182 research outputs found
Origin of craters on Phoebe: comparison with Cassini's data
Phoebe is one of the irregular satellites of Saturn; the images taken by
Cassini-Huygens spacecraft allowed us to analyze its surface and the craters on
it. We study the craters on Phoebe produced by Centaur objects from the
Scattered Disk (SD) and plutinos escaped from the 3:2 mean motion resonance
with Neptune and compare our results with the observations by Cassini. We use
previous simulations on trans-Neptunian Objects and a method that allows us to
obtain the number of craters and the cratering rate on Phoebe. We obtain the
number of craters and the greatest crater on Phoebe produced by Centaurs in the
present configuration of the Solar System. Moreover, we obtain a present
normalized rate of encounters of Centaurs with Saturn of per year, from which we can infer the current cratering rate on
Phoebe for each crater diameter. Our study and the comparison with the
observations suggest that the main crater features on Phoebe are unlikely to
have been produced in the present configuration of the Solar System and that
they must have been acquired when the SD were depleted in the early Solar
System. If this is what happened and the craters were produced when Phoebe was
a satellite of Saturn, then it had to be captured, very early in the evolution
of the Solar System.Comment: Accepted for publication in Astronomy & Astrophysic
Radiation induced warping of protostellar accretion disks
We examine the consequences of radiatively driven warping of accretion disks
surrounding pre-main-sequence stars. These disks are stable against warping if
the luminosity arises from a steady accretion flow, but are unstable at late
times when the intrinsic luminosity of the star overwhelms that provided by the
disk. Warps can be excited for stars with luminosities of around 10 solar
luminosities or greater, with larger and more severe warps in the more luminous
systems. A twisted inner disk may lead to high extinction towards stars often
viewed through their disks. After the disk at all radii becomes optically thin,
the warp decays gradually on the local viscous timescale, which is likely to be
long. We suggest that radiation induced warping may account for the origin of
the warped dust disk seen in Beta Pictoris, if the star is only around 10-20
Myr old, and could lead to non-coplanar planetary systems around higher mass
stars.Comment: 12 pages, including 3 figures. ApJ Letters, in pres
Dynamical evolution of escaped plutinos, another source of Centaurs
It was shown in previous works the existence of weakly chaotic orbits in the
plutino population that diffuse very slowly. These orbits correspond to
long-term plutino escapers and then represent the plutinos that are escaping
from the resonance at present. In this paper we perform numerical simulations
in order to explore the dynamical evolution of plutinos recently escaped from
the resonance. The numerical simulations were divided in two parts. In the
first one we evolved 20,000 test particles in the resonance in order to detect
and select the long-term escapers. In the second one, we numerically integrate
the selected escaped plutinos in order to study their dynamical post escaped
behavior. Our main results include the characterization of the routes of escape
of plutinos and their evolution in the Centaur zone. We obtained a present rate
of escape of plutinos between 1 and 10 every 10 years. The escaped plutinos
have a mean lifetime in the Centaur zone of 108 Myr and their contribution to
the Centaur population would be a fraction of less than 6 % of the total
Centaur population. In this way, escaped plutinos would be a secondary source
of Centaurs.Comment: Accepted for publication in A&
A method to ingest GPS-TEC into the NeQuick ionospheric model
This paper presents a method to ingest Total Electron Content measurements from ground-based GPS receivers into the empirical NeQuick model. The method here presented relies upon optimizing the parameter that primarily drives the NeQuick profile, i.e., the electron density of the F2 peak, NmF2. The effectiveness of the method is assessed in a rather benevolent ionospheric scenario: a midlatitude region and quiet geomagnetic days that cover solstices and equinoxes conditions during a medium-high solar activity year. Thus, the procedure demonstrated to be capable of improving the climatological value of NmF2 computed from the Radioscience Section of the International Telecommunication Union (ITU-R) database. This capability was assessed by comparing the ITU-R value and the corrected value produced by our method to the value measured with a Digisonde. The result of this comparison was an overall reduction of the error of the NmF2 parameter to approximately half of its original value.Facultad de Ciencias Astronómicas y Geofísica
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
Bioecologia de Alabama argillacea II. Evolução populacional em seis regiões do Estado de São Paulo, com base em suas exigências térmicas
The cotton leafworm (Alabama argillacea Hübner (1818) has, each year, a constant period of occurence during the cotton growing season, in the State of São Paulo, Brazil. Surveys with light traps in six localities of the State (Assis, Campinas, Mococa, Piracicaba, Ribeirão Preto and Valinhos) were made in order to find out those periods. With its thermal requirements it was possible to know the number of its generations all through the year in order to provide basic data concerning this pest, as a prediction of its outbreak, and for improving control procedures in the crop. The thermal requirements of the cotton leafworm were determined based on the biological results obtained in climatic chambers set at 20, 25, 30 and 35°C (60 ± 10% relative humidity and a 14-hour photoperiod) with IAC-17 cultivar leaves. The threshold of development for the egg, larval, pre-pupal and pupal phases was calculated by the coefficient of variation method; values of 12.6; 9.3; 14.9 and 13.5°C, respectively, were found. The formula for obtaining those thermal requirement data was: K = y (t - a). Values of 37.20, 195.97, 10.10 and 110.35 degree-days for