292 research outputs found
A Monte Carlo Study of the 6.4 keV Emission at the Galactic Center
Strong fluorescent Fe line emission at 6.4 keV has been observed from the Sgr
B2 giant molecular cloud located in the Galactic Center region. The large
equivalent width of this line and the lack of an apparent illuminating nearby
object indicate that a time-dependent source, currently in a low-activity
state, is causing the fluorescent emission. It has been suggested that this
illuminator is the massive black hole candidate, Sgr A*, whose X-ray luminosity
has declined by an unprecedented six orders of magnitude over the past 300
years. We here report the results of our Monte Carlo simulations for producing
this line under a variety of source configurations and characteristics. These
indicate that the source may in fact be embedded within Sgr B2, although
external sources give a slightly better fit to the data. The weakened
distinction between the internal and external illuminators is due in part to
the instrument response function, which accounts for an enhanced equivalent
width of the line by folding some of the continuum radiation in with the
intrinsic line intensity. We also point out that although the spectrum may be
largely produced by K emission in cold gas, there is some evidence in
the data to suggest the presence of warm (~10^5 K) emitting material near the
cold cloud.Comment: 11 pages, 4 figure
Hypervelocity Stars: Predicting the Spectrum of Ejection Velocities
The disruption of binary stars by the tidal field of the black hole in the
Galactic Center can produce the hypervelocity stars observed in the halo. We
use numerical models to simulate the full spectrum of observable velocities of
stars ejected into the halo by this binary disruption process. Our model
includes a range of parameters for binaries with 3-4 M_Solar primaries,
consideration of radial orbits of the ejected stars through an approximate mass
distribution for the Galaxy, and the impact of stellar lifetimes. We calculate
the spectrum of ejection velocities and reproduce previous results for the mean
ejection velocity at the Galactic center. The model predicts that the full
population of ejected stars includes both the hypervelocity stars with
velocities large enough to escape from the Galaxy and a comparable number of
ejected, but bound, stars of the same stellar type. The predicted median speeds
of the population of ejected stars as a function of distance in the halo are
consistent with current observations. Combining the model with the data also
shows that interesting constraints on the properties of binaries in the
Galactic Center and on the mass distribution in the Galaxy can be obtained even
with modest samples of ejected stars.Comment: 26 pages, including 6 figures, accepted for publication in the
Astrophysical Journa
Ocean acidification affects iron speciation during a coastal seawater mesocosm experiment
Rising atmospheric CO2 is acidifying the surface ocean, a process which is expected to greatly influence the chemistry and biology of the future ocean. Following the development of iron-replete phytoplankton blooms in a coastal mesocosm experiment at 350, 700, and 1050 μatm pCO2, we observed significant increases in dissolved iron concentrations, Fe(II) concentrations, and Fe(II) half-life times during and after the peak of blooms in response to CO2 enrichment and concomitant lowering of pH, suggesting increased iron bioavailability. If applicable to the open ocean this may provide a negative feedback mechanism to the rising atmospheric CO2 by stimulating marine primary production
Tidal Disruption of a Star By a Black Hole : Observational Signature
We have modeled the time-variable profiles of the Halpha emission line from
the non-axisymmetric disk and debris tail created in the tidal disruption of a
solar-type star by a million solar mass black hole. Two tidal disruption event
simulations were carried out using a three dimensional relativistic
smooth-particle hydrodynamic code, to describe the early evolution of the
debris during the first fifty to ninety days. We have calculated the physical
conditions and radiative processes in the debris using the photoionization code
CLOUDY. We model the emission line profiles in the period immediately after the
accretion rate onto the black hole became significant. We find that the line
profiles at these very early stages of the evolution of the post-disruption
debris do not resemble the double peaked profiles expected from a rotating disk
since the debris has not yet settled into such a stable structure. As a result
of the uneven distribution of the debris and the existence of a ``tidal tail''
(the stream of returning debris), the line profiles depend sensitively on the
orientation of the tail relative to the line of sight. Moreover, the predicted
line profiles vary on fairly short time scales (of order hours to days). Given
the accretion rate onto the black hole we also model the Halpha light curve
from the debris and the evolution of the Halpha line profiles in time.Comment: 20 pages, 9 figures, to appear in ApJ, 1 August 2004 issue; mpeg
simulations of tidal disruption available at
http://www.astro.psu.edu/users/tamarab/tdmovies.htm
Build-up and decline of organic matter during PeECE III
Increasing atmospheric carbon dioxide (CO2) concentrations due to anthropogenic fossil fuel combustion are currently changing the ocean's chemistry. Increasing oceanic [CO2] and consequently decreasing seawater pH have the potential to significantly impact marine life. Here we describe and analyze the build-up and decline of a natural phytoplankton bloom initiated during the 2005 mesocosm Pelagic Ecosystem CO2 Enrichment study (PeECE III). The draw-down of inorganic nutrients in the upper surface layer of the mesocosms was reflected by a concomitant increase of organic matter until day t11, the peak of the bloom. From then on, biomass standing stocks steadily decreased as more and more particulate organic matter was lost into the deeper layer of the mesocosms. We show that organic carbon export to the deeper layer was significantly enhanced at elevated CO2. This phenomenon might have impacted organic matter remineralization leading to decreased oxygen concentrations in the deeper layer of the high CO2 mesocosms as indicated by deep water ammonium concentrations. This would have important implications for our understanding of pelagic ecosystem functioning and future carbon cycling
