4,392 research outputs found
High-Temperature Processing of Solids Through Solar Nebular Bow Shocks: 3D Radiation Hydrodynamics Simulations with Particles
A fundamental, unsolved problem in Solar System formation is explaining the
melting and crystallization of chondrules found in chondritic meteorites.
Theoretical models of chondrule melting in nebular shocks has been shown to be
consistent with many aspects of thermal histories inferred for chondrules from
laboratory experiments; but, the mechanism driving these shocks is unknown.
Planetesimals and planetary embryos on eccentric orbits can produce bow shocks
as they move supersonically through the disk gas, and are one possible source
of chondrule-melting shocks. We investigate chondrule formation in bow shocks
around planetoids through 3D radiation hydrodynamics simulations. A new
radiation transport algorithm that combines elements of flux-limited diffusion
and Monte Carlo methods is used to capture the complexity of radiative
transport around bow shocks. An equation of state that includes the rotational,
vibrational, and dissociation modes of H is also used. Solids are followed
directly in the simulations and their thermal histories are recorded. Adiabatic
expansion creates rapid cooling of the gas, and tail shocks behind the embryo
can cause secondary heating events. Radiative transport is efficient, and bow
shocks around planetoids can have luminosities few
L. While barred and radial chondrule textures could be produced in
the radiative shocks explored here, porphyritic chondrules may only be possible
in the adiabatic limit. We present a series of predicted cooling curves that
merit investigation in laboratory experiments to determine whether the solids
produced by bow shocks are represented in the meteoritic record by chondrules
or other solids.Comment: Accepted for publication in ApJ. Images have been resized to conform
to arXiv limits, but are all readable upon adjusting the zoom. Changes from
v1: Corrected typos discovered in proofs. Most changes are in the appendi
Physics at Future Linear Colliders
This article summarises the physics at future linear colliders. It will be
shown that in all studied physics scenarios a 1 TeV linear collider in addition
to the LHC will enhance our knowledge significantly and helps to reconstruct
the model of new physics nature has chosen.Comment: Invited talk at the Lepton Photon Symposium 2005, Upsala, Sweden,
July 2005, V2: minor improvement
The Precision of Higgs Boson Measurements and Their Implications
The prospects for a precise exploration of the properties of a single or many
observed Higgs bosons at future accelerators are summarized, with particular
emphasis on the abilities of a Linear Collider (LC). Some implications of these
measurements for discerning new physics beyond the Standard Model (SM) are also
discussed.Comment: Summary report of the Precision Higgs Working Group P1WG2 at Snowmass
200
Interaction of Supernova Ejecta with Nearby Protoplanetary Disks
The early Solar System contained short-lived radionuclides such as 60Fe (t1/2
= 1.5 Myr) whose most likely source was a nearby supernova. Previous models of
Solar System formation considered a supernova shock that triggered the collapse
of the Sun's nascent molecular cloud. We advocate an alternative hypothesis,
that the Solar System's protoplanetary disk had already formed when a very
close (< 1 pc) supernova injected radioactive material directly into the disk.
We conduct the first numerical simulations designed to answer two questions
related to this hypothesis: will the disk be destroyed by such a close
supernova; and will any of the ejecta be mixed into the disk? Our simulations
demonstrate that the disk does not absorb enough momentum from the shock to
escape the protostar to which it is bound. Only low amounts (< 1%) of mass loss
occur, due to stripping by Kelvin-Helmholtz instabilities across the top of the
disk, which also mix into the disk about 1% of the intercepted ejecta. These
low efficiencies of destruction and injectation are due to the fact that the
high disk pressures prevent the ejecta from penetrating far into the disk
before stalling. Injection of gas-phase ejecta is too inefficient to be
consistent with the abundances of radionuclides inferred from meteorites. On
the other hand, the radionuclides found in meteorites would have condensed into
dust grains in the supernova ejecta, and we argue that such grains will be
injected directly into the disk with nearly 100% efficiency. The meteoritic
abundances of the short-lived radionuclides such as 60Fe therefore are
consistent with injection of grains condensed from the ejecta of a nearby (< 1
pc) supernova, into an already-formed protoplanetary disk.Comment: 57 pages, 16 figure
Cooling of Dense Gas by H2O Line Emission and an Assessment of its Effects in Chondrule-Forming Shocks
We consider gas at densities appropriate to protoplanetary disks and
calculate its ability to cool due to line radiation emitted by H2O molecules
within the gas. Our work follows that of Neufeld & Kaufman (1993; ApJ, 418,
263), expanding on their work in several key aspects, including use of a much
expanded line database, an improved escape probability formulism, and the
inclusion of dust grains, which can absorb line photons. Although the escape
probabilities formally depend on a complicated combination of optical depth in
the lines and in the dust grains, we show that the cooling rate including dust
is well approximated by the dust-free cooling rate multiplied by a simple
function of the dust optical depth. We apply the resultant cooling rate of a
dust-gas mixture to the case of a solar nebula shock pertinent to the formation
of chondrules, millimeter-sized melt droplets found in meteorites. Our aim is
to assess whether line cooling can be neglected in chondrule-forming shocks or
if it must be included. We find that for typical parameters, H2O line cooling
shuts off a few minutes past the shock front; line photons that might otherwise
escape the shocked region and cool the gas will be absorbed by dust grains.
During the first minute or so past the shock, however, line photons will cool
the gas at rates ~ 10,000 K/hr, dropping the temperature of the gas (and most
likely the chondrules within the gas) by several hundred K. Inclusion of H2O
line cooling therefore must be included in models of chondrule formation by
nebular shocks.Comment: Accepted for publication in The Astrophysical Journa
Annealing of Silicate Dust by Nebular Shocks at 10 AU
Silicate dust grains in the interstellar medium are known to be mostly
amorphous, yet crystalline silicate grains have been observed in many
long-period comets and in protoplanetary disks. Annealing of amorphous silicate
grains into crystalline grains requires temperatures > 1000 K, but exposure of
dust grains in comets to such high temperatures is incompatible with the
generally low temperatures experienced by comets. This has led to the proposal
of models in which dust grains were thermally processed near the protoSun, then
underwent considerable radial transport until they reached the gas giant planet
region where the long-period comets originated. We hypothesize instead that
silicate dust grains were annealed in situ, by shock waves triggered by
gravitational instabilities. We assume a shock speed of 5 km/s, a plausible
value for shocks driven by gravitational instabilities. We calculate the peak
temperatures of micron and submicron amorphous pyroxene grains of chondritic
composition under conditions typical in protoplanetary disks at 5 - 10 AU. Our
results also apply to chondritic amorphous olivine grains. We show that {\it in
situ} thermal annealing of submicron and micron-sized silicate dust grains can
occur, obviating the need for large-scale radial transport.Comment: 12 pages; includes 1 figure, 1 table; accepted by ApJ Letter
Voyager 1 Planetary Radio Astronomy Observations Near Jupiter
Results are reported from the first low frequency radio receiver to be transported into the Jupiter magnetosphere. Dramatic new information was obtained both because Voyager was near or in Jupiter's radio emission sources and also because it was outside the relatively dense solar wind plasma of the inner solar system. Extensive radio arcs, from above 30 MHz to about 1 MHz, occurred in patterns correlated with planetary longitude. A newly discovered kilometric wavelength radio source may relate to the plasma torus near Io's orbit. In situ wave resonances near closest approach define an electron density profile along the Voyager trajectory and form the basis for a map of the torus. Studies in progress are outlined briefly
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