4,444 research outputs found
A constraint on a varying proton--electron mass ratio 1.5 billion years after the Big Bang
A molecular hydrogen absorber at a lookback time of 12.4 billion years,
corresponding to 10 of the age of the universe today, is analyzed to put a
constraint on a varying proton--electron mass ratio, . A high resolution
spectrum of the J14432724 quasar, which was observed with the Very Large
Telescope, is used to create an accurate model of 89 Lyman and Werner band
transitions whose relative frequencies are sensitive to , yielding a limit
on the relative deviation from the current laboratory value of
.Comment: Accepted for publication in PRL. Includes supplemental materia
DichroMatch: a website for similarity searching of circular dichroism spectra
Circular dichroism (CD) spectroscopy is a widely used method for examining the structure, folding and conformational changes of proteins. A new online CD analysis server (DichroMatch) has been developed for identifying proteins with similar spectral characteristics by detecting possible structurally and functionally related proteins and homologues. DichroMatch includes six different methods for determining the spectral nearest neighbours to a query protein spectrum and provides metrics of how similar these spectra are and, if corresponding crystal structures are available for the closest matched proteins, information on their secondary structures and fold classifications. By default, DichroMatch uses all the entries in the Protein Circular Dichroism Data Bank (PCDDB) for its comparison set, providing the broadest range of publicly available protein spectra to match with the unknown protein. Alternatively, users can download or create their own specialized data sets, thereby enabling comparisons between the structures of related proteins such as wild-type versus mutants or homologues or a series of spectra of the same protein under different conditions. The DichroMatch server is freely available at http://dichromatch.cryst.bbk.ac.uk
First Starbursts at high redshift: Formation of globular clusters
Numerical simulations of a Milky Way-size galaxy demonstrate that globular
clusters with the properties similar to observed can form naturally at z > 3 in
the concordance Lambda-CDM cosmology. The clusters in our model form in the
strongly baryon-dominated cores of supergiant molecular clouds. The first
clusters form at z = 12, while the peak formation appears to be at z = 3-5. The
zero-age mass function of globular clusters can be approximated by a power-law
dN/dM ~ M^-2, in agreement with observations of young massive star clusters.Comment: 4 pages, proceedings of the "Multi-Wavelength Cosmology" meeting,
June 200
The Infrared Massive Stellar Content of M83
We present an analysis of archival Spitzer images and new ground-based and
Hubble Space Telescope (HST) near-infrared (IR) and optical images of the field
of M83 with the goal of identifying rare, dusty, evolved massive stars. We
present point source catalogs consisting of 3778 objects from
Infrared Array Camera (IRAC) Band 1 (3.6 m) and Band 2 (4.5 m), and
975 objects identified in Magellan 6.5m FourStar near-IR and
images. A combined catalog of coordinate matched near- and mid-IR point sources
yields 221 objects in the field of M83. Using this photometry we identify 185
massive evolved stellar candidates based on their location in color-magnitude
and color-color diagrams. We estimate the background contamination to our
stellar candidate lists and further classify candidates based on their
appearance in Wide Field Camera 3 (WFC3) observations of M83. We find 49
strong candidates for massive stars which are very promising objects for
spectroscopic follow-up. Based on their location in a versus
diagram, we expect at least 24, or roughly 50%, to be confirmed as red
supergiants.Comment: 32 pages, 23 figures, accepted for publication in A&
ValiDichro: a website for validating and quality control of protein circular dichroism spectra
Circular dichroism (CD) spectroscopy is widely used in structural biology as a technique for examining the structure, folding and conformational changes of proteins. A new server, ValiDichro, has been developed for checking the quality and validity of CD spectral data and metadata, both as an aid to data collection and processing and as a validation procedure for spectra to be included in publications. ValiDichro currently includes 25 tests for data completeness, consistency and quality. For each test that is done, not only is a validation report produced, but the user is also provided with suggestions for correcting or improving the data. The ValiDichro server is freely available at http://valispec.cryst.bbk.ac.uk/circularDichroism/ValiDichro/upload.html
The Globular Cluster System in the Inner Region of M87
1057 globular cluster candidates have been identified in a WFPC2 image of the
inner region of M87. The Globular Cluster Luminosity Function (GCLF) can be
well fit by a Gaussian profile with a mean value of m_V^0=23.67 +/- 0.07 mag
and sigma=1.39 +/- 0.06 mag (compared to m_V^0=23.74 mag and sigma=1.44 mag
from an earlier study using the same data by Whitmore it et al. 1995). The GCLF
in five radial bins is found to be statistically the same at all points,
showing no clear evidence of dynamical destruction processes based on the
luminosity function (LF), in contradiction to the claim by Gnedin (1997).
Similarly, there is no obvious correlation between the half light radius of the
clusters and the galactocentric distance. The core radius of the globular
cluster density distribution is R_c=56'', considerably larger than the core of
the stellar component (R_c=6.8''). The mean color of the cluster candidates is
V-I=1.09 mag which corresponds to an average metallicity of Fe/H = -0.74 dex.
The color distribution is bimodal everywhere, with a blue peak at V-I=0.95 mag
and a red peak at V-I=1.20 mag. The red population is only 0.1 magnitude bluer
than the underlying galaxy, indicating that these clusters formed late in the
metal enrichment history of the galaxy and were possibly created in a burst of
star/cluster formation 3-6 Gyr after the blue population. We also find that
both the red and the blue cluster distributions have a more elliptical shape
(Hubble type E3.5) than the nearly spherical galaxy. The average half light
radius of the clusters is ~2.5 pc which is comparable to the 3 pc average
effective radius of the Milky Way clusters, though the red candidates are ~20%
smaller than the blue ones.Comment: 40 pages, 17 figures, 4 tables, latex, accepted for publication in
the Ap
Fluid Induced Particle Size Segregation in Sheared Granular Assemblies
We perform a two-dimensional molecular-dynamics study of a model for sheared
bidisperse granular systems under conditions of simple shear and Poiseuille
flow. We propose a mechanism for particle-size segregation based on the
observation that segregation occurs if the viscous length scale introduced by a
liquid in the system is smaller than of the order of the particle size. We show
that the ratio of shear rate to viscosity must be small if one wants to find
size segregation. In this case the particles in the system arrange themselves
in bands of big and small particles oriented along the direction of the flow.
Similarly, in Poiseuille flow we find the formation of particle bands. Here, in
addition, the variety of time scales in the flow leads to an aggregation of
particles in the zones of low shear rate and can suppress size segregation in
these regions. The results have been verified against simulations using a full
Navier-Stokes description for the liquid.Comment: 11 pages, REVTEX format, ps figures compressed uuencoded separately
or by e-mail from [email protected]. A postscript version of the
paper will be available from
http://www.ica1.uni-stuttgart.de/local/WWW/papers/papers.htm
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