8,693 research outputs found
Open cluster candidates in the VVVX area: VVVX CL 076 and CL 077
We are reporting some basic parameters of two newly discovered clusters, VVVX CL 076 and CL 077, recently discovered in the galactic disk area covered by the VISTA Variables in the Via Lactea eXtended (VVVX) ESO Public Survey. The preliminary analysis shows that both clusters are young and relatively close to the Sun.Peer reviewedFinal Published versio
The State of the Circumstellar Medium Surrounding Gamma-Ray Burst Sources and its Effect on the Afterglow Appearance
We present a numerical investigation of the contribution of the presupernova
ejecta of Wolf-Rayet stars to the environment surrounding gamma-ray bursts
(GRBs), and describe how this external matter can affect the observable
afterglow characteristics. An implicit hydrodynamic calculation for massive
stellar evolution is used here to provide the inner boundary conditions for an
explicit hydrodynamical code to model the circumstellar gas dynamics. The
resulting properties of the circumstellar medium are then used to calculate the
deceleration of a relativistic, gas-dynamic jet and the corresponding afterglow
light curve produced as the shock wave propagates through the shocked-wind
medium. We find that variations in the stellar wind drive instabilities that
may produce radial filaments in the shocked-wind region. These comet-like tails
of clumps could give rise to strong temporal variations in the early afterglow
lightcurve. Afterglows may be expected to differ widely among themselves,
depending on the angular anisotropy of the jet and the properties of the
stellar progenitor; a wide diversity of behaviors may be the rule, rather than
the exception.Comment: 17 pages, 7 figures, ApJ in pres
A theoretical analysis of Ballistic Electron Emission Microscopy: k-space distributions and spectroscopy
We analyze BEEM experiments. At low temperatures and low voltages, near the
threshold value of the Schottky barrier, the BEEM current is dominated by the
elastic component. Elastic scattering by the lattice results in the formation
of focused beams and narrow lines in real space. To obtain the current injected
in the semiconductor, we compute the current distribution in reciprocal space
and, assuming energy and conservation. Our results show an
important focalization of the injected electron beam and explain the similarity
between BEEM currents for Au/Si(111) and Au/Si(100).Comment: 17 pages, 5 figures (postscript), Latex, APS,
http://www.icmm.csic.es/Pandres/pedro.htm. Appl. Surf. Sci. (in press
Compton Echoes from Gamma-Ray Bursts: Unveiling Misaligned Jets in Nearby Type Ib/c Supernovae
There is now compelling evidence of a link between long-duration gamma-ray
bursts (GRBs) and Type Ib/c supernovae (SNe). These core-collapse explosions
are conjectured to radiate an anisotropic, beamed component associated with a
decelerating, relativistic outflow and an unbeamed, isotropic component
associated with the slowly expanding stellar debris. The anisotropic emission
remains at a very low level until the Doppler cone of the beam intersects the
observer's line of sight, making off-axis GRB jets directly detectable only at
long wavelengths and late times. Circumstellar material, however, will Compton
scatter the prompt gamma-ray and afterglow radiation flux and give rise to a
reflection echo. We show that the Compton echo of a misaligned GRB carries an
X-ray luminosity that may exceed by many orders of magnitude that produced by
the underlying subrelativistic SN during the first few weeks. Bright scattering
echoes may therefore provide a means for detecting a population of misaligned
GRBs associated with nearby Type Ib/c SNe and yield crucial information on the
environment surrounding a massive star at the time of its death. The question
of whether the interpretation of GRB980425 as an ordinary GRB observed off-axis
is consistent with the lack of an X-ray echo is addressed, along with the
constraints derived on the possible existence of misaligned GRB jets in
SN1993J, SN1994I, SN1999em, and SN2002ap.Comment: 9 pages, 2 figures, to appear in the ApJ Letter
Characterization of bespoke force sensors for tailored applications
Bespoke force sensors made with active polymer
composites are inexpensive, thin and flexible, hence popular in
wearable electronics, however their wider application is limited
due to the lack of literature studying their voltage response related
errors. We present the voltage response characterization of
bespoke force sensors made with an active polymer composite,
silver coated fabric, stainless steel thread and silver epoxy.
Characterization of the effects of static and dynamic loading was
completed with a mechanical testing machine. Static tests
consisted of loading and unloading at 0.01, 0.1, 0.5 and 1 N/s, and
drift tests for 120 minutes up to 10 N every 1 N. Dynamic tests
consisted of a sinusoidal load of 5 N ± 1 N applied at 0.05, 0.1 and
0.5 Hz for 60 minutes. The force-voltage relationships were
modelled using an exponential function. Maximum mean drift
error was observed when applying different static loads for 120
minutes each. Drift error is minimal at 5 s (<1%)and at 60 (< 5%)
minutes with loads under 1 N. Maximum hysteresis of 18% was
observed at a 1 N/s loading rate. The maximum drift error after
one hour of dynamic loading was observed at 0.5 Hz and is
minimal (-0.00004%). The cost of fabricating these sensors is very
low compared with commercially available options. These sensors
can be fabricated in any shape and size with the added advantage
of being able to set the location of the electronic connections as
desired
Proper motions of the HH1 jet
We describe a new method for determining proper motions of extended objects,
and a pipeline developed for the application of this method. We then apply this
method to an analysis of four epochs of [S~II] HST images of the HH~1 jet
(covering a period of ~yr).
We determine the proper motions of the knots along the jet, and make a
reconstruction of the past ejection velocity time-variability (assuming
ballistic knot motions). This reconstruction shows an "acceleration" of the
ejection velocities of the jet knots, with higher velocities at more recent
times. This acceleration will result in an eventual merging of the knots in
~yr and at a distance of from the outflow source, close to
the present-day position of HH~1.Comment: 12 pages, 8 figure
Critical behavior of 2 and 3 dimensional ferro- and antiferromagnetic spin ice systems in the framework of the Effective Field Renormalization Group technique
In this work we generalize and subsequently apply the Effective Field
Renormalization Group technique to the problem of ferro- and
antiferromagnetically coupled Ising spins with local anisotropy axes in
geometrically frustrated geometries (kagome and pyrochlore lattices). In this
framework, we calculate the various ground states of these systems and the
corresponding critical points. Excellent agreement is found with exact and
Monte Carlo results. The effects of frustration are discussed. As pointed out
by other authors, it turns out that the spin ice model can be exactly mapped to
the standard Ising model but with effective interactions of the opposite sign
to those in the original Hamiltonian. Therefore, the ferromagnetic spin ice is
frustrated, and does not order. Antiferromagnetic spin ice (in both 2 and 3
dimensions), is found to undergo a transition to a long range ordered state.
The thermal and magnetic critical exponents for this transition are calculated.
It is found that the thermal exponent is that of the Ising universality class,
whereas the magnetic critical exponent is different, as expected from the fact
that the Zeeman term has a different symmetry in these systems. In addition,
the recently introduced Generalized Constant Coupling method is also applied to
the calculation of the critical points and ground state configurations. Again,
a very good agreement is found with both exact, Monte Carlo, and
renormalization group calculations for the critical points. Incidentally, we
show that the generalized constant coupling approach can be regarded as the
lowest order limit of the EFRG technique, in which correlations outside a
frustrated unit are neglected, and scaling is substituted by strict equality of
the thermodynamic quantities.Comment: 28 pages, 9 figures, RevTeX 4 Some minor changes in the conclussions.
One reference adde
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