3,614 research outputs found
Performance of two transferred modules in the Lagunera Region: Water relations
Water policy / Performance / Privatization / Irrigation systems / Operations / Maintenance / Irrigation efficiency / Water users' associations / Water rights / Water allocation / Water supply / Water distribution
Lifetime Measurement of the 8s Level in Francium
We measure the lifetime of the 8s level on a magneto-optically trapped sample
of ^{210}Fr atoms with time-correlated single-photon counting. The 7P_{1/2}
state serves as the resonant intermediate level for two-photon excitation of
the 8s level completed with a 1300 nm laser. Analysis of the fluorescence decay
through the the 7P_{3/2} level gives 53.30 +- 0.44 ns for the 8s level
lifetime.Comment: 4 pages, 4 figure
Statistics of Core Lifetimes in Numerical Simulations of Turbulent, Magnetically Supercritical Molecular Clouds
We present measurements of the mean dense core lifetimes in numerical
simulations of magnetically supercritical, turbulent, isothermal molecular
clouds, in order to compare with observational determinations. "Prestellar"
lifetimes (given as a function of the mean density within the cores, which in
turn is determined by the density threshold n_thr used to define them) are
consistent with observationally reported values, ranging from a few to several
free-fall times. We also present estimates of the fraction of cores in the
"prestellar", "stellar'', and "failed" (those cores that redisperse back into
the environment) stages as a function of n_thr. The number ratios are measured
indirectly in the simulations due to their resolution limitations. Our approach
contains one free parameter, the lifetime of a protostellar object t_yso (Class
0 + Class I stages), which is outside the realm of the simulations. Assuming a
value t_yso = 0.46 Myr, we obtain number ratios of starless to stellar cores
ranging from 4-5 at n_thr = 1.5 x 10^4 cm^-3 to 1 at n_thr = 1.2 x 10^5 cm^-3,
again in good agreement with observational determinations. We also find that
the mass in the failed cores is comparable to that in stellar cores at n_thr =
1.5 x 10^4 cm^-3, but becomes negligible at n_thr = 1.2 x 10^5 cm^-3, in
agreement with recent observational suggestions that at the latter densities
the cores are in general gravitationally dominated. We conclude by noting that
the timescale for core contraction and collapse is virtually the same in the
subcritical, ambipolar diffusion-mediated model of star formation, in the model
of star formation in turbulent supercritical clouds, and in a model
intermediate between the previous two, for currently accepted values of the
clouds' magnetic criticality.Comment: 25 pages, 8 figures, ApJ accepted. Fig.1 animation is at
http://www.astrosmo.unam.mx/~e.vazquez/turbulence/movies/Galvan_etal07/Galvan_etal07.htm
Beta-delayed-neutron studies of Sb and I performed with trapped ions
Beta-delayed-neutron (n) spectroscopy was performed using the
Beta-decay Paul Trap and an array of radiation detectors. The n
branching ratios and energy spectra for Sb and I were
obtained by measuring the time of flight of recoil ions emerging from the
trapped ion cloud. These nuclei are located at the edge of an isotopic region
identified as having n branching ratios that impact the r-process
abundance pattern around the A~130 peak. For Sb and I,
n branching ratios of 14.6(11)%, 17.6(28)%, and 7.6(28)% were
determined, respectively. The n energy spectra obtained for Sb
and I are compared with results from direct neutron measurements, and
the n energy spectrum for Sb has been measured for the first
time
Dynamic Regulation of T Cell Immunity by CD43
During a viral response, Ag-specific effector T cells show dramatically increased binding by the mAb 1B11 and the lectin peanut agglutinin (PNA). We investigated the contribution of CD43 expression to 1B11 and PNA binding as well as its role in generation and maintenance of a CD8 T cell response. Analysis of CD43(-/-) mice revealed no increased 1B11 binding and reduced PNA binding on virus-specific CD8 T cells from -/- mice compared with +/+ mice. Furthermore, we examined the role of CD43 in the kinetics of an immune response. We show that CD43 expression modestly effects generation of a primary virus-specific CD8 T cell response in vivo but plays a more significant role in trafficking of CD8 T cells to tissues such as the brain. More interestingly, CD43 plays a role in the contraction of the immune response, with CD43(-/-) mice showing increased numbers of Ag-specific CD8 T cells following initial expansion. Following the peak of expansion, Ag-specific CD8 T cells from -/- mice show similar proliferation but demonstrate increased Bcl-2 levels and decreased apoptosis of Ag-specific effector CD8 T cells in vitro. Consistent with a delay in the down-modulation of the immune response, following chronic viral infection CD43(-/-) mice show increased morbidity. These data suggest a dynamic role of CD43 during an immune response: a positive regulatory role in costimulation and trafficking of T cells to the CNS and a negative regulatory role in the down-modulation of an immune response
