6,828 research outputs found
Placing the Sun in Galactic Chemical Evolution: Mainstream SiC Particles
We examine the consequences and implications of the possibility that the
best-fit =4/3 line of the silicon isotopic ratios measured in mainstream SiC
grains is identical or parallel to to the mean ISM evolution line of the
silicon isotopes. Even though the mean ISM evolution proceeds along a line of
unity slope when deviations are expressed in terms of the native representation
(the mean ISM), the evolution line can become a slope 4/3 line in the solar
representation, provided that the solar composition is displaced from the mean
ISM evolution. During the course of this analysis, we introduce new methods for
relating the solar composition to that of the mean ISM at the time of solar
birth. These new developments offer a unique view on the meaning of the
mainstream SiC particles, and affords a new way of quantitatively answering the
question whether the sun has a special composition relative to the mean ISM at
solar birth. If the correlation slope of the silicon isotopes in the mean ISM
could be decisively established, then its value would quantify the difference
between the solar and mean ISM silicon abundances. Our formalism details the
transformations between the two representations, and applies not only to
Si and Si, but to any two purely secondary isotopes of any
element (O, Ne, Mg, and perhaps S). Both the advantages and disadvantages of
this technique are critically reviewed.Comment: 19 pages including 4 figures, ApJ in pres
Galactic Evolution of Silicon Isotopes: Applications to Presolar SiC Grains from Meteorites
We calculate and discuss the chemical evolution of the isotopic silicon abundances in the interstellar medium (ISM) at distances and times appropriate to the birth of the solar system. This has several objectives, some of which are related to anomalous silicon isotope ratios within presolar grains extracted from meteorites; namely: (1) What is the relative importance for silicon isotopic compositions in the bulk ISM of Type II supernovae, Type Ia supernovae, and AGB stars? (2) Are 29Si and 30Si primary or secondary nucleosynthesis products? (3) In what isotopic direction in a three-isotope plot do core-collapse supernovae of different mass move the silicon isotopic composition? (4) Why do present calcu-lations not reproduce the solar ratios for silicon isotopes, and what does that impose upon studies of anomalous Si isotopes in meteoritic silicon carbide grains? (5) Are chemical-evolution features recorded in the anomalous SiC grains? Our answers are formulated on the basis of the Woosley & Weaver super-nova yield survey. Renormalization with the calculated interstellar medium silicon isotopic composition and solar composition is as an important and recurring concept of this paper. Possible interpretations of the silicon isotope anomalies measured in single SiC grains extracted from carbonaceous meteorites are then presented. The calculations suggest that the temporal evolution of the isotopic silicon abundances in the interstellar medium may be recorded in these grains
Integral throat entrance development, qualification and production for the Antares 3 nozzle
Although design analyses of a G-90 graphite integral throat entrance for the Antares 3 solid rocket motor nozzle indicated acceptable margins of safety, the nozzle throat insert suffered a thermostructural failure during the first development firing. Subsequent re-analysis using properties measured on material from the same billet as the nozzle throat insert showed negative margins. Carbon-carbon was investigated and found to result in large positive margins of safety. The G-90 graphite was replaced by SAI fast processed 4-D material which uses Hercules HM 10000 fiber as the reinforcement. Its construction allows powder filling of the interstices after preform fabrication which accelerates the densification process. Allied 15V coal tar pitch is then used to complete densification. The properties were extensively characterized on this material and six nozzles were subjected to demonstration, development and qualification firings
Galactic Evolution of Silicon Isotopes: Application to Presolar SiC Grains From Meteorites
We calculate and discuss the chemical evolution of the isotopic silicon
abundances in the interstellar medium at distances and times appropriate to the
birth of the solar system. This has several objectives, some of which are
related to anomalous silicon isotope ratios within presolar grains extracted
from meteorites; namely: (1) What is the relative importance for silicon
isotopic compositions in the bulk ISM of Type II supernovae, Type Ia
supernovae, and AGB stars? (2) Are Si and Si primary or secondary
nucleosynthesis products? (3) In what isotopic direction in a three-isotope
plot do core-collapse supernovae of different mass move the silicon isotopic
composition? (4) Why do present calculations not reproduce the solar ratios for
silicon isotopes, and what does that impose upon studies of anomalous Si
isotopes in meteoritic silicon carbide grains? (5) Are chemical-evolution
features recorded in the anomalous SiC grains? Our answers are formulated on
the basis of the Woosley \& Weaver (1995) supernova yield survey.
