445 research outputs found
Impedance adaptation for optimal robotâenvironment interaction
In this paper, impedance adaptation is investigated for robots interacting with unknown environments. Impedance control is employed for the physical interaction between robots and environments, subject to unknown and uncertain environments dynamics. The unknown environments are described as linear systems with unknown dynamics, based on which the desired impedance model is obtained. A cost function that measures the tracking error and interaction force is defined, and the critical impedance parameters are found to minimize it. Without requiring the information of the environments dynamics, the proposed impedance adaptation is feasible in a large number of applications where robots physically interact with unknown environments. The validity of the proposed method is verified through simulation studies
The position of graptolites within Lower Palaeozoic planktic ecosystems.
An integrated approach has been used to assess the palaeoecology of graptolites both as a discrete group and also as a part of the biota present within Ordovician and Silurian planktic realms. Study of the functional morphology of graptolites and comparisons with recent ecological analogues demonstrates that graptolites most probably filled a variety of niches as primary consumers, with modes of life related to the colony morphotype. Graptolite coloniality was extremely ordered, lacking any close morphological analogues in Recent faunas. To obtain maximum functional efficiency, graptolites would have needed varying degrees of coordinated automobility. A change in lifestyle related to ontogenetic changes was prevalent within many graptolite groups. Differing lifestyle was reflected by differing reproductive strategies, with synrhabdosomes most likely being a method for rapid asexual reproduction. Direct evidence in the form of graptolithophage 'coprolitic' bodies, as well as indirect evidence in the form of probable defensive adaptations, indicate that graptolites comprised a food item for a variety of predators. Graptolites were also hosts to a variety of parasitic organisms and provided an important nutrient source for scavenging organisms
The luminosities of protostars in the spitzer c2d and gould belt legacy clouds
Journal ArticlePublished version available online at the Astronomical Journal, Volume 145, Number 4, Article 94; doi: doi: 10.1088/0004-6256/145/4/94Motivated by the long-standing "luminosity problem" in low-mass star formation whereby protostars are underluminous compared to theoretical expectations, we identify 230 protostars in 18 molecular clouds observed by two Spitzer Space Telescope Legacy surveys of nearby star-forming regions. We compile complete spectral energy distributions, calculate L bol for each source, and study the protostellar luminosity distribution. This distribution extends over three orders of magnitude, from 0.01 L ÈŻ to 69 L ÈŻ, and has a mean and median of 4.3 L ÈŻ and 1.3 L ÈŻ, respectively. The distributions are very similar for Class 0 and Class I sources except for an excess of low luminosity (L bol âČ 0.5 L) Class I sources compared to Class 0. 100 out of the 230 protostars (43%) lack any available data in the far-infrared and submillimeter (70 ÎŒm <λ < 850 ÎŒm) and have L bol underestimated by factors of 2.5 on average, and up to factors of 8-10 in extreme cases. Correcting these underestimates for each source individually once additional data becomes available will likely increase both the mean and median of the sample by 35%-40%. We discuss and compare our results to several recent theoretical studies of protostellar luminosities and show that our new results do not invalidate the conclusions of any of these studies. As these studies demonstrate that there is more than one plausible accretion scenario that can match observations, future attention is clearly needed. The better statistics provided by our increased data set should aid such future work. © 2013. The American Astronomical Society. All rights reserved..National Science FoundationNational Aeronautics and Space AdministrationJet Propulsion Laboratory, California Institute of Technolog
Fermi acceleration in astrophysical jets
We consider the acceleration of energetic particles by Fermi processes (i.e.,
diffusive shock acceleration, second order Fermi acceleration, and gradual
shear acceleration) in relativistic astrophysical jets, with particular
attention given to recent progress in the field of viscous shear acceleration.
We analyze the associated acceleration timescales and the resulting particle
distributions, and discuss the relevance of these processes for the
acceleration of charged particles in the jets of AGNs, GRBs and microquasars,
showing that multi-component powerlaw-type particle distributions are likely to
occur.Comment: 6 pages, one figure; based on talk at "The multimessenger approach to
unidentified gamma-ray sources", Barcelona/Spain, July 2006; accepted for
publication in Astrophysics and Space Scienc
Can tonne-scale direct detection experiments discover nuclear dark matter?
Models of nuclear dark matter propose that the dark sector contains large
composite states consisting of dark nucleons in analogy to Standard Model
nuclei. We examine the direct detection phenomenology of a particular class of
nuclear dark matter model at the current generation of tonne-scale liquid noble
experiments, in particular DEAP-3600 and XENON1T. In our chosen nuclear dark
matter scenario distinctive features arise in the recoil energy spectra due to
the non-point-like nature of the composite dark matter state. We calculate the
number of events required to distinguish these spectra from those of a standard
point-like WIMP state with a decaying exponential recoil spectrum. In the most
favourable regions of nuclear dark matter parameter space, we find that a few
tens of events are needed to distinguish nuclear dark matter from WIMPs at the
level in a single experiment. Given the total exposure time of
DEAP-3600 and XENON1T we find that at best a distinction is
possible by these experiments individually, while sensitivity is
reached for a range of parameters by the combination of the two experiments. We
show that future upgrades of these experiments have potential to distinguish a
large range of nuclear dark matter models from that of a WIMP at greater than
.Comment: 23 pages, 7 multipanel figure
The PHENIX Experiment at RHIC
The physics emphases of the PHENIX collaboration and the design and current
status of the PHENIX detector are discussed. The plan of the collaboration for
making the most effective use of the available luminosity in the first years of
RHIC operation is also presented.Comment: 5 pages, 1 figure. Further details of the PHENIX physics program
available at http://www.rhic.bnl.gov/phenix
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in âs = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fbâ1 of protonâproton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
Stochastic Acceleration by Turbulence
The subject of this paper is stochastic acceleration by plasma turbulence, a
process akin to the original model proposed by Fermi. We review the relative
merits of different acceleration models, in particular the so called first
order Fermi acceleration by shocks and second order Fermi by stochastic
processes, and point out that plasma waves or turbulence play an important role
in all mechanisms of acceleration. Thus, stochastic acceleration by turbulence
is active in most situations. We also show that it is the most efficient
mechanism of acceleration of relatively cool non relativistic thermal
background plasma particles. In addition, it can preferentially accelerate
electrons relative to protons as is needed in many astrophysical radiating
sources, where usually there are no indications of presence of shocks. We also
point out that a hybrid acceleration mechanism consisting of initial
acceleration by turbulence of background particles followed by a second stage
acceleration by a shock has many attractive features. It is demonstrated that
the above scenarios can account for many signatures of the accelerated
electrons, protons and other ions, in particular He and He, seen
directly as Solar Energetic Particles and through the radiation they produce in
solar flares.Comment: 29 pages 7 figures for proceedings of ISSI-Bern workshop on Particle
Acceleration 201
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