4,093 research outputs found
Continuous spectra in high-harmonic generation driven by multicycle laser pulses
We present observations of the emission of XUV continua in the 20-37 eV
region by high harmonic generation (HHG) with - pulses
focused onto a Kr gas jet. The underlying mechanism relies on coherent control
of the relative delays and phases between individually generated attosecond
pulse, achievable by adjusting the chirp of the driving pulses and the
interaction geometry. Under adequate negative chirp and phase matching
conditions, the resulting interpulse interference yields a continuum XUV
spectrum, which is due to both microscopic and macroscopic (propagation)
contributions. This technique opens the route for modifying the phase of
individual attosecond pulses and for the coherent synthesis of XUV continua
from multicycle driving laser pulses without the need of an isolated attosecond
burst.Comment: 14 pages, 5 figures. Submitted to Physical Review
A mid-IR survey of the L 1641-N region with ISOCAM
We present an analysis of the L 1641 outflow region using broad-band and
narrow-band imaging data at mid-infrared wavelengths from ISOCAM. We detect a
total of 34 sources in the x region covered by
the broad-band filters. Four of these sources have no reported detection in
previous studies of the region. We find that the source previously identified
as the near-IR counter-part to the IRAS detected point-source (IRAS 05338-0624)
is not the brightest source in the wavelength region of the IRAS 12 \micron\
filter. We find instead that a nearby object (within the beam of IRAS and not
detected at near-IR wavelengths) outshines all others sources in the area by a
factor of 2. We submit that this source is likely to be the IRAS detected
point source. A comparison of the near-IR (J-H vs H-K) and mid-IR (J-K vs [6.7
um]-[14 um]) color-color plots shows only four sources with excess emission at
near-IR wavelengths, but atleast 85% of all sources show excess emission at
mid-IR wavelengths. The CVF spectra suggest a range of evolutionary status in
the program stars ranging from embedded YSOs to the young disks. When combined
with optical and near-IR age estimates, these results show active current
star-formation in the region that has been on-going for at least 2 Myr.Comment: Submitted to ApJ. Abstracted edited for arXiv submission Replaced by
version accepted by Ap
Probing halo nucleus structure through intermediate energy elastic scattering
This work addresses the question of precisely what features of few body
models of halo nuclei are probed by elastic scattering on protons at high
centre-of-mass energies. Our treatment is based on a multiple scattering
expansion of the proton-projectile transition amplitude in a form which is well
adapted to the weakly bound cluster picture of halo nuclei. In the specific
case of Li scattering from protons at 800 MeV/u we show that because
core recoil effects are significant, scattering crosssections can not, in
general, be deduced from knowledge of the total matter density alone.
We advocate that the optical potential concept for the scattering of halo
nuclei on protons should be avoided and that the multiple scattering series for
the full transition amplitude should be used instead.Comment: 8 pages REVTeX, 1 eps figure, accepted for publication in Phys. Rev.
The Luminous Blue Variable RMC127 as seen with ALMA and ATCA
We present ALMA and ATCA observations of the luminous blue variable \rmc. The
radio maps show for the first time the core of the nebula and evidence that the
nebula is strongly asymmetric with a Z-pattern shape. Hints of this morphology
are also visible in the archival \emph{HST} image, which overall
resembles the radio emission. The emission mechanism in the outer nebula is
optically thin free-free in the radio. At high frequencies, a component of
point-source emission appears at the position of the star, up to the ALMA
frequencies. The rising flux density distribution () of this object suggests thermal emission from the ionized
stellar wind and indicates a departure from spherical symmetry with
. We examine different scenarios to explain this excess
of thermal emission from the wind and show that this can arise from a bipolar
outflow, supporting the suggestion by other authors that the stellar wind of
\rmc is aspherical. We fit the data with two collimated ionized wind models and
we find that the mass-loss rate can be a factor of two or more smaller than in
the spherical case. We also fit the photometry obtained by IR space telescopes
and deduce that the mid- to far-IR emission must arise from extended, cool
() dust within the outer ionized nebula. Finally we discuss two
possible scenarios for the nebular morphology: the canonical single star
expanding shell geometry, and a precessing jet model assuming presence of a
companion star.Comment: Accepted for publication in ApJ (minor revision included
Gamma-widths, lifetimes and fluctuations in the nuclear quasi-continuum
Statistical -decay from highly excited states is determined by the
nuclear level density (NLD) and the -ray strength function
(SF). These average quantities have been measured for several nuclei
using the Oslo method. For the first time, we exploit the NLD and SF to
evaluate the -width in the energy region below the neutron binding
energy, often called the quasi-continuum region. The lifetimes of states in the
quasi-continuum are important benchmarks for a theoretical description of
nuclear structure and dynamics at high temperature. The lifetimes may also have
