De onde vem a população de baixas inclinações do
cinturão de Kuiper?
(Review: Gomes 2009 CMDA 104, 39-51)
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A região do cinturão de Kuiper (40-50 AU) precisava ser 'vazia' para que
Netuno não migrasse até 50 AU (Gomes et al. 2004).
Existe um mecanismo de transporte da população de altas inclinações de
um disco truncado em 30 AU até o cinturão de Kuiper atual (Gomes 2003).
Não haveria também um mecanismo de transporte da população de baixas
inclinações?
Um mecanismo pré modelo de Nice
Levison & Morbidelli (2003) Nature 426 419
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Diferentemente do modelo de produção da população de altas inclinações,
este mecanismo não funciona para o modelo de Nice, já que exige uma
migração suave por um trecho longo.
Transporte da população de baixas inclinações pelo
modelo de Nice
Levison et al. 2008 Icarus 196, 258
Variação das excentricidades de objetos espalhados por Netuno
devido à sua alta excentricidade
Exemplo semelhante vindo de uma simulação do
modelo de Nice
Simulação 'mista': Netuno excêntrico e objetos espalhados
vindos de uma integração original; circularização de Netuno
artificial com partículas colocadas com massa zero.
O mesmo para outro conjunto de condições iniciais
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Principal problema com este mecanismo: excentricidades ainda muito altas:
mediana entre 0.1 e 0.13 dependendo das condições iniciais; mediana das
excentricidades para as órbitas reais: 0.07.
Controvérsia sobre origem da população de
baixas inclinações
Gomes 2009 CMDA 104, 39-51
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L1. The initial Kuiper conjecture: a ‘real’ Kuiper belt should display
low inclination/ low eccentricity orbits as the cold population does.
L2. The fact that it is really a different population from the hot one (by
their different sizes / colors). A totally different origin for cold and hot
population is naturally consistent with a local origin for the local
population.
Controvérsia
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T1. As Gomes et al. (2004) showed, it is difficult to keep both an original
massive enough Kuiper belt at its present location and keep Neptune at 30
AU at the time of planetary migration.
T2. The outer border of the Kuiper belt suggests the transporting scenario.
For the local formation scenario the border must be original, thus it could be
virtually anywhere. For the transporting scenario it must be near the 1:2
resonance with Neptune (Levison et al. 2008a). So the fact that the outer
border of the KB is located where the transporting scenario (which is
consistent with other important features of the KB) predicts and where it
would just by chance be located by the local formation scenario surely gives
extra support to the transporting scenario.
T3. The fact that both the cold and hot populations have about the same
mass argues against a local origin for the cold population. If the hot
population was originated by the evaders mechanism and thus had a low
mass from the beginning, it must be considered a coincidence that the cold
population was eroded from a much larger mass to about the same mass of
the hot population. Also consider Petit and Mousis (2004) that strengthens
the case for a dynamical cold population implantation.
Mais argumentos para local
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As to point L1, it must be noted that the eccentricities of the cold
population are in average a little too high for an undisturbed
primordial population. But the excitation of the eccentricities could be
achieved by any migration scenario, including the Nice model. In this
case, the temporary passage of Neptune’s aphelion at the Kuiper
region could do the job. As to point L2, it must be noted that the Nice
model (Levison et al. 2008a) naturally brings the inner portion of the
planetesimal disk as a hot population and the outer portion as a cold
population. This might account for the differences in sizes and colors
between both populations. The correlations are not, however, quite
nicely established as in the case of the locally formed cold population.
Mis argumentos para transporte
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As to point T1, one can counter-argue with the idea that when
Neptune approached its present position, the Kuiper region was
already sufficiently depleted so as not to induce any further outer
displacement for Neptune. This may need a vigorous mass erosion
through collisions, which may be difficult to accept, unless two
hypothesis are assumed: (1) the Nice model, that would give time for
the erosion in the Kuiper belt to take place before Neptune reached
30AU; (2) a size distribution of objects concentrated in smaller ones,
to make the grinding process efficient enough to deplete the Kuiper
region of mass so as to stop Neptune when it reached the KB inner
border (see Kenyon et al. 2008, The Solar System beyond Neptune,
pp. 293–313.). This size distribution would have to be quite different
from the one in the inner portion on the planetesimal disk, where a
more evolved size distribution must be assumed so as to create big
enough bodies to populate the Oort cloud and the trans-Neptunian
region.