8,051 research outputs found
Emulating Simulations of Cosmic Dawn for 21cm Power Spectrum Constraints on Cosmology, Reionization, and X-ray Heating
Current and upcoming radio interferometric experiments are aiming to make a
statistical characterization of the high-redshift 21cm fluctuation signal
spanning the hydrogen reionization and X-ray heating epochs of the universe.
However, connecting 21cm statistics to underlying physical parameters is
complicated by the theoretical challenge of modeling the relevant physics at
computational speeds quick enough to enable exploration of the high dimensional
and weakly constrained parameter space. In this work, we use machine learning
algorithms to build a fast emulator that mimics expensive simulations of the
21cm signal across a wide parameter space to high precision. We embed our
emulator within a Markov-Chain Monte Carlo framework, enabling it to explore
the posterior distribution over a large number of model parameters, including
those that govern the Epoch of Reionization, the Epoch of X-ray Heating, and
cosmology. As a worked example, we use our emulator to present an updated
parameter constraint forecast for the Hydrogen Epoch of Reionization Array
experiment, showing that its characterization of a fiducial 21cm power spectrum
will considerably narrow the allowed parameter space of reionization and
heating parameters, and could help strengthen Planck's constraints on
. We provide both our generalized emulator code and its
implementation specifically for 21cm parameter constraints as publicly
available software.Comment: 22 pages, 9 figures; accepted to Ap
Controlling quantum systems by embedded dynamical decoupling schemes
A dynamical decoupling method is presented which is based on embedding a
deterministic decoupling scheme into a stochastic one. This way it is possible
to combine the advantages of both methods and to increase the suppression of
undesired perturbations of quantum systems significantly even for long
interaction times. As a first application the stabilization of a quantum memory
is discussed which is perturbed by one-and two-qubit interactions
Slx5/Slx8-dependent ubiquitin hotspots on chromatin contribute to stress tolerance
Chromatin is a highly regulated environment, and protein association with chromatin is often controlled by post-translational modifications and the corresponding enzymatic machinery. Specifically, SUMO-targeted ubiquitin ligases (STUbLs) have emerged as key players in nuclear quality control, genome maintenance, and transcription. However, how STUbLs select specific substrates among myriads of SUMOylated proteins on chromatin remains unclear. Here, we reveal a remarkable co-localization of the budding yeast STUbL Slx5/Slx8 and ubiquitin at seven genomic loci that we term "ubiquitin hotspots". Ubiquitylation at these sites depends on Slx5/Slx8 and protein turnover on the Cdc48 segregase. We identify the transcription factor-like Ymr111c/Euc1 to associate with these sites and to be a critical determinant of ubiquitylation. Euc1 specifically targets Slx5/Slx8 to ubiquitin hotspots via bipartite binding of Slx5 that involves the Slx5 SUMO-interacting motifs and an additional, novel substrate recognition domain. Interestingly, the Euc1-ubiquitin hotspot pathway acts redundantly with chromatin modifiers of the H2A.Z and Rpd3L pathways in specific stress responses. Thus, our data suggest that STUbL-dependent ubiquitin hotspots shape chromatin during stress adaptation
NASA metrology information system: A NEMS subsystem
the NASA Metrology Information Systems (NMIS) is being developed as a standardized tool in managing the NASA field Center's instrument calibration programs. This system, as defined by the NASA Metrology and Calibration Workshop, will function as a subsystem of the newly developed NASA Equipment Management System (NEMS). The Metrology Information System is designed to utilize and update applicable NEMS data fields for controlled property and to function as a stand alone system for noncontrolled property. The NMIS provides automatic instrument calibration recall control, instrument historical performance data storage and analysis, calibration and repair labor and parts cost data, and instrument user and location data. Nineteen standardized reports were developed to analyze calibration system operations
Structural phase transitions of vortex matter in an optical lattice
We consider the vortex structure of a rapidly rotating trapped atomic
Bose-Einstein condensate in the presence of a co-rotating periodic optical
lattice potential. We observe a rich variety of structural phases which reflect
the interplay of the vortex-vortex and vortex-lattice interactions. The lattice
structure is very sensitive to the ratio of vortices to pinning sites and we
observe structural phase transitions and domain formation as this ratio is
varied.Comment: 4 pages, 3 figure
Robust Control Of Flexible Structures Using Multiple Shape Memory Alloy Actuators
