5,570 research outputs found
Effect of Core Cooling on the Radius of Sub-Neptune Planets
Sub-Neptune planets are very common in our galaxy and show a large diversity
in their mass-radius relation. In sub-Neptunes most of the planet mass is in
the rocky part (hereafter core) which is surrounded by a modest hydrogen-helium
envelope. As a result, the total initial heat content of such a planet is
dominated by that of the core. Nonetheless, most studies contend that the core
cooling will only have a minor effect on the radius evolution of the gaseous
envelope, because the core's cooling is in sync with the envelope, i.e., most
of the initial heat is released early on timescales of about 10-100 Myr. In
this Letter we examine the importance of the core cooling rate for the thermal
evolution of the envelope. Thus, we relax the early core cooling assumption and
present a model where the core is characterized by two parameters: the initial
temperature and the cooling time. We find that core cooling can significantly
enhance the radius of the planet when it operates on a timescale similar to the
observed age, i.e. several Gyr. Consequently, the interpretation of
sub-Neptunes' mass-radius observations depends on the assumed core thermal
properties and the uncertainty therein. The degeneracy of composition and core
thermal properties can be reduced by obtaining better estimates of the planet
ages (in addition to their radii and masses) as envisioned by future
observations.Comment: Accepted for publication in A&A Letter
Point trajectory planning of flexible redundant robot manipulators using genetic algorithms
The paper focuses on the problem of point-to-point trajectory planning for flexible redundant robot manipulators (FRM) in joint space. Compared with irredundant flexible manipulators, a FRM possesses additional possibilities during point-to-point trajectory planning due to its kinematics redundancy. A trajectory planning method to minimize vibration and/or executing time of a point-to-point motion is presented for FRMs based on Genetic Algorithms (GAs). Kinematics redundancy is integrated into the presented method as planning variables. Quadrinomial and quintic polynomial are used to describe the segments that connect the initial, intermediate, and final points in joint space. The trajectory planning of FRM is formulated as a problem of optimization with constraints. A planar FRM with three flexible links is used in simulation. Case studies show that the method is applicable
Collinear Four-Wave Mixing of Two-Component Matter Waves
We demonstrate atomic four-wave mixing of two-component matter waves in a
collinear geometry. Starting from a single-species Bose-Einstein condensate,
seed and pump modes are prepared through microwave state transfer and
state-selective Kapitza-Dirac diffraction. Four-wave mixing then populates the
initially empty output modes. Simulations based on a coupled-mode expansion of
the Gross-Pitaevskii equation are in very good agreement with the experimental
data. We show that four-wave mixing can play an important role in studies of
bosonic mixtures in optical lattices. Moreover our system should be of interest
in the context of quantum atom optics.Comment: 4 pages, 4 figures; revised version, essentially as publishe
TRANSLATIONS OF GREEK LYRIC POETRY
Sappho 1
Deathless Aphrodite, child of god, wearing
your wiles upon your ornate throne, Ibeg you:
Don't hurt me, and don't overwhelm my heart with
pain and grief, mistress
Compression Behaviour of Porous Dust Agglomerates
The early planetesimal growth proceeds through a sequence of sticking
collisions of dust agglomerates. Very uncertain is still the relative velocity
regime in which growth rather than destruction can take place. The outcome of a
collision depends on the bulk properties of the porous dust agglomerates.
Continuum models of dust agglomerates require a set of material parameters that
are often difficult to obtain from laboratory experiments. Here, we aim at
determining those parameters from ab-initio molecular dynamics simulations. Our
goal is to improveon the existing model that describe the interaction of
individual monomers. We use a molecular dynamics approach featuring a detailed
micro-physical model of the interaction of spherical grains. The model includes
normal forces, rolling, twisting and sliding between the dust grains. We
present a new treatment of wall-particle interaction that allows us to perform
customized simulations that directly correspond to laboratory experiments. We
find that the existing interaction model by Dominik & Tielens leads to a too
soft compressive strength behavior for uni and omni-directional compression.
Upon making the rolling and sliding coefficients stiffer we find excellent
agreement in both cases. Additionally, we find that the compressive strength
curve depends on the velocity with which the sample is compressed. The modified
interaction strengths between two individual dust grains will lead to a
different behaviour of the whole dust agglomerate. This will influences the
sticking probabilities and hence the growth of planetesimals. The new parameter
set might possibly lead to an enhanced sticking as more energy can be stored in
the system before breakup.Comment: 11 pages, 14 figures, accepted for publication in A&
The Physics Case for the New Muon (g-2) Experiment
This White Paper briefly reviews the present status of the muon (g-2)
experiment and the physics motivation for a new effort. The present comparison
between experiment and theory indicates a tantalizing deviation.
