4,264 research outputs found
Coordination polymers and isomerism; a study using silver(I) and a ∏-stacked ligand
The ligand 2,5-bis(2-pyridylmethylsulfanylmethyl)pyrazine (L) was prepared by the base coupling of 2-(sulfanylmethyl)pyridine and 2,5-bis(chloromethyl)pyrazine. This new ligand was treated with AgClO₄ in a 1 1 metal-to-ligand ratio and with AgNO₃in a 2 1 metal-to-ligand ratio to give coordination polymers. The crystal structures of {[Ag(L)]ClO₄}∞ ( 1) and {[Ag₂(L)](NO₃)₂}∞ ( 2) were determined. The Ag(I) ions in the one-dimensional polymeric chains of 1 adopted square-pyramidal geometries with the pyridine and pyrazine N donors coordinated in an extremely bent fashion. The structure of 2 revealed two isomeric polymer chains in the one crystal forming a single supramolecular array. The isomeric polymers differed in the donor atoms about the Ag(I) ions and in the arrangement of adjacent ligands along the chain. A feature of both structures was that L adopted a three-layer ∏-stacked arrangement
Empirical relations for the accurate estimation of stellar masses and radii
In this work, we have taken advantage of the most recent accurate stellar
characterizations carried out using asteroseismology, eclipsing binaries and
interferometry to evaluate a comprehensive set of empirical relations for the
estimation of stellar masses and radii. We have gathered a total of 934 stars
-- of which around two-thirds are on the Main Sequence -- that are
characterized with different levels of precision, most of them having estimates
of M, R, Teff, L, g, density, and [Fe/H]. We have deliberately used a
heterogeneous sample (in terms of characterizing techniques and spectroscopic
types) to reduce the influence of possible biases coming from the observation,
reduction, and analysis methods used to obtain the stellar parameters. We have
studied a total of 576 linear combinations of Teff, L, g, density, and [Fe/H]
(and their logarithms) to be used as independent variables to estimate M or R.
We have used an error-in-variables linear regression algorithm to extract the
relations and to ensure the fair treatment of the uncertainties. We present a
total of 38 new or revised relations that have an adj-R2 regression statistic
higher than 0.85, and a relative accuracy and precision better than 10% for
almost all the cases. The relations cover almost all the possible combinations
of observables, ensuring that, whatever list of observables is available, there
is at least one relation for estimating the stellar mass and radius.Comment: 49 Pages, 17 figures, 11 tables, accepted for publication in ApJ
Changing the Scaling Relation: The Need For a Mean Molecular Weight Term
The scaling relations that relate the average asteroseismic parameters
and to the global properties of stars are used quite
extensively to determine stellar properties. While the scaling
relation has been examined carefully and the deviations from the relation have
been well documented, the scaling relation has not been examined
as extensively. In this paper we examine the scaling relation
using a set of stellar models constructed to have a wide range of mass,
metallicity, and age. We find that as with , does not
follow the simple scaling relation. The most visible deviation is because of a
mean molecular weight term and a term that are commonly ignored. The
remaining deviation is more difficult to address. We find that the influence of
the scaling relation errors on asteroseismically derived values of are
well within uncertainties. The influence of the errors on mass and radius
estimates is small for main sequence and subgiants, but can be quite large for
red giants.Comment: 15 pages, 14 figures, accepted for publication in Ap
K2P A photometry pipeline for the K2 mission
With the loss of a second reaction wheel, resulting in the inability to point
continuously and stably at the same field of view, the NASA Kepler satellite
recently entered a new mode of observation known as the K2 mission. The data
from this redesigned mission present a specific challenge; the targets
systematically drift in position on a ~6 hour time scale, inducing a
significant instrumental signal in the photometric time series --- this greatly
impacts the ability to detect planetary signals and perform asteroseismic
analysis. Here we detail our version of a reduction pipeline for K2 target
pixel data, which automatically: defines masks for all targets in a given
frame; extracts the target's flux- and position time series; corrects the time
series based on the apparent movement on the CCD (either in 1D or 2D) combined
with the correction of instrumental and/or planetary signals via the KASOC
filter (Handberg & Lund 2014), thus rendering the time series ready for
asteroseismic analysis; computes power spectra for all targets, and identifies
potential contaminations between targets. From a test of our pipeline on a
sample of targets from the K2 campaign 0, the recovery of data for multiple
targets increases the amount of potential light curves by a factor .