The design and implementation of control strategies for large, flexible smart structures presents challenging problems. To demonstrate the capabilities of shape-memory-alloy actuators, we have designed and fabricated a three-mass test article with multiple shape-memory-alloy (NiTiNOL) actuators. The force and moment actuators were implemented on the structure to examine the effects of control structure interaction and to increase actuation force. These SMA actuators exhibit nonlinear effects due to dead band and saturation. The first step in the modeling process was the experimental determination of the transfer function matrix derived from frequency response data. A minimal state space representation was determined based on this transfer function matrix. Finally in order to reduce the order of the controller, a reduced order state space model was derived from the minimal state space representation. The simplified analytical models are compared with models developed by structural identification techniques based on vibration test data. From the reduced order model, a controller was designed to dampen vibrations in the test bed. To minimize the effects of uncertainties on the closed-loop system performance of smart structures, a LQG/LTR control methodology has been utilized. An initial standard LQG/LTR controller was designed; however, this controller could not achieve the desired performance robustness due to saturation effects. Therefore, a modified LQG/LTR design methodology was implemented to accommodate for the limited control force provided by the actuators. The closed-loop system response of the multiple input-multiple output (MIMO) test article with robustness verification has been experimentally obtained and presented in the paper. The modified LQG/LTR controller demonstrated performance and stability robustness to both sensor noise and parameter variations
Satellite passive microwave sea-ice concentration data set inter-comparison for Arctic summer conditions
We report on results of a systematic inter-comparison of 10 global sea-ice concentration (SIC) data products at 12.5 to 50.0 km grid resolution from satellite passive microwave (PMW) observations for the Arctic during summer. The products are compared against SIC and net ice surface fraction (ISF) - SIC minus the per-grid-cell melt pond fraction (MPF) on sea ice - as derived from MODerate resolution Imaging Spectroradiometer (MODIS) satellite observations and observed from ice-going vessels. Like in Kern et al. (2019), we group the 10 products based on the concept of the SIC retrieval used. Group I consists of products of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Ocean and Sea Ice Satellite Application Facility (OSI SAF) and European Space Agency (ESA) Climate Change Initiative (CCI) algorithms. Group II consists of products derived with the Comiso bootstrap algorithm and the National Oceanographic and Atmospheric Administration (NOAA) National Snow and Ice Data Center (NSIDC) SIC climate data record (CDR). Group III consists of Arctic Radiation and Turbulence Interaction Study (ARTIST) Sea Ice (ASI) and National Aeronautics and Space Administration (NASA) Team (NT) algorithm products, and group IV consists of products of the enhanced NASA Team algorithm (NT2). We find widespread positive and negative differences between PMW and MODIS SIC with magnitudes frequently reaching up to 20 %-25 % for groups I and III and up to 30 %-35 % for groups II and IV. On a pan-Arctic scale these differences may cancel out: Arctic average SIC from group I products agrees with MODIS within 2 %-5 % accuracy during the entire melt period from May through September. Group II and IV products overestimate MODIS Arctic average SIC by 5 %-10 %. Out of group III, ASI is similar to group I products while NT SIC underestimates MODIS Arctic average SIC by 5 %-10 %. These differences, when translated into the impact computing Arctic sea-ice area (SIA), match well with the differences in SIA between the four groups reported for the summer months by Kern et al. (2019). MODIS ISF is systematically overestimated by all products; NT provides the smallest overestimations (up to 25 %) and group II and IV products the largest overestimations (up to 45 %). The spatial distribution of the observed overestimation of MODIS ISF agrees reasonably well with the spatial distribution of the MODIS MPF and we find a robust linear relationship between PMW SIC and MODIS ISF for group I and III products during peak melt, i.e. July and August. We discuss different cases taking into account the expected influence of ice surface properties other than melt ponds, i.e. wet snow and coarse-grained snow/refrozen surface, on brightness temperatures and their ratios used as input to the SIC retrieval algorithms. Based on this discussion we identify the mismatch between the actually observed surface properties and those represented by the ice tie points as the most likely reason for (i) the observed differences between PMW SIC and MODIS ISF and for (ii) the often surprisingly small difference between PMW and MODIS SIC in areas of high melt pond fraction. We conclude that all 10 SIC products are highly inaccurate during summer melt. We hypothesize that the unknown number of melt pond signatures likely included in the ice tie points plays an important role - particularly for groups I and II - and recommend conducting further research in this field
Silicon surface with giant spin-splitting
We demonstrate the induction of a giant Rashba-type spin-splitting on a
semiconducting substrate by means of a Bi trimer adlayer on a Si(111) wafer.
