4,197 research outputs found
Photoassociative spectroscopy at long range in ultracold strontium
We report photoassociative spectroscopy of Sr in a magneto-optical
trap operating on the intercombination line at 689 nm.
Photoassociative transitions are driven with a laser red-detuned by 600-2400
MHz from the atomic resonance at 461 nm. Photoassociation
takes place at extremely large internuclear separation, and the
photoassociative spectrum is strongly affected by relativistic retardation. A
fit of the transition frequencies determines the atomic lifetime
( ns) and resolves a discrepancy between experiment and
recent theoretical calculations.Comment: 4 pages, 4 figures, submitte
Self-organized criticality in the hysteresis of the Sherrington - Kirkpatrick model
We study hysteretic phenomena in random ferromagnets. We argue that the angle
dependent magnetostatic (dipolar) terms introduce frustration and long range
interactions in these systems. This makes it plausible that the Sherrington -
Kirkpatrick model may be able to capture some of the relevant physics of these
systems. We use scaling arguments, replica calculations and large scale
numerical simulations to characterize the hysteresis of the zero temperature SK
model. By constructing the distribution functions of the avalanche sizes,
magnetization jumps and local fields, we conclude that the system exhibits
self-organized criticality everywhere on the hysteresis loop.Comment: 4 pages, 4 eps figure
Infrared Emission from the Nearby Cool Core Cluster Abell 2597
We observed the brightest central galaxy (BCG) in the nearby (z=0.0821) cool
core galaxy cluster Abell 2597 with the IRAC and MIPS instruments on board the
Spitzer Space Telescope. The BCG was clearly detected in all Spitzer
bandpasses, including the 70 and 160 micron wavebands. We report aperture
photometry of the BCG. The spectral energy distribution exhibits a clear excess
in the FIR over a Rayleigh-Jeans stellar tail, indicating a star formation rate
of ~4-5 solar masses per year, consistent with the estimates from the UV and
its H-alpha luminosity. This large FIR luminosity is consistent with that of a
starburst or a Luminous Infrared Galaxy (LIRG), but together with a very
massive and old population of stars that dominate the energy output of the
galaxy. If the dust is at one temperature, the ratio of 70 to 160 micron fluxes
indicate that the dust emitting mid-IR in this source is somewhat hotter than
the dust emitting mid-IR in two BCGs at higher-redshift (z~0.2-0.3) and higher
FIR luminosities observed earlier by Spitzer, in clusters Abell 1835 and Zwicky
3146.Comment: Accepted at Ap
Nonuniversal scaling behavior of Barkhausen noise
We simulate Barkhausen avalanches on fractal clusters in a two-dimensional
diluted Ising ferromagnet with an effective Gaussian random field. We vary the
concentration of defect sites and find a scaling region for moderate
disorder, where the distribution of avalanche sizes has the form . The exponents for size
and for length distribution, and the fractal dimension of
avalanches satisfy the scaling relation .
For fixed disorder the exponents vary with driving rate in agreement with
experiments on amorphous Si-Fe alloys.Comment: 5 pages, Latex, 4 PostScript figures include
Evolution of Bilayer Quantum Hall Ferromagnet
The natures of the ground state in a bilayer quantum Hall
system at a variety of layer spacing are investigated. At small layer
separations the system exhibits spontaneous interlayer phase coherence. It is
claimed that the Halperin's (1,1,1) state is not relevant in the incompressible
regime near the incompressible to compressible transition point in which the
Josephson-like effect was observed. The two-particle correlation function shows
the deflated correlation hole at this regime. An effective model that can give
a good approximation to the ground state is proposed. A connection to the
modified composite fermion theory is discussed
Quantum degeneracy and interaction effects in spin-polarized Fermi-Bose mixtures
Various features of spin-polarized Fermi gases confined in harmonic traps are
discussed, taking into account possible perspectives of experimental
measurements. The mechanism of the expansion of the gas is explicitly
investigated and compared with the one of an interacting Bose gas. The role of
interactions on the equilibrium and non equilibrium behaviour of the fermionic
component in Fermi-Bose mixtures is discussed. Special emphasis is given to the
case of potassium isotopes mixtures.Comment: 5 pages, 3 figures, revtex, to be published in J. Phys.
The ACS Virgo Cluster Survey III. Chandra and HST Observations of Low-Mass X-Ray Binaries and Globular Clusters in M87
The ACIS instrument on board the Chandra X-ray Observatory has been used to
carry out the first systematic study of low-mass X-ray binaries (LMXBs) in M87.
We identify 174 X-ray point-sources, of which ~150 are likely LMXBs. This LMXB
catalog is combined with deep F475W and F850LP images taken with ACS on HST to
examine the connection between LMXBs and globular clusters in M87. Of the 1688
globular clusters in our catalog, f_X = 3.6 +- 0.5% contain a LMXB and we find
that the metal-rich clusters are 3 +- 1 times more likely to harbor a LMXB than
their metal-poor counterparts. In agreement with previous findings for other
galaxies, we find that brighter, more metal-rich clusters are more likely to
contain a LMXB. For the first time, however, we are able to demonstrate that
the probability, p_X, that a given cluster will contain a LMXB depends
sensitively on the dynamical properties of the host cluster. Specifically, we
use the HST images to measure the half-light radius, concentration index and
central density, \rho_0, for each globular, and define a parameter, \Gamma,
which is related to the tidal capture and binary-neutron star exchange rate.