Our pipeline could be applied to the upcoming TESS (Ricker et al. 2014) and
PLATO 2.0 (Rauer et al. 2013) missions.Comment: 14 pages, 20 figures, Accepted for publication in The Astrophysical
Journal (Apj
Observations of the Structure and Dynamics of the Inner M87 Jet
M87 is the best source in which to study a jet at high resolution in
gravitational units because it has a very high mass black hole and is nearby.
The angular size of the black hole is second only to Sgr A*, which does not
have a strong jet. The jet structure is edge brightened with a wide opening
angle base and a weak counterjet. We have roughly annual observations for 17
years plus intensive monitoring at three week intervals for a year and five day
intervals for 2.5 months made with the Very Long Baseline Array (VLBA) at 43
GHz. The inner jet shows very complex dynamics, with apparent motions both
along and across the jet. Speeds from zero to over 2c are seen, with
acceleration observed over the first 3 milli-arcseconds. The counterjet
decreases in brightness much more rapidly than the main jet, as is expected
from relativistic beaming in an accelerating jet oriented near the
line-of-sight. Details of the structure and dynamics are discussed. The roughly
annual observations show side-to-side motion of the whole jet with a
characteristic time scale of about 9 years.Comment: 11 pages, 7 figures. Published in a special issue of Galaxies which
is the proceedings of "Blazars through Sharp Multi-Wavelength Eyes" edited by
J. L. Gomez, A. P. Marscher, and S. G. Jorsta
Spatial incoherence of solar granulation: a global analysis using BiSON 2B data
A poor understanding of the impact of convective turbulence in the outer
layers of the Sun and Sun-like stars challenges the advance towards an improved
understanding of their internal structure and dynamics. Assessing and
calibrating these effects is therefore of great importance. Here we study the
spatial coherence of granulation noise and oscillation modes in the Sun, with
the aim of exploiting any incoherence to beat-down observed granulation noise,
hence improving the detection of low-frequency p-modes. Using data from the
BiSON 2B instrument, we assess the coherence between different atmospheric
heights and between different surface regions. We find that granulation noise
from the different atmospheric heights probed is largely incoherent; frequency
regions dominated by oscillations are almost fully coherent. We find a
randomised phase difference for the granulation noise, and a near zero
difference for the evanescent oscillations. A reduction of the incoherent
granulation noise is shown by application of the cross-spectrum.Comment: 8 pages, 7 figures, MNRAS in pres
A synthetic sample of short-cadence solar-like oscillators for TESS
NASA's Transiting Exoplanet Survey Satellite (TESS) has begun a two-year
survey of most of the sky, which will include lightcurves for thousands of
solar-like oscillators sampled at a cadence of two minutes. To prepare for this
steady stream of data, we present a mock catalogue of lightcurves, designed to
realistically mimic the properties of the TESS sample. In the process, we also
present the first public release of the asteroFLAG Artificial Dataset
Generator, which simulates lightcurves of solar-like oscillators based on input
mode properties. The targets are drawn from a simulation of the Milky Way's
populations and are selected in the same way as TESS's true Asteroseismic
Target List. The lightcurves are produced by combining stellar models,
pulsation calculations and semi-empirical models of solar-like oscillators. We
describe the details of the catalogue and provide several examples. We provide
pristine lightcurves to which noise can be added easily. This mock catalogue
will be valuable in testing asteroseismology pipelines for TESS and our methods
can be applied in preparation and planning for other observatories and
observing campaigns.Comment: 14 pages, 6 figures, accepted for publication in ApJS. Archives
containing the mock catalogue are available at
https://doi.org/10.5281/zenodo.1470155 and the pipeline to produce it at
https://github.com/warrickball/s4tess . The first public release of the
asteroFLAG Artificial Dataset Generator v3 (AADG3) is described at
https://warrickball.github.io/AADG3
A fatal case of infective endocarditis complicated by acute COVID-19 pneumonia.
