1,726 research outputs found
Non-overlapping block smoothers for the Stokes equations
Overlapping block smoothers efficiently damp the error contributions from
highly oscillatory components within multigrid methods for the Stokes equations
but they are computationally expensive. This paper is concentrated on the
development and analysis of new block smoothers for the Stokes equations that
are discretized on staggered grids. These smoothers are non-overlapping and
therefore desirable due to reduced computational costs. Traditional geometric
multigrid methods are based on simple pointwise smoothers. However, the
efficiency of multigrid methods for solving more difficult problems such as the
Stokes equations lead to computationally more expensive smoothers, e.g.,
overlapping block smoothers. Non-overlapping smoothers are less expensive, but
have been considered less efficient in the literature. In this paper, we
develop new non-overlapping smoothers, the so-called triad-wise smoothers, and
show their efficiency within multigrid methods to solve the Stokes equations.
In addition, we compare overlapping and non-overlapping smoothers by measuring
their computational costs and analyzing their behavior by the use of local
Fourier analysis.Comment: 17 pages, 34 figure
Carbon Abundances in Starburst Galaxies of the Local Universe
The cosmological origin of carbon, the fourth most abundant element in the
Universe, is not well known and matter of heavy debate. We investigate the
behavior of C/O to O/H in order to constrain the production mechanism of
carbon. We measured emission-line intensities in a spectral range from 1600 to
10000 \AA\ on Space Telescope Imaging Spectrograph (STIS) long-slit spectra of
18 starburst galaxies in the local Universe. We determined chemical abundances
through traditional nebular analysis and we used a Markov Chain Monte Carlo
(MCMC) method to determine where our carbon and oxygen abundances lie in the
parameter space. We conclude that our C and O abundance measurements are
sensible. We analyzed the behavior of our sample in the [C/O] vs. [O/H] diagram
with respect to other objects such as DLAs, neutral ISM measurements, and disk
and halo stars, finding that each type of object seems to be located in a
specific region of the diagram. Our sample shows a steeper C/O vs. O/H slope
with respect to other samples, suggesting that massive stars contribute more to
the production of C than N at higher metallicities, only for objects where
massive stars are numerous; otherwise intermediate-mass stars dominate the C
and N production.Comment: Accepted for publication in Ap
Sparse Approximate Multifrontal Factorization with Butterfly Compression for High Frequency Wave Equations
We present a fast and approximate multifrontal solver for large-scale sparse
linear systems arising from finite-difference, finite-volume or finite-element
discretization of high-frequency wave equations. The proposed solver leverages
the butterfly algorithm and its hierarchical matrix extension for compressing
and factorizing large frontal matrices via graph-distance guided entry
evaluation or randomized matrix-vector multiplication-based schemes. Complexity
analysis and numerical experiments demonstrate
computation and memory complexity when applied to an sparse system arising from 3D high-frequency Helmholtz and Maxwell problems
Comparison of Theoretical Starburst Photoionisation Models for Optical Diagnostics
We study and compare different examples of stellar evolutionary synthesis
input parameters used to produce photoionisation model grids using the MAPPINGS
V modelling code. The aim of this study is to (a) explore the systematic
effects of various stellar evolutionary synthesis model parameters on the
interpretation of emission lines in optical strong-line diagnostic diagrams,
(b) characterise the combination of parameters able to reproduce the spread of
local galaxies located in the star-forming region in the Sloan Digital Sky
Survey, and (c) investigate the emission from extremely metal-poor galaxies
using photoionisation models. We explore and compare the stellar input ionising
spectrum (stellar population synthesis code [Starburst99, SLUG, BPASS], stellar
evolutionary tracks, stellar atmospheres, star-formation history, sampling of
the initial mass function) as well as parameters intrinsic to the H II region
(metallicity, ionisation parameter, pressure, H II region boundedness). We also
perform a comparison of the photoionisation codes MAPPINGS and CLOUDY. On the
variations in the ionising spectrum model parameters, we find that the
differences in strong emission-line ratios between varying models for a given
input model parameter are small, on average ~0.1 dex. An average difference of
~0.1 dex in emission-line ratio is also found between models produced with
MAPPINGS and CLOUDY. Large differences between the emission-line ratios are
found when comparing intrinsic H II region parameters. We find that
low-metallicity galaxies are better explained by a density-bounded H II region
and higher pressures better encompass the spread of galaxies at high redshift.Comment: 33 pages, 26 figures, accepted for publication in Ap
HST/WFC3 Observations of an Off-Nuclear Superbubble in Arp 220
We present a high spatial resolution optical and infrared study of the
circumnuclear region in Arp 220, a late-stage galaxy merger. Narrowband imaging
using HST/WFC3 has resolved the previously observed peak in H+[NII]
emission into a bubble-shaped feature. This feature measures 1.6" in diameter,
or 600 pc, and is only 1" northwest of the western nucleus. The bubble is
aligned with the western nucleus and the large-scale outflow axis seen in
X-rays. We explore several possibilities for the bubble origin, including a jet
or outflow from a hidden active galactic nucleus (AGN), outflows from high
levels of star formation within the few hundred pc nuclear gas disk, or an
ultraluminous X-ray source. An obscured AGN or high levels of star formation
within the inner 100 pc of the nuclei are favored based on the alignment
of the bubble and energetics arguments.Comment: Accepted for publication in ApJ. 12 pages, 10 figure
An inosine triphosphate pyrophosphatase safeguards plant nucleic acids from aberrant purine nucleotides
In plants, inosine is enzymatically introduced in some tRNAs, but not in other RNAs or DNA. Nonetheless, our data show that RNA and DNA from Arabidopsis thaliana contain (deoxy)inosine, probably derived from nonenzymatic adenosine deamination in nucleic acids and usage of (deoxy)inosine triphosphate (dITP and ITP) during nucleic acid synthesis. We combined biochemical approaches, LCâMS, as well as RNA-Seq to characterize a plant INOSINE TRIPHOSPHATE PYROPHOSPHATASE (ITPA) from A. thaliana, which is conserved in many organisms, and investigated the sources of deaminated purine nucleotides in plants. Inosine triphosphate pyrophosphatase dephosphorylates deaminated nucleoside di- and triphosphates to the respective monophosphates. ITPA loss-of-function causes inosine di- and triphosphate accumulation in vivo and an elevated inosine and deoxyinosine content in RNA and DNA, respectively, as well as salicylic acid (SA) accumulation, early senescence, and upregulation of transcripts associated with immunity and senescence. Cadmium-induced oxidative stress and biochemical inhibition of the INOSINE MONOPHOSPHATE DEHYDROGENASE leads to more IDP and ITP in the wild-type (WT), and this effect is enhanced in itpa mutants, suggesting that ITP originates from ATP deamination and IMP phosphorylation. Inosine triphosphate pyrophosphatase is part of a molecular protection system in plants, preventing the accumulation of (d)ITP and its usage for nucleic acid synthesis
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