130 research outputs found
Hierarchical fractional-step approximations and parallel kinetic Monte Carlo algorithms
We present a mathematical framework for constructing and analyzing parallel
algorithms for lattice Kinetic Monte Carlo (KMC) simulations. The resulting
algorithms have the capacity to simulate a wide range of spatio-temporal scales
in spatially distributed, non-equilibrium physiochemical processes with complex
chemistry and transport micro-mechanisms. The algorithms can be tailored to
specific hierarchical parallel architectures such as multi-core processors or
clusters of Graphical Processing Units (GPUs). The proposed parallel algorithms
are controlled-error approximations of kinetic Monte Carlo algorithms,
departing from the predominant paradigm of creating parallel KMC algorithms
with exactly the same master equation as the serial one.
Our methodology relies on a spatial decomposition of the Markov operator
underlying the KMC algorithm into a hierarchy of operators corresponding to the
processors' structure in the parallel architecture. Based on this operator
decomposition, we formulate Fractional Step Approximation schemes by employing
the Trotter Theorem and its random variants; these schemes, (a) determine the
communication schedule} between processors, and (b) are run independently on
each processor through a serial KMC simulation, called a kernel, on each
fractional step time-window.
Furthermore, the proposed mathematical framework allows us to rigorously
justify the numerical and statistical consistency of the proposed algorithms,
showing the convergence of our approximating schemes to the original serial
KMC. The approach also provides a systematic evaluation of different processor
communicating schedules.Comment: 34 pages, 9 figure
A Novel Color Parameter As A Luminosity Calibrator for Type Ia Supernovae
Type Ia supernovae (SNe Ia) provide us with a unique tool for measuring
extragalactic distances and determining cosmological parameters. As a result,
the precise and effective calibration for peak luminosities of SNe Ia becomes
extremely crucial and thus is critically scrutinized for cosmological
explorations. In this Letter, we reveal clear evidence for a tight linear
correlation between peak luminosities of SNe Ia and their colors days after the maximum denoted by . By introducing such
a novel color parameter, , this empirical correlation allows us
to uniformly standardize SNe Ia with decline rates in the range
of and to reduce scatters in estimating their peak
luminosities from mag to the levels of 0.18 and 0.12 mag in the
and bands, respectively. For a sample of SNe Ia with insignificant
reddenings of host galaxies [e.g., E(B-V)_{host}\lsim 0.06 mag], the scatter
drops further to only 0.07 mag (or 3-4% in distance), which is comparable to
observational accuracies and is better than other calibrations for SNe Ia. This
would impact observational and theoretical studies of SNe Ia and cosmological
scales and parameters.Comment: 13 pages, including 3 figures. To appear in ApJL (2005 Feb issue
Molecular Basis of the Differentiation and Function of Virus Specific Follicular Helper CD4+ T Cells
During viral infection, virus-specific follicular helper T cells provide important help to cognate B cells for their survival, consecutive proliferation and mutation and eventual differentiation into memory B cells and antibody-secreting plasma cells. Similar to Tfh cells generated in other conditions, the differentiation of virus-specific Tfh cells can also be characterized as a process involved multiple factors and stages, however, which also exhibits distinct features. Here, we mainly focus on the current understanding of Tfh fate commitment, functional maturation, lineage maintenance and memory transition and formation in the context of viral infection
The Early Asymmetries of Supernova 2008D / XRF 080109
Spectropolarimetry of the Type Ib SN 2008D, associated with the XRF 080109,
at two separate epochs, are presented. The epochs of these observations
correspond to V-band light curve maximum and 15 days after light curve maximum
(or 21 and 36 days after the XRF). We find SN 2008D to be significantly
polarized, although the largest contribution is due to the interstellar
polarization component of Q_ISP=0+/-0.1% and U_ISP=-1.2+/-0.1%. At the two
epochs, the spectropolarimetry of SN 2008D is classified as being
D1+L(HeI)+L(Ca II). The intrinsic polarization of continuum wavelength regions
is <0.4%, at both epochs, implying an asymmetry of the photosphere of <10%.
Similar to other Type Ibc SNe, such as 2005bf, 2006aj and 2007gr, we observed
significant polarization corresponding to the spectral features of Ca II, He I,
Mg I, Fe II and, possibly, O I 7774, about a close-to-spherically-symmetric
photosphere. We introduce a new plot showing the chemically distinct line
forming regions in the ejecta and comment on the apparent ubiquity of highly
polarized high-velocity Ca II features in Type Ibc SNe. The polarization angle
of Ca II IR triplet was significantly different, at both epochs, to those of
the other species, suggesting high-velocity Ca II forms in a separate part of
the ejecta. The apparent structure in the outer layers of SN 2008D has
implications for the interpretation of the early-time X-ray emission associated
with shock break-out. (abridged)Comment: ApJ Subm., 45 pages, 13 figure
Gattini 2010: Cutting Edge Science at the Bottom of the World
The high altitude Antarctic sites of Dome A and the South Pole offer intriguing locations for future large scale optical astronomical
Observatories. The Gattini project was created to measure the optical
sky brightness, large area cloud cover and aurora of the winter-time
sky above such high altitude Antarctic sites. The Gattini-DomeA camera
was installed on the PLATO instrument module as part of the Chinese-led
traverse to the highest point on the Antarctic plateau in January 2008.
This single automated wide field camera contains a suite of Bessel
photometric filters (B, V, R) and a long-pass red filter for the
detection and monitoring of OH emission. We have in hand one complete
winter-time dataset (2009) from the camera that was recently returned
in April 2010.
The Gattini-South Pole UV camera is a wide-field optical camera that in
2011 will measure for the first time the UV properties of the
winter-time sky above the South Pole dark sector. This unique dataset
will consist of frequent images taken in both broadband U and B filters
in addition to high resolution (R similar to 5000) long slit
spectroscopy over a narrow bandwidth of the central field. The camera
is a proof of concept for the 2m-class Antarctic Cosmic Web Imager
telescope, a dedicated experiment to directly detect and map the
redshifted lyman alpha fluorescence or Cosmic Web emission we believe
possible due to the unique geographical qualities of the site.
We present the current status of both projects
Photometry of Variable Stars from Dome A, Antarctica
Dome A on the Antarctic plateau is likely one of the best observing sites on
Earth thanks to the excellent atmospheric conditions present at the site during
the long polar winter night. We present high-cadence time-series aperture
photometry of 10,000 stars with i<14.5 mag located in a 23 square-degree region
centered on the south celestial pole. The photometry was obtained with one of
the CSTAR telescopes during 128 days of the 2008 Antarctic winter.
We used this photometric data set to derive site statistics for Dome A and to
search for variable stars. Thanks to the nearly-uninterrupted synoptic
coverage, we find 6 times as many variables as previous surveys with similar
magnitude limits. We detected 157 variable stars, of which 55% are
unclassified, 27% are likely binaries and 17% are likely pulsating stars. The
latter category includes delta Scuti, gamma Doradus and RR Lyrae variables. One
variable may be a transiting exoplanet.Comment: Accepted for publication in the Astronomical Journal. PDF version
with high-resolution figures available at
http://faculty.physics.tamu.edu/lmacri/papers/wang11.pd
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