2,210 research outputs found
Astrometry of mutual approximations between natural satellites. Application to the Galilean moons
Typically we can deliver astrometric positions of natural satellites with
errors in the 50-150 mas range. Apparent distances from mutual phenomena, have
much smaller errors, less than 10 mas. However, this method can only be applied
during the equinox of the planets. We developed a method that can provide
accurate astrometric data for natural satellites -- the mutual approximations.
The method can be applied when any two satellites pass close by each other in
the apparent sky plane. The fundamental parameter is the central instant
of the passage when the distances reach a minimum.
We applied the method for the Galilean moons. All observations were made with
a 0.6 m telescope with a narrow-band filter centred at 889 nm with width of 15
nm which attenuated Jupiter's scattered light. We obtained central instants for
14 mutual approximations observed in 2014-2015. We determined with an
average precision of 3.42 mas (10.43 km). For comparison, we also applied the
method for 5 occultations in the 2009 mutual phenomena campaign and for 22
occultations in the 2014-2015 campaign. The comparisons of determined by
our method with the results from mutual phenomena show an agreement by less
than 1-sigma error in , typically less than 10 mas. This new method is
particularly suitable for observations by small telescopes.Comment: 13 pages, 11 figures and 8 tables. Based on observations made at the
Laborat\'orio Nacional de Astrof\'isica (LNA), Itajub\'a-MG, Brazi
Triplicity and Physical Characteristics of Asteroid (216) Kleopatra
To take full advantage of the September 2008 opposition passage of the M-type
asteroid (216) Kleopatra, we have used near-infrared adaptive optics (AO)
imaging with the W.M. Keck II telescope to capture unprecedented high
resolution images of this unusual asteroid. Our AO observations with the W.M.
Keck II telescope, combined with Spitzer/IRS spectroscopic observations and
past stellar occultations, confirm the value of its IRAS radiometric radius of
67.5 km as well as its dog-bone shape suggested by earlier radar observations.
Our Keck AO observations revealed the presence of two small satellites in orbit
about Kleopatra (see Marchis et al., 2008). Accurate measurements of the
satellite orbits over a full month enabled us to determine the total mass of
the system to be 4.64+/-0.02 10^18 Kg. This translates into a bulk density of
3.6 +/-0.4 g/cm3, which implies a macroscopic porosity for Kleopatra of ~
30-50%, typical of a rubble-pile asteroid. From these physical characteristics
we measured its specific angular momentum, very close to that of a spinning
equilibrium dumbbell.Comment: 35 pages, 3 Tables, 9 Figures. In press to Icaru
Orbit determination of Transneptunian objects and Centaurs for the prediction of stellar occultations
The prediction of stellar occultations by Transneptunian objects and Centaurs
is a difficult challenge that requires accuracy both in the occulted star
position as for the object ephemeris. Until now, the most used method of
prediction involving tens of TNOs/Centaurs was to consider a constant offset
for the right ascension and for the declination with respect to a reference
ephemeris. This offset is determined as the difference between the most recent
observations of the TNO and the reference ephemeris. This method can be
successfully applied when the offset remains constant with time. This paper
presents an alternative method of prediction based on a new accurate orbit
determination procedure, which uses all the available positions of the TNO from
the Minor Planet Center database plus sets of new astrometric positions from
unpublished observations. The orbit determination is performed through a
numerical integration procedure (NIMA), in which we develop a specific
weighting scheme. The NIMA method was applied for 51 selected TNOs/Centaurs.
For this purpose, we have performed about 2900 new observations during
2007-2014. Using NIMA, we succeed in predicting the stellar occultations of 10
TNOs and 3 Centaurs between 2013 and 2015. By comparing the NIMA and JPL
ephemerides, we highlighted the variation of the offset between them with time.
Giving examples, we show that the constant offset method could not accurately
predict 6 out of the 13 observed positive occultations successfully predicted
by NIMA. The results indicate that NIMA is capable of efficiently refine the
orbits of these bodies. Finally, we show that the astrometric positions given
by positive occultations can help to further refine the orbit of the TNO and
consequently the future predictions. We also provide the unpublished
observations of the 51 selected TNOs and their ephemeris in a usable format by
the SPICE library.Comment: 12 pages, 9 figures, accepted in A&
Teste de modelo agrometeorológico de monitoramento e de estimativa de produtividade do cafeeiro (Coffea arabica L.). no Estado de São Paulo.