The in-plane inversion symmetry is broken so that the in-plane potential
gradient induces a giant spin-splitting with a Rashba energy of about 140 meV,
which is more than an order of magnitude larger than what has previously been
reported for any semiconductor heterostructure. The separation of the
electronic states is larger than their lifetime broadening, which has been
directly observed with angular resolved photoemission spectroscopy. The
experimental results are confirmed by relativistic first-principles
calculations. We envision important implications for basic phenomena as well as
for the semiconductor based technology
Polarized Redundant-Baseline Calibration for 21 cm Cosmology Without Adding Spectral Structure
21 cm cosmology is a promising new probe of the evolution of visible matter
in our universe, especially during the poorly-constrained Cosmic Dawn and Epoch
of Reionization. However, in order to separate the 21 cm signal from bright
astrophysical foregrounds, we need an exquisite understanding of our telescopes
so as to avoid adding spectral structure to spectrally-smooth foregrounds. One
powerful calibration method relies on repeated simultaneous measurements of the
same interferometric baseline to solve for the sky signal and for instrumental
parameters simultaneously. However, certain degrees of freedom are not
constrained by asserting internal consistency between redundant measurements.
In this paper, we review the origin of these "degeneracies" of
redundant-baseline calibration and demonstrate how they can source unwanted
spectral structure in our measurement and show how to eliminate that
additional, artificial structure. We also generalize redundant calibration to
dual-polarization instruments, derive the degeneracy structure, and explore the
unique challenges to calibration and preserving spectral smoothness presented
by a polarized measurement.Comment: 12 pages, 3 figures, updated to match the published MNRAS versio
Hierarchical Triggering of Star Formation by Superbubbles in W3/W4
It is generally believed that expanding superbubbles and mechanical feedback
from massive stars trigger star formation, because there are numerous examples
of superbubbles showing secondary star formation at their edges. However, while
these systems show an age sequence, they do not provide strong evidence of a
causal relationship. The W3/W4 Galactic star-forming complex suggests a
three-generation hierarchy: the supergiant shell structures correspond to the
oldest generation; these triggered the formation of IC 1795 in W3, the
progenitor of a molecular superbubble; which in turn triggered the current
star-forming episodes in the embedded regions W3-North, W3-Main, and W3-OH. We
present UBV photometry and spectroscopic classifications for IC 1795, which
show an age of 3 - 5 Myr. This age is intermediate between the reported 6 - 20
Myr age of the supergiant shell system, and the extremely young ages (10^4 -
10^5 yr) for the embedded knots of ultracompact HII regions, W3-North, W3-Main,
and W3-OH. Thus, an age sequence is indeed confirmed for the entire W3/W4
hierarchical system. This therefore provides some of the first convincing
evidence that superbubble action and mechanical feedback are indeed a
triggering mechanism for star formation.Comment: 10 pages, 6 figures; accepted to the Astronomical Journal. Figure 2
included in this submission as JPE
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