8,919 research outputs found
Quantum inequalities and `quantum interest' as eigenvalue problems
Quantum inequalities (QI's) provide lower bounds on the averaged energy
density of a quantum field. We show how the QI's for massless scalar fields in
even dimensional Minkowski space may be reformulated in terms of the positivity
of a certain self-adjoint operator - a generalised Schroedinger operator with
the energy density as the potential - and hence as an eigenvalue problem. We
use this idea to verify that the energy density produced by a moving mirror in
two dimensions is compatible with the QI's for a large class of mirror
trajectories. In addition, we apply this viewpoint to the `quantum interest
conjecture' of Ford and Roman, which asserts that the positive part of an
energy density always overcompensates for any negative components. For various
simple models in two and four dimensions we obtain the best possible bounds on
the `quantum interest rate' and on the maximum delay between a negative pulse
and a compensating positive pulse. Perhaps surprisingly, we find that - in four
dimensions - it is impossible for a positive delta-function pulse of any
magnitude to compensate for a negative delta-function pulse, no matter how
close together they occur.Comment: 18 pages, RevTeX. One new result added; typos fixed. To appear in
Phys. Rev.
Formation of '3D' multiplanet systems by dynamical disruption of multiple-resonance configurations
Assuming that giant planets are formed in thin protoplanetary discs, a '3D'
system can form, provided that the mutual inclination is excited by some
dynamical mechanism. Resonant interactions and close planetary encounters are
thought to be the primary inclination-excitation mechanisms, resulting in a
resonant and non-resonant system, respectively. Here we propose an alternative
formation scenario, starting from a system composed of three giant planets in a
nearly coplanar configuration. As was recently shown for the case of the Solar
system, planetary migration in the gas disc (Type II migration) can force the
planets to become trapped in a multiply resonant state. We simulate this
process, assuming different values for the planetary masses and mass ratios. We
show that such a triple resonance generally becomes unstable as the resonance
excites the eccentricities of all planets and planet-planet scattering sets in.
One of the three planets is typically ejected from the system, leaving behind a
dynamically 'hot' (but stable) two-planet configuration. The resulting
two-planet systems typically have large values of semimajor axial ratios (a1/a2
< 0.3), while the mutual inclination can be as high as 70{\deg}, with a median
of \sim30{\deg}. A small fraction of our two-planet systems (\sim5 per cent)
ends up in the stability zone of the Kozai resonance. In a few cases, the
triple resonance can remain stable for long times and a '3D' system can form by
resonant excitation of the orbital inclinations; such a three-planet system
could be stable if enough eccentricity damping is exerted on the planets.
Finally, in the single-planet resulting systems, which are formed when two
planets are ejected from the system, the inclination of the planet's orbital
plane with respect to the initial invariant plane -presumably the plane
perpendicular to the star's spin axis- can be as large as \sim40{\deg}.Comment: 9 pages, 5 figures, published in MNRA
Simon Says (Fall 2014)
In this issue: New Electronic Databases Einstein\u27s Now Open - More Renovations in the Works New Music Acquisitions New Archival Acquisitions Research Clinic CSU ePress New Science Librarian - Paul Luft Upcoming Library Forum Events Upcoming Library Exhibits CSU Libraries Connectedhttps://csuepress.columbusstate.edu/library_newsletters/1015/thumbnail.jp
Formation, Survival, and Detectability of Planets Beyond 100 AU
Direct imaging searches have begun to detect planetary and brown dwarf
companions and to place constraints on the presence of giant planets at large
separations from their host star. This work helps to motivate such planet
searches by predicting a population of young giant planets that could be
detectable by direct imaging campaigns. Both the classical core accretion and
the gravitational instability model for planet formation are hard-pressed to
form long-period planets in situ. Here, we show that dynamical instabilities
among planetary systems that originally formed multiple giant planets much
closer to the host star could produce a population of giant planets at large
(~100 AU - 100000 AU) separations. We estimate the limits within which these
planets may survive, quantify the efficiency of gravitational scattering into
both stable and unstable wide orbits, and demonstrate that population analyses
must take into account the age of the system. We predict that planet scattering
creates a population of detectable giant planets on wide orbits that decreases
in number on timescales of ~10 Myr. We demonstrate that several members of such
populations should be detectable with current technology, quantify the
prospects for future instruments, and suggest how they could place interesting
constraints on planet formation models.Comment: 13 pages (emulateapj format), 10 figures, accepted for publication in
Ap
Secular Evolution of HD 12661: A System Caught at an Unlikely Time
The eccentricity evolution of multiple planet systems can provide valuable
constraints on planet formation models. Unfortunately, the inevitable
uncertainties in the current orbital elements can lead to significant
ambiguities in the nature of the secular evolution. Integrating any single set
of orbital elements inadequately describes the full range of secular evolutions
consistent with current observations. Thus, we combine radial velocity
observations of HD 12661 with Markov Chain Monte Carlo sampling to generate
ensembles of initial conditions for direct n-body integrations. We find that
any mean motion resonances are quite weak and do not significantly impact the
secular evolution, and that current observations indicate circulation or large
amplitude libration of the periapses. The eccentricity of the outer planet
undergoes large oscillations for nearly all of the allowed two-planet orbital
solutions. This type of secular evolution would arise if planet c had been
impulsively perturbed, perhaps due to strong scattering of an additional planet
that was subsequently accreted onto the star. Finally, we note that the secular
evolution implied by the current orbital configuration implies that planet c
spends ~96% of the time following an orbit more eccentric than that presently
observed. Either this system is being observed during a relatively rare state,
or additional planets are affecting the observed radial velocities and/or the
system's secular eccentricity evolution.Comment: 5 pages, 2 figures, 1 table, accepted for publication in ApJ
Effects of Turbulence, Eccentricity Damping, and Migration Rate on the Capture of Planets into Mean Motion Resonance
Pairs of migrating extrasolar planets often lock into mean motion resonance
as they drift inward. This paper studies the convergent migration of giant
planets (driven by a circumstellar disk) and determines the probability that
they are captured into mean motion resonance. The probability that such planets
enter resonance depends on the type of resonance, the migration rate, the
eccentricity damping rate, and the amplitude of the turbulent fluctuations.
