1,286 research outputs found
Studies of multivariable manual control systems - Two-axis compensatory systems with separated displays and controls
Human controller behavior in multivariable control system
Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12
Explaining the origin of the orbit of 2000 CR105 (a ~ 230AU, q ~ 45AU) is a
major test for our understanding of the primordial evolution of the outer Solar
System. Gladman et al. (2001) showed that this objects could not have been a
normal member of the scattered disk that had its perihelion distance increased
by chaotic diffusion. In this paper we explore four seemingly promising
mechanisms for explaining the origin of the orbit of this peculiar object: (i)
the passage of Neptune through a high-eccentricity phase, (ii) the past
existence of massive planetary embryos in the Kuiper belt or the scattered
disk, (iii) the presence of a massive trans-Neptunian disk at early epochs
which exerted tides on scattered disk objects, and (iv) encounters with other
stars. Of all these mechanisms, the only one giving satisfactory results is the
passage of a star. Indeed, our simulations show that the passage of a solar
mass star at about 800 AU only perturbs objects with semi-major axes larger
than roughly 200 AU to large perihelion distances. This is in good agreement
with the fact that 2000 CR105 has a semi-major axis of 230AU and no other
bodies with similar perihelion distances but smaller semi-major axes have yet
been discovered. The discovery of 2003 VB12, (a=450AU, q=75AU) announced a few
days before the submission of this paper, strengthen our conclusions.Comment: AJ submitted. 27 pages, 6 figure
Henri Temianka correspondence, Gurs
https://digitalcommons.chapman.edu/beach_gurs_et_al_correspondence/1004/thumbnail.jp
A Lagrangian Integrator for Planetary Accretion and Dynamics (LIPAD)
We presented the first particle based, Lagrangian code that can follow the
collisional/accretional/dynamical evolution of a large number of km-sized
planetesimals through the entire growth process to become planets. We refer to
it as the 'Lagrangian Integrator for Planetary Accretion and Dynamics' or
LIPAD. LIPAD is built on top of SyMBA, which is a symplectic -body
integrator. In order to handle the very large number of planetesimals required
by planet formation simulations, we introduce the concept of a `tracer'
particle. Each tracer is intended to represent a large number of disk particles
on roughly the same orbit and size as one another, and is characterized by
three numbers: the physical radius, the bulk density, and the total mass of the
disk particles represented by the tracer. We developed statistical algorithms
that follow the dynamical and collisional evolution of the tracers due to the
presence of one another. The tracers mainly dynamically interact with the
larger objects (`planetary embryos') in the normal N-body way. LIPAD's greatest
strength is that it can accurately model the wholesale redistribution of
planetesimals due to gravitational interaction with the embryos, which has
recently been shown to significantly affect the growth rate of planetary
embryos . We verify the code via a comprehensive set of tests which compare our
results with those of Eulerian and/or direct N-body codes.Comment: Accepted to the Astronomical Journal. See
http://www.boulder.swri.edu/~hal/LIPAD.html for more detail including
animation
Modeling the Formation of Giant Planet Cores I: Evaluating Key Processes
One of the most challenging problems we face in our understanding of planet
formation is how Jupiter and Saturn could have formed before the the solar
nebula dispersed. The most popular model of giant planet formation is the
so-called 'core accretion' model. In this model a large planetary embryo formed
first, mainly by two-body accretion. This is then followed by a period of
inflow of nebular gas directly onto the growing planet. The core accretion
model has an Achilles heel, namely the very first step. We have undertaken the
most comprehensive study of this process to date. In this study we numerically
integrate the orbits of a number of planetary embryos embedded in a swarm of
planetesimals. In these experiments we have included: 1) aerodynamic gas drag,
2) collisional damping between planetesimals, 3) enhanced embryo cross-sections
due to their atmospheres, 4) planetesimal fragmentation, and 5) planetesimal
driven migration. We find that the gravitational interaction between the
embryos and the planetesimals lead to the wholesale redistribution of material
- regions are cleared of material and gaps open near the embryos. Indeed, in
90% of our simulations without fragmentation, the region near that embryos is
cleared of planetesimals before much growth can occur. The remaining 10%,
however, the embryos undergo a burst of outward migration that significantly
increases growth. On timescales of ~100,000 years, the outer embryo can migrate
~6 AU and grow to roughly 30 Earth-masses. We also find that the inclusion of
planetesimal fragmentation tends to inhibit growth.Comment: Accepted to AJ, 62 pages 11 figure
Which Radial Velocity Exoplanets Have Undetected Outer Companions?
