1,184 research outputs found
Range of orbital angular momenta available for complete fusion between heavy ions
The same compound nucleus, 158Er, has been formed through three different entrance channels, with projectiles 16O, 40Ar and 84Kr. Excitation functions for reactions (HI, 5n) and (HI, 6n) are well fitted by statistical model calculations, provided that a certain window in orbital angular momentum should be taken in order to produce complete fusion in the case of Ar ions and Kr ions. Curiously enough, low l-waves should be avoided. It implies that, during the interaction leading to complete fusion, the energy dissipation by tangential friction should be rather large
Conditions determining the morphology and nanoscale magnetism of Co nanoparticles: Experimental and numerical studies
Co-based nanostructures ranging from core-shell to hollow nanoparticles were
produced by varying the reaction time and the chemical environment during the
thermal decomposition of Co2(CO)8. Both structural characterization and kinetic
model simulation illustrate that the diffusivities of Co and oxygen determine
the growth ratio and the final morphology of the nanoparticles. Exchange
coupling between Co and Co-oxide in core/shell nanoparticles induced a shift of
field-cooled hysteresis loops that is proportional to the shell thickness, as
verified by numerical studies. The increased nanocomplexity when going from
core/shell to hollow particles, also leads to the appearance of hysteresis
above 300 K due to an enhancement of the surface anisotropy resulting from the
additional spin-disordered surfaces.Comment: 29 pages including 11 figures embedded. Submitted to Phys. Rev.
Spectrum and Variability of Mrk501 as observed by the CAT Imaging Telescope
The CAT Imaging Telescope has observed the BL Lac object Markarian 501
between March and August 1997. We report here on the variability over this time
including several large flares. We present also preliminary spectra for all
these data, for the low emission state, and for the largest flare.Comment: 4 pages, 4 figures, Late
Very High Energy Gamma-ray spectral properties of Mrk 501 from CAT Cerenkov telescope observations in 1997
The BL Lac object Mrk 501 went into a very high state of activity during
1997, both in VHE gamma-rays and X-rays. We present here results from
observations at energies above 250 GeV carried out between March and October
1997 with the CAT Cerenkov imaging Telescope. The average differential spectrum
between 30 GeV and 13 TeV shows significant curvature and is well represented
by phi_0 * E_TeV^{-(alpha + beta*log10(E_TeV))}, with: phi_0 = 5.19 +/- 0.13
{stat} +/- 0.12 {sys-MC} +1.66/-1.04 {sys-atm} * 10^-11 /cm^2/s/TeV alpha =
2.24 +/- 0.04 {stat} +/- 0.05 {sys} beta = 0.50 +/- 0.07 {stat} (negligible
systematics). The TeV spectral energy distribution of Mrk 501 clearly peaks in
the range 500 GeV-1 TeV. Investigation of spectral variations shows a
significant hardness-intensity correlation with no measurable effect on the
curvature. This can be described as an increase of the peak TeV emission energy
with intensity. Simultaneous and quasi-simultaneous CAT VHE gamma-ray and
BeppoSAX hard X-ray detections for the highest recorded flare on 16th April and
for lower-activity states of the same period show correlated variability with a
higher luminosity in X-rays than in gamma-rays. The observed spectral energy
distribution and the correlated variability between X-rays and gamma-rays, both
in amplitude and in hardening of spectra, favour a two-component emission
scheme where the low and high energy components are attributed to synchrotron
and inverse Compton (IC) radiation, respectively.Comment: Submitted to Astronomy and Astrophysics, 8 pages including 6 figures.
Published with minor change
Observation of the Crab Nebula Gamma-Ray Emission Above 220 Gev by the Cat Cherenkov Imaging Telescope
The CAT imaging telescope, recently built on the site of the former solar
plant Themis (French Pyrenees), observed gamma-rays from the Crab nebula from
October 1996 to March 1997. This steady source, often considered as the
standard candle of very-high-energy gamma-ray astronomy, is used as a test-beam
to probe the performances of the new telescope, particularly its energy
threshold (220 GeV at 20 degrees zenith angle) and the stability of its
response. Due to the fine-grain camera, an accurate analysis of the
longitudinal profiles of shower images is performed, yielding the source
position in two dimensions for each individual shower.Comment: 5 pages, 3 figures, Tex, contribution to 25th ICRC Durba
Detection of Vhe Gamma-Rays from MRK 501 with the Cat Imaging Telescope
The CAT imaging telescope on the site on the former solar plant Themis has
been observing gamma-rays from Mrk501 above 220 GeV in March and April 1997.
This source is shown to be highly variable and the light curve is presented.
