28,956 research outputs found
The Relative Age of the Thin and Thick Galactic Disks
We determine the relative ages of the open cluster NGC 188 and selected
Hipparcos field stars by isochrone fitting, and compare them to the age of the
thick disk globular cluster 47 Tuc. The best fit age for NGC 188 was determined
to be Gyr. The solar metallicity Hipparcos field stars yielded a
slightly older thin disk age, Gyr. Two slightly metal-poor (\feh
= -0.22) field stars whose kinematic and orbital parameters indicate that they
are members of the thin disk were found to have an age of Gyr. The
age for 47 Tuc was determined to be Gyr. All errors are internal
errors due to the uncertainty in the values of metallicity and reddening. Thus,
the oldest stars dated in the thin disk are found to be Gyr
younger than 47 Tuc. Furthermore, as discussed by \citet{Chb99} 47 Tuc has a
similar age to three globular clusters located in the inner part of the
Galactic halo, implying that star formation in the thin disk started within
Gyr of star formation in the halo.Comment: 26 pages, 11 figures, to appear in Ap
Resolved Mid-Infrared Emission Around AB Aur and V892 Tau with Adaptive Optics Nulling Interferometric Observations
We present the results of adaptive optics nulling interferometric
observations of two Herbig Ae stars, AB Aur and V892 Tau. Our observations at
10.3 microns show resolved circumstellar emission from both sources. Further
analysis of the AB Aur emission suggests that there is an inclined disk
surrounding the star. The diameter of the disk is derived to be 24 to 30 AU
with an inclination of 45 to 65 degrees from face-on, and a major-axis PA of 30
+/- 15 degrees (E of N). Differences in the physical characteristics between
the mid-IR emission and emission at other wavelengths (near-IR and millimeter),
found in previous studies, suggest a complex structure for AB Aur's
circumstellar environment, which may not be explained by a disk alone. The
similarity in the observed size of AB Aur's resolved emission and that of
another Herbig Ae star, HD 100546, is likely coincidental, as their respective
evolutionary states and spectral energy distributions suggest significantly
different circumstellar environments.Comment: 11 pages, 1 figure, Accepted for publication in ApJ Letter
Determination of an Interaction Network between an Extracellular Bacterial Pathogen and the Human Host
A major gap in understanding infectious diseases is the lack of information about molecular interaction networks between pathogens and the human host. Haemophilus ducreyi causes the genital ulcer disease chancroid in adults and is a leading cause of cutaneous ulcers in children in the tropics. We developed a model in which human volunteers are infected on the upper arm with H. ducreyi until they develop pustules. To define the H. ducreyi and human interactome, we determined bacterial and host transcriptomic and host metabolomic changes in pustules. We found that in vivo H. ducreyi transcripts were distinct from those in the inocula, as were host transcripts in pustule and wounded control sites. Many of the upregulated H. ducreyi genes were found to be involved in ascorbic acid and anaerobic metabolism and inorganic ion/nutrient transport. The top 20 significantly expressed human pathways showed that all were involved in immune responses. We generated a bipartite network for interactions between host and bacterial gene transcription; multiple positively correlated networks contained H. ducreyi genes involved in anaerobic metabolism and host genes involved with the immune response. Metabolomic studies showed that pustule and wounded samples had different metabolite compositions; the top ion pathway involved ascorbate and aldarate metabolism, which correlated with the H. ducreyi transcriptional response and upregulation of host genes involved in ascorbic acid recycling. These data show that an interactome exists between H. ducreyi and the human host and suggest that H. ducreyi exploits the metabolic niche created by the host immune response.IMPORTANCE Dual RNA sequencing (RNA-seq) offers the promise of determining an interactome at a transcriptional level between a bacterium and the host but has yet to be done on any bacterial infection in human tissue. We performed dual RNA-seq and metabolomics analyses on wounded and infected sites following experimental infection of the arm with H. ducreyi Our results suggest that H. ducreyi survives in an abscess by utilizing l-ascorbate as an alternative carbon source, possibly taking advantage of host ascorbic acid recycling, and that H. ducreyi also adapts by upregulating genes involved in anaerobic metabolism and inorganic ion and nutrient transport. To our knowledge, this is the first description of an interaction network between a bacterium and the human host at a site of infection
Bumps and rings in a two-dimensional neural field: splitting and rotational instabilities
In this paper we consider instabilities of localised solutions in planar neural field firing rate models of Wilson-Cowan or Amari type. Importantly we show that angular perturbations can destabilise spatially localised solutions. For a scalar model with Heaviside firing rate function we calculate symmetric one-bump and ring solutions explicitly and use an Evans function approach to predict the point of instability and the shapes of the dominant growing modes. Our predictions are shown to be in excellent agreement with direct numerical simulations. Moreover, beyond the instability our simulations demonstrate the emergence of multi-bump and labyrinthine patterns.
With the addition of spike-frequency adaptation, numerical simulations of the resulting vector model show that it is possible for structures without rotational symmetry, and in particular multi-bumps, to undergo an instability to a rotating wave. We use a general argument, valid for smooth firing rate functions, to establish the conditions necessary to generate such a rotational instability. Numerical continuation of the rotating wave is used to quantify the emergent angular velocity as a bifurcation parameter is varied. Wave stability is found via the numerical evaluation of an associated eigenvalue problem
Neutron scattering study of magnetic phase separation in nanocrystalline LaCaMnO
We demonstrate that magnetic phase separation and competing spin order in the
colossal magnetoresistive (CMR) manganites can be directly explored via tuning
strain in bulk samples of nanocrystalline LaCaMnO. Our results
show that strain can be reversibly frozen into the lattice in order to
stabilize coexisting antiferromagnetic domains within the nominally
ferromagnetic metallic state of LaCaMnO. The measurement of
tunable phase separation via magnetic neutron powder diffraction presents a
direct route of exploring the correlated spin properties of phase separated
charge/magnetic order in highly strained CMR materials and opens a potential
avenue for realizing intergrain spin tunnel junction networks with enhanced CMR
behavior in a chemically homogeneous material.Comment: 6 pages, 4 figures. New figure and text added to manuscrip
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