A 74-year-old man with no co-morbidities presented to hospital with a 3-day history of diarrhoea and vomiting. He met the modified Duke's criteria for definite infective endocarditis and was immediately started on an intravenous antibiotic. Over Days 1-9, he developed renal failure. On Day 10, he was transferred to a tertiary hospital for mitral valve replacement. However, he tested positive for SARS-CoV-2 on arrival at the tertiary hospital, which delayed his surgery. He underwent bi-weekly nasopharyngeal swabs for SARS-CoV-2 with a plan to operate as soon as he tested negative, or as soon as his incubation period for COVID-19 pneumonia had elapsed. Unfortunately, he died on Day 31 from acute respiratory distress syndrome secondary to COVID-19 pneumonia. We describe the challenges in deciding on the optimal timing for valve replacement. We conclude by suggesting that earlier valve replacement may result in better outcomes
Majority Voting Approach to Ransomware Detection
Crypto-ransomware remains a significant threat to governments and companies
alike, with high-profile cyber security incidents regularly making headlines.
Many different detection systems have been proposed as solutions to the
ever-changing dynamic landscape of ransomware detection. In the majority of
cases, these described systems propose a method based on the result of a single
test performed on either the executable code, the process under investigation,
its behaviour, or its output. In a small subset of ransomware detection
systems, the concept of a scorecard is employed where multiple tests are
performed on various aspects of a process under investigation and their results
are then analysed using machine learning. The purpose of this paper is to
propose a new majority voting approach to ransomware detection by developing a
method that uses a cumulative score derived from discrete tests based on
calculations using algorithmic rather than heuristic techniques. The paper
describes 23 candidate tests, as well as 9 Windows API tests which are
validated to determine both their accuracy and viability for use within a
ransomware detection system. Using a cumulative score calculation approach to
ransomware detection has several benefits, such as the immunity to the
occasional inaccuracy of individual tests when making its final classification.
The system can also leverage multiple tests that can be both comprehensive and
complimentary in an attempt to achieve a broader, deeper, and more robust
analysis of the program under investigation. Additionally, the use of multiple
collaborative tests also significantly hinders ransomware from masking or
modifying its behaviour in an attempt to bypass detection.Comment: 17 page
Possible Detection of OVI from the LMC Superbubble N70
We present FUSE observations toward four stars in the LMC superbubble N70 and
compare these spectra to those of four comparison targets located in nearby
field and diffuse regions. The N70 sight lines show OVI 1032 absorption that is
consistently stronger than the comparison sight lines by ~60%. We attribute the
excess column density (logN_OVI=14.03 cm^-2) to hot gas within N70, potentially
the first detection of OVI associated with a superbubble. In a survey of 12 LMC
sight lines, Howk et al. (2002a) concluded that there was no correlation
between ISM morphology and N_OVI. We present a reanalysis of their measurements
combined with our own and find a clear difference between the superbubble and
field samples. The five superbubbles probed to date with FUSE show a
consistently higher mean N_OVI than the 12 non-superbubble sight lines, though
both samples show equivalent scatter from halo variability. Possible ionization
mechanisms for N70 are discussed, and we conclude that the observed OVI could
be the product of thermal conduction at the interface between the hot, X-ray
emitting gas inside the superbubble and the cooler, photoionized material
making up the shell seen prominently in Halpha. We calculate the total hydrogen
density n_H implied by our OVI measurements and find a value consistent with
expectations. Finally, we discuss emission-line observations of OVI from N70.Comment: 9 pages in emulateapj style. Accepted to Ap
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