10,102 research outputs found
The effect of placebo and neurophysiological involvements
Placebo and placebo effect are important issues related to the drug therapy for clinical and scientific meanings. The rates of placebo
may get as many as 50% for analgesic drugs in headache. The high answer to placebo brings questions on pathophysiology of headache. Answers may offer a new strategy in the implementation of trials and new insight in neurophysiology of headache. Current knowledge on
placebo and placebo effect will be analysed and dicussed looking for new direction in headache field
The luminosity of GRB afterglows as distance estimator
We investigate the clustering of afterglow light curves observed at X-ray and
optical wavelengths. We have constructed a sample of 61 bursts with known
distance and X-ray afterglow. GRB sources can be divided in three classes,
namely optical and X-ray bright afterglows, optical and X-ray dim one s, and
optically bright -X-ray dim ones. We argue that this clustering is related to
the fireball total energy, the external medium density, the fraction of
fireball energy going in relativistic electrons and magnetic fields. We propose
a method for the estimation of the GRB source redshift based on the observe d
X-ray flux one day after the burst and optical properties. We tested this
method on three recently detected SWIFT GRBs with known redshift, and found it
i n good agreement with the reported distance from optical spectroscopy.Comment: 6 pages, proceeding of the PCHE session at the Journees de la SF2
Three-dimensional structure of the Upper Scorpius association with the Gaia first data release
Using new proper motion data from recently published catalogs, we revisit the
membership of previously identified members of the Upper Scorpius association.
We confirmed 750 of them as cluster members based on the convergent point
method, compute their kinematic parallaxes and combined them with Gaia
parallaxes to investigate the 3D structure and geometry of the association
using a robust covariance method. We find a mean distance of ~pc
and show that the morphology of the association defined by the brightest (and
most massive) stars yields a prolate ellipsoid with dimensions of
~pc, while the faintest cluster members define a more
elongated structure with dimensions of ~pc. We
suggest that the different properties of both populations is an imprint of the
star formation history in this region.Comment: 5 pages, 1 figure, MNRAS letters (in press
Magnetic Diffusion in Star Formation
Magnetic diffusion plays a vital role in star formation. We trace its
influence from interstellar cloud scales down to star-disk scales. On both
scales, we find that magnetic diffusion can be significantly enhanced by the
buildup of strong gradients in magnetic field structure. Large scale nonlinear
flows can create compressed cloud layers within which ambipolar diffusion
occurs rapidly. However, in the flux-freezing limit that may be applicable to
photoionized molecular cloud envelopes, supersonic motions can persist for long
times if driven by an externally generated magnetic field that corresponds to a
subcritical mass-to-flux ratio. In the case of protostellar accretion, rapid
magnetic diffusion (through Ohmic dissipation with additional support from
ambipolar diffusion) near the protostar causes dramatic magnetic flux loss. By
doing so, it also allows the formation of a centrifugal disk, thereby avoiding
the magnetic braking catastrophe.Comment: 5 pages, 4 figures. Conference proceedings of IAU Symposium 270,
Computational Star Formation (eds. Alves, Elmegreen, Girart, Trimble
Low noise tunnel diode receivers for satellite application
Low noise tunnel diode receivers for satellite application
Delayed Recombination and Cosmic Parameters
Current cosmological constraints from Cosmic Microwave Background (CMB)
anisotropies are typically derived assuming a standard recombination scheme,
however additional resonance and ionizing radiation sources can delay
recombination, altering the cosmic ionization history and the cosmological
inferences drawn from CMB data. We show that for recent observations of CMB
anisotropy, from the Wilkinson Microwave Anisotropy Probe satellite mission
5-year survey (WMAP5) and from the ACBAR experiment, additional resonance
radiation is nearly degenerate with variations in the spectral index, n_s, and
has a marked effect on uncertainties in constraints on the Hubble constant, age
of the universe, curvature and the upper bound on the neutrino mass. When a
