7,336 research outputs found
Local heat flux and energy loss in a 2D vibrated granular gas
We performed event-driven simulations of a two-dimensional granular gas
between two vibrating walls and directly measured the local heat flux and
energy dissipation rate in the stationary state. Describing the local heat flux
as a function of the coordinate x in the direction perpendicular to the driving
walls, we use a generalization of Fourier's law, q_x(x) = kappa d_x T(x) + mu
d_x rho(x), to relate the local heat flux to the local gradients of the
temperature and density. This ansatz accounts for the fact that density
gradients also generate heat flux, not only temperature gradients. The
transport coefficients kappa and mu are assumed to be independent of x, and we
check the validity of this assumption in the simulations. Both kappa and mu are
determined for different system parameters, in particular, for a wide range of
coefficients of restitution. We also compare our numerical results to existing
hydrodynamic theories. Agreement is found for kappa for very small
inelasticities only. Beyond this region, kappa and mu exhibit a striking
non-monotonic behavior.Comment: 8 pages, 5 figure
Indirect observation of unobservable interstellar molecules
It is suggested that the abundances of neutral non-polar interstellar molecules unobservable by radio astronomy can be systematically determined by radio observation of the protonated ions. As an example, observed N2H(+) column densities are analyzed to infer molecular nitrogen abundances in dense interstellar clouds. The chemistries and expected densities of the protonated ions of O2, C2, CO2, C2H2 and CH4 are then discussed. Microwave transition frequencies fo HCO2(+) and C2H3(+) are estimated, and a preliminary astronomical search for HCO2(+) is described
Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
We present a unique, homogeneous database of photometric measurements for
Classical T Tauri stars extending up to 20 years. The database contains more
than 21,000 UBVR observations of 72 CTTs. All the data were collected within
the framework of the ROTOR-program at Mount Maidanak Observatory (Uzbekistan)
and together they constitute the longest homogeneous, accurate record of TTS
variability ever assembled. We characterize the long term photometric
variations of 49 CTTs with sufficient data to allow a robust statistical
analysis and propose an empirical classification scheme. Several patterns of
long term photometric variability are identified. The most common pattern,
exhibited by a group of 15 stars which includes T Tau itself, consists of low
level variability (Delta(V)<=0.4mag) with no significant changes occurring from
season to season over many years. A related subgroup of 22 stars exhibits a
similar stable long term variability pattern, though with larger amplitudes (up
to Delta(V)~1.6 mag). Besides these representative groups, we identify three
smaller groups of 3-5 stars each which have distinctive photometric properties.
The long term variability of most CTTs is fairly stable and merely reflects
shorter term variability due to cold and hot surface spots. Only a small
fraction of CTTs undergo significant brightness changes on the long term
(months, years), which probably arise from slowly varying circumstellar
extinction.Comment: 16 pages, 11 figures. Astron. Astrophys., in pres
Testing the Disk-Locking Paradigm: An Association Between U-V Excess and Rotation in NGC 2264
We present some results from a UVI photometric study of a field in the young
open cluster NGC 2264 aimed, in part, at testing whether accretion in pre-main
sequence stars is linked to rotation. We confirm that U-V excess is well
correlated with H-alpha equivalent width for the stars in our sample. We show
that for the more massive stars in the cluster sample (roughly 0.4-1.2 M_sun)
there is also a significant association between U-V excess and rotation, in the
sense that slow rotators are more likely to show excess U-band emission and
variability. This constitutes significant new evidence in support of the
disk-locking paradigm.Comment: Accepted by ApJ Letter
Experimental Test bed to De-Risk the Navy Advanced Development Model
This paper presents a reduced scale demonstration test-bed at the University of Texas’ Center for Electromechanics (UT-CEM) which is well equipped to support the development and assessment of the anticipated Navy Advanced Development Model (ADM). The subscale ADM test bed builds on collaborative power management experiments conducted as part of the Swampworks Program under the US/UK Project Arrangement as well as non-military applications. The system includes the required variety of sources, loads, and controllers as well as an Opal-RT digital simulator. The test bed architecture is described and the range of investigations that can be carried out on it is highlighted; results of preliminary system simulations and some initial tests are also provided. Subscale ADM experiments conducted on the UT-CEM microgrid can be an important step in the realization of a full-voltage, full-power ADM three-zone demonstrator, providing a test-bed for components, subsystems, controls, and the overall performance of the Medium Voltage Direct Current (MVDC) ship architecture.Center for Electromechanic
