34,357 research outputs found
Accretion Disk Assembly During Common Envelope Evolution: Implications for Feedback and LIGO Binary Black Hole Formation
During a common envelope episode in a binary system, the engulfed companion
spirals to tighter orbital separations under the influence of drag from the
surrounding envelope material. As this object sweeps through material with a
steep radial gradient of density, net angular momentum is introduced into the
flow, potentially leading to the formation of an accretion disk. The presence
of a disk would have dramatic consequences for the outcome of the interaction
because accretion might be accompanied by strong, polar outflows with enough
energy to unbind the entire envelope. Without a detailed understanding of the
necessary conditions for disk formation during common envelope, therefore, it
is difficult to accurately predict the population of merging compact binaries.
This paper examines the conditions for disk formation around objects embedded
within common envelopes using the `wind tunnel' formalism developed by MacLeod
et al. (2017). We find that the formation of disks is highly dependent on the
compressibility of the envelope material. Disks form only in the most
compressible of stellar envelope gas, found in envelopes' outer layers in zones
of partial ionization. These zones are largest in low-mass stellar envelopes,
but comprise small portions of the envelope mass and radius in all cases. We
conclude that disk formation and associated accretion feedback in common
envelope is rare, and if it occurs, transitory. The implication for LIGO black
hole binary assembly is that by avoiding strong accretion feedback, common
envelope interactions should still result in the substantial orbital tightening
needed to produce merging binaries.Comment: 12 pages, 10 figures, submitted to Ap
A Survey of Techniques For Improving Energy Efficiency in Embedded Computing Systems
Recent technological advances have greatly improved the performance and
features of embedded systems. With the number of just mobile devices now
reaching nearly equal to the population of earth, embedded systems have truly
become ubiquitous. These trends, however, have also made the task of managing
their power consumption extremely challenging. In recent years, several
techniques have been proposed to address this issue. In this paper, we survey
the techniques for managing power consumption of embedded systems. We discuss
the need of power management and provide a classification of the techniques on
several important parameters to highlight their similarities and differences.
This paper is intended to help the researchers and application-developers in
gaining insights into the working of power management techniques and designing
even more efficient high-performance embedded systems of tomorrow
Wireless Communications in the Era of Big Data
The rapidly growing wave of wireless data service is pushing against the
boundary of our communication network's processing power. The pervasive and
exponentially increasing data traffic present imminent challenges to all the
aspects of the wireless system design, such as spectrum efficiency, computing
capabilities and fronthaul/backhaul link capacity. In this article, we discuss
the challenges and opportunities in the design of scalable wireless systems to
embrace such a "bigdata" era. On one hand, we review the state-of-the-art
networking architectures and signal processing techniques adaptable for
managing the bigdata traffic in wireless networks. On the other hand, instead
of viewing mobile bigdata as a unwanted burden, we introduce methods to
capitalize from the vast data traffic, for building a bigdata-aware wireless
network with better wireless service quality and new mobile applications. We
highlight several promising future research directions for wireless
communications in the mobile bigdata era.Comment: This article is accepted and to appear in IEEE Communications
Magazin
Investigation into intermodulation distortion in HEMTs using a quasi-2-D physical model
The need for both linear and efficient pseudomorphic high electron-mobility transistors (pHEMTs) for modern wireless handsets necessitates a thorough understanding of the origins of intermodulation distortion at the device level. For the first time, the dynamic large-signal internal physical behavior of a pHEMT is examined using a quasi-two-dimensional physical device model.
The model accounts fully for device-circuit interaction and is validated experimentally for a two-tone experiment around 5 GHz
Constrained Simulations of the Magnetic Field in the Local Universe and the Propagation of UHECRs
We use simulations of LSS formation to study the build-up of magnetic fields
(MFs) in the ICM. Our basic assumption is that cosmological MFs grow in a MHD
amplification process driven by structure formation out of a seed MF present at
high z. Our LCDM initial conditions for the density fluctuations have been
statistically constrained by the observed galaxies, based on the IRAS 1.2-Jy
all-sky redshift survey. As a result, prominent galaxy clusters in our
simulation coincide closely with their real counterparts. We find excellent
agreement between RMs of our simulated clusters and observational data. The
improved resolution compared to previous work also allows us to study the MF in
large-scale filaments, sheets and voids. By tracing the propagation of UHE
protons in the simulated MF we construct full-sky maps of expected deflection
angles of protons with arrival energies E=1e20eV and 4e19eV, respectively.
Strong deflections are only produced if UHE protons cross clusters, however
covering only a small area on the sky. Multiple crossings of sheets and
filaments over larger distances may give rise to noticeable deflections,
depending on the model adopted for the magnetic seed field. Based on our
results we argue that over a large fraction of the sky the deflections are
likely to remain smaller than the present experimental angular sensitivity.
Therefore, we conclude that forthcoming air shower experiments should be able
to locate sources of UHE protons and shed more light on the nature of
cosmological MFs.Comment: 3revised version, JCAP, accepte
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