Renormalization with the calculated interstellar medium silicon isotopic
composition and solar composition is as an important and recurring concept of
this paper. Possible interpretations of the silicon isotope anomalies measured
in single SiC grains extracted from carbonaceous meteorites are then presented.
The calculations suggest that the temporal evolution of the isotopic silicon
abundances in the interstellar medium may be recorded in these grains.Comment: Accepted ApJ. 41 pages including 13 figures and 2 tables. uuencoded
gzipped postscrip
Collisional cross sections and momentum distributions in astrophysical plasmas: dynamics and statistical mechanics link
We show that, in stellar core plasmas, the one-body momentum distribution
function is strongly dependent, at least in the high velocity regime, on the
microscopic dynamics of ion elastic collisions and therefore on the effective
collisional cross sections, if a random force field is present. We take into
account two cross sections describing ion-dipole and ion-ion screened
interactions. Furthermore we introduce a third unusual cross section, to link
statistical distributions and a quantum effect originated by the
energy-momentum uncertainty owing to many-body collisions, and propose a
possible physical interpretation in terms of a tidal-like force. We show that
each collisional cross section gives rise to a slight peculiar correction on
the Maxwellian momentum distribution function in a well defined velocity
interval. We also find a possible link between microscopical dynamics of ions
and statistical mechanics interpreting our results in the framework of
non-extensive statistical mechanics.Comment: 8 page
S wave velocity structure below central Mexico using high-resolution surface wave tomography
Shear wave velocity of the crust below central Mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense, 500 km long linear seismic network. Vertical components of regional records from 90 well-located earthquakes were used to compute Rayleigh-wave group-velocity dispersion curves. A tomographic inversion, with high resolution in a zone close to the array, obtained for periods between 5 and 50 s reveals significant differences relative to a reference model, especially at larger periods (>30 s). A 2-D S wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions. The results show large differences, especially in the lower crust, among back-arc, volcanic arc, and fore-arc regions; they also show a well-resolved low-velocity zone just below the active part of the Trans Mexican Volcanic Belt (TMVB) suggesting the presence of a mantle wedge. Low densities in the back arc, inferred from the low shear wave velocities, can provide isostatic support for the TMVB
Addemdum to: ''The Mathematical Structure of Quantum Superspace as a Consequence of Time Asymmetry''
In this paper we improve the results of sec. VI of paper [M. Castagnino,
Phys. Rev. D 57, 750 (1998)] by considering that the main source of entropy
production are the photospheres of the stars
Effect of Long-lived Strongly Interacting Relic Particles on Big Bang Nucleosynthesis
It has been suggested that relic long-lived strongly interacting massive
particles (SIMPs, or particles) existed in the early universe. We study
effects of such long-lived unstable SIMPs on big bang nucleosynthesis (BBN)
assuming that such particles existed during the BBN epoch, but then decayed
long before they could be detected. The interaction strength between an
particle and a nucleon is assumed to be similar to that between nucleons. We
then calculate BBN in the presence of the unstable neutral charged
particles taking into account the capture of particles by nuclei to form
-nuclei. We also study the nuclear reactions and beta decays of -nuclei.
We find that SIMPs form bound states with normal nuclei during a relatively
early epoch of BBN. This leads to the production of heavy elements which remain
attached to them. Constraints on the abundance of particles during BBN
are derived from observationally inferred limits on the primordial light
element abundances. Particle models which predict long-lived colored particles
with lifetimes longer than 200 s are rejected based upon these
constraints.Comment: 19 pages, 4 figure
The Ever Changing Circumstellar Nebula Around UW Centauri
We present new images of the reflection nebula surrounding the R Coronae
Borealis Star, UW Cen. This nebula, first detected in 1990, has changed its
appearance significantly. At the estimated distance of UW Cen, this nebula is
approximately 0.6 ly in radius so the nebula cannot have physically altered in
only 8 years. Instead, the morphology of the nebula appears to change as
different parts are illuminated by light from the central star modulated by
shifting thick dust clouds near its surface. These dust clouds form and
dissipate at irregular intervals causing the well-known declines in the R
Coronae Borealis (RCB) stars. In this way, the central star acts like a
lighthouse shining through holes in the dust clouds and lighting up different
portions of the nebula. The existence of this nebula provides clues to the
evolutionary history of RCB stars possibly linking them to the Planetary
Nebulae and the final helium shell flash stars.Comment: To be published in ApJ Letters. 5 pages, 3 figures (2 in color
- âŠ