92 research outputs found
Atomic data from the IRON Project. I. Electron-impact scattering of Fe17+ using <I>R</I>-matrix theory with intermediate coupling
We present results for electron-impact excitation of F-like Fe calculated using R-matrix theory where an intermediate-coupling frame transformation (ICFT) is used to obtain level-resolved collision strengths. Two such calculations are performed, the first expands the target using 2s2 2p5, 2s 2p6, 2s2 2p4 3l, 2s 2p5 3l, and 2p6 3l configurations while the second calculation includes the 2s2 2p4 4l, 2s 2p5 4l, and 2p6 4l configurations as well. The effect of the additional structure in the latter calculation on the n=3 resonances is explored and compared with previous calculations. We find strong resonant enhancement of the effective collision strengths to the 2s2 2p4 3s levels. A comparison with a Chandra X-ray observation of Capella shows that the n=4 R-matrix calculation leads to good agreement with observation</p
Empirical comparison of correlation measures and pruning levels in complex networks representing the global climate system
Climate change is an issue of growing economic, social, and political concern. Continued rise in the average temperatures of the Earth could lead to drastic climate change or an increased frequency of extreme events, which would negatively affect agriculture, population, and global health. One way of studying the dynamics of the Earth's changing climate is by attempting to identify regions that exhibit similar climatic behavior in terms of long-term variability. Climate networks have emerged as a strong analytics framework for both descriptive analysis and predictive modeling of the emergent phenomena. Previously, the networks were constructed using only one measure of similarity, namely the (linear) Pearson cross correlation, and were then clustered using a community detection algorithm. However, nonlinear dependencies are known to exist in climate, which begs the question whether more complex correlation measures are able to capture any such relationships. In this paper, we present a systematic study of different univariate measures of similarity and compare how each affects both the network structure as well as the predictive power of the clusters. © 2011 IEEE
Emission-Line Properties of the Optical Filaments of NGC 1275
Extended nebular filaments are seen at optical wavelengths in NGC 1275, the
central galaxy in the Perseus cluster. The agents responsible for the
excitation of these filaments remain poorly understood. In this paper we
investigate possible mechanisms for powering the filaments, using measurements
from an extensive spectroscopic data set acquired at the Lick Observatory 3-m
Shane telescope. The results show that the filaments are in an extremely low
ionization and excitation state. The high signal-to-noise ratio of the spectra
allows us to measure or place sensitive upper limits on weak but important
diagnostic lines. We compare the observed line intensity ratios to the
predictions of various ionization models, including photoionization by an
active galactic nucleus, shock heating, stellar photoionization, and
photoionization by the intracluster medium. We also investigate possible roles
for cluster extreme-ultraviolet emission, and filtering of cluster soft X-ray
emission by an ionized screen, in the energetics of the filaments. None of
these mechanisms provides an entirely satisfactory explanation for the physical
state of the nebulae. Heating and ionization by reconnection of the
intracluster magnetic field remains a potentially viable alternative, which
merits further investigation through Faraday rotation studies.Comment: Accepted for publication in Ap
The coronal line regions of planetary nebulae NGC6302 and NGC6537: 3-13um grating and echelle spectroscopy
We report on advances in the study of the cores of NGC6302 and NGC6537 using
infrared grating and echelle spectroscopy. In NGC6302, emission lines from
species spanning a large range of ionization potential, and in particular
[SiIX]3.934um, are interpreted using photoionization models (including CLOUDY),
which allow us to reestimate the central star's temperature to be about
250000K. All of the detected lines are consistent with this value, except for
[AlV] and [AlVI]. Aluminium is found to be depleted to one hundredth of the
solar abundance, which provides further evidence for some dust being mixed with
the highly ionized gas (with photons harder than 154eV). A similar depletion
pattern is observed in NGC6537. Echelle spectroscopy of IR coronal ions in
NGC6302 reveals a stratified structure in ionization potential, which confirms
photoionization to be the dominant ionization mechanism. The lines are narrow
(< 22km/s FWHM), with no evidence of the broad wings found in optical lines
from species with similar ionization potentials, such as [NeV]3426A. We note
the absence of a hot bubble, or a wind blown bipolar cavity filled with a hot
plasma, at least on 1'' and 10km/s scales. We also provide accurate new
wavelengths for several of the infrared coronal lines observed with the
echelle.Comment: Accepted for publication in MNRA
The gene-rich genome of the scallop Pecten maximus.
