found
No Heavy Element Dispersion in the Globular Cluster M92
Although there have been recent claims that there is a large dispersion in
the abundances of the heavy neutron capture elements in the old Galactic
globular cluster M92, we show that the measured dispersion for the absolute
abundances of four of the rare earth elements within a sample of 12 luminous
red giants in M92 (less than or equal to 0.07 dex) does not exceed the relevant
sources of uncertainty. As expected from previous studies, the heavy elements
show the signature of the r-process. Their abundance ratios are essentially
identical to those of M30, another nearby globular cluster of similar
metallicity.Comment: Accepted for publication in ApJ Letter
Characterizing the Chemistry of the Milky Way Stellar Halo: Detailed Chemical Analysis of a Metal-Poor Stellar Stream
We present the results of a detailed abundance analysis of one of the
confirmed building blocks of the Milky Way stellar halo, a
kinematically-coherent metal-poor stellar stream. We have obtained high
resolution and high S/N spectra of 12 probable stream members using the MIKE
spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory and the
2dCoude spectrograph on the Smith Telescope at McDonald Observatory. We have
derived abundances or upper limits for 51 species of 46 elements in each of
these stars. The stream members show a range of metallicity (-3.4 < [Fe/H] <
-1.5) but are otherwise chemically homogeneous, with the same star-to-star
dispersion in [X/Fe] as the rest of the halo. This implies that, in principle,
a significant fraction of the Milky Way stellar halo could have formed from
accreted systems like the stream. The stream stars show minimal evolution in
the alpha or Fe-group elements over the range of metallicity. This stream is
enriched with material produced by the main and weak components of the rapid
neutron-capture process and shows no evidence for enrichment by the slow
neutron-capture process.Comment: v2: Removed references to M15 after learning that the source
kinematic data for M15 were incorrect in an earlier paper. M15 is not related
to this stream. (ApJ, accepted; 31 pages, 18 figures, 11 tables
Slow movement of a random walk on the range of a random walk in the presence of an external field
In this article, a localisation result is proved for the biased random walk
on the range of a simple random walk in high dimensions (d \geq 5). This
demonstrates that, unlike in the supercritical percolation setting, a slowdown
effect occurs as soon a non-trivial bias is introduced. The proof applies a
decomposition of the underlying simple random walk path at its cut-times to
relate the associated biased random walk to a one-dimensional random walk in a
random environment in Sinai's regime
Chemical composition of AY Ceti: A flaring, spotted star with a white dwarf companion
The detailed chemical composition of the atmosphere AY Cet (HD 7672) is
determined from a high-resolution spectrum in the optical region. The main
atmospheric parameters and the abundances of 22 chemical elements, including
key species such as 12C, 13C, N, and O, are determined. A differential line
analysis gives T_eff=5080 K, log g=3.0, [Fe/H]=-0.33, [C/Fe]=-0.17,
[N/Fe]=0.17, [O/Fe]=0.05, C/N=1.58, and 12C/13C=21. Despite the high
chromospheric activity, the optical spectrum of AY Cet provides a chemical
composition typical for first ascent giants after the first dredge-up.Comment: 11 figure
The chemical composition of nearby young associations: s-process element abundances in AB Doradus, Carina-Near, and Ursa Major
Recently, several studies have shown that young, open clusters are
characterised by a considerable over-abundance in their barium content. In
particular, D'Orazi et al. (2009) reported that in some younger clusters
[Ba/Fe] can reach values as high as ~0.6 dex. The work also identified the
presence of an anti-correlation between [Ba/Fe] and cluster age. For clusters
in the age range ~4.5 Gyr-500 Myr, this is best explained by assuming a higher
contribution from low-mass asymptotic giant branch stars to the Galactic
chemical enrichment. The purpose of this work is to investigate the ubiquity of
the barium over-abundance in young stellar clusters. We analysed
high-resolution spectroscopic data, focusing on the s-process elemental
abundance for three nearby young associations, i.e. AB Doradus, Carina-Near,
and Ursa Major. The clusters have been chosen such that their age spread would
complement the D'Orazi et al. (2009) study. We find that while the s-process
elements Y, Zr, La, and Ce exhibit solar ratios in all three associations, Ba
is over-abundant by ~0.2 dex. Current theoretical models can not reproduce this
abundance pattern, thus we investigate whether this unusually large Ba content
might be related to chromospheric effects. Although no correlation between
[Ba/Fe] and several activity indicators seems to be present, we conclude that
different effects could be at work which may (directly or indirectly) be
related to the presence of hot stellar chromospheres.Comment: Accepted for publication in MNRA
A computationally inspired in-vivo approach identifies a link between amygdalar transcriptional heterogeneity, socialization and anxiety
Pharmaceutical breakthroughs for anxiety have been lackluster in the last half-century. Converging behavior and limbic
molecular heterogeneity has the potential to revolutionize biomarker-driven interventions. However, current in vivo models
too often deploy artificial systems including directed evolution, mutations and fear induction, which poorly mirror clinical
manifestations. Here, we explore transcriptional heterogeneity of the amygdala in isogenic mice using an unbiased multidimensional
computational approach that segregates intra-cohort reactions to moderate situational adversity and intersects
it with high content molecular profiling. We show that while the computational approach stratifies known features of
clinical anxiety including nitric oxide, opioid and corticotropin signaling, previously unrecognized druggable biomarkers
emerge, such as calpain11 and scand1. Through ingenuity pathway analyses, we further describe a role for neurosteroid
estradiol signaling, heat shock proteins, ubiquitin ligases and lipid metabolism. In addition, we report a remarkable
behavioral pattern that maps to molecular features of anxiety in mice through counterphobic social attitudes, which
manifest as increased, yet spatially distant socialization. These findings provide an unbiased approach for interrogating