O desenvolvimento de modelo agrometeorológico que possibilite a estimativa de quebra de produtividade antecipada é importante para subsidiar programas de previsão de safras de café. Uma boa estimativa da produtividade implica na utilização de modelos que considerem os efeitos ambientais aos processos fisiológicos determinantes da produção. Este trabalho teve o objetivo de testar um modelo matemático agrometeorológico de estimativa de produtividade do cafeeiro para três diferentes escalas produtivas ?talhão?, ?propriedade? e ?município? em diferentes regiões do Estado de São Paulo. Dados meteorológicos e de produtividade foram coletados no IAC e CATI para o período de 2000 a 2004. O modelo se baseia na penalização da produtividade potencial da cultura em função do déficit hídrico quantificado através da relação [1(ETr/ETp)] ajustadas por diferentes coeficientes de sensibilidade da cultura (Ky) ocorridos em diferentes fases fenológicas. O modelo considera também os efeitos da produtividade do ano anterior e temperaturas adversas ocorridas em fases fenológicas críticas. Os modelos parametrizados, nas três escalas produtivas consideradas, apresentaram ajustes satisfatórios entre valores observados e estimados, com valores do índice ?d? variando de 0,88 a 0,92, ?R? variando de 0,81 a 0,87 e erros aleatórios relativamente baixos de 5,1 a 9,4 sacas.ha 1 e sistemáticos de 3,5 a 7,1sacas.ha 1. O modelo apresentou pequena tendência a superestimar as produtividades estimadas. Os resultados indicam que o modelo parametrizado em diferentes escalas produtivas tem potencial para estimar a produtividade do café, podendo servir como subsídio aos trabalhos de previsão de safra
Análise das condições agrometeorológicas e fenológicas do cafeeiro arábica em Guaxupé, MG, no ano agrícola 2005-2006.
Neste trabalho são apresentados o balanço hídrico seqüencial decendial e uma análise dos principais eventos agrometeorológicos e fenológicos do cafeeiro arábica ocorridos no ano agrícola 2005-2006 no município de Guaxupé, localizado na região cafeeira do Sul de Minas Gerais. Pode-se verificar que o ano agrícola 2005-2006 foi caracterizado por uma temperatura média anual de 20,5°C, um índice pluviométrico de 1.117 mm, uma deficiência hídrica de 146 mm, um excedente hídrico de 348 mm e uma taxa de armazenamento médio de água no solo de 65 mm. De maneira geral, este ano agrícola foi menos chuvoso e mais frio que a média histórica (1960-2003). Os eventos fenológicos mais marcantes neste ano agrícola foram: a ocorrência de chuvas irregulares no início da primavera, que provocou vários florescimentos na região, acarretando o desenvolvimento irregular dos frutos do cafeeiro, e o longo período sem chuvas significativas no mês de janeiro, que prejudicou o crescimento vegetativo do cafeeiro, trazendo conseqüências à granação dos frutos. As perdas decorrentes do estresse provocado pelo veranico de janeiro na região de Guaxupé chegaram a 12%
Parallaxes of southern extremely cool objects III : 118 L and T dwarfs
We present new results from the Parallaxes of Southern Extremely Cool dwarfs program to measure parallaxes, proper motions and multiepoch photometry of L and early T dwarfs. The observations were made on 108 nights over the course of 8 yr using the Wide Field Imager on the ESO 2.2m telescope. We present 118 new parallaxes of L and T dwarfs of which 52 have no published values and 24 of the 66 published values are preliminary estimates from this program. The parallax precision varies from 1.0 to 15.5mas with a median of 3.8mas. We find evidence for two objects with long term photometric variation and 24 new moving group candidates. We cross-match our sample to published photometric catalogues and find standard magnitudes in up to 16 pass-bands from which we build spectral energy distributions and H-R diagrams. This allows us to confirm the theoretically anticipated minimum in radius between stars and brown dwarfs across the hydrogen burning minimum mass. We find the minimum occurs between L2 and L6 and verify the predicted steep dependence of radius in the hydrogen burning regime and the gentle rise into the degenerate brown dwarf regime. We find a relatively young age of ~2 Gyr from the kinematics of our sample.Peer reviewedFinal Accepted Versio
Astrometric positions for 18 irregular satellites of giant planets from 23 years of observations
The irregular satellites of the giant planets are believed to have been
captured during the evolution of the solar system. Knowing their physical
parameters, such as size, density, and albedo is important for constraining
where they came from and how they were captured. The best way to obtain these
parameters are observations in situ by spacecrafts or from stellar occultations
by the objects. Both techniques demand that the orbits are well known. We aimed
to obtain good astrometric positions of irregular satellites to improve their
orbits and ephemeris. We identified and reduced observations of several
irregular satellites from three databases containing more than 8000 images
obtained between 1992 and 2014 at three sites (Observat\'orio do Pico dos Dias,
Observatoire de Haute-Provence, and European Southern Observatory - La Silla).
We used the software PRAIA (Platform for Reduction of Astronomical Images
Automatically) to make the astrometric reduction of the CCD frames. The UCAC4
catalog represented the International Celestial Reference System in the
reductions. Identification of the satellites in the frames was done through
their ephemerides as determined from the SPICE/NAIF kernels. Some procedures
were followed to overcome missing or incomplete information (coordinates,
date), mostly for the older images. We managed to obtain more than 6000
positions for 18 irregular satellites: 12 of Jupiter, 4 of Saturn, 1 of Uranus
(Sycorax), and 1 of Neptune (Nereid). For some satellites the number of
obtained positions is more than 50\% of what was used in earlier orbital
numerical integrations. Comparison of our positions with recent JPL ephemeris
suggests there are systematic errors in the orbits for some of the irregular
satellites. The most evident case was an error in the inclination of Carme.Comment: 9 pages, with 3 being online materia
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