54 research outputs found
General relativistic effects on neutrino-driven wind from young, hot neutron star and the r-process nucleosynthesis
Neutrino-driven wind from young hot neutron star, which is formed by
supernova explosion, is the most promising candidate site for r-process
nucleosynthesis. We study general relativistic effects on this wind in
Schwarzschild geometry in order to look for suitable conditions for a
successful r-process nucleosynthesis. It is quantitatively discussed that the
general relativistic effects play a significant role in increasing entropy and
decreasing dynamic time scale of the neutrino-driven wind. Exploring wide
parameter region which determines the expansion dynamics of the wind, we find
interesting physical conditions which lead to successful r-process
nucleosynthesis. The conditions which we found realize in the neutrino-driven
wind with very short dynamic time scale ms and
relatively low entropy . We carry out the -process and
r-process nucleosynthesis calculation on these conditions by the use of our
single network code including over 3000 isotopes, and confirm quantitatively
that the second and third r-process abundance peaks are produced in the
neutrino-driven wind.Comment: Accepted for publication in Ap
Supernovae versus Neutron Star Mergers as the Major r-Process Sources
I show that recent observations of r-process abundances in metal-poor stars
are difficult to explain if neutron star mergers (NSMs) are the major r-process
sources. In contrast, such observations and meteoritic data on Hf182 and I129
in the early solar system support a self-consistent picture of r-process
enrichment by supernovae (SNe). While further theoretical studies of r-process
production and enrichment are needed for both SNe and NSMs, I emphasize two
possible direct observational tests of the SN r-process model: gamma rays from
decay of r-process nuclei in SN remnants and surface contamination of the
companion by SN r-process ejecta in binaries.Comment: 5 pages, to appear in ApJ
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HF echoes from ionization potentially produced by high-altitude discharges
In this paper the authors report on recent radar measurements taken during the month of October 1994 with the LDG HF radar in the Ivory Coast, Africa as part of the International Equatorial Electrojet Year. The purpose of this experimental effort in part was to study the effects of thunderstorms on the ionosphere. At the same time, the authors decided to carry out a set of experiments of an exploratory nature to look for echoes that could potentially arise from ionization produced in the mesosphere. The two leading candidates for producing transient ionization in the mesosphere are meteors and high-altitude discharges. Each is discussed in the context of these measurements
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Optical, radio and x-ray radiation of red sprites produced by runaway air breakdown
The authors use the runaway air breakdown model of upward discharges to calculate optical, radio, and X-ray radiation generated by red sprites. Red sprites are high altitude (up to 90 km) lightning discharges. Aircraft based observations show that sprites are predominantly red in color at altitudes above {approximately}55 km with faint blue tendrils, which extend downward to an altitude of 40 km; the duration of a single sprite is less than 17 ms, their maximum brightness is about 600 kR, and estimated total optical energy is about 1--5 kJ per event. The ground based observations show similar results, and provide some additional information on spatial and temporal structure of sprites, and on sprite locations. One difference between aircraft and ground-based observations is that blue tendrils are rarely observed from the ground. Sprites usually occur above the anvils of large mesoscale convective systems and correlate with strong positive cloud to ground discharge. Upward discharges are the most probable source of X-ray emission observed above large thunderstorm complexes by the Compton Gamma-ray Observatory. To escape the atmosphere these {gamma}-rays must originate above 25 km altitude. Red sprites are usually observed at altitudes higher than 50 km, and are therefore a likely source of this x-ray emission
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Generation of elves by sprites and jets
Recent years of observations of the upper atmosphere and the lower ionosphere brought a fascinating collection of new phenomena including optical, radio, and gamma-ray emissions originating in the 20 to 90 km altitude range. Up to now, the most diverse phenomenology has emerged from the optical observations which have led to the identification of red sprites, blue jets, blue starts, and elves. Most of the studies have concentrated on relating such phenomena in the upper atmosphere to regular lightning discharges in the troposphere. The sprite/jet discharge itself can be caused by the runaway air breakdown, or regular air breakdown. The standard theory for optical airglow transients in the lower ionosphere above the thunderstorms also known as elves suggests that they are produced during interaction of electromagnetic pulses (EMP) from lightning with the lower ionosphere. Heating of the ambient electrons by the EMP in the D region can result in excitation of optical emissions once the optical excitation thresholds are reached. In this paper the authors suggest that in addition to this mechanism elves can be caused by an EMP generated by sprites and jets
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High altitude atmospheric discharges according to the runaway air breakdown mechanism
