6,895 research outputs found
Sympathetic cooling of He ions in a radiofrequency trap
We have generated Coulomb crystals of ultracold He ions in a linear
radiofrequency trap, by sympathetic cooling via laser--cooled Be.
Stable crystals containing up to 150 localized He ions at 20 mK were
obtained. Ensembles or single ultracold He ions open up interesting
perspectives for performing precision tests of QED and measurements of nuclear
radii. The present work also indicates the feasibility of cooling and
crystallizing highly charged atomic ions using Be as coolant.Comment: 4 pages, 2 figure
Mu and Tau Neutrino Thermalization and Production in Supernovae: Processes and Timescales
We investigate the rates of production and thermalization of and
neutrinos at temperatures and densities relevant to core-collapse
supernovae and protoneutron stars. Included are contributions from electron
scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and
nucleon scattering. For the scattering processes, in order to incorporate the
full scattering kinematics at arbitrary degeneracy, the structure function
formalism developed by Reddy et al. (1998) and Burrows and Sawyer (1998) is
employed. Furthermore, we derive formulae for the total and differential rates
of nucleon-nucleon bremsstrahlung for arbitrary nucleon degeneracy in
asymmetric matter. We find that electron scattering dominates nucleon
scattering as a thermalization process at low neutrino energies
( MeV), but that nucleon scattering is always faster
than or comparable to electron scattering above MeV. In
addition, for g cm, MeV, and
neutrino energies MeV, nucleon-nucleon bremsstrahlung always
dominates electron-positron annihilation as a production mechanism for
and neutrinos.Comment: 29 pages, LaTeX (RevTeX), 13 figures, submitted to Phys. Rev. C. Also
to be found at anonymous ftp site http://www.astrophysics.arizona.edu; cd to
pub/thompso
Neutrino Signatures and the Neutrino-Driven Wind in Binary Neutron Star Mergers
We present VULCAN/2D multigroup flux-limited-diffusion radiation-hydrodynamics simulations of binary neutron star mergers, using the Shen equation of state, covering âł 100 ms, and starting from azimuthal-averaged two-dimensional slices obtained from three-dimensional smooth-particle-hydrodynamics simulations of Rosswog & Price for 1.4Mâ (baryonic) neutron stars with no initial spins, co-rotating spins, or counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a multiangle neutrino-transport solver. We find polar-enhanced neutrino luminosities, dominated by ¯νe and âνΟâ neutrinos at the peak, although νe emission may be stronger at late times. We obtain typical peak neutrino energies for νe, ¯νe, and âνΟâ of âź12, âź16, and âź22 MeV, respectively. The supermassive neutron star (SMNS) formed from the merger has a cooling timescale of âž 1 s. Charge-current neutrino reactions lead to the formation of a thermally driven bipolar wind with (M¡) âź 10^â3 Mâ s^â1 and baryon-loading in the polar regions, preventing any production of a Îł-ray burst prior to black hole formation. The large budget of rotational free energy suggests that magneto-rotational effects could produce a much-greater polar mass loss. We estimate that âž 10^â4 Mâ of material with an electron fraction in the range 0.1â0.2 becomes unbound during this SMNS phase as a result of neutrino heating. We present a new formalism to compute the νi ¯νi annihilation rate based on moments of the neutrino-specific intensity computed with our multiangle solver. Cumulative annihilation rates, which decay as âźt^â1.8, decrease over our 100 ms window from a few Ă1050 to âź 1049 erg sâ1, equivalent to a few Ă10^54 to âź10^53 eâe+ pairs per second
Shear horizontal (SH) ultrasound wave propagation around smooth corners
Shear horizontal (SH) ultrasound guided waves are being used in an increasing number of non-destructive testing (NDT) applications. One advantage SH waves have over some wave types, is their ability to propagate around curved surfaces with little energy loss; to understand the geometries around which they could propagate, the wave reflection must be quantified. A 0.83 mm thick aluminium sheet was placed in a bending machine, and a shallow bend was introduced. Periodically-poled magnet (PPM) electromagnetic acoustic transducers (EMATs), for emission and reception of SH waves, were placed on the same side of the bend, so that reflected waves were received. Additional bending of the sheet demonstrated a clear relationship between bend angles and the reflected signal. Models suggest that the reflection is a linear superposition of the reflections from each bend segment, such that sharp turns lead to a larger peak-to-peak amplitude, in part due to increased phase coherence
z'-band Ground-Based Detection of the Secondary Eclipse of WASP-19b
We present the ground-based detection of the secondary eclipse of the
transiting exoplanet WASP-19b. The observations were made in the Sloan z'-band
using the ULTRACAM triple-beam CCD camera mounted on the NTT. The measurement
shows a 0.088\pm0.019% eclipse depth, matching previous predictions based on H-
and K-band measurements. We discuss in detail our approach to the removal of
errors arising due to systematics in the data set, in addition to fitting a
model transit to our data. This fit returns an eclipse centre, T0, of
2455578.7676 HJD, consistent with a circular orbit. Our measurement of the
secondary eclipse depth is also compared to model atmospheres of WASP-19b, and
is found to be consistent with previous measurements at longer wavelengths for
the model atmospheres we investigated.Comment: 20 pages, 10 figures. Published in the ApJ Supplement serie
On homogeneous statistical distributions exoplanets for their dynamic parameters
Correct distributions of extrasolar systems for their orbital parameters
(semi-major axes, period, eccentricity) and physical characteristics (mass,
spectral type of parent star) are received. Orbital resonances in extrasolar
systems are considered. It is shown, that the account of more thin effects,
including with use of wavelet methods, in obviously incorrectly reduced
distributions it is not justified, to what the homogeneous statistical
distributions for dynamic parameters of exoplanets, received in the present
work, testify.Comment: 9 pages, 15 figures; International Conference "100 years since
Tunguska phenomenon: Past, present and future", (June 26-28, 2008. Russia,
Moscow), Lomonosov readings 2009 (Moscow State University
GRB Energetics in the Swift Era
We examine the rest frame energetics of 76 gamma-ray bursts (GRBs) with known
redshift that were detected by the Swift spacecraft and monitored by the
satellite's X-ray Telescope (XRT). Using the bolometric fluence values
estimated in Butler et al. 2007b and the last XRT observation for each event,
we set a lower limit the their collimation corrected energy Eg and find that a
68% of our sample are at high enough redshift and/or low enough fluence to
accommodate a jet break occurring beyond the last XRT observation and still be
consistent with the pre-Swift Eg distribution for long GRBs. We find that
relatively few of the X-ray light curves for the remaining events show evidence
for late-time decay slopes that are consistent with that expected from post jet
break emission. The breaks in the X-ray light curves that do exist tend to be
shallower and occur earlier than the breaks previously observed in optical
light curves, yielding a Eg distribution that is far lower than the pre-Swift
distribution. If these early X-ray breaks are not due to jet effects, then a
small but significant fraction of our sample have lower limits to their
collimation corrected energy that place them well above the pre-Swift Eg
distribution. Either scenario would necessitate a much wider post-Swift Eg
distribution for long cosmological GRBs compared to the narrow standard energy
deduced from pre-Swift observations. We note that almost all of the pre-Swift
Eg estimates come from jet breaks detected in the optical whereas our sample is
limited entirely to X-ray wavelengths, furthering the suggestion that the
assumed achromaticity of jet breaks may not extend to high energies.Comment: 30 pages, 10 figures, Accepted to Ap
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