BACKGROUND: The king scallop, Pecten maximus, is distributed in shallow waters along the Atlantic coast of Europe. It forms the basis of a valuable commercial fishery and plays a key role in coastal ecosystems and food webs. Like other filter feeding bivalves it can accumulate potent phytotoxins, to which it has evolved some immunity. The molecular origins of this immunity are of interest to evolutionary biologists, pharmaceutical companies, and fisheries management. FINDINGS: Here we report the genome assembly of this species, conducted as part of the Wellcome Sanger 25 Genomes Project. This genome was assembled from PacBio reads and scaffolded with 10X Chromium and Hi-C data. Its 3,983 scaffolds have an N50 of 44.8 Mb (longest scaffold 60.1 Mb), with 92% of the assembly sequence contained in 19 scaffolds, corresponding to the 19 chromosomes found in this species. The total assembly spans 918.3 Mb and is the best-scaffolded marine bivalve genome published to date, exhibiting 95.5% recovery of the metazoan BUSCO set. Gene annotation resulted in 67,741 gene models. Analysis of gene content revealed large numbers of gene duplicates, as previously seen in bivalves, with little gene loss, in comparison with the sequenced genomes of other marine bivalve species. CONCLUSIONS: The genome assembly of P. maximus and its annotated gene set provide a high-quality platform for studies on such disparate topics as shell biomineralization, pigmentation, vision, and resistance to algal toxins. As a result of our findings we highlight the sodium channel gene Nav1, known to confer resistance to saxitoxin and tetrodotoxin, as a candidate for further studies investigating immunity to domoic acid
The 44Ti-powered spectrum of SN 1987A
SN 1987A provides a unique opportunity to study the evolution of a supernova
from explosion into very late phases. Due to the rich chemical structure, the
multitude of physical process involved, and extensive radiative transfer
effects, detailed modeling is needed to interpret the emission from this and
other supernovae. In this paper, we analyze the late-time (~8 years) HST
spectrum of the SN 1987A ejecta, where 44Ti is the dominant power source. Based
on an explosion model for a 19 Msun progenitor, we compute a model spectrum by
calculating the degradation of positrons and gamma-rays from the radioactive
decays, solving the equations governing temperature, ionization balance and
NLTE level populations, and treating the radiative transfer with a Monte Carlo
technique. We obtain a UV/optical/NIR model spectrum which is found to
reproduce most of the lines in the observed spectrum to good accuracy. We find
non-local radiative transfer in atomic lines to be an important process also at
this late stage of the supernova, with ~30% of the emergent flux in the optical
and NIR coming from scattering/fluorescence. We investigate the question of
where the positrons deposit their energy, and favor the scenario where they are
locally trapped in the Fe/He clumps by a magnetic field. Energy deposition into
these largely neutral Fe/He clumps makes Fe I lines prominent in the emergent
spectrum. Using the best available estimates for the dust extinction, we
determine the amount of 44Ti produced in the explosion to 1.5\pm0.5 * 10^-4
Msun.Comment: 23 pages, 9 figures. 44Ti mass updated from 1.4E-4 to 1.5E-4 Msu
A chromosomal reference genome sequence for the malaria mosquito Anopheles gambiae, Giles, 1902, Ifakara strain
We present a genome assembly from an individual female Anopheles gambiae (the malaria mosquito; Arthropoda; Insecta; Diptera; Culicidae), Ifakara strain. The genome sequence is 264 megabases in span. Most of the assembly is scaffolded into three chromosomal pseudomolecules with the X sex chromosome assembled. The complete mitochondrial genome was also assembled and is 15.4 kilobases in length
Population genomics of the critically endangered kākāpō
Summary The kākāpō is a flightless parrot endemic to New Zealand. Once common in the archipelago, only 201 individuals remain today, most of them descending from an isolated island population. We report the first genome-wide analyses of the species, including a high-quality genome assembly for kākāpō, one of the first chromosome-level reference genomes sequenced by the Vertebrate Genomes Project (VGP). We also sequenced and analyzed 35 modern genomes from the sole surviving island population and 14 genomes from the extinct mainland population. While theory suggests that such a small population is likely to have accumulated deleterious mutations through genetic drift, our analyses on the impact of the long-term small population size in kākāpō indicate that present-day island kākāpō have a reduced number of harmful mutations compared to mainland individuals. We hypothesize that this reduced mutational load is due to the island population having been subjected to a combination of genetic drift and purging of deleterious mutations, through increased inbreeding and purifying selection, since its isolation from the mainland ∼10,000 years ago. Our results provide evidence that small populations can survive even when isolated for hundreds of generations. This work provides key insights into kākāpō breeding and recovery and more generally into the application of genetic tools in conservation efforts for endangered species
A mid-infrared spectroscopic survey of starburst galaxies: excitation and abundances
We present spectroscopy of MIR emission lines in twelve starburst regions,
located in eleven starburst galaxies, for which a significant number of lines
between 2.38 and 45micron were observed with the ISO Short Wavelength
Spectrometer, with the intention of providing a reference resource for MIR
spectra of starburst galaxies. The observation apertures were centred on
actively star forming regions, including those which are inaccessible at
optical wavelengths due to high levels of obscuration. We use this data set,
which includes fine structure and hydrogen recombination lines, to investigate
excitation and to derive gas phase abundances of neon, argon, and sulphur of
the starburst galaxies. The derived Ne abundances span approximately an order
of magnitude, up to values of ~3 times solar. The excitation ratios measured
from the Ne and Ar lines correlate well with each other (positively) and with
abundances (negatively). Both in excitation and abundance, a separation of
objects with visible Wolf-Rayet features (high excitation, low abundance) is
noted from those without (low excitation, high abundance). For a given
abundance, the starbursts are of relatively lower excitation than a comparative
sample of HII regions, possibly due to ageing stellar populations. By
considering the abundance ratios of S with Ne and Ar we find that, in our
higher metallicity systems, S is relatively underabundant by a factor of ~3. We
discuss the origin of this deficit and favour depletion of S onto dust grains
as a likely explanation. This weakness of the MIR fine structure lines of S has
ramifications for future infrared missions such as SIRTF and Herschel since it
indicates that the S lines are less favourable tracers of star formation than
is suggested by nebular models which do not consider this effect.Comment: 20 pages, 23 figures, Accepted for publication in A&
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