1,033 research outputs found
The interstellar gas experiment
The Interstellar Gas Experiment (IGE) exposed thin metallic foils to collect neutral interstellar gas particles. These particles penetrate the solar system due to their motion relative to the sun. Thus, it is possible to entrap them in the collecting foils along with precipitating magnetospheric and perhaps some ambient atmospheric particles. For the entire duration of the Long Duration Exposure Facility (LDEF) mission, seven of these foils collected particles arriving from seven different directions as seen from the spacecraft. In the mass spectroscopic analysis of the noble gas component of these particles, we have detected the isotopes of He-3, He-4, Ne-20, and Ne-22. In the foil analyses carried out so far, we find a distribution of particle arrival directions which shows that a significant part of the trapped particles are indeed interstellar atoms. The analysis needed to subtract the competing fluxes of magnetospheric and atmospheric particles is still in progress
The Interstellar Gas Experiment: Analysis in progress
The Interstellar Gas Experiment (IGE) exposed thin metallic foils aboard the LDEF spacecraft in low Earth orbit in order to collect neutral interstellar particles which penetrate the solar system due to their motion relative to the sun. By mechanical penetration these atoms were imbedded in the collecting foils along with precipitating magnetospheric ions and, possibly, with ambient atmospheric atoms. During the entire LDEF mission, seven of these foils collected particles arriving from seven different directions as seen from the spacecraft. After the foils were returned to Earth, a mass spectrometric analysis of the noble gas component of the trapped particles was begun. The isotopes of He-3, He-4, Ne-20, and Ne-22 were detected. We have given a first account of the experiment. In order to infer the isotopic ratios in the interstellar medium from the concentrations found in the foils, several lines of investigation had to be initiated. The flux of ambient atmospheric noble gas atoms moving toward the foils due to the orbital motion of LDEF was estimated by detailed calculations. Any of these particles which evaded the baffles in the IGE collector could be entrapped in the foils as a background flux. However, the calculations have shown that this flux is negligible, which was the intent of the experiment hardware design. This conclusion is supported by the measurements. However, both the concentration of trapped helium and its impact energy indicate that the flux of magnetospheric ions which was captured was larger than had been expected. In fact, it appears that the magnetospheric particles constitute the largest fraction of the particles in the foils. Since little is known about this particle flux, their presence in the IGE foils appears fortunate. The analysis of these particles provides information about their isotropic composition and average flux
Fractional smoothness and applications in finance
This overview article concerns the notion of fractional smoothness of random
variables of the form , where is a certain
diffusion process. We review the connection to the real interpolation theory,
give examples and applications of this concept. The applications in stochastic
finance mainly concern the analysis of discrete time hedging errors. We close
the review by indicating some further developments.Comment: Chapter of AMAMEF book. 20 pages
Isotopic Composition of Solar Wind Calcium: First in Situ Measurement by CELIAS/MTOF on Board SOHO
We present first results on the Ca isotopic abundances derived from the high
resolution Mass Time-of-Flight (MTOF) spectrometer of the charge, element, and
isotope analysis system (CELIAS) experiment on board the Solar and Heliospheric
Observatory (SOHO). We obtain isotopic ratios 40Ca/42Ca = (128+-47) and
40Ca/44Ca = (50+-8), consistent with terrestrial values. This is the first in
situ determination of the solar wind calcium isotopic composition and is
important for studies of stellar modeling and solar system formation since the
present-day solar Ca isotopic abundances are unchanged from their original
isotopic composition in the solar nebula.Comment: 14 pages, 3 figure
Effects of thermohaline instability and rotation-induced mixing on the evolution of light elements in the Galaxy : D, 3He and 4He
Recent studies of low- and intermediate-mass stars show that the evolution of
the chemical elements in these stars is very different from that proposed by
standard stellar models. Rotation-induced mixing modifies the internal chemical
structure of main sequence stars, although its signatures are revealed only
later in the evolution when the first dredge-up occurs. Thermohaline mixing is
likely the dominating process that governs the photospheric composition of
low-mass red giant branch stars and has been shown to drastically reduce the
net 3He production in these stars. The predictions of these new stellar models
need to be tested against galaxy evolution. In particular, the resulting
evolution of the light elements D, 3He and 4He should be compared with their
primordial values inferred from the Wilkinson Microwave Anisotropy Probe data
and with the abundances derived from observations of different Galactic
regions. We study the effects of thermohaline mixing and rotation-induced
mixing on the evolution of the light elements in the Milky Way. We compute
Galactic evolutionary models including new yields from stellar models computed
with thermohaline instability and rotation-induced mixing. We discuss the
effects of these important physical processes acting in stars on the evolution
of the light elements D, 3He, and 4He in the Galaxy. Galactic chemical
evolution models computed with stellar yields including thermohaline mixing and
rotation fit better observations of 3He and 4He in the Galaxy than models
computed with standard stellar yields. The inclusion of thermohaline mixing in
stellar models provides a solution to the long-standing "3He problem" on a
Galactic scale. Stellar models including rotation-induced mixing and
thermohaline instability reproduce also the observations of D and 4He.Comment: 12 pages, 9 figures, accepted for publication in A&
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