Evidence for Differential Rotation on a T Tauri Star
Five years of photometric monitoring of the T Tauri star HBC 338 in NGC 1333
has revealed that it is a periodic variable, but the period has changed
significantly with time. From 2000-2003, a period near 5.6 days was observed,
while in the last two seasons, the dominant period is near 4.6 days. No other T
Tauri star has been seen to change its period by such a large percentage. We
propose a model in which a differentially rotating star is seen nearly
equator-on and a high latitude spot has gradually been replaced by a low
latitude spot. We show that this model provides an excellent fit to the
observed shapes of the light curves at each epoch. The amplitude and sense of
the inferred differential rotation is similar to what is seen on the Sun. This
may be surprising given the likely high degree of magnetic surface activity on
the star relative to the Sun but we note that HBC 338 is clearly an exceptional
T Tauri star.Comment: Acepted for publication in PAS
From structure to ae tiology: a new window on the biology of leucine-rich repeat kinase 2 and Parkinson's disease
Since the discovery of mutations in leucine-rich repeat kinase 2 (LRRK2) as an underlying genetic cause for the development of Parkinson's disease (PD) in 2004 (Neuron 44, 601–607; Neuron 44, 595–600), and subsequent efforts to develop LRRK2 kinase inhibitors as a therapy for Parkinson's (Expert Opin. Ther. Targets 21, 751–753), elucidating the atomic resolution structure of LRRK2 has been a major goal of research into this protein. At over 250 kDa, the large size and complicated domain organisation of LRRK2 has made this a highly challenging target for structural biologists, however, a number of recent studies using both in vitro and in situ approaches (Nature 588, 344–349; Cell 182, 1508–1518.e1516; Cell 184, 3519–3527.e3510) have provided important new insights into LRRK2 structure and the complexes formed by this protein
Effective superpotentials for B-branes in Landau-Ginzburg models
We compute the partition function for the topological Landau-Ginzburg B-model
on the disk. This is done by treating the worldsheet superpotential
perturbatively. We argue that this partition function as a function of bulk and
boundary perturbations may be identified with the effective D-brane
superpotential in the target spacetime. We point out the relationship of this
approach to matrix factorizations. Using these methods, we prove a conjecture
for the effective superpotential of Herbst, Lazaroiu and Lerche for the A-type
minimal models. We also consider the Landau-Ginzburg theory of the cubic torus
where we show that the effective superpotential, given by the partition
function, is consistent with the one obtained by summing up disk instantons in
the mirror A-model. This is done by explicitly constructing the open-string
mirror map.Comment: 57p, 7 figs, harvma
Simulation System for the Wendelstein 7-X Safety Control System
The Wendelstein 7-X (W7-X) Safety Instrumented System (SIS) ensures personal
safety and investment protection. The development and implementation of the SIS
are based on the international safety standard for the process industry sector,
IEC 61511. The SIS exhibits a distributed and hierarchical organized
architecture consisting of a central Safety System (cSS) on the top and many
local Safety Systems (lSS) at the bottom. Each technical component or
diagnostic system potentially hazardous for the staff or for the device is
equipped with an lSS. The cSS is part of the central control system of W7-X.
Whereas the lSSs are responsible for the safety of each individual component,
the cSS ensures safety of the whole W7-X device. For every operation phase of
the W7-X experiment hard- and software updates for the SIS are mandatory. New
components with additional lSS functionality and additional safety signals have
to be integrated. Already established safety functions must be adapted and new
safety functions have to be integrated into the cSS. Finally, the safety
programs of the central and local safety systems have to be verified for every
development stage and validated against the safety requirement specification.
This contribution focuses on the application of a model based simulation system
for the whole SIS of W7-X. A brief introduction into the development process of
the SIS and its technical realization will be give followed by a description of
the design and implementation of the SIS simulation system using the framework
SIMIT (Siemens). Finally, first application experiences of this simulation
system for the preparation of the SIS for the upcoming operation phase OP 1.2b
of W7-X will be discussed
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