High altitude optical transients - red sprites, blue jets, and elves - are modeled in the context of the relativistic electron runaway air breakdown mechanism. These emissions are usually associated with large mesoscale convective systems (hereafter MCS). In thunderstorms cloud electrification proceeds over a time scale long enough to permit the conducting atmosphere above the cloud to polarize and short out the thunderstorm electric field. When a lightning strike rapidly neutralizes a cloud charge layer runaway driving fields can develop in the stratosphere and mesosphere. According to present simulations of the full runaway process the variety of observed optical emissions are due to the nature of the normal lightning event in the MCS that kick starts the runaway avalanche. In this paper the authors describe some details of the model, present the results of the evolution of the primary electron population, and summarize the initial conditions necessary for different types of discharges. Two companion papers present (a) the predicted optical, gamma ray, and radio emissions caused by these electrical discharges, and (b) the time evolution of the secondary electron population and its implications in terms of observables
Video and Photometric Observations of a Sprite in Coincidence with a Meteor-triggered Jet Event
Video and photometric observations of a meteor-triggered âjetâ event in association with the occurrence of a sprite were collected during the SPRITES \u2798 campaign. The event raises interest in the question of possible meteoric triggering of upper atmospheric transients as originally suggested by Muller [1995]. The event consisted of three stages: (1) the observation of a moderately bright meteor, (2) the development of a sprite in the immediate vicinity of the meteor as the meteor reached no lower than âź70 km altitude, and (3) a slower-forming jet of luminosity that appeared during the late stages of the sprite and propagated back up the ionization trail of the meteor. The event is analyzed in terms of its geometry, its relevance to the meteor, and the implications to existing theories for sprite formation
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Alignment of leading-edge and peak-picking time of arrival methods to obtain accurate source locations
The location of a radiating source can be determined by time-tagging the arrival of the radiated signal at a network of spatially distributed sensors. The accuracy of this approach depends strongly on the particular time-tagging algorithm employed at each of the sensors. If different techniques are used across the network, then the time tags must be referenced to a common fiducial for maximum location accuracy. In this report we derive the time corrections needed to temporally align leading-edge, time-tagging techniques with peak-picking algorithms. We focus on broadband radio frequency (RF) sources, an ionospheric propagation channel, and narrowband receivers, but the final results can be generalized to apply to any source, propagation environment, and sensor. Our analytic results are checked against numerical simulations for a number of representative cases and agree with the specific leading-edge algorithm studied independently by Kim and Eng (1995) and Pongratz (2005 and 2007)
Relativistic electron beams above thunderclouds
Non-luminous relativistic electron beams above thunderclouds have been detected by the radio signals of low frequency &sim;40â400 kHz which they radiate. The electron beams occur &sim;2â9 ms after positive cloud-to-ground lightning discharges at heights between &sim;22â72 km above thunderclouds. Intense positive lightning discharges can also cause sprites which occur either above or prior to the electron beam. One electron beam was detected without any luminous sprite which suggests that electron beams may also occur independently of sprites. Numerical simulations show that beams of electrons partially discharge the lightning electric field above thunderclouds and thereby gain a mean energy of &sim;7 MeV to transport a total charge of &sim;â10 mC upwards. The impulsive current &sim;3 &times; 10<sup>&minus;3</sup> Am<sup>â2</sup> associated with relativistic electron beams above thunderclouds is directed downwards and needs to be considered as a novel element of the global atmospheric electric circuit
Neutrino signatures and the neutrino-driven wind in Binary Neutron Star Mergers
We present VULCAN/2D multi-group flux-limited-diffusion radiation
hydrodynamics simulations of binary neutron star (BNS) mergers, using the Shen
equation of state, covering ~100 ms, and starting from azimuthal-averaged 2D
slices obtained from 3D SPH simulations of Rosswog & Price for 1.4 Msun
(baryonic) neutron stars with no initial spins, co-rotating spins, and
counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a
multi-angle neutrino-transport solver. We find polar-enhanced neutrino
luminosities, dominated by and ``'' neutrinos at peak,
although emission may be stronger at late times. We obtain typical peak
neutrino energies for , , and ``'' of ~12, ~16,
and ~22 MeV. The super-massive neutron star (SMNS) formed from the merger has a
cooling timescale of ~1 s. Charge-current neutrino reactions lead to the
formation of a thermally-driven bipolar wind with ~10
Msun/s, baryon-loading the polar regions, and preventing any production of a
GRB prior to black-hole formation. The large budget of rotational free energy
suggests magneto-rotational effects could produce a much greater polar mass
loss. We estimate that ~10 Msun of material with electron fraction in
the range 0.1-0.2 become unbound during this SMNS phase as a result of neutrino
heating. We present a new formalism to compute the
annihilation rate based on moments of the neutrino specific intensity computed
with our multi-angle solver. Cumulative annihilation rates, which decay as
, decrease over our 100 ms window from a few 10 to ~10
erg/s, equivalent to a few 10 to ~10 pairs per second.Comment: 23 pages, 20 figures, 2 tables, submitted to ApJ, high resolution
version of the paper available at http://hermes.as.arizona.edu/~luc/ms.pd
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