(Abridged) The observed radial velocity (RV) eccentricity distribution for
extrasolar planets in single-planet systems shows that a significant fraction
of planets are eccentric (). Here we investigate the effects on an RV
planet's eccentricity produced by undetected outer companions. We have carried
out Monte Carlo simulations of mock RV data to understand this effect and
predict its impact on the observed distribution. We first quantify the
statistical effect of undetected outer companions and show that this alone
cannot explain the observed distribution. We then modify the simulations to
consist of two populations, one of zero-eccentricity planets in double-planet
systems and the other of single planets drawn from an eccentric distribution.
Our simulations show that a good fit to the observed distribution is obtained
with 45% zero-eccentricity double-planets and 55% single eccentric planets.
Matching the observed distribution allows us to determine the probability that
a known RV planet's orbital eccentricity has been biased by an undetected
wide-separation companion. Our simulations show that moderately-eccentric
planets, with and , have a and probability, respectively, of having an undetected outer companion. We
encourage both high-contrast direct imaging and RV follow-up surveys of known
RV planets with moderate eccentricities to test our predictions and look for
previously undetected outer companions.Comment: 23 pages (12 text, 2 tables, 9 figures). Accepted to the
Astrophysical Journal 30 June 200
Modeling low-flow bedrock springs providing ecological habitats with climate change scenarios
Groundwater discharge areas, including low-flow bedrock aquifer springs, are ecologically important and can be impacted by climate change. The development of and results from a groundwater modeling study simulating fractured bedrock spring flow are presented. This was conducted to produce hydrological data for an ecohydrological study of an endangered species, Allegheny Mountain Dusky Salamanders (Desmognathus ochrophaeus), in southern Quebec, Canada. The groundwater modeling approach in terms of scale and complexity was strongly driven by the need to produce hydrological data for the related ecohydrological modeling. Flows at four springs at different elevations were simulated for recent past conditions (2006–2010) and for reference (1971–2000) and future (2041–2070) periods using precipitation and temperature data from ten climate scenarios. Statistical analyses of spring flow parameters including activity periods and duration of flow were conducted. Flow rates for the four simulated springs, located at different elevations, are predicted to increase between 2% and 46% and will be active (flowing) 1–2% longer in the future. A significant change (predominantly an increase) looking at the seasonality of the number of active days occurs in the winter (2–4.9%) and spring seasons (−0.6–6.5%). Greatest flow rates were produced from springs at elevations where sub-horizontal fractures intersect the ground surface. These results suggest an intensification of the spring activity at the study site in context of climate change by 2050, which provides a positive habitat outlook for the endangered salamanders residing in the springs for the future
A comparison of head and manual control for a position-control pursuit tracking task
Head control was compared with manual control in a pursuit tracking task involving proportional controlled-element dynamics. An integrated control/display system was used to explore tracking effectiveness in horizontal and vertical axes tracked singly and concurrently. Compared with manual tracking, head tracking resulted in a 50 percent greater rms error score, lower pilot gain, greater high-frequency phase lag and greater low-frequency remnant. These differences were statistically significant, but differences between horizontal- and vertical-axis tracking and between 1- and 2-axis tracking were generally small and not highly significant. Manual tracking results were matched with the optimal control model using pilot-related parameters typical of those found in previous manual control studies. Head tracking performance was predicted with good accuracy using the manual tracking model plus a model for head/neck response dynamics obtained from the literature
- …