6,736 research outputs found
Radioactive nuclei from cosmochronology to habitability
In addition to long-lived radioactive nuclei like U and Th isotopes, which
have been used to measure the age of the Galaxy, also radioactive nuclei with
half-lives between 0.1 and 100 million years (short-lived radionuclides, SLRs)
were present in the early Solar System (ESS), as indicated by high-precision
meteoritic analysis. We review the most recent meteoritic data and describe the
nuclear reaction processes responsible for the creation of SLRs in different
types of stars and supernovae. We show how the evolution of radionuclide
abundances in the Milky Way Galaxy can be calculated based on their stellar
production. By comparing predictions for the evolution of galactic abundances
to the meteoritic data we can build up a time line for the nucleosynthetic
events that predated the birth of the Sun, and investigate the lifetime of the
stellar nursery where the Sun was born. We then review the scenarios for the
circumstances and the environment of the birth of the Sun within such a stellar
nursery that have been invoked to explain the abundances in the ESS of the SLRs
with the shortest lives - of the order of million years or less. Finally, we
describe how the heat generated by radioactive decay and in particular by the
abundant 26Al in the ESS had important consequences for the thermo-mechanical
and chemical evolution of planetesimals, and discuss possible implications on
the habitability of terrestrial-like planets. We conclude with a set of open
questions and future directions related to our understanding of the
nucleosynthetic processes responsible for the production of SLRs in stars,
their evolution in the Galaxy, the birth of the Sun, and the connection with
the habitability of extra-solar planets.Comment: Review published in Progress in Particle and Nuclear Physics. The
article is being published Open Access, access to the full article is not
restricted in any way. Please download the final version of the paper at
https://doi.org/10.1016/j.ppnp.2018.05.00
Stellar and nuclear-physics constraints on two r-process components in the early Galaxy
Proceedings of "Nuclei in the Cosmos 2000", Aarhus, DanmarkComment: 3 pages, 2 figures; to be publ. in Nucl. Phys.
Infrared Spectra of Meteoritic SiC Grains
We present here the first infrared spectra of meteoritic SiC grains. The
mid-infrared transmission spectra of meteoritic SiC grains isolated from the
Murchison meteorite were measured in the wavelength range 2.5--16.5 micron, in
order to make available the optical properties of presolar SiC grains. These
grains are most likely stellar condensates with an origin predominately in
carbon stars. Measurements were performed on two different extractions of
presolar SiC from the Murchison meteorite. The two samples show very different
spectral appearance due to different grain size distributions. The spectral
feature of the smaller meteoritic SiC grains is a relatively broad absorption
band found between the longitudinal and transverse lattice vibration modes
around 11.3 micron, supporting the current interpretation about the presence of
SiC grains in carbon stars. In contrast to this, the spectral feature of the
large (> 5 micron) grains has an extinction minimum around 10 micron. The
obtained spectra are compared with commercially available SiC grains and the
differences are discussed. This comparison shows that the crystal structure
(e.g., beta-SiC versus alpha-SiC) of SiC grains plays a minor role on the
optical signature of SiC grains compared to e.g. grain size.Comment: 7 pages, 6 figures. To appear in A&
Itinerant-electron Ferromagnetism in W(Nb)O3-d
The crystal structure and the magnetic properties of the W1-xNbxO3-d,
(x<0.03) system have been investigated. In contrast to the orthorhombic
diamagnetic WO3, the material with x=0.01 is paramagnetic down to 5 K.
Introducing of 2.5 at. % of Nb into WO3 leads to a tetragonal structure and to
a weak itinerant ferromagnetic ordering below TC= 225 K. The saturation
magnetic moment at 5 K is 1.07*10-3 mB, whereas the paramagnetic effective
moment is 0.06 mB per mole. This high ratio indicates itinerant ferromagnetism
in W0.975Nb0.025O3-d.Comment: accepted to Physica
New attempts to understand nanodiamond stardust
We report on a concerted effort aimed at understanding the origin and history
of the pre-solar nanodiamonds in meteorites including the astrophysical sources
of the observed isotopic abundance signatures. This includes measurement of
light elements by secondary ion mass spectrometry (SIMS), analysis of
additional heavy trace elements by accelerator mass spectrometry (AMS) and
dynamic calculations of r-process nucleosynthesis with updated nuclear
properties. Results obtained indicate: a) there is no evidence for the former
presence of now extinct 26Al and 44Ti in our diamond samples other than what
can be attributed to silicon carbide and other "impurities"; this does not
offer support for a supernova (SN) origin but neither does it negate it; b)
analysis by AMS of platinum in "bulk diamond" yields an overabundance of r-only
198Pt that at face value seems more consistent with the neutron burst than with
the separation model for the origin of heavy trace elements in the diamonds,
although this conclusion is not firm given analytical uncertainties; c) if the
Xe-H pattern was established by an unadulterated r-process, it must have been a
strong variant of the main r-process, which possibly could also account for the
new observations in platinum.Comment: Workshop on Astronomy with Radioactvities VII; Publications of the
Astronomical Society of Australia, accepte
s-Process Xe and Kr and Ne-E in a ^(13)C Rich Murchison Sample; Noble Gas Analysis by Stepped Combustion
Some of the more interesting isotopic anomalies in meteorites are located in minor phases that are extremely resistant to chemical treatments. Among these are Ne-E (Black and Pepin, 1969; Eberhardt, 1978; Alaerts et al., 1980) and s-process Kr and Xe (Srinivasan and Anders, 1978; Alaerts et al., 1980), which have been detected in samples that at the same time contain a carbon component enriched in ^(13)C by about a factor of two (Swart et al., 1983). Swart et al. (1983) have concluded that s-Xe and Ne-E (L)- that subcomponent of Ne-E that is released at low temperature in pyrolysis (Eberhardt, 1978)- are related to isotopically heavy carbon. Their argument involved a comparison of stability in pyrolysis (for noble gas retention) and combustion (carbon). Since it is preferrable to compare stability under identical conditions, we have analyzed for its noble gases by stepped combustion Murchison sample CFP. This sample had been prepared using HF/HC1 and HC10_4 and analyzed for carbon by Yang and Epstein (1984), who found a δ^(13)C of + 977‰ in the highest temperature step. We followed their schedule except for an intermediate combustion step at 1000°C
Intermittency in Two-Dimensional Turbulence with Drag
We consider the enstrophy cascade in forced two-dimensional turbulence with a
linear drag force. In the presence of linear drag, the energy wavenumber
spectrum drops with a power law faster than in the case without drag, and the
vorticity field becomes intermittent, as shown by the anomalous scaling of the
vorticity structure functions. Using a previous theory, we compare numerical
simulation results with predictions for the power law exponent of the energy
wavenumber spectrum and the scaling exponents of the vorticity structure
functions obtained in terms of the distribution of finite time
Lyapunov exponents. We also study, both by numerical experiment and theoretical
analysis, the multifractal structure of the viscous enstrophy dissipation in
terms of its R\'{e}nyi dimension spectrum and singularity spectrum
. We derive a relation between and , and discuss
its relevance to a version of the refined similarity hypothesis. In addition,
we obtain and compare theoretically and numerically derived results for the
dependence on separation of the probability distribution of
\delta_{\V{r}}\omega, the difference between the vorticity at two points
separated by a distance . Our numerical simulations are done on a grid.Comment: 18 pages, 17 figure
Dynamics and gravitational wave signature of collapsar formation
We perform 3+1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase. We extract gravitational waves from the spacetime curvature and identify a unique gravitational wave signature associated with the early phase of collapsar formatio
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