1,750 research outputs found
An H2CO 6cm Maser Pinpointing a Possible Circumstellar Torus in IRAS18566+0408
We report observations of 6cm, 3.6cm, 1.3cm, and 7mm radio continuum,
conducted with the Very Large Array towards IRAS18566+0408, one of the few
sources known to harbor H2CO 6cm maser emission. Our observations reveal that
the emission is dominated by an ionized jet at cm wavelengths. Spitzer/IRAC
images from GLIMPSE support this interpretation, given the presence of 4.5um
excess emission at approximately the same orientation as the cm continuum. The
7mm emission is dominated by thermal dust from a flattened structure almost
perpendicular to the ionized jet, thus, the 7mm emission appears to trace a
torus associated with a young massive stellar object. The H2CO 6cm maser is
coincident with the center of the torus-like structure. Our observations rule
out radiative pumping via radio continuum as the excitation mechanism for the
H2CO 6cm maser in IRAS18566+0408.Comment: 20 pages, 4 figures, ApJ (in press
A measurement of the differential cross section for the two-body photodisintegration of 3He at theta_LAB = 90deg using tagged photons in the energy range 14 -- 31 MeV
The two-body photodisintegration of 3He has been investigated using tagged
photons with energies from 14 -- 31 MeV at MAX-lab in Lund, Sweden. The
two-body breakup channel was unambiguously identified by the (nonsimultaneous)
detection of both protons and deuterons. This approach was made feasible by the
over-determined kinematic situation afforded by the tagged-photon technique.
Proton- and deuteron-energy spectra were measured using four silicon
surface-barrier detector telescopes located at a laboratory angle of 90deg with
respect to the incident photon-beam direction. Average statistical and
systematic uncertainties of 5.7% and 6.6% in the differential cross section
were obtained for 11 photon-energy bins with an average width of 1.2 MeV. The
results are compared to previous experimental data measured at comparable
photon energies as well as to the results of two recent Faddeev calculations
which employ realistic potential models and take into account three-nucleon
forces and final-state interactions. Both the accuracy and precision of the
present data are improved over the previous measurements. The data are in good
agreement with most of the previous results, and favor the inclusion of
three-nucleon forces in the calculations.Comment: 12 pages, 13 figures; further Referee comments addresse
Creation, doubling, and splitting, of vortices in intracavity second harmonic generation
We demonstrate generation and frequency doubling of unit charge vortices in a
linear astigmatic resonator. Topological instability of the double charge
harmonic vortices leads to well separated vortex cores that are shown to
rotate, and become anisotropic, as the resonator is tuned across resonance
A Search for H2CO 6cm Emission toward Young Stellar Objects III: VLA Observations
We report the results of our third survey for formaldehyde (H2CO) 6cm maser
emission in the Galaxy. Using the Very Large Array, we detected two new H2CO
maser sources (G23.01-0.41 and G25.83-0.18), thus increasing the sample of
known H2CO maser regions in the Galaxy to seven. We review the characteristics
of the G23.01-0.41 and G25.83-0.18 star forming regions. The H2CO masers in
G23.01-0.41 and G25.83-0.18 share several properties with the other known H2CO
masers, in particular, emission from rich maser environments and close
proximity to very young massive stellar objects.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
Noisy Monte Carlo: Convergence of Markov chains with approximate transition kernels
Monte Carlo algorithms often aim to draw from a distribution by
simulating a Markov chain with transition kernel such that is
invariant under . However, there are many situations for which it is
impractical or impossible to draw from the transition kernel . For instance,
this is the case with massive datasets, where is it prohibitively expensive to
calculate the likelihood and is also the case for intractable likelihood models
arising from, for example, Gibbs random fields, such as those found in spatial
statistics and network analysis. A natural approach in these cases is to
replace by an approximation . Using theory from the stability of
Markov chains we explore a variety of situations where it is possible to
quantify how 'close' the chain given by the transition kernel is to
the chain given by . We apply these results to several examples from spatial
statistics and network analysis.Comment: This version: results extended to non-uniformly ergodic Markov chain
