784 research outputs found
On the survivability and detectability of terrestrial meteorites on the moon
Materials blasted into space from the surface of early Earth may preserve a unique record of our planet's early surface environment. Armstrong et al. (2002) pointed out that such materials, in the form of terrestrial meteorites, may exist on the Moon and be of considerable astrobiological interest if biomarkers from early Earth are preserved within them. Here, we report results obtained via the AUTODYN hydrocode to calculate the peak pressures within terrestrial meteorites on the lunar surface to assess their likelihood of surviving the impact. Our results confirm the order-of-magnitude estimates of Armstrong et al. (2002) that substantial survivability is to be expected, especially in the case of relatively low velocity (ca. 2.5 km/s) or oblique (≤45°) impacts, or both. We outline possible mechanisms for locating such materials on the Moon and conclude that searching for them would be a scientifically valuable activity for future lunar exploration
Exceptionally Slow Rise in Differential Reflectivity Spectra of Excitons in GaN: Effect of Excitation-induced Dephasing
Femtosecond pump-probe (PP) differential reflectivity spectroscopy (DRS) and
four-wave mixing (FWM) experiments were performed simultaneously to study the
initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats
between the A-B excitons were found \textit{only for positive time delay} in
both PP and FWM experiments. The rise time at negative time delay for the
differential reflection spectra was much slower than the FWM signal or PP
differential transmission spectroscopy (DTS) at the exciton resonance. A
numerical solution of a six band semiconductor Bloch equation model including
nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS
results from excitation induced dephasing (EID), that is, the strong density
dependence of the dephasing time which changes with the laser excitation
energy.Comment: 8 figure
Discontinuity Induced Bifurcations in a Model of Saccharomyces cerevisiae
We perform a bifurcation analysis of the mathematical model of Jones and
Kompala [K.D. Jones and D.S. Kompala, Cybernetic model of the growth dynamics
of Saccharomyces cerevisiae in batch and continuous cultures, J. Biotech.,
71:105-131, 1999]. Stable oscillations arise via Andronov-Hopf bifurcations and
exist for intermediate values of the dilution rate as has been noted from
experiments previously. A variety of discontinuity induced bifurcations arise
from a lack of global differentiability. We identify and classify discontinuous
bifurcations including several codimension-two scenarios. Bifurcation diagrams
are explained by a general unfolding of these singularities.Comment: 23 pages, 7 figure
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