Our preferred form for p_X is then p_X \propto \Gamma \rho_0^{-0.42\pm0.11}
(Z/Z_{\odot})^{0.33\pm0.1}. We argue that if the form of p_X is determined by
dynamical processes, then the observed metallicity dependence is a consequence
of an increased number of neutron stars per unit mass in metal-rich globular
clusters. Finally, we find no compelling evidence for a break in the luminosity
distribution of resolved X-ray point sources. Instead, the LMXB luminosity
function is well described by a power law with an upper cutoff at L_X ~ 10^39
erg/s. (abridged)Comment: 23 pages, 21 figures. Accepted for publication in ApJ. Also available
at http://www.physics.rutgers.edu/~pcote/acs/publications.htm
Transport of Proteins into Mitochondria
The mitochondrial ADP/ATP carrier is an integral transmembrane protein of the inner membrane. It is synthesized on cytoplasmic ribosomes. Kinetic data suggested that this protein is transferred into mitochondria in a posttranslational manner. The following results provide further evidence for such a mechanism and provide information on its details.
1. In homologous and heterologous translation systems the newly synthesized ADP/ATP carrier protein is present in the postribosomal supernatant.
2. Analysis by density gradient centrifugation and gel filtration shows, that the ADP/ATP carrier molecules in the postribosomal fraction are present as soluble complexes with apparent molecular weights of about 120000 and 500000 or larger. The carrier binds detergents such as Triton X-100 and deoxycholate forming mixed micelles with molecular weights of about 200000–400000.
3. Incubation of a postribosomal supernatant of a reticulocyte lysate containing newly synthesized ADP/ATP carrier with mitochondria isolated from Neurospora spheroplasts results in efficient transfer of the carrier into mitochondria. About 20–30% of the transferred carrier are resistant to proteinase in whole mitochondria. The authentic mature protein is also largely resistant to proteinase in whole mitochondria and sensitive after lysis of mitochondria with detergent. Integrity of mitochondria is a prerequisite for translocation into proteinase resistant position.
4. The transfer in vitro into a proteinase-resistant form is inhibited by the uncoupler carbonyl-cyanide m-chlorophenylhydrazone but not the proteinase-sensitive binding.
These observations suggest that the posttranslational transfer of ADP/ATP carrier occurs via the cytosolic space through a soluble oligomeric precursor form. This precursor is taken up by intact mitochondria into an integral position in the membrane. These findings are considered to be of general importance for the intracellular transfer of insoluble membrane proteins. They support the view that such proteins can exist in a water-soluble form its precursors and upon integration into the membrane undergo a conformational change. Uptake into the membrane may involve the cleavage of an additional sequence in some proteins, but this appears not to be a prerequisite as demonstrated by the ADP/ATP carrier protein
Comparing eDNA metabarcoding and conventional pelagic netting to inform biodiversity monitoring in deep ocean environments
The performance of environmental DNA (eDNA) metabarcoding has rarely been evaluated against conventional sampling methods in deep
ocean mesopelagic environments. We assessed the biodiversity patterns generated with eDNA and two co-located conventional methods,
oblique midwater trawls and vertical multinets, to compare regional and sample-level diversity. We then assessed the concordance of ecological
patterns across water column habitats and evaluated how DNA markers and the level of sampling effort influenced the inferred community. We
found eDNA metabarcoding characterized regional diversity well, detecting more taxa while identifying similar ecological patterns as conventional
samples. Within sampling locations, eDNA metabarcoding rarely detected taxa across more than one replicate. While more taxa were found in
eDNA than oblique midwater trawls within sample stations, fewer were found compared to vertical multinets. Our simulations show greater
eDNA sampling effort would improve concordance with conventional methods. We also observed that using taxonomic data from multiple
markers generated ecological patterns most similar to those observed with conventional methods. Patterns observed with Exact Sequence
Variants were more stable across markers suggesting they are more powerful for detecting change. eDNA metabarcoding is a valuable tool for
identifying and monitoring biological hotspots but some methodological adjustments are recommended for deep ocean environments
Different Transport Pathways of Individual Precursor Proteins in Mitochondria
Transport of mitochondrial precursor proteins into mitochondria of Neurospora crassa was studied in a cellfree reconstituted system. Precursors were synthesized in a reticulocyte lysate programmed with Neurospora mRNA and transported into isolated mitochondria in the absence of protein synthesis. Uptake of the following precursors was investigated: apocytochrome c, ADP/ATP carrier and subunit 9 of the oligomycin-sensitive ATPase.
Addition of high concentrations of unlabelled chemically prepared apocytochrome c (1–10 μM) inhibited the appearance in the mitochondrial of labelled cytochrome c synthesized in vitro because the unlabelled protein dilutes the labelled one and because the translocation system has a limited capacity [apparent V is 1–3 pmol × min−1× (mg mitochondrial protein)−1]. Concentrations of added apocytochrome c exceeding the concentrations of precursor proteins synthesized in vitro by a factor of about 104 did not inhibit the transfer of ADP/ATP carrier or ATPase subunit 9 into mitochondria. Carbonylcyanide m-chlorophenylhydrazone, an uncoupler of oxidative phosphorylation, inhibited transfer in vitro of ADP/ATP carrier and of ATPase subunit 9, but not of cytochrome c.
These findings suggest that cytochrome c and the other two proteins have different import pathways into mitochondria. It can be inferred from the data presented that different 'receptors' on the mitochondrial surface mediate the specific recognition of precursor proteins by mitochondria as a first step in the transport process
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