Polarization From A Radially Stratified Off-Axis GRB Outflow
While the dominant radiation mechanism gamma-ray bursts (GRBs) remains a
question of debate, synchrotron emission is one of the foremost candidates to
describe the multi-wavelength afterglow observations. As such, it is expected
that GRBs should present some degree of polarization across their evolution -
presenting a feasible means of probing these bursts' energetic and angular
properties. Although obtaining polarization data is difficult due to the
inherent complexities regarding GRB observations, advances are being made, and
theoretical modeling of synchrotron polarization is now more relevant than
ever. In this manuscript, we present the polarization for a fiduciary model
where the synchrotron forward-shock emission evolving in the
radiative-adiabatic regime is described by a radially stratified off-axis
outflow. This is parameterized with a power-law velocity distribution and
decelerated in a constant-density and wind-like external environment. We apply
this theoretical polarization model for selected bursts presenting evidence of
off-axis afterglow emission, including the nearest orphan GRB candidates
observed by the Neil Gehrels Swift Observatory and a few Gravitational Wave
(GWs) events that could generate electromagnetic emission. In the case of GRB
170817A, we require the available polarimetric upper limits in radio
wavelengths to constrain its magnetic field geometry.Comment: In submission. 18 pages, 7 figures, 3 table
Afterglow Polarization from Off-Axis GRB Jets
As we further our studies on Gamma-ray bursts (GRBs), both on theoretical
models and observational tools, more and more options begin to open for
exploration of its physical properties. As transient events primarily dominated
by synchrotron radiation, it is expected that the synchrotron photons emitted
by GRBs should present some degree of polarization throughout the evolution of
the burst. Whereas observing this polarization can still be challenging due to
the constraints on observational tools, especially for short GRBs, it is
paramount that the groundwork is laid for the day we have abundant data. In
this work, we present a polarization model linked with an off-axis spreading
top-hat jet synchrotron scenario in a stratified environment with a density
profile . We present this model's expected temporal
polarization evolution for a realistic set of afterglow parameters constrained
within the values observed in the GRB literature for four degrees of
stratification and two magnetic field
configurations with high extreme anisotropy. We apply this model and predict
polarization from a set of GRBs exhibiting off-axis afterglow emission. In
particular, for GRB 170817A, we use the available polarimetric upper limits to
rule out the possibility of a extremely anisotropic configuration for the
magnetic field.Comment: 17 pages, 3 tables, 6 figures. Accepted for publication in the
Astrophysical Journal (ApJ
Modeling Gamma-ray burst Afterglow observations with an Off-axis Jet emission
Gamma-ray bursts (GRBs) are fascinating extragalactic objects. They represent
a fantastic opportunity to investigate unique properties not exhibited in other
sources. Multi-wavelength afterglow observations from some short- and
long-duration GRBs reveal an atypical long-lasting emission that evolves
differently from the canonical afterglow light curves favoring the off-axis
emission. We present an analytical synchrotron afterglow scenario, and the
hydrodynamical evolution of an off-axis top-hat jet decelerated in a stratified
surrounding environment. The analytical synchrotron afterglow model is shown
during the coasting, deceleration (off- and on-axis emission), and the
post-jet-break decay phases, and the hydrodynamical evolution is computed by
numerical simulations showing the time evolution of the Doppler factor, the
half-opening angle, the bulk Lorentz factor, and the deceleration radius. We
show that numerical simulations are in good agreement with those derived with
our analytical approach. We apply the current synchrotron model and describe
successfully the delayed non-thermal emission observed in a sample of long and
short GRBs with evidence of off-axis emission. Furthermore, we provide
constraints on the possible afterglow emission by requiring the
multi-wavelength upper limits derived for the closest Swift-detected GRBs and
promising gravitational-wave events.Comment: 36 pages, 16 figures, accepted for publication in Ap
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