Profiling microRNA expression in murine bone healing and non-union formation: Role of miR-140 during the early stage of bone healing
Although cellular and molecular mechanisms during the course of bone healing have been
thoroughly investigated, the regulation of gene expression by microRNA during bone regen eration is still poorly understood. We hypothesized that nonunion formation is associated
with different microRNA expression patterns and that target proteins of these microRNAs
are differently expressed in callus tissue of nonunions compared to physiologically healing
bones. In a well-established femoral osteotomy model in CD-1 mice osteotomies were
induced which result either in healing or in nonunion formation. MicroRNA and target protein
expression was evaluated by microarray, quantitative real-time polymerase chain reaction
(qrt-PCR) and Western blot. Microarray analyses demonstrated 44 microRNAs to be rele vant for nonunion formation compared to physiological bone healing. In nonunions qrt-PCR
could validate a higher expression of microRNA-140-3p and microRNA-140-5p. This was
associated with a reduced expression of Dnpep and stromal cell-derived factor (SDF)-1α,
which are both known to be target proteins of microRNA-140 and also to be involved in the
process of bone healing. These data suggest that an increased expression of microRNA 140-3p and microRNA-140-5p markedly contributes to the development of nonunions, most
probably by affecting bone morphogenetic protein (BMP)-2 function during the early stage
of healing due to a reduced SDF-1α expression
High-resolution Laboratory Measurements of K-shell X-ray Line Polarization and Excitation Cross Sections in Heliumlike S XV Ions
We report measurements of electron-impact excitation cross sections for the
strong K-shell n=2-1 transitions in S XV using the LLNL EBIT-I electron beam
ion trap, two crystal spectrometers, and the EBIT Calorimeter Spectrometer. The
cross sections are determined by direct normalization to the well known cross
sections of radiative electron capture, measured simultaneously. Using
contemporaneous polarization measurements with the two crystal spectrometers,
whose dispersion planes are oriented parallel and perpendicular to the electron
beam direction, the polarization of the direct excitation line emission is
determined, and in turn the isotropic total cross sections are extracted. We
further experimentally investigate various line-formation mechanisms, finding
that radiative cascades and collisional inner-shell ionization dominate the
degree of linear polarization and total line-emission cross sections of the
forbidden line .Comment: 16 Pages, 9 Figures, published in Ap
Apolipoprotein E-dependent load of white matter hyperintensities in Alzheimer’s disease: a voxel-based lesion mapping study
Introduction: White matter (WM) magnetic resonance imaging (MRI) hyperintensities are common in Alzheimer’s disease (AD), but their pathophysiological relevance and relationship to genetic factors are unclear. In the present study, we investigated potential apolipoprotein E (APOE)-dependent effects on the extent and cognitive impact of WM hyperintensities in patients with AD. Methods: WM hyperintensity volume on fluid-attenuated inversion recovery images of 201 patients with AD (128 carriers and 73 non-carriers of the APOE ε4 risk allele) was determined globally as well as regionally with voxel-based lesion mapping. Clinical, neuropsychological and MRI data were collected from prospective multicenter trials conducted by the German Dementia Competence Network. Results: WM hyperintensity volume was significantly greater in non-carriers of the APOE ε4 allele. Lesion distribution was similar among ε4 carriers and non-carriers. Only ε4 non-carriers showed a correlation between lesion volume and cognitive performance. Conclusion: The current findings indicate an increased prevalence of WM hyperintensities in non-carriers compared with carriers of the APOE ε4 allele among patients with AD. This is consistent with a possibly more pronounced contribution of heterogeneous vascular risk factors to WM damage and cognitive impairment in patients with AD without APOE ε4-mediated risk
Evaluating Depressive Symptoms in Schizophrenia: A Psychometric Comparison of the Calgary Depression Scale for Schizophrenia and the Hamilton Depression Rating Scale
Background: The aim of this study was to compare two measures of depression in patients with schizophrenia and schizophrenia spectrum disorder, including patients with delusional and schizoaffective disorder, to conclude implications for their application. Sampling and Methods: A total of 278 patients were assessed using the Calgary Depression Scale for Schizophrenia (CDSS) and the Hamilton Depression Rating Scale (HAMD-17). The Positive and Negative Syndrome Scale (PANSS) was also applied. At admission and discharge, a principal component analysis was performed with each depression scale. The two depression rating scales were furthermore compared using correlation and regression analyses. Results: Three factors were revealed for the CDSS and HAMD-17 factor component analysis. A very similar item loading was found for the CDSS at admission and discharge, whereas results of the loadings of the HAMD-17 items were less stable. The first two factors of the CDSS revealed correlations with positive, negative and general psychopathology. In contrast, multiple significant correlations were found for the HAMD-17 factors and the PANSS sub-scores. Multiple regression analyses demonstrated that the HAMD-17 accounted more for the positive and negative symptom domains than the CDSS. Conclusions:The present results suggest that compared to the HAMD-17, the CDSS is a more specific instrument to measure depressive symptoms in schizophrenia and schizophrenia spectrum disorder, especially in acutely ill patients. Copyright (c) 2012 S. Karger AG, Base
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