2,389 research outputs found
Constraints on the Spin Evolution of Young Planetary-Mass Companions
Surveys of young star-forming regions have discovered a growing population of
planetary-mass (<13 M_Jup) companions around young stars. There is an ongoing
debate as to whether these companions formed like planets (that is, from the
circumstellar disk), or if they represent the low-mass tail of the star
formation process. In this study we utilize high-resolution spectroscopy to
measure rotation rates of three young (2-300 Myr) planetary-mass companions and
combine these measurements with published rotation rates for two additional
companions to provide a look at the spin distribution of these objects. We
compare this distribution to complementary rotation rate measurements for six
brown dwarfs with masses <20 M_Jup, and show that these distributions are
indistinguishable. This suggests that either that these two populations formed
via the same mechanism, or that processes regulating rotation rates are
independent of formation mechanism. We find that rotation rates for both
populations are well below their break-up velocities and do not evolve
significantly during the first few hundred million years after the end of
accretion. This suggests that rotation rates are set during late stages of
accretion, possibly by interactions with a circumplanetary disk. This result
has important implications for our understanding of the processes regulating
the angular momentum evolution of young planetary-mass objects, and of the
physics of gas accretion and disk coupling in the planetary-mass regime.Comment: 31 pages, 10 figures, published in Nature Astronomy,
DOI:10.1038/s41550-017-0325-
On the nature of the boundary resonance error in numerical homogenization and its reduction
Numerical homogenization of multiscale equations typically requires taking an
average of the solution to a microscale problem. Both the boundary conditions
and domain size of the microscale problem play an important role in the
accuracy of the homogenization procedure. In particular, imposing naive
boundary conditions leads to a error in the
computation, where is the characteristic size of the microscopic
fluctuations in the heterogeneous media, and is the size of the
microscopic domain. This so-called boundary, or ``cell resonance" error can
dominate discretization error and pollute the entire homogenization scheme.
There exist several techniques in the literature to reduce the error. Most
strategies involve modifying the form of the microscale cell problem. Below we
present an alternative procedure based on the observation that the resonance
error itself is an oscillatory function of domain size . After rigorously
characterizing the oscillatory behavior for one dimensional and quasi-one
dimensional microscale domains, we present a novel strategy to reduce the
resonance error. Rather than modifying the form of the cell problem, the
original problem is solved for a sequence of domain sizes, and the results are
averaged against kernels satisfying certain moment conditions and regularity
properties. Numerical examples in one and two dimensions illustrate the utility
of the approach
Certifiable Robustness to Adversarial State Uncertainty in Deep Reinforcement Learning
Deep Neural Network-based systems are now the state-of-the-art in many
robotics tasks, but their application in safety-critical domains remains
dangerous without formal guarantees on network robustness. Small perturbations
to sensor inputs (from noise or adversarial examples) are often enough to
change network-based decisions, which was recently shown to cause an autonomous
vehicle to swerve into another lane. In light of these dangers, numerous
algorithms have been developed as defensive mechanisms from these adversarial
inputs, some of which provide formal robustness guarantees or certificates.
This work leverages research on certified adversarial robustness to develop an
online certifiably robust for deep reinforcement learning algorithms. The
proposed defense computes guaranteed lower bounds on state-action values during
execution to identify and choose a robust action under a worst-case deviation
in input space due to possible adversaries or noise. Moreover, the resulting
policy comes with a certificate of solution quality, even though the true state
and optimal action are unknown to the certifier due to the perturbations. The
approach is demonstrated on a Deep Q-Network policy and is shown to increase
robustness to noise and adversaries in pedestrian collision avoidance scenarios
and a classic control task. This work extends one of our prior works with new
performance guarantees, extensions to other RL algorithms, expanded results
aggregated across more scenarios, an extension into scenarios with adversarial
behavior, comparisons with a more computationally expensive method, and
visualizations that provide intuition about the robustness algorithm.Comment: arXiv admin note: text overlap with arXiv:1910.1290
Star-Formation in the Ultraluminous Infrared Galaxy F00183-7111
We report the detection of molecular CO(1-0) gas in F00183-7111, one of the
most extreme Ultra-Luminous Infrared Galaxies known, with the Australia
Telescope Compact Array. We measure a redshift of 0.3292 for F00183-7111 from
the CO(1-0) line and estimate the mass of the molecular gas in 00183 to be 1
10 M. We find that F00183-7111 is predominately
powered by the AGN and only 14 per cent of the total luminosity is
contributed by star-formation (SFR 220 M yr). We also
present an optical image of F00183-7111, which shows an extension to the East.
