7,093 research outputs found
Testing the inversion of asteroids' Gaia photometry combined with ground-based observations
We investigated the reliability of the genetic algorithm which will be used
to invert the photometric measurements of asteroids collected by the European
Space Agency Gaia mission. To do that, we performed several sets of simulations
for 10 000 asteroids having different spin axis orientations, rotational
periods and shapes. The observational epochs used for each simulation were
extracted from the Gaia mission simulator developed at the Observatoire de la
C\^{o}te d'Azur, while the brightness was generated using a Z-buffer standard
graphic method. We also explored the influence on the inversion results of
contaminating the data set with Gaussian noise with different values.
The research enabled us to determine a correlation between the reliability of
the inversion method and the asteroid's pole latitude. In particular, the
results are biased for asteroids having quasi-spherical shapes and low pole
latitudes. This effect is caused by the low lightcurve amplitude observed under
such circumstances, as the periodic signal can be lost in the photometric
random noise when both values are comparable, causing the inversion to fail.
Such bias might be taken into account when analysing the inversion results, not
to mislead it with physical effects such as non-gravitational forces. Finally,
we studied what impact on the inversion results has combining a full lightcurve
and Gaia photometry collected simultaneously. Using this procedure we have
shown that it is possible to reduce the number of wrong solutions for asteroids
having less than 50 data points. The latter will be of special importance for
planning ground-based observations of asteroids aiming to enhance the
scientific impact of Gaia on Solar system science.Comment: Accepted in MNRA
Frequency correlation requirements on the biphoton wavefunction in an induced coherence experiment between separate sources
There is renewed interest in using the coherence between beams generated in
separate down-converter sources for new applications in imaging, spectroscopy,
microscopy and optical coherence tomography (OCT). These schemes make use of
continuous wave (CW) pumping in the low parametric gain regime, which produces
frequency correlations, and frequency entanglement, between signal-idler pairs
generated in each single source. But can induced coherence still be observed if
there is no frequency correlation, so the biphoton wavefunction is factorable?
We will show that this is the case, and this might be an advantage for OCT
applications. High axial resolution requires a large bandwidth. For CW pumping
this requires the use of short nonlinear crystals. This is detrimental since
short crystals generate small photon fluxes. We show that the use of ultrashort
pump pulses allows improving axial resolution even for long crystal that
produce higher photon fluxes
Magnetite and its transformation to hematite in a soil derived from steatite.
O presente trabalho objetivou caracterizar o mineral magnetico e identificar suas rotas pedogeneticas de transformacao em um solo formado sobre esteatito, de Minas Gerais, Brasil. O oxido de ferro isoestrutural ao espinelio foi identificado e caracterizado por analises quimicas, difracao de raios X, espectroscopia Mossbauer e medidas de magnetizacao de saturacao. Na rocha fresca, foi encontrada magnetita estequiometrica e bem cristalizada, com parametro da rede cubica, ao = 0.8407(5) nm. Nas fracoes areia e silte, foram detectadas magnetita parcialmente alterada e hematita estequiometrica e bem cristalizada, com parametros de rede hexagonal, a = 0.5036(3) nm e c = 1.375(4)nm. A ocorrencia dessas hematitas deveu-se principalmente a oxidacao do Fe2+ a Fe3+, no sitio octaedrico de magnetita, durante a pedogenese. Esse processo foi caracterizado pelo aparecimento de pequena quantidade de Fe3+ eletronicamente desacoplada, encontrada nas magnetitas parcialmente oxidadas, cujas formulas para as diferentes estequiometrias foram propostas. Verificou-se tambem pequena quantidade de ilmenita nas amostras de rocha e de solo
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