The detected gamma-ray rate for the most intense flare is in excess of 10 per
minute.Comment: 5 pages, 4 figures, Tex, contribution to 25th ICRC Durba
Efficient and reliable nonlocal damage models
We present an efficient and reliable approach for the numerical modelling of failure with nonlocal damage models. The two major numerical challenges––the strongly nonlinear, highly localized and parameter-dependent structural response of quasi-brittle materials, and the interaction between nonadjacent finite elements associated to nonlocality––are addressed in detail. Reliability of the numerical results is ensured by an h-adaptive strategy based on error estimation. We use a residual-type error estimator for nonlinear FE analysis based on local computations, which, at the same time, accounts for the nonlocality of the damage model. Efficiency is achieved by a proper combination of load-stepping control technique and iterative solver for the nonlinear equilibrium equations. A major issue is the computation of the consistent tangent matrix, which is nontrivial due to nonlocal interaction between Gauss points. With computational efficiency in mind, we also present a new nonlocal damage model based on the nonlocal average of displacements. For this new model, the consistent tangent matrix is considerably simpler to compute than for current models. The various ideas discussed in the paper are illustrated by means of three application examples: the uniaxial tension test, the three-point bending test and the single-edge notched beam test.Peer ReviewedPostprint (author’s final draft
Transmission spectroscopy of the lowest-density gas giant: metals and a potential extended outflow in HAT-P-67b
Extremely low-density exoplanets are tantalizing targets for atmospheric
characterization because of their promisingly large signals in transmission
spectroscopy. We present the first analysis of the atmosphere of the
lowest-density gas giant currently known, HAT-P-67 b. This inflated Saturn-mass
exoplanet sits at the boundary between hot and ultrahot gas giants, where
thermal dissociation of molecules begins to dominate atmospheric composition.
We observed a transit of HAT-P-67 b at high spectral resolution with CARMENES
and searched for atomic and molecular species using cross-correlation and
likelihood mapping. Furthermore, we explored potential atmospheric escape by
targeting H and the metastable helium line. We detect Ca II and Na I
with significances of 13.2 and 4.6, respectively. Unlike in
several ultrahot Jupiters, we do not measure a day-to-night wind. The large
line depths of Ca II suggest that the upper atmosphere may be more ionized than
models predict. We detect strong variability in H and the helium
triplet during the observations. These signals suggest the possible presence of
an extended planetary outflow that causes an early ingress and late egress. In
the averaged transmission spectrum, we measure redshifted absorption at the
and level in the H and He I triplet lines,
respectively. From an isothermal Parker wind model, we derive a mass loss rate
of and an outflow temperature of . However, due to the lack of a longer out-of-transit baseline in
our data, additional observations are needed to rule out stellar variability as
the source of the H and He signals.Comment: The Astronomical Journal, in press. 17 pages, 9 figure
Measured Spin-Orbit Alignment of Ultra-Short Period Super-Earth 55 Cancri e
A planet's orbital alignment places important constraints on how a planet
formed and consequently evolved. The dominant formation pathway of ultra-short
period planets ( day) is particularly mysterious as such planets most
likely formed further out, and it is not well understood what drove their
migration inwards to their current positions. Measuring the orbital alignment
is difficult for smaller super-Earth/sub-Neptune planets, which give rise to
smaller amplitude signals. Here we present radial velocities across two
transits of 55 Cancri e, an ultra-short period Super-Earth, observed with the
Extreme Precision Spectrograph (EXPRES). Using the classical
Rossiter-McLaughlin (RM) method, we measure 55 Cnc e's sky-projected stellar
spin-orbit alignment (i.e., the projected angle between the planet's orbital
axis and its host star's spin axis) to be with an unprojected angle of . The best-fit RM model to the EXPRES data has a radial velocity
semi-amplitude of just . The spin-orbit
alignment of 55 Cnc e favors dynamically gentle migration theories for
ultra-short period planets, namely tidal dissipation through low-eccentricity
planet-planet interactions and/or planetary obliquity tides.Comment: 12 pages, 4 figures, published in Nature Astronom
The CAT Imaging Telescope for Very-High-Energy Gamma-Ray Astronomy
The CAT (Cherenkov Array at Themis) imaging telescope, equipped with a
very-high-definition camera (546 fast phototubes with 0.12 degrees spacing
surrounded by 54 larger tubes in two guard rings) started operation in Autumn
1996 on the site of the former solar plant Themis (France). Using the
atmospheric Cherenkov technique, it detects and identifies very high energy
gamma-rays in the range 250 GeV to a few tens of TeV. The instrument, which has
detected three sources (Crab nebula, Mrk 421 and Mrk 501), is described in
detail.Comment: 24 pages, 15 figures. submitted to Elsevier Preprin
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