anxiolytics, and hint toward biomarkers underpinning behavioral and social patterns that merit further exploration
The two metallicity groups of the globular cluster M22: a chemical perspective
We present a detailed chemical composition analysis of 35 red giant stars in
the globular cluster M22. High resolution spectra for this study were obtained
at five observatories, and analyzed in a uniform manner. We have determined
abundances of representative light proton-capture, alpha, Fe-peak and
neutron-capture element groups. Our aim is to better understand the peculiar
chemical enrichment history of this cluster, in which two stellar groups are
characterized by a different content in iron, neutron capture elements Y, Zr
and Ba, and alpha element Ca. The principal results of this study are: (i)
substantial star-to-star metallicity scatter (-2.0<[Fe/H]<-1.6); (ii)
enhancement of s-process/r-process neutron-capture abundance ratios in a
fraction of giants, positively correlated with metallicity; (iii) sharp
separation between the s-process rich and s-process poor groups by [La/Eu]
ratio; (iv) possible increase of [Cu/Fe] ratios with increasing [Fe/H],
suggesting that this element also has a significant s-process component; and
(v) presence of Na-O and C-N anticorrelations in both the stellar groups.Comment: 26 pages, 19 figures. Accepted for publication in A&
On The Power of Tree Projections: Structural Tractability of Enumerating CSP Solutions
The problem of deciding whether CSP instances admit solutions has been deeply
studied in the literature, and several structural tractability results have
been derived so far. However, constraint satisfaction comes in practice as a
computation problem where the focus is either on finding one solution, or on
enumerating all solutions, possibly projected to some given set of output
variables. The paper investigates the structural tractability of the problem of
enumerating (possibly projected) solutions, where tractability means here
computable with polynomial delay (WPD), since in general exponentially many
solutions may be computed. A general framework based on the notion of tree
projection of hypergraphs is considered, which generalizes all known
decomposition methods. Tractability results have been obtained both for classes
of structures where output variables are part of their specification, and for
classes of structures where computability WPD must be ensured for any possible
set of output variables. These results are shown to be tight, by exhibiting
dichotomies for classes of structures having bounded arity and where the tree
decomposition method is considered
Electromagnetic Counterparts of Compact Object Mergers Powered by the Radioactive Decay of R-process Nuclei
The most promising astrophysical sources of kHz gravitational waves (GWs) are
the inspiral and merger of binary neutron star(NS)/black hole systems.
Maximizing the scientific return of a GW detection will require identifying a
coincident electro-magnetic (EM) counterpart. One of the most likely sources of
isotropic EM emission from compact object mergers is a supernova-like transient
powered by the radioactive decay of heavy elements synthesized in ejecta from
the merger. We present the first calculations of the optical transients from
compact object mergers that self-consistently determine the radioactive heating
by means of a nuclear reaction network; using this heating rate, we model the
light curve with a one dimensional Monte Carlo radiation transfer calculation.
For an ejecta mass ~1e-2 M_sun[1e-3 M_sun] the resulting light curve peaks on a
timescale ~ 1 day at a V-band luminosity nu L_nu ~ 3e41[1e41] ergs/s (M_V =
-15[-14]); this corresponds to an effective "f" parameter ~3e-6 in the
Li-Paczynski toy model. We argue that these results are relatively insensitive
to uncertainties in the relevant nuclear physics and to the precise early-time
dynamics and ejecta composition. Due to the rapid evolution and low luminosity
of NS merger transients, EM counterpart searches triggered by GW detections
will require close collaboration between the GW and astronomical communities.
NS merger transients may also be detectable following a short-duration
Gamma-Ray Burst or "blindly" with present or upcoming optical transient
surveys. Because the emission produced by NS merger ejecta is powered by the
formation of rare r-process elements, current optical transient surveys can
directly constrain the unknown origin of the heaviest elements in the Universe.Comment: 14 pages, 7 figures; accepted to MNRAS; title changed to highlight
r-process connection and new figure added
Abundances in giant stars of the globular cluster NGC 6752
Recent theoretical yields and chemical evolution models demonstrate that
intermediate-mass AGB stars cannot reproduce the observed abundance
distributions of O, Na, Mg, and Al. As a further observational test of this
finding, we present elemental abundance ratios [X/Fe] for 20 elements in 38
bright giants of the globular cluster NGC 6752. Our mean abundance ratios
[X/Fe] are in good agreement with previous studies of this cluster and are also
consistent with other globular clusters and field stars at the same
metallicity. The mean abundance ratios [Ba/Eu] and [La/Eu] exhibit values, in
agreement with field stars at the same metallicity, that lie approximately
midway between the pure r-process and the solar (s-process + r-process) mix,
indicating that AGB stars have played a role in the chemical evolution of the
proto-cluster gas.
For the first time, we find possible evidence for an abundance variation for
elements heavier than Al in this cluster. We find a correlation between [Si/Fe]
and [Al/Fe] which is consistent with the abundance anomalies being synthesized
via proton captures at high temperatures. Leakage from the Mg-Al chain into
28Si may explain the Si excess in stars with the highest [Al/Fe]. We identify
correlations between [Y/Fe] and [Al/Fe], [Zr/Fe] and [Al/Fe], and [Ba/Fe] and
[Al/Fe] suggesting that Y, Zr, and Ba abundances may increase by about 0.1 dex
as Al increases by about 1.3 dex. While the correlations are statistically
significant, the amplitudes of the variations are small. If the small
variations in Y, Zr, and Ba are indeed real, then the synthesis of the Al
anomalies must have taken place within an unknown class of stars that also ran
the s-process. [Abridged]Comment: Accepted for publication in A&
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