the phases of egg, larva, pre-pupa and pupa, respectively, were obtained. Basing on the total sum of degree-days per "péntada" for the six localities, two to seven generations per year may occur.O curuquerê-do-algodoeiro (Alabama argillacea Hübner (1818) tem uma época definida de surto nas diferentes áreas do Estado de São Paulo. Tomando-se por base levantamentos realizados com armadilhas luminosas nas regiões de Assis, Campinas, Mococa, Piracicaba, Ribeirão Preto e Valinhos, determinou-se, em função destas épocas de seu aparecimento e das suas necessidades térmicas, o provável número de gerações desta praga nestas áreas durante o ano, numa tentativa de prever e controlar a sua evolução populacional dentro da cultura. As necessidades térmicas do curuquerê foram determinadas com base nos resultados biológicos obtidos em câmaras climatizadas reguladas a 20, 25, 30 e 35°C (UR de 60 ± 10% e fotófase de 14 h), com folhas da cultivar IAC-17, estimando-se a temperatura-base para as fases de ovo, lagarta, pré-pupa e pupa, pelo método do coeficiente de variação e encontrando-se valores, respectivamente, de 12,6; 9,3; 14,9 e 13,5°C. A constante térmica em graus dias (GD)foi estimada pela fórmula K = y (t - a), sendo obtidos os valores de 37, 20 GD, 195,97 GD, 10,10 GD e 110,35 GD para as fases de ovo, lagarta, pré-pupa e pupa, respectivamente. A partir destes resultados e de graus dias acumulados por pêntada, utilizando-se as normais de cada localidade, estimou-se que o número de gerações pode variar de duas a sete para as regiões analisadas
Origin and Dynamical Evolution of Neptune Trojans - I: Formation and Planetary Migration
We present the results of detailed dynamical simulations of the effect of the
migration of the four giant planets on both the transport of pre-formed Neptune
Trojans, and the capture of new Trojans from a trans-Neptunian disk. We find
that scenarios involving the slow migration of Neptune over a large distance
(50Myr to migrate from 18.1AU to its current location) provide the best match
to the properties of the known Trojans. Scenarios with faster migration (5Myr),
and those in which Neptune migrates from 23.1AU to its current location, fail
to adequately reproduce the current day Trojan population. Scenarios which
avoid disruptive perturbation events between Uranus and Neptune fail to yield
any significant excitation of pre-formed Trojans (transported with efficiencies
between 30 and 98% whilst maintaining the dynamically cold nature of these
objects). Conversely, scenarios with periods of strong Uranus-Neptune
perturbation lead to the almost complete loss of such pre-formed objects. In
these cases, a small fraction (~0.15%) of these escaped objects are later
recaptured as Trojans prior to the end of migration, with a wide range of
eccentricities (<0.35) and inclinations (<40 deg). In all scenarios (including
those with such disruptive interaction between Uranus and Neptune) the capture
of objects from the trans-Neptunian disk (through which Neptune migrates) is
achieved with efficiencies between ~0.1 and ~1%. The captured Trojans display a
wide range of inclinations (<40 deg for slow migration, and <20 deg for rapid
migration) and eccentricities (<0.35), and we conclude that, given the vast
amount of material which undoubtedly formed beyond the orbit of Neptune, such
captured objects may be sufficient to explain the entire Neptune Trojan
population. (Shortened version)Comment: 25 pages, 6 figure
On the formation of hot Neptunes and super-Earths
The discovery of short-period Neptune-mass objects, now including the
remarkable system HD69830 (Lovis et al. 2006) with three Neptune analogues,
raises difficult questions about current formation models which may require a
global treatment of the protoplanetary disc. Several formation scenarios have
been proposed, where most combine the canonical oligarchic picture of core
accretion with type I migration (e.g. Terquem & Papaloizou 2007) and planetary
atmosphere physics (e.g. Alibert et al. 2006). To date, published studies have
considered only a small number of progenitors at late times. This leaves
unaddressed important questions about the global viability of the models. We
seek to determine whether the most natural model -- namely, taking the
canonical oligarchic picture of core accretion and introducing type I migration
-- can succeed in forming objects of 10 Earth masses and more in the innermost
parts of the disc. This problem is investigated using both traditional
semianalytic methods for modelling oligarchic growth as well as a new parallel
multi-zone N-body code designed specifically for treating planetary formation
problems with large dynamic range (McNeil & Nelson 2009). We find that it is
extremely difficult for oligarchic tidal migration models to reproduce the
observed distribution. Even under many variations of the typical parameters, we
form no objects of mass greater than 8 Earth masses. By comparison, it is
relatively straightforward to form icy super-Earths. We conclude that either
the initial conditions of the protoplanetary discs in short-period Neptune
systems were substantially different from the standard disc models we used, or
there is important physics yet to be understood.Comment: 19 pages, 18 figures. Final version accepted to MNRAS 30 September
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Planetesimal fragmentation and giant planet formation: the role of planet migration
In the standard model of core accretion, the cores of the giant planets form by the accretion of planetesimals. In this scenario, the evolution of the planetesimal population plays an important role in the formation of massive cores. Recently, we studied the role of planetesimal fragmentation in the in situ formation of a giant planet. However, the exchange of angular momentum between the planet and the gaseous disk causes the migration of the planet in the disk. In this new work, we incorporate the migration of the planet and study the role of planet migration in the formation of a massive core when the population of planetesimals evolves by planet accretion, migration, and fragmentatio
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