7,635 research outputs found
Reassessing the Source of Long-Period Comets
We present numerical simulations to model the production of observable
long-period comets (LPCs) from the Oort Cloud, a vast reservoir of icy bodies
surrounding the Sun. We show that inner Oort Cloud objects can penetrate
Jupiter's orbit via a largely unexplored dynamical pathway, and they are an
important, if not the dominant, source of known LPCs. We use this LPC
production to place observationally motivated constraints on the population and
mass of the inner Oort Cloud, which are consistent with giant planet formation
theory. These constraints indicate that only one comet shower producing late
Eocene bombardment levels has likely occurred since the Cambrian Explosion,
making these phenomena an improbable cause of additional extinction events.Comment: Originally published in Science (9/4/09), 30 pages, 9 figures, main
article and online material combine
The Formation of the Oort Cloud in Open Cluster Environments
We study the influence of an open cluster environment on the formation and
current structure of the Oort cloud. To do this, we have run 19 different
simulations of the formation of the Oort Cloud for 4.5 Gyrs. In each
simulation, the solar system spends its first 100 Myrs in a different open
cluster environment before transitioning to its current field environment. We
find that, compared to forming in the field environment, the inner Oort Cloud
is preferentially loaded with comets while the Sun resides in the open cluster
and that most of this material remains locked in the interior of the cloud for
the next 4.4 Gyrs. In addition, the outer Oort Cloud trapping efficiencies we
observe in our simulations are lower than previous formation models by about a
factor of 2, possibly implying an even more massive early planetesimal disk.
Furthermore, some of our simulations reproduce the orbits of observed extended
scattered disk objects, which may serve as an observational constraint on the
Sun's early environment. Depending on the particular open cluster environment,
the properties of the inner Oort Cloud and extended scattered disk can vary
widely. On the other hand, the outer portions of the Oort Cloud in each of our
simulations are all similar.Comment: 65 pages, 14 figures, 3 tables, Accepted to Icaru
The evolution of dwarf galaxy satellites with different dark matter density profiles in the ErisMod simulations. I. The early infalls
We present the first simulations of tidal stirring of dwarf galaxies in the
Local Group carried out in a cosmological context. We use the ErisDARK
simulation of a MW-sized galaxy to identify some of the most massive subhalos
() that fall into the main host before .
Subhalos are replaced before infall with high-resolution models of dwarf
galaxies comprising a faint stellar disk embedded in a dark matter halo. The
set of models contains cuspy halos as well as halos with "cored" profiles (with
asymptotic inner slope ). The simulations are then run to
with as many as 54 million particles and resolution as small as pc
using the N-Body code ChaNGa. The stellar components of all satellites are
significantly affected by tidal stirring, losing stellar mass and undergoing a
morphological transformation towards a pressure supported spheroidal system.
However, while some remnants with cuspy halos maintain significant rotational
flattening and disk-like features, all the shallow halo models achieve
and round shapes typical of dSph satellites of the MW and M31.
Mass loss is also enhanced in the latter, and remnants can reach luminosities
and velocity dispersions as low as those of Ultra Faint Dwarfs (UFDs). We argue
that cuspy progenitors must be the exception rather than the rule among
satellites of the MW since all the MW and M31 satellites in the luminosity
range of our remnants are dSphs, a result matched only in the simulation with
"cored" models
Solar dynamic modules for Space Station Freedom: The relationship between fine-pointing control and thermal loading of the aperture plate
Dynamic simulations of Space Station Freedom (SSF) configured with solar dynamic (SD) power modules were performed. The structure was subjected to Space Shuttle docking disturbances, while being controlled with a 'natural' vibration and tracking control approach. Three control cases were investigated for the purpose of investigating the relationship between actuator effort, SD pointing, and thermal loading on the receiver aperture plate. Transient, one-dimensional heat transfer analyses were performed to conservatively predict temperatures of the multi-layered receiver aperture plate assembly and thermal stresses in its shield layer. Results indicate that the proposed aperture plate is tolerant of concentrated flux impingement during short-lived structural disturbances. Pointing requirements may be loosened and the requirement control torques lessened from that previously specified. Downsizing and simplifying the joint drive system should result in a considerable savings mass
High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability
The water content and habitability of terrestrial planets are determined
during their final assembly, from perhaps a hundred 1000-km "planetary embryos"
and a swarm of billions of 1-10 km "planetesimals." During this process, we
assume that water-rich material is accreted by terrestrial planets via impacts
of water-rich bodies that originate in the outer asteroid region. We present
analysis of water delivery and planetary habitability in five high-resolution
simulations containing about ten times more particles than in previous
simulations (Raymond et al 2006a, Icarus, 183, 265-282). These simulations
formed 15 terrestrial planets from 0.4 to 2.6 Earth masses, including five
planets in the habitable zone. Every planet from each simulation accreted at
least the Earth's current water budget; most accreted several times that amount
(assuming no impact depletion). Each planet accreted at least five water-rich
embryos and planetesimals from past 2.5 AU; most accreted 10-20 water-rich
bodies.
We present a new model for water delivery to terrestrial planets in
dynamically calm systems, with low-eccentricity or low-mass giant planets --
such systems may be very common in the Galaxy. We suggest that water is
accreted in comparable amounts from a few planetary embryos in a "hit or miss"
way and from millions of planetesimals in a statistically robust process.
Variations in water content are likely to be caused by fluctuations in the
number of water-rich embryos accreted, as well as from systematic effects such
as planetary mass and location, and giant planet properties.Comment: Astrobiology, in pres
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