impact on reaction rates for the rapid neutron-capture process, now
demonstrated to take place in neutron star mergers.Comment: CGS16, Shanghai 2017, Proceedings, 5 pages, 3 figure
Grain size limits derived from 3.6 {\mu}m and 4.5 {\mu}m coreshine
Recently discovered scattered light from molecular cloud cores in the
wavelength range 3-5 {\mu}m (called "coreshine") seems to indicate the presence
of grains with sizes above 0.5 {\mu}m. We aim to analyze 3.6 and 4.5 {\mu}m
coreshine from molecular cloud cores to probe the largest grains in the size
distribution. We analyzed dedicated deep Cycle 9 Spitzer IRAC observations in
the 3.6 and 4.5 {\mu}m bands for a sample of 10 low-mass cores. We used a new
modeling approach based on a combination of ratios of the two background- and
foreground-subtracted surface brightnesses and observed limits of the optical
depth. The dust grains were modeled as ice-coated silicate and carbonaceous
spheres. We discuss the impact of local radiation fields with a spectral slope
differing from what is seen in the DIRBE allsky maps. For the cores L260,
ecc806, L1262, L1517A, L1512, and L1544, the model reproduces the data with
maximum grain sizes around 0.9, 0.5, 0.65, 1.5, 0.6, and > 1.5 {\mu}m,
respectively. The maximum coreshine intensities of L1506C, L1439, and L1498 in
the individual bands require smaller maximum grain sizes than derived from the
observed distribution of band ratios. Additional isotropic local radiation
fields with a spectral shape differing from the DIRBE map shape do not remove
this discrepancy. In the case of Rho Oph 9, we were unable to reliably
disentangle the coreshine emission from background variations and the strong
local PAH emission. Considering surface brightness ratios in the 3.6 and 4.5
{\mu}m bands across a molecular cloud core is an effective method of
disentangling the complex interplay of structure and opacities when used in
combination with observed limits of the optical depth.Comment: 23 pages, 18 figures, accepted for publication in A&
Star Formation and Feedback in Dwarf Galaxies
We examine the star formation history and stellar feedback effects of dwarf
galaxies under the influence of extragalactic ultraviolet radiation. We
consider the dynamical evolution of gas in dwarf galaxies using a
one-dimensional, spherically symmetric, Lagrangian numerical scheme to compute
the effects of radiative transfer and photoionization. We include a
physically-motivated star formation recipe and consider the effects of
feedback. Our results indicate that star formation in the severe environment of
dwarf galaxies is a difficult and inefficient process. For intermediate mass
systems, such as the dSphs around the Galaxy, star formation can proceed with
in early cosmic epochs despite the intense background UV flux. Triggering
processes such as merger events, collisions, and tidal disturbance can lead to
density enhancements, reducing the recombination timescale, allowing gas to
cool and star formation to proceed. However, the star formation and gas
retention efficiency may vary widely in galaxies with similar dark matter
potentials, because they depend on many factors, such as the baryonic fraction,
external perturbation, IMF, and background UV intensity. We suggest that the
presence of very old stars in these dwarf galaxies indicates that their initial
baryonic to dark matter content was comparable to the cosmic value. This
constraint suggests that the initial density fluctuation of baryonic matter may
be correlated with that of the dark matter. For the more massive dwarf
elliptical galaxies, the star formation efficiency and gas retention rate is
much higher. Their mass to light ratio is regulated by star formation feedback,
and is expected to be nearly independent of their absolute luminosity. The
results of our theoretical models reproduce the observed correlation.Comment: 35 pages, 13 figure
Controlled Anisotropic Deformation of Ag Nanoparticles by Si Ion Irradiation
The shape and alignment of silver nanoparticles embedded in a glass matrix is
controlled using silicon ion irradiation. Symmetric silver nanoparticles are
transformed into anisotropic particles whose larger axis is along the ion beam.
Upon irradiation, the surface plasmon resonance of symmetric particles splits
into two resonances whose separation depends on the fluence of the ion
irradiation. Simulations of the optical absorbance show that the anisotropy is
caused by the deformation and alignment of the nanoparticles, and that both
properties are controlled with the irradiation fluence.Comment: Submitted to Phys. Rev. Lett. (October 14, 2005
Study of MicroPattern Gaseous detectors with novel nanodiamond based photocathodes for single photon detection in EIC RICH
Identification of high momentum hadrons at the future EIC is crucial, gaseous
RICH detectors are therefore viable option. Compact collider setups impose to
construct RICHes with small radiator length, hence significantly limiting the
number of detected photons. More photons can be detected in the far UV region,
using a windowless RICH approach. QE of CsI degrades under strong irradiation
and air contamination. Nanodiamond based photocathodes (PCs) are being
developed as an alternative to CsI. Recent development of layers of
hydrogenated nanodiamond powders as an alternative photosensitive material and
their performance, when coupled to the THick Gaseous Electron Multipliers
(THGEM)-based detectors, are the objects of an ongoing R\&D. We report about
the initial phase of our studies.Comment: 3 pages, 5 figures, RICH2018 conference proceedin
- …