An improvement in precision on this comparison by a factor of 2--with the
central value remaining unchanged--will exceed the ``discovery'' threshold,
with a sensitivity above . The 2.5-fold reduction improvement goal of
the new Brookhaven E969 experiment, along with continued steady reduction of
the standard model theory uncertainty, will achieve this more definitive test.
Already, the (g-2) result is arguably the most compelling indicator of
physics beyond the standard model and, at the very least, it represents a major
constraint for speculative new theories such as supersymmetry or extra
dimensions. In this report, we summarize the present experimental status and
provide an up-to-date accounting of the standard model theory, including the
expectations for improvement in the hadronic contributions, which dominate the
overall uncertainty. Our primary focus is on the physics case that motivates
improved experimental and theoretical efforts. Accordingly, we give examples of
specific new-physics implications in the context of direct searches at the LHC
as well as general arguments about the role of an improved (g-2) measurement. A
brief summary of the plans for an upgraded effort complete the report.Comment: 18 pages, 7 figure
The composition and size distribution of the dust in the coma of comet Hale-Bopp
We discuss the composition and size distribution of the dust in the coma of
comet Hale-Bopp. We do this by fitting simultaneously the infrared emission
spectrum measured by the infrared space observatory (ISO) and the measured
degree of linear polarization of scattered light at various phase angles and 12
different wavelengths. The effects of particle shape on the modeled optical
properties of the dust grains are taken into account. We constrain our fit by
forcing the abundances of the major rock forming chemical elements to be solar.
The infrared spectrum at long wavelengths reveals that large grains are needed
in order to fit the spectral slope. The size and shape distribution we employ
allows us to estimate the sizes of the crystalline silicates. The ratios of the
strength of various forsterite features show that the crystalline silicate
grains in Hale-Bopp must be submicron sized. We exclude the presence of large
crystalline silicate grains in the coma. Because of this lack of large
crystalline grains combined with the fact that we do need large amorphous
grains to fit the emission spectrum at long wavelengths, we need only
approximately 4% of crystalline silicates by mass. After correcting for
possible hidden crystalline material included in large amorphous grains, our
best estimate of the total mass fraction of crystalline material is
approximately 7.5%, significantly lower than deduced in previous studies in
which the typical derived crystallinity is 20-30%. The implications of this on
the possible origin and evolution of the comet are discussed. The crystallinity
we observe in Hale-Bopp is consistent with the production of crystalline
silicates in the inner solar system by thermal annealing and subsequent radial
mixing to the comet forming region.Comment: Accepted for publication in Icaru
E/E-product data management in consideration of model-based systems engineering
This paper presents objectives for permeable electric/electronics product data management for mechatronic products in consideration of model-based systems engineering from the early product development phase till a lifecycle management. Idiosyncrasies of mechatronic products, requirements engineering, model-based systems engineering, artifact-orientation, and interconnections of artifacts are evaluated and postulate objectives, how artifacts have to be designed in order to support the linkage of model-based systems engineering and product data management (PDM). The objectives, derived from the different theories and requirements to foster permeable PDM, are: i) Identify all existing norms for the development of mechanical, electronic, and software aspects and elaborate how information artifacts have to be defined. ii) (Textual) Requirements have to be technically feasible to be linked to information artifacts and system models already in the early development phase. iii) System models have to be aligned to information artifacts from the models' creation onwards and standardization in exchange formats has to be ensured. iv) Information artifacts with own lifecycles shall alleviate PDM in the early product development phase. v) Interconnections shall ameliorate associativity through capturing process information between single artifacts. A first concept is presented, visualizing the aforementioned objectives and their contribution in the early development process of mechatronic products, how a permeable PDM might be achieved
Steady-state crystallization of Rydberg excitations in an optically driven lattice gas
We study resonant optical excitations of atoms in a one-dimensional lattice
to the Rydberg states interacting via the van der Waals potential which
suppresses simultaneous excitation of neighboring atoms. Considering two- and
three-level excitation schemes, we analyze the dynamics and stationary state of
the continuously-driven, dissipative many-body system employing time-dependent
density-matrix renormalization group (t-DMRG) simulations. We show that
two-level atoms can exhibit only nearest neighbor correlations, while
three-level atoms under dark-state resonant driving can develop finite-range
crystalline order of Rydberg excitations. We present an approximate rate
equation model whose analytic solution yields qualitative understanding of the
numerical results.Comment: 5 pages,3 figure
- …