modified recombination scheme is considered, the redshift of recombination is
constrained to z_*=1078\pm11, with uncertainties in the measurement weaker by
one order of magnitude than those obtained under the assumption of standard
recombination while constraints on the shift parameter are shifted by 1-sigma
to R=1.734\pm0.028. Although delayed recombination limits the precision of
parameter estimation from the WMAP satellite, we demonstrate that this should
not be the case for future, smaller angular scales measurements, such as those
by the Planck satellite mission.Comment: 9 pages, 9 figure
Thermal evolution of the primordial clouds in warm dark matter models with keV sterile neutrinos
We analyze the processes relevant for star formation in a model with dark
matter in the form of sterile neutrinos. Sterile neutrino decays produce an
X-ray background radiation that has a two-fold effect on the collapsing clouds
of hydrogen. First, the X-rays ionize the gas and cause an increase in the
fraction of molecular hydrogen, which makes it easier for the gas to cool and
to form stars. Second, the same X-rays deposit a certain amount of heat, which
could, in principle, thwart the cooling of gas. We find that, in all the cases
we have examined, the overall effect of sterile dark matter is to facilitate
the cooling of gas. Hence, we conclude that dark matter in the form of sterile
neutrinos can help the early collapse of gas clouds and the subsequent star
formation.Comment: aastex, 31 pages, 4 figures; one figure and some references added,
minor changes in the text; to appear in Astrophysical Journa
Nonaxisymmetric Evolution of Magnetically Subcritical Clouds: Bar Growth, Core Elongation, and Binary Formation
We have begun a systematic numerical study of the nonlinear growth of
nonaxisymmetric perturbations during the ambipolar diffusion-driven evolution
of initially magnetically subcritical molecular clouds, with an eye on the
formation of binaries, multiple stellar systems and small clusters. In this
initial study, we focus on the (or bar) mode, which is shown to be
unstable during the dynamic collapse phase of cloud evolution after the central
region has become magnetically supercritical. We find that, despite the
presence of a strong magnetic field, the bar can grow fast enough that for a
modest initial perturbation (at 5% level) a large aspect ratio is obtained
during the isothermal phase of cloud collapse. The highly elongated bar is
expected to fragment into small pieces during the subsequent adiabatic phase.
Our calculations suggest that the strong magnetic fields observed in some
star-forming clouds and envisioned in the standard picture of single star
formation do not necessarily suppress bar growth and fragmentation; on the
contrary, they may actually promote these processes, by allowing the clouds to
have more than one (thermal) Jeans mass to begin with without collapsing
promptly. Nonlinear growth of the bar mode in a direction perpendicular to the
magnetic field, coupled with flattening along field lines, leads to the
formation of supercritical cores that are triaxial in general. It removes a
longstanding objection to the standard scenario of isolated star formation
involving subcritical magnetic field and ambipolar diffusion based on the
likely prolate shape inferred for dense cores. Continuted growth of the bar
mode in already elongated starless cores, such as L1544, may lead to future
binary and multiple star formation.Comment: 5 pages, 2 figures, accepted by ApJ
Inhomogeneous holographic thermalization
The sudden injection of energy in a strongly coupled conformal field theory
and its subsequent thermalization can be holographically modeled by a shell
falling into anti-de Sitter space and forming a black brane. For a homogeneous
shell, Bhattacharyya and Minwalla were able to study this process analytically
using a weak field approximation. Motivated by event-by-event fluctuations in
heavy ion collisions, we include inhomogeneities in this model, obtaining
analytic results in a long wavelength expansion. In the early-time window in
which our approximations can be trusted, the resulting evolution matches well
with that of a simple free streaming model. Near the end of this time window,
we find that the stress tensor approaches that of second-order viscous
hydrodynamics. We comment on possible lessons for heavy ion phenomenology.Comment: 53 pages, 10 figures; v2: references adde
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