We searched for star-formation in this extension using radio continuum
observations but do not detect any. This suggests that the star-formation is
likely to be predominately nuclear. These observations provide additional
support for a model in which the radio emission from ULIRGs is powered by an
intense burst of star-formation and by a radio-loud AGN embedded in its
nucleus, both triggered by a merger of gas-rich galaxies.Comment: 5 pages, 2 figures, Accepted for publication in MNRAS Letters
Accepted 2014 January 19. Received 2013 December 30; in original form 2013
November 2
Lepton-mediated electroweak baryogenesis
We investigate the impact of the tau and bottom Yukawa couplings on the
transport dynamics for electroweak baryogenesis in supersymmetric extensions of
the Standard Model. Although it has generally been assumed in the literature
that all Yukawa interactions except those involving the top quark are
negligible, we find that the tau and bottom Yukawa interaction rates are too
fast to be neglected. We identify an illustrative "lepton-mediated electroweak
baryogenesis" scenario in which the baryon asymmetry is induced mainly through
the presence of a left-handed leptonic charge. We derive analytic formulae for
the computation of the baryon asymmetry that, in light of these effects, are
qualitatively different from those in the established literature. In this
scenario, for fixed CP-violating phases, the baryon asymmetry has opposite sign
compared to that calculated using established formulae.Comment: 26 pages, 5 figure
FLUORESCENCE AND CIRCULAR DICHROISM STUDIES ON THE PHYCOERYTHROCYANINS FROM THE CYANOBACTERIUM
Two phycoerythrocyanin (PEC) fractions have been obtained from the phycobilisomes of the cyanobac-terium Westiellopsis prolifica ARM 365. They have been characterized by absorption, fluorescence and circular dichroism spectroscopy. One of them is spectroscopically similar to a PEC trimer known from other organisms. Whereas efficient energy transfer from its violin (α-84) to the cyanin (β-84, 155) chromophores is efficient in the trimer (αβ it is impeded after dissociation to the monomer (α,β). A second fraction of PEC which we earlier termed PEC(X) (Maruthi Sai et al., Photochem. Photobiol. 55,119–124, 1992), exhibited the spectral properties similar to that of the α-subunit of PEC from Mastigocladus laminosus. With this highly photoactive fraction, the circular dichroism spectra of the violobilin chromophore in both photoreversible states were obtained
Efficient Uncertainty Quantification and Variance-Based Sensitivity Analysis in Epidemic Modelling Using Polynomial Chaos
The use of epidemic modelling in connection with spread of diseases plays an important role in understanding dynamics and providing forecasts for informed analysis and decision-making. In this regard, it is crucial to quantify the effects of uncertainty in the modelling and in model-based predictions to trustfully communicate results and limitations. We propose to do efficient uncertainty quantification in compartmental epidemic models using the generalized Polynomial Chaos (gPC) framework. This framework uses a suitable polynomial basis that can be tailored to the underlying distribution for the parameter uncertainty to do forward propagation through efficient sampling via a mathematical model to quantify the effect on the output. By evaluating the model in a small number of selected points, gPC provides illuminating statistics and sensitivity analysis at a low computational cost. Through two particular case studies based on Danish data for the spread of Covid-19, we demonstrate the applicability of the technique. The test cases consider epidemic peak time estimation and the dynamics between superspreading and partial lockdown measures. The computational results show the efficiency and feasibility of the uncertainty quantification techniques based on gPC, and highlight the relevance of computational uncertainty quantification in epidemic modelling.Peer reviewe
Imaging skeletal muscle using second harmonic generation and coherent anti-Stokes Raman scattering microscopy
We describe experimental results on label free imaging of striated skeletal muscle using second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) microscopy. The complementarity of the SHG and CARS data makes it possible to clearly identify the main sarcomere sub-structures such as actin, myosin, acto-myosin, and the intact T-tubular system as it emanates from the sarcolemma. Owing to sub-micron spatial resolution and the high sensitivity of the CARS microscopy technique we were able to resolve individual myofibrils. In addition, key organelles such as mitochondria, cell nuclei and their structural constituents were observed revealing the entire structure of the muscle functional units. There is a noticeable difference in the CARS response of the muscle structure within actin, myosin and t-tubule areas with respect to laser polarization. We attribute this to a preferential alignment of the probed molecular bonds along certain directions. The combined CARS and SHG microscopy approach yields more extensive and complementary information and has a potential to become an indispensable method for live skeletal muscle characterization
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