850 research outputs found
The formation of the solar system
The solar system started to form about 4.56 Gyr ago and despite the long
intervening time span, there still exist several clues about its formation. The
three major sources for this information are meteorites, the present solar
system structure and the planet-forming systems around young stars. In this
introduction we give an overview of the current understanding of the solar
system formation from all these different research fields. This includes the
question of the lifetime of the solar protoplanetary disc, the different stages
of planet formation, their duration, and their relative importance. We consider
whether meteorite evidence and observations of protoplanetary discs point in
the same direction. This will tell us whether our solar system had a typical
formation history or an exceptional one. There are also many indications that
the solar system formed as part of a star cluster. Here we examine the types of
cluster the Sun could have formed in, especially whether its stellar density
was at any stage high enough to influence the properties of today's solar
system. The likelihood of identifying siblings of the Sun is discussed.
Finally, the possible dynamical evolution of the solar system since its
formation and its future are considered.Comment: 36 pages, 7 figures, invited review in Physica Script
EXPERIMENTAL INVESTIGATION OF A PARAFFIN BASED HYBRID ROCKET
Hybrid rockets are known to be simpler, safer, environmentally friend, and, more importantly, cheaper than most of the technologies for propulsion devices used today. Hybrid rockets can be applied as the propulsion system in satellites launch vehicles, micro-satellites and tactical missiles. This paper deals with combustion of ultra-high molecular weight polyethylene (UHMWPE) and paraffin as the solid fuels burning with gaseous oxygen (GOX) as well as N O as the oxidizer in lab scale hybrid rocket motors. A test 2 stand was built to carry out the experiments. The main objectives were to investigate the ignition of the solid fuels, burning performance and regression rates for different operating conditions. With paraffin-based fuel the hybrid motor had the regression rate enhanced two to three folds compared to the UHMWPE, as reported in the literature. The overall performance of the motor, with paraffin as the fuel, is comparable to other technologies. Paraffin-based hybrid rockets can, then, be a safer and cheaper alternative to satellite launch vehicles for the Brazilian space program
Degeneracy in the characterization of non-transiting planets from transit timing variations
The transit timing variation (TTV) method allows the detection of
non-transiting planets through their gravitational perturbations. Since TTVs
are strongly enhanced in systems close to mean-motion resonances (MMR), even a
low mass planet can produce an observable signal. This technique has thus been
proposed to detect terrestrial planets. In this letter, we analyse TTV signals
for systems in or close to MMR in order to illustrate the difficulties arising
in the determination of planetary parameters. TTVs are computed numerically
with an n-body integrator for a variety of systems close to MMR. The main
features of these TTVs are also derived analytically. Systems deeply inside MMR
do not produce particularly strong TTVs, while those close to MMR generate
quasiperiodic TTVs characterised by a dominant long period term and a low
amplitude remainder. If the remainder is too weak to be detected, then the
signal is strongly degenerate and this prevents the determination of the
planetary parameters. Even though an Earth mass planet can be detected by the
TTV method if it is close to a MMR, it may not be possible to assert that this
planet is actually an Earth mass planet. On the other hand, if the system is
right in the center of a MMR, the high amplitude oscillation of the TTV signal
vanishes and the detection of the perturber becomes as difficult as it is far
from MMR.Comment: 5 pages, 3 figures, submitted to MNRA
Transiting Disintegrating Planetary Debris around WD 1145+017
More than a decade after astronomers realized that disrupted planetary
material likely pollutes the surfaces of many white dwarf stars, the discovery
of transiting debris orbiting the white dwarf WD 1145+017 has opened the door
to new explorations of this process. We describe the observational evidence for
transiting planetary material and the current theoretical understanding (and in
some cases lack thereof) of the phenomenon.Comment: Invited review chapter. Accepted March 23, 2017 and published October
7, 2017 in the Handbook of Exoplanets. 15 pages, 10 figure
Frações de N do solo cultivado com milho e plantas de cobertura.
O milho é uma cultura de relevância no país e exige quantidades elevadas de nitrogênio (N) durante o seu crescimento. A sua disponibilidade para as plantas depende tanto do teor imediatamente disponível no solo, representada pelas formas inorgânicas, quanto do N das formas orgânicas, mineralizáveis durante o seu ciclo. O objetivo deste trabalho foi avaliar as alterações de frações do nitrogênio sob cultivo de plantas de cobertura na cultura do milho. O experimento está sendo conduzido há oito anos em Latossolo Vermelho sob milho cultivado em sistema plantio direto em sucessão às seguintes plantas de cobertura: Urochloa ruziziensis, Canavalia brasiliensis, Cajanus cajan e Sorghum bicolor. O delineamento experimental foi de blocos ao acaso com parcelas subdivididas e três repetições. Nas parcelas foram semeadas as plantas de cobertura e nas subparcelas foi feita a adubação fertilização nitrogenada em cobertura (com e sem N) na cultura do milho. Em Abril de 2013, após a colheita do milho foi realizada a coleta do solo nas profundidades de 0-10 e 10-20 cm. As plantas de cobertura apresentaram comportamento diferenciado em relação às camadas de solo. O solo sob U. ruziziensis apresentou maior N total e particulado que aquele sob C. cajan
Amperometric and spectrophotometric determination of carbaryl in natural waters and commercial formulations
The work presented describes the development
and evaluation of two flow-injection analysis (FIA) systems
for the automated determination of carbaryl in spiked
natural waters and commercial formulations. Samples are
injected directly into the system where they are subjected
to alkaline hydrolysis thus forming 1-naphthol. This product
is readily oxidised at a glassy carbon electrode. The
electrochemical behaviour of 1-naphthol allows the development
of an FIA system with an amperometric detector
in which 1-naphthol determination, and thus measurement
of carbaryl concentration, can be performed. Linear response
over the range 1.0×10–7 to 1.0×10–5 mol L–1, with a
sampling rate of 80 samples h–1, was recorded. The detection
limit was 1.0×10–8 mol L–1. Another FIA manifold was
constructed but this used a colorimetric detector. The methodology
was based on the coupling of 1-naphthol with phenylhydrazine
hydrochloride to produce a red complex which
has maximum absorbance at 495 nm. The response was
linear from 1.0×10–5 to 1.5×10–3 mol L–1 with a detection
limit of 1.0×10–6 mol L–1. Sample-throughput was about
60 samples h–1. Validation of the results provided by the
two FIA methodologies was performed by comparing
them with results from a standard HPLC–UV technique.
The relative deviation was <5%. Recovery trials were also
carried out and the values obtained ranged from 97.0 to
102.0% for both methods. The repeatability (RSD, %) of
12 consecutive injections of one sample was 0.8% and
1.6% for the amperometric and colorimetric systems, respectively
Stellar archaeology with Gaia: the Galactic white dwarf population
Gaia will identify several 1e5 white dwarfs, most of which will be in the
solar neighborhood at distances of a few hundred parsecs. Ground-based optical
follow-up spectroscopy of this sample of stellar remnants is essential to
unlock the enormous scientific potential it holds for our understanding of
stellar evolution, and the Galactic formation history of both stars and
planets.Comment: Summary of a talk at the 'Multi-Object Spectroscopy in the Next
Decade' conference in La Palma, March 2015, to be published in ASP Conference
Series (editors Ian Skillen & Scott Trager
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