This problem is studied both through direct integrations of the full 3-body
problem, and via semi-analytic model equations. In general, the probability of
resonance decreases with increasing migration rate, and with increasing levels
of turbulence, but increases with eccentricity damping. Previous work has shown
that the distributions of orbital elements (eccentricity and semimajor axis)
for observed extrasolar planets can be reproduced by migration models with
multiple planets. However, these results depend on resonance locking, and this
study shows that entry into -- and maintenance of -- mean motion resonance
depends sensitively on migration rate, eccentricity damping, and turbulence.Comment: 43 pages including 14 figures; accepted for publication in The
Astrophysical Journa
Simon Says (Spring 2011)
In this issue: Library 2.0 & CSU Libraries Tenth Annual Faculty Research Forums Information Commons Move Department Spotlight: Reference Blog Highlights Films on Demand National Library Week Interlibrary Loan Electronic Article Delivery CSU Libraries Offer Two New JSTOR Collections Rite of Passage Convocations Return New Employees Join the CSU Libraries On the Road with CSU Libraries African American Read-In Faculty Emeritus Designationshttps://csuepress.columbusstate.edu/library_newsletters/1014/thumbnail.jp
Simon Says (Spring 2008)
Inside this issue: CSU History Showcased in Digital Collections Expanded LIBR 1105: In Full Swing African American Read-In 2008-2013 CSU Libraries Strategic Plan Columbus State University Archives Receives Conservation Bookshelf Library Staff Development Day: Road Trip! Libraries Host Rite of Passage Convocations Georgia Depository Meeting Midland Middle School Tour Collection Analysis for a Trimmer Figure New Faculty/Staff WilsonWeb OmniFile Full Text Faculty Research Forum Series 2008: Seven Faculty Present their Research Findingshttps://csuepress.columbusstate.edu/library_newsletters/1010/thumbnail.jp
The effects of fly-bys on planetary systems
Most of the observed extrasolar planets are found on tight and often
eccentric orbits. The high eccentricities are not easily explained by
planet-formation models, which predict that planets should be on rather
circular orbits. Here we explore whether fly-bys involving planetary systems
with properties similar to those of the gas giants in the solar system, can
produce planets with properties similar to the observed planets. Using
numerical simulations, we show that fly-bys can cause the immediate ejection of
planets, and sometimes also lead to the capture of one or more planets by the
intruder. More common, however, is that fly-bys only perturb the orbits of
planets, sometimes leaving the system in an unstable state. Over time-scales of
a few million to several hundred million years after the fly-by, this
perturbation can trigger planet-planet scatterings, leading to the ejection of
one or more planets. For example, in the case of the four gas giants of the
solar system, the fraction of systems from which at least one planet is ejected
more than doubles in 10^8 years after the fly-by. The remaining planets are
often left on more eccentric orbits, similar to the eccentricities of the
observed extrasolar planets. We combine our results of how fly-bys effect
solar-system-like planetary systems, with the rate at which encounters in young
stellar clusters occur. For example, we measure the effects of fly-bys on the
four gas giants in the solar system. We find, that for such systems, between 5
and 15 per cent suffer ejections of planets in 10^8 years after fly-bys in
typical open clusters. Thus, encounters in young stellar clusters can
significantly alter the properties of any planets orbiting stars in clusters.
As a large fraction of stars which populate the solar neighbourhood form in
stellar clusters, encounters can significantly affect the properties of the
observed extrasolar planets.Comment: 22 pages, 15 figures, 5 tables. Accepted for publication in MNRA
Long period planets from dynamical relaxation
Recent imaging campaigns indicate the likely existence of massive planets (~
1-10 MJ) on ~1000 year orbits about a few percent of stars. Such objects are
not easily explained in most current planet formation models. In this Letter we
use ensembles of 100 N-body simulations to evaluate the potential for planet
scattering during relaxation of dynamically active systems to explain the
population of giant planets with projected separations up to a few 100 AU. We
find that such a mechanism could indeed be at play, and that statistical
samples of long period planets could place interesting constraints on early
stage planet formation scenarios. Results from direct imaging and microlensing
surveys are complementary probes of this dynamical relaxation process.Comment: Accepted for publication in ApJ Letters, this is final versio
- …