Probing Electronic Correlations in Actinide Materials Using Multipolar Transitions
We report nonresonant inelastic x-ray scattering from the semi-core 5d levels
of several actinide compounds. Dipole-forbidden, high-multipole features form a
rich bound-state spectrum dependent on valence electron configuration and
spin-orbit and Coulomb interactions. Cross-material comparisons, together with
the anomalously high Coulomb screening required for agreement between atomic
multiplet theory and experiment, demonstrate sensitivity to the neighboring
electronic environment, such as is needed to address long-standing questions of
electronic localization and bonding in 5f compounds.Comment: LA-UR 09-0782
How limit cycles and quasi-cycles are related in systems with intrinsic noise
Fluctuations and noise may alter the behavior of dynamical systems
considerably. For example, oscillations may be sustained by demographic
fluctuations in biological systems where a stable fixed point is found in the
absence of noise. We here extend the theoretical analysis of such stochastic
effects to models which have a limit cycle for some range of the model
parameters. We formulate a description of fluctuations about the periodic orbit
which allows the relation between the stochastic oscillations in the fixed
point phase and the oscillations in the limit cycle phase to be elucidated. In
the case of the limit cycle, a suitable transformation into a co-moving frame
allow fluctuations transverse and longitudinal with respect to the limit cycle
to be effectively decoupled. While longitudinal fluctuations are of a diffusive
nature, those in the transverse direction follow a stochastic path more akin to
an Ornstein-Uhlenbeck process. Their power spectrum is computed analytically
within a van Kampen expansion in the inverse system size. This is carried out
in two different ways, and the subsequent comparison with numerical simulations
illustrates the effects that can occur due to diffusion in the longitudinal
direction.Comment: 15 pages, 14 figure
Performance of clinical guidelines compared with molecular tumour screening methods in identifying possible Lynch syndrome among colorectal cancer patients: a Norwegian population-based study
Do Aphid Colonies Amplify their Emission of Alarm Pheromone?
When aphids are attacked by natural enemies, they emit alarm pheromone to alert conspecifics. For most aphids tested, (E)-ÎČ-farnesene (EBF) is the main, or only, constituent of the alarm pheromone. In response to alarm pheromone, alerted aphids drop off the plant, walk away, or attempt to elude predators. However, under natural conditions, EBF concentration might be low due to the low amounts emitted, to rapid air movement, or to oxidative degradation. To ensure that conspecifics are warned, aphids might conceivably amplify the alarm signal by emitting EBF in response to EBF emitted by other aphids. To examine whether such amplification occurs, we synthesized deuterated EBF (DEBF), which allowed us to differentiate between applied and aphid-derived chemical. Colonies of Acyrthosiphon pisum were treated with DEBF, and headspace volatiles were collected and analyzed for evidence of aphid-derived EBF. No aphid-derived EBF was detected, suggesting that amplification of the alarm signal does not occur. We discuss the disadvantages of alarm signal reinforcement
Ligand-Receptor Interactions
The formation and dissociation of specific noncovalent interactions between a
variety of macromolecules play a crucial role in the function of biological
systems. During the last few years, three main lines of research led to a
dramatic improvement of our understanding of these important phenomena. First,
combination of genetic engineering and X ray cristallography made available a
simultaneous knowledg of the precise structure and affinity of series or
related ligand-receptor systems differing by a few well-defined atoms. Second,
improvement of computer power and simulation techniques allowed extended
exploration of the interaction of realistic macromolecules. Third, simultaneous
development of a variety of techniques based on atomic force microscopy,
hydrodynamic flow, biomembrane probes, optical tweezers, magnetic fields or
flexible transducers yielded direct experimental information of the behavior of
single ligand receptor bonds. At the same time, investigation of well defined
cellular models raised the interest of biologists to the kinetic and mechanical
properties of cell membrane receptors. The aim of this review is to give a
description of these advances that benefitted from a largely multidisciplinar
approach
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