4,626 research outputs found
Modeling UV Radiation Feedback from Massive Stars: I. Implementation of Adaptive Ray Tracing Method and Tests
We present an implementation of an adaptive ray tracing (ART) module in the
Athena hydrodynamics code that accurately and efficiently handles the radiative
transfer involving multiple point sources on a three-dimensional Cartesian
grid. We adopt a recently proposed parallel algorithm that uses non-blocking,
asynchronous MPI communications to accelerate transport of rays across the
computational domain. We validate our implementation through several standard
test problems including the propagation of radiation in vacuum and the
expansions of various types of HII regions. Additionally, scaling tests show
that the cost of a full ray trace per source remains comparable to that of the
hydrodynamics update on up to processors. To demonstrate
application of our ART implementation, we perform a simulation of star cluster
formation in a marginally bound, turbulent cloud, finding that its star
formation efficiency is when both radiation pressure forces and
photoionization by UV radiation are treated. We directly compare the radiation
forces computed from the ART scheme with that from the M1 closure relation.
Although the ART and M1 schemes yield similar results on large scales, the
latter is unable to resolve the radiation field accurately near individual
point sources.Comment: 20 pages, 14 figures; accepted for publication in Ap
Optimal Integrated Abundances for Chemical Tagging of Extragalactic Globular Clusters
High resolution integrated light (IL) spectroscopy provides detailed
abundances of distant globular clusters whose stars cannot be resolved.
Abundance comparisons with other systems (e.g. for chemical tagging) require
understanding the systematic offsets that can occur between clusters, such as
those due to uncertainties in the underlying stellar population. This paper
analyses high resolution IL spectra of the Galactic globular clusters 47 Tuc,
M3, M13, NGC 7006, and M15 to (1) quantify potential systematic uncertainties
in Fe, Ca, Ti, Ni, Ba, and Eu and (2) identify the most stable abundance ratios
that will be useful in future analyses of unresolved targets. When stellar
populations are well-modelled, uncertainties are ~0.1-0.2 dex based on
sensitivities to the atmospheric parameters alone; in the worst case scenarios,
uncertainties can rise to 0.2-0.4 dex. The [Ca I/Fe I] ratio is identified as
the optimal integrated [alpha/Fe] indicator (with offsets <0.1 dex), while [Ni
I/Fe I] is also extremely stable to within <0.1 dex. The [Ba II/Eu II] ratios
are also stable when the underlying populations are well modelled and may also
be useful for chemical tagging.Comment: 28 pages, 4 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
Non-local nuclear spin quieting in quantum dot molecules: Optically-induced extended two-electron spin coherence time
We demonstrate the extension of coherence between all four two-electron spin
ground states of an InAs quantum dot molecule (QDM) via non-local suppression
of nuclear spin fluctuations in both constituent quantum dots (QDs), while
optically addressing only the upper QD transitions. Long coherence times are
revealed through dark-state spectroscopy as resulting from nuclear spin locking
mediated by the exchange interaction between the QDs. Lineshape analysis
provides the first measurement of the quieting of the Overhauser field
distribution correlating with reduced nuclear spin fluctuations.Comment: Supplementary materials can be found on the publication page of our
website. http://research.physics.lsa.umich.edu/dst/Publications.htm
Anonymity, Privacy, and Disclosure (APD) Triad on Social Networking Applications
While the average time people spend on their mobile apps continues to increase, the life cycle of using new social networking apps (SNA) remains relatively short, mostly due to privacy concern. For SNA users, it is important to know how the perception of anonymity and privacy concern determine the depth of disclosed information. For many SNA developers and practitioners, understanding the actual engagement of users on the platform is critical for measuring success of the app. Previous research has evaluated motivations/preventions of app usage and consequences of continuing usage. Despite efforts to understand the engagement with mobile devices and other users, there is little work in the Information Systems (IS) field to simultaneously investigate the triad of anonymity, privacy concern, and disclosure (APD) on continuous engagement with SNAs. Through the lens of contextual integrity of privacy, this research proposes a research model to investigate APD relationships with perceived and actual engagements with a new SNA. The research model is tested using a survey and actual usage data captured from users’ log files provided by mobile app developers. Results demonstrate how privacy is significantly related with actual engagement while anonymity relationship with actual engagement is fully mediated by perceived engagement
The sharp interface limit of an Ising game
The Ising model of statistical physics has served as a keystone example of
phase transitions, thermodynamic limits, scaling laws, and many other phenomena
and mathematical methods. We introduce and explore an Ising game, a variant of
the Ising model that features competing agents influencing the behavior of the
spins. With long-range interactions, we consider a mean-field limit resulting
in a nonlocal potential game at the mesoscopic scale. This game exhibits a
phase transition and multiple constant Nash-equilibria in the supercritical
regime.
Our analysis focuses on a sharp interface limit for which potential
minimizing solutions to the Ising game concentrate on two of the constant
Nash-equilibria. We show that the mesoscopic problem can be recast as a mixed
local/nonlocal space-time Allen-Cahn type minimization problem. We prove, using
a -convergence argument, that the limiting interface minimizes a
space-time anisotropic perimeter type energy functional. This macroscopic scale
problem could also be viewed as a problem of optimal control of interface
motion. Sharp interface limits of Allen-Cahn type functionals have been well
studied. We build on that literature with new techniques to handle a mixture of
local derivative terms and nonlocal interactions. The boundary conditions
imposed by the game theoretic considerations also appear as novel terms and
require special treatment
Exploring Planets with Directed Aerial Robot Explorers
Global Aerospace Corporation (GAC) is developing a revolutionary system architecture for exploration of planetary atmospheres and surfaces from atmospheric altitudes. The work is supported by the NASA Institute for Advanced Concepts (NIAC). The innovative system architecture relies upon the use of Directed Aerial Robot Explorers (DAREs), which essentially are long-duration-flight autonomous balloons with trajectory control capabilities that can deploy swarms of miniature probes over multiple target areas. Balloon guidance capabilities will offer unprecedented opportunities in high-resolution, targeted observations of both atmospheric and surface phenomena. Multifunctional microprobes will be deployed from the balloons once over the target areas, and perform a multitude of functions, such as atmospheric profiling or surface exploration, relaying data back to the balloons or an orbiter. This architecture will enable low-cost, low-energy, long-term global exploration of planetary atmospheres and surfaces. This paper focuses on a conceptual analysis of the DARE architecture capabilities and science applications for Venus, Titan and Jupiter. Preliminary simulations with simplified atmospheric models show that a relatively small trajectory control wing can enable global coverage of the atmospheres of Venus and Titan by a single balloon over a 100-day mission. This presents unique opportunities for global in situ sampling of the atmospheric composition and dynamics, atmospheric profiling over multiple sites with small dropsondes and targeted deployment of surface microprobes. At Jupiter, path guidance capabilities of the DARE platforms permits targeting localized regions of interest, such as "hot spots" or the Great Red Spot. A single DARE platform at Jupiter can sample major types of the atmospheric flows (zones and belts) over a 100-day mission. Observations by deployable probes would reveal if the differences exist in radiative, dynamic and compositional environments at these sites
Optimal integrated abundances for chemical tagging of extragalactic globular clusters
High-resolution integrated light (IL) spectroscopy provides detailed abundances of distant globular clusters whose stars cannot be resolved. Abundance comparisons with other systems (e.g. for chemical tagging) require understanding the systematic offset
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