1,002 research outputs found
Magnetically-driven explosions of rapidly-rotating white dwarfs following Accretion-Induced Collapse
We present 2D multi-group flux-limited diffusion magnetohydrodynamics (MHD)
simulations of the Accretion-Induced Collapse (AIC) of a rapidly-rotating white
dwarf. We focus on the dynamical role of MHD processes after the formation of a
millisecond-period protoneutron star. We find that including magnetic fields
and stresses can lead to a powerful explosion with an energy of a few Bethe,
rather than a weak one of at most 0.1 Bethe, with an associated ejecta mass of
~0.1Msun, instead of a few 0.001Msun. The core is spun down by ~30% within
500ms after bounce, and the rotational energy extracted from the core is
channeled into magnetic energy that generates a strong magnetically-driven
wind, rather than a weak neutrino-driven wind. Baryon loading of the ejecta,
while this wind prevails, precludes it from becoming relativistic. This
suggests that a GRB is not expected to emerge from such AICs during the early
protoneutron star phase, except in the unlikely event that the massive white
dwarf has sufficient mass to lead to black hole formation. In addition, we
predict both negligible 56Ni-production (that should result in an
optically-dark, adiabatically-cooled explosion) and the ejection of 0.1Msun of
material with an electron fraction of 0.1-0.2. Such pollution by neutron-rich
nuclei puts strong constraints on the possible rate of such AICs. Moreover,
being free from ``fallback,'' such highly-magnetized millisecond-period
protoneutron stars may later become magnetars, and the magnetically-driven
winds may later transition to Poynting-flux-dominated, relativistic winds,
eventually detectable as GRBs at cosmological distances. However, the low
expected event rate of AICs will constrain them to be, at best, a small subset
of GRB and/or magnetar progenitors.Comment: 16 pages, 8 figures, paper accepted to ApJ; High resolution version
available at http://hermes.as.arizona.edu/~luc/aic_mhd/aic_mhd.htm
Parallel Wavelet Tree Construction
We present parallel algorithms for wavelet tree construction with
polylogarithmic depth, improving upon the linear depth of the recent parallel
algorithms by Fuentes-Sepulveda et al. We experimentally show on a 40-core
machine with two-way hyper-threading that we outperform the existing parallel
algorithms by 1.3--5.6x and achieve up to 27x speedup over the sequential
algorithm on a variety of real-world and artificial inputs. Our algorithms show
good scalability with increasing thread count, input size and alphabet size. We
also discuss extensions to variants of the standard wavelet tree.Comment: This is a longer version of the paper that appears in the Proceedings
of the IEEE Data Compression Conference, 201
The Proto-neutron Star Phase of the Collapsar Model and the Route to Long-soft Gamma-ray Bursts and Hypernovae
Recent stellar evolutionary calculations of low-metallicity massive
fast-rotating main-sequence stars yield iron cores at collapse endowed with
high angular momentum. It is thought that high angular momentum and black hole
formation are critical ingredients of the collapsar model of long-soft
gamma-ray bursts (GRBs). Here, we present 2D multi-group,
flux-limited-diffusion MHD simulations of the collapse, bounce, and immediate
post-bounce phases of a 35-Msun collapsar-candidate model of Woosley & Heger.
We find that, provided the magneto-rotational instability (MRI) operates in the
differentially-rotating surface layers of the millisecond-period neutron star,
a magnetically-driven explosion ensues during the proto-neutron star phase, in
the form of a baryon-loaded non-relativistic jet, and that a black hole,
central to the collapsar model, does not form. Paradoxically, and although much
uncertainty surrounds stellar mass loss, angular momentum transport, magnetic
fields, and the MRI, current models of chemically homogeneous evolution at low
metallicity yield massive stars with iron cores that may have too much angular
momentum to avoid a magnetically-driven, hypernova-like, explosion in the
immediate post-bounce phase. We surmise that fast rotation in the iron core may
inhibit, rather than enable, collapsar formation, which requires a large
angular momentum not in the core but above it. Variations in the angular
momentum distribution of massive stars at core collapse might explain both the
diversity of Type Ic supernovae/hypernovae and their possible association with
a GRB. A corollary might be that, rather than the progenitor mass, the angular
momentum distribution, through its effect on magnetic field amplification,
distinguishes these outcomes.Comment: 5 pages, 1 table, 2 figures, accepted to ApJ
GPU-Accelerated BWT Construction for Large Collection of Short Reads
Advances in DNA sequencing technology have stimulated the development of
algorithms and tools for processing very large collections of short strings
(reads). Short-read alignment and assembly are among the most well-studied
problems. Many state-of-the-art aligners, at their core, have used the
Burrows-Wheeler transform (BWT) as a main-memory index of a reference genome
(typical example, NCBI human genome). Recently, BWT has also found its use in
string-graph assembly, for indexing the reads (i.e., raw data from DNA
sequencers). In a typical data set, the volume of reads is tens of times of the
sequenced genome and can be up to 100 Gigabases. Note that a reference genome
is relatively stable and computing the index is not a frequent task. For reads,
the index has to computed from scratch for each given input. The ability of
efficient BWT construction becomes a much bigger concern than before. In this
paper, we present a practical method called CX1 for constructing the BWT of
very large string collections. CX1 is the first tool that can take advantage of
the parallelism given by a graphics processing unit (GPU, a relative cheap
device providing a thousand or more primitive cores), as well as simultaneously
the parallelism from a multi-core CPU and more interestingly, from a cluster of
GPU-enabled nodes. Using CX1, the BWT of a short-read collection of up to 100
Gigabases can be constructed in less than 2 hours using a machine equipped with
a quad-core CPU and a GPU, or in about 43 minutes using a cluster with 4 such
machines (the speedup is almost linear after excluding the first 16 minutes for
loading the reads from the hard disk). The previously fastest tool BRC is
measured to take 12 hours to process 100 Gigabases on one machine; it is
non-trivial how BRC can be parallelized to take advantage a cluster of
machines, let alone GPUs.Comment: 11 page
Better quality score compression through sequence-based quality smoothing
Current NGS techniques are becoming exponentially cheaper. As a result, there is an exponential growth of genomic data unfortunately not followed by an exponential growth of storage, leading to the necessity of compression. Most of the entropy of NGS data lies in the quality values associated to each read. Those values are often more diversified than necessary. Because of that, many tools such as Quartz or GeneCodeq, try to change (smooth) quality scores in order to improve compressibility without altering the important information they carry for downstream analysis like SNP calling
- …