5,312 research outputs found
On the Influence of Magnetic Fields on the Structure of Protostellar Jets
We here present the first results of fully three-dimensional (3-D) MHD
simulations of radiative cooling pulsed (time-variable) jets for a set of
parameters which are suitable for protostellar outflows. Considering different
initial magnetic field topologies in approximate with the
thermal gas, i.e., (i) a longitudinal, and (ii) a helical field, both of which
permeating the jet and the ambient medium; and (iii) a purely toroidal field
permeating only the jet, we find that the overall morphology of the pulsed jet
is not very much affected by the presence of the different magnetic field
geometries in comparison to a nonmagnetic calculation. Instead, the magnetic
fields tend to affect essentially the detailed structure and emission
properties behind the shocks at the head and at the pulse-induced internal
knots, particularly for the helical and toroidal geometries. In these cases, we
find, for example, that the emissivity behind the internal knots can
be about three to four times larger than that of the purely hydrodynamical jet.
We also find that some features, like the nose cones that often develop at the
jet head in 2-D calculations involving toroidal magnetic fields, are smoothed
out or absent in the 3-D calculations.Comment: 13 pages, 3 figures, Accepted by ApJ Letters after minor corrections
(for high resolution figures, see http://www.iagusp.usp.br/~adriano/h.tar
Systematic Errors in Cosmic Microwave Background Interferometry
Cosmic microwave background (CMB) polarization observations will require
superb control of systematic errors in order to achieve their full scientific
potential, particularly in the case of attempts to detect the B modes that may
provide a window on inflation. Interferometry may be a promising way to achieve
these goals. This paper presents a formalism for characterizing the effects of
a variety of systematic errors on interferometric CMB polarization
observations, with particular emphasis on estimates of the B-mode power
spectrum. The most severe errors are those that couple the temperature
anisotropy signal to polarization; such errors include cross-talk within
detectors, misalignment of polarizers, and cross-polarization. In a B mode
experiment, the next most serious category of errors are those that mix E and B
modes, such as gain fluctuations, pointing errors, and beam shape errors. The
paper also indicates which sources of error may cause circular polarization
(e.g., from foregrounds) to contaminate the cosmologically interesting linear
polarization channels, and conversely whether monitoring of the circular
polarization channels may yield useful information about the errors themselves.
For all the sources of error considered, estimates of the level of control that
will be required for both E and B mode experiments are provided. Both
experiments that interfere linear polarizations and those that interfere
circular polarizations are considered. The fact that circular experiments
simultaneously measure both linear polarization Stokes parameters in each
baseline mitigates some sources of error.Comment: 19 pages, 9 figures, submitted to Phys. Rev.
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The Performance of US Bond Mutual Funds
We evaluate the performance of the US bond mutual fund industry using a comprehensive sample of bond funds over a long time period from January 1998 to February 2017. In this one study, we evaluate bond fund selectivity, market timing and performance persistence. We evaluate bond funds relative to their self-declared benchmarks and in terms of both gross-of-fee returns and net-of-fee returns. We document considerable abnormal performance among funds both to the fund (gross returns) and to the investor (net returns). Bond fund performance is found to be superior in the post financial crisis period. However, past strong performance cannot be relied upon to predict future performance. Finally, while some funds exhibit market timing ability; we find a predominance of negative market timing among US bond mutual funds
Self-field effects upon the critical current density of flat superconducting strips
We develop a general theory to account self-consistently for self-field
effects upon the average transport critical current density Jc of a flat
type-II superconducting strip in the mixed state when the bulk pinning is
characterized by a field-dependent depinning critical current density Jp(B),
where B is the local magnetic flux density. We first consider the possibility
of both bulk and edge-pinning contributions but conclude that bulk pinning
