7,397 research outputs found
On the Stability of Tidal Streams
We explore the stability of tidal streams to perturbations, motivated by
recent claims that the clumpy structure of the stellar streams surrounding the
globular cluster Palomar 5 are the result of gravitational instability. We
calculate the Jeans length of tidal streams by treating them as a thin
expanding cylinder of collisionless matter. We also find a general relation
between the density and the velocity dispersion inside a stream, which is used
to determine the longitudinal Jeans criterion. Our analytic results are checked
by following the time evolution of the phase space density within streams using
numerical simulations. We conclude that tidal streams within our galactic halo
are stable on all length scales and over all timescales.Comment: Accepted for publication in MNRA
A Physical Model-based Correction for Charge Traps in the Hubble Space Telescope's Wide Field Camera 3 Near-IR Detector and Applications to Transiting Exoplanets and Brown Dwarfs
The Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) near-IR channel
is extensively used in time-resolved observations, especially for transiting
exoplanet spectroscopy and brown dwarf and directly imaged exoplanet rotational
phase mapping. The ramp effect is the dominant source of systematics in the
WFC3 for time-resolved observations, which limits its photometric precision.
Current mitigation strategies are based on empirical fits and require
additional orbits "to help the telescope reach a thermal equilibrium". We show
that the ramp effect profiles can be explained and corrected with high fidelity
using charge trapping theories. We also present a model for this process that
can be used to predict and to correct charge trap systematics. Our model is
based on a very small number of parameters that are intrinsic to the detector.
We find that these parameters are very stable between the different datasets,
and we provide best-fit values. Our model is tested with more than 120 orbits
( visits) of WFC3 observations and is proved to be able to provide near
photon noise limited corrections for observations made with both staring and
scanning modes of transiting exoplanets as well as for starting-mode
observations of brown dwarfs. After our model correction, the light curve of
the first orbit in each visit has the same photometric precision as subsequent
orbits, so data from the first orbit need no longer be discarded. Near IR
arrays with the same physical characteristics (e.g., JWST/NIRCam) may also
benefit from the extension of this model, if similar systematic profiles are
observed.Comment: 16 pages, 13 figures, accepted to Astronomical Journa
The developmental state in Brazil: comparative and historical perspectives
The record of successful developmental states in East Asia and the partial successes of developmental states in Latin America suggest several common preconditions for effective state intervention including a Weberian bureaucracy, monitoring of implementation, reciprocity (subsidies in exchange for performance), and collaborative relations between government and business. Although Brazil failed to develop the high technology manufacturing industry and exports that have fueled sustained growth in East Asia, its developmental state had a number of important, and often neglected, successes, especially in steel, automobiles, mining, ethanol, and aircraft manufacturing. Where Brazil's developmental state was less successful was in promoting sectors like information technology and nuclear energy, as well as overall social and regional equality. In addition, some isolated initiatives by state governments were also effective in promoting particular local segments of industry and agriculture. Comparisons with East Asia, highlight the central role of state enterprises in Brazil that in effect internalized monitoring and reciprocity and bypassed collaboration between business and government (that was overall rarer in Brazil)
Impact of Dark Matter Microhalos on Signatures for Direct and Indirect Detection
Detecting dark matter as it streams through detectors on Earth relies on
knowledge of its phase space density on a scale comparable to the size of our
solar system. Numerical simulations predict that our Galactic halo contains an
enormous hierarchy of substructures, streams and caustics, the remnants of the
merging hierarchy that began with tiny Earth mass microhalos. If these bound or
coherent structures persist until the present time, they could dramatically
alter signatures for the detection of weakly interacting elementary particle
dark matter (WIMP). Using numerical simulations that follow the coarse grained
tidal disruption within the Galactic potential and fine grained heating from
stellar encounters, we find that microhalos, streams and caustics have a
negligible likelihood of impacting direct detection signatures implying that
dark matter constraints derived using simple smooth halo models are relatively
robust. We also find that many dense central cusps survive, yielding a small
enhancement in the signal for indirect detection experiments.Comment: 6 pages, revision in response to referees report. Now accepted by
Phys. Rev D., in pres
On the stability of tidal streams
We explore the stability of tidal streams to perturbations, motivated by recent claims that the clumpy structure of the stellar streams surrounding the globular cluster Palomar 5 are the result of gravitational instability. We calculate the Jeans length of tidal streams by treating them as a thin expanding cylinder of collisionless matter. We also find a general relation between the density and the velocity dispersion inside a stream, which is used to determine the longitudinal Jeans criterion. Our analytic results are checked by following the time evolution of the phase space density within streams using numerical simulations. We conclude that tidal streams within our Galactic halo are stable on all length scales and over all time-scale
Business-Government Interaction in Policy Councils in Latin America - Cheap Talk, Expensive Exchanges, or Collaborative Learning?
While effective industrial policy requires close cooperation between government and business, there is little agreement on what makes that cooperation work best. This paper analyzes institutional arrangements for public-private cooperation and the character of private sector representation. Questions on institutional design focus on three main issues: i) maximizing the benefits of dialogue and information exchange; ii) motivating participation through authoritative allocation; and iii) minimizing unproductive rent seeking. Key elements in the nature of business representation through associations are the quality of research staff and internal mechanisms for reconciling divergent preferences within associations. The empirical analysis also disaggregates councils by scope (economy-wide versus targeted), function (trade, upgrading, technology, etc.), sector (agriculture, industry, services), and level (national, provincial, and municipal).Industrial policy, Business-Government relations, Rent seeking
Network structure determines patterns of network reorganization during adult neurogenesis
New cells are generated throughout life and integrate into the hippocampus
via the process of adult neurogenesis. Epileptogenic brain injury induces many
structural changes in the hippocampus, including the death of interneurons and
altered connectivity patterns. The pathological neurogenic niche is associated
with aberrant neurogenesis, though the role of the network-level changes in
development of epilepsy is not well understood. In this paper, we use
computational simulations to investigate the effect of network environment on
structural and functional outcomes of neurogenesis. We find that small-world
networks with external stimulus are able to be augmented by activity-seeking
neurons in a manner that enhances activity at the stimulated sites without
altering the network as a whole. However, when inhibition is decreased or
connectivity patterns are changed, new cells are both less responsive to
stimulus and the new cells are more likely to drive the network into bursting
dynamics. Our results suggest that network-level changes caused by
epileptogenic injury can create an environment where neurogenic reorganization
can induce or intensify epileptic dynamics and abnormal integration of new
cells.Comment: 28 pages, 10 figure
The Failure of Mandated Disclosure
This article explores the spectacular prevalence, and failure, of the single most common technique for protecting personal autonomy in modern society: mandated disclosure. The article has four sections:
(1) A comprehensive summary of the recurring use of mandated disclosures, in many forms and circumstances, in the areas of consumer and borrower protection, patient informed consent, contract formation, and constitutional rights;
(2) A survey of the empirical literature documenting the failure of the mandated disclosure regime in informing people and in improving their decisions;
(3) An account of the multitude of reasons mandated disclosures fail, focusing on the political dynamics underlying the enactments of these mandates, the incentives of disclosers to carry them out, and, most importantly, on the ability of disclosees to use them;
(4) An argument that mandated disclosure not only fails to achieve its stated goal but also leads to unintended consequences that often harm the very people it intends to serve
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