dominates over geometrical edge-barrier effects in state-of-the-art YBCO films
and prototype second-generation coated conductors. We apply our theory using
the Kim model, JpK(B) = JpK(0)/(1+|B|/B0), as an example. We calculate Jc(Ba)
as a function of a perpendicular applied magnetic induction Ba and show how
Jc(Ba) is related to JpK(B). We find that Jc(Ba) is very nearly equal to
JpK(Ba) when Ba > Ba*, where Ba* is the value of Ba that makes the net flux
density zero at the strip's edge. However, Jc(Ba) is suppressed relative to
JpK(Ba) at low fields when Ba < Ba*, with the largest suppression occurring
when Ba*/B0 is of order unity or larger.Comment: 9 pages, 4 figures, minor revisions to add four reference
Mosaicking with cosmic microwave background interferometers
Measurements of cosmic microwave background (CMB) anisotropies by
interferometers offer several advantages over single-dish observations. The
formalism for analyzing interferometer CMB data is well developed in the
flat-sky approximation, valid for small fields of view. As the area of sky is
increased to obtain finer spectral resolution, this approximation needs to be
relaxed. We extend the formalism for CMB interferometry, including both
temperature and polarization, to mosaics of observations covering arbitrarily
large areas of the sky, with each individual pointing lying within the flat-sky
approximation. We present a method for computing the correlation between
visibilities with arbitrary pointing centers and baselines and illustrate the
effects of sky curvature on the l-space resolution that can be obtained from a
mosaic.Comment: 9 pages; submitted to Ap
Unsettling sustainability: the poetics of discomfort
peerreview_statement: The publishing and review policy for this title is described in its Aims & Scope. aims_and_scope_url: http://www.tandfonline.com/action/journalInformation?show=aimsScope&journalCode=rgrl2
Ticarcillin hypersusceptibility in pseudomonas aeruginosa in cystic fibrosis
Background: A subpopulation of Pseudomonas aeruginosa (PsA) exists in cysticfibrosis (CF) patients that is hypersusceptible to ticarcillin, a carboxypenicillin, in vitro (Tichs strain) defined as a minimum inhibitory concentration (MIC) ≤4μg/ml. Methods: In a retrospective cohort study, isolates of PsA from CF (23), non-cystic fibrosis bronchiectasis (NCFB) (17) and control (18) patients were analysed. MICs for each isolate were determined using agar dilution against six antibiotics and interpreted using EUCAST breakpoints. Prevalence of Tichs in each cohort was calculated. A point prevalence survey was conducted in CF to review the patients’ clinical progress following PsA isolation. Results: Prevalence of the Tichs strain in PsA was 48%, 76% and 0% in the CF, NCFB and control cohorts respectively. A statistically significant difference in geometric mean MIC was seen between the Tichs and non-Tichs cohorts in CF for ticarcillin (as expected) and temocillin (p=0.041and p=0.036 respectively). A similar trend was observed in NCFB for ticarcillin (p=0.038) and temocillin (p=0.067), although statistical significance was not reached for the latter.In CF, the Tichs strain demonstrated lower MICs to all antibiotics tested apart from gentamicin compared to their non-Tichs counterparts. Those who had the Tichs strain in CF had fewer antibiotics (13.9 days versus 23.5 days, Tichs and non-Tichs respectively) although this result was not statistically significant p=0.202. Conclusion: Our data supports the existence of a Tichs strain of PsA in our CF and NCFB patient populations. This strain correlated with reduced MICs to temocillin in CF, to which PsA would normally be resistant, which may be of clinical relevance.</p
A microfabricated sensor for thin dielectric layers
We describe a sensor for the measurement of thin dielectric layers capable of
operation in a variety of environments. The sensor is obtained by
microfabricating a capacitor with interleaved aluminum fingers, exposed to the
dielectric to be measured. In particular, the device can measure thin layers of
solid frozen from a liquid or gaseous medium. Sensitivity to single atomic
layers is achievable in many configurations and, by utilizing fast, high
sensitivity capacitance read out in a feedback system onto environmental
parameters, coatings of few layers can be dynamically maintained. We discuss
the design, read out and calibration of several versions of the device
optimized in different ways. We specifically dwell on the case in which
atomically thin solid xenon layers are grown and stabilized, in cryogenic
conditions, from a liquid xenon bath
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