4,544 research outputs found
Further investigation of the interference minimums in the low-frequency electromagnetic fields produced by a submerged vertical magnetic dipole
The quasi-static electromagnetic fields generated along the sea surface by a submerged vertical magnetic dipole are evaluated numerically using exact expressions and the results are plotted in a parametric form for source depths varying from 2 to 14 seawater skin depths δ. The curves show that there is a minimum in the amplitude of the vertical component of the magnetic field for horizontal distances from the source in the range 9–14δ and for dipole depths ranging from 2 to 8δ, with the deepest minimum occurring at a horizontal distance of pmin ≃ 11.07δ when the dipole is at a critical depth of dc ≃ 4.22δ. There also exists a similar minimum point in the variation along the surface of the amplitude of the total electric field for horizontal distances from the source in the range 10–20δ and dipole depths ranging from 4 to 23δ, with the deepest minimum occurring at a horizontal distance of pmin ≃ 12.95δ when the dipole is at a depth of dc ≃ 9.38δ. Both minimums are due to the strong destructive interference between the direct and the lateral wave components of the fields. No such minimum point exists for the variation of the amplitude of the horizontal component of the magnetic field
ULF/ELF electromagnetic fields produced in a conducting medium of infinite extent by linear current sources of infinite length
A previous analysis of a linear current source of finite length embedded in a conducting medium of infinite extent is extended to linear current sources of (1) infinite length and (2) semi-infinite length. Electric and magnetic field expressions are derived, and the results are numerically evaluated for frequencies in the ULF/ELF bands. For convenience, some of the results are presented in a dimensionless form. A comparison is made between the electromagnetic fields produced by linear current sources of finite and infinite length, and it is shown that there is a relative enhancement in the electric field near the source of finite length. It is also found that an optimum frequency exists for the electric field produced by a linear current source of infinite length at which the field amplitude is a maximum at a fixed observation point. Some practical applications of our results are suggested
ULF/ELF electromagnetic fields generated along the sea floor interface by a straight current source of infinite length
Propagation of ULF/ELF electromagnetic fields along the seafloor interface (assumed to be a plane boundary separating two semi-infinite conducting media) is considered. Earlier expressions for the electromagnetic fields generated by a straight current source of infinite length are applied to the sea/seabed interface. The field components are calculated numerically and are compared to the field components in seawater of infinite extent. At the seafloor boundary, the fields can propagate longer distances because of the lower seabed conductivities. The new horizontal component of the magnetic field generated as a result of the existence of the sea/seabed interface becomes larger than the vertical component of the magnetic field at large distances; it is also more sensitive to the conductivity of the seabed at low frequencies. The results indicate that there is an optimal frequency at which two of the field components have a maximum field intensity at a certain distance from the source. Some practical applications are discussed
Seabed propagation of ULF/ELF electromagnetic fields from harmonic dipole sources located on the seafloor
The amplitudes of the quasi-static electromagnetic fields generated at points on the seafloor by harmonic dipole sources (vertically directed magnetic dipoles, horizontally directed magnetic dipoles, vertically directed electric dipoles, and horizontally directed electric dipoles) also located on the seafloor are computed using a numerical integration technique. The primary purpose of these computations is to obtain field amplitudes that can be used in undersea communication studies. An important secondary purpose is to examine the enhancements of the fields produced at moderate to large distances by the presence of the relatively less conducting seafloor, as compared with the fields produced at the same distances in a sea of infinite extent, for frequencies in the ULF/ELF bands (frequencies less than 3 kHz). These latter enhancements can be surprisingly large, with increases of 4 orders of magnitude or more being typical at distances of 20 seawater skin depths
Light drinking in pregnancy and mid-childhood mental health and learning outcomes
OBJECTIVE:
To investigate whether light drinking in pregnancy is associated with adverse child mental health and academic outcomes.
DESIGN:
Using data from the prospective, population-based Avon Longitudinal Study of Parents and Children (ALSPAC), we investigated the associations between light drinking in pregnancy (<1 glass per week in the first trimester) and child mental health (using both parent and teacher rated Strengths and Difficulties Questionnaires (SDQs)) and academic outcomes based on Key Stage 2 examination results at age 11 years.
PARTICIPANTS:
11-year-old children from ALSPAC with parent (n=6587) and teacher (n=6393) completed SDQs and data from Key Stage 2 examination results (n=10 558).
RESULTS:
39% of women had consumed <1 glass per week and 16% ≥1 glass per week of alcohol during the first trimester (45% abstaining). After adjustment, relative to abstainers, there was no effect of light drinking on teacher-rated SDQ scores or examination results. In girls, although there was a suggestion of worse outcomes (adjusted regression coefficient=0.38; 95% CI 0.01 to 0.74) on the parent-rated total SDQ score in those exposed to light drinking compared to abstainers, no dose-response relationship was evident.
CONCLUSIONS:
Although the pattern of findings involving parent ratings for girls exposed to light drinking is consistent with earlier findings from this cohort, the overall lack of any adverse effects of light drinking is similar to findings from other recent cohort studies. Light drinking in pregnancy does not appear to be associated with clinically important adverse effects for mental health and academic outcomes at the age of 11 years
Comparison of ethnic group classification using naming analysis and routinely collected data: application to cancer incidence trends in children and young people
Objective: Inpatient Hospital Episode Statistics (HES) ethnicity data are available but not always collected and data quality can be unreliable. This may have implications when assessing outcomes by ethnicity. An alternative method for assigning ethnicity is using naming algorithms. We investigate if the association between ethnicity and cancer incidence varied dependent upon how ethnic group was assigned. Design: Population-based cancer registry cohort study Setting: Yorkshire, UK Participants: Cancer registrations from 1998-2009 in children and young people (0-29 years) from a specialist cancer register in Yorkshire, UK (N=3998) were linked to inpatient HES data to obtain recorded ethnicity. Patient?s names, recorded in the cancer register, were matched to an ethnic group using the naming algorithm software Onomap. Each source of ethnicity was categorised as White, South Asian (SA) or Other and a further two indicators were defined based on the combined ethnicities of HES and Onomap, one prioritising HES results, the other prioritising Onomap. Outcomes: Incidence rate ratios (IRR) between ethnic groups were compared using Poisson regression for all cancers combined, leukaemia, lymphoma and central nervous system (CNS) tumours. Results: Depending on the indicator used, 7.1% to 8.6% of the study population were classified as SA. For all cancers there were no statistically significant differences between White and SA groups using any indicator, however for lymphomas significant differences were only evident using one of the ?Combined? indicators (IRR=1.36 (95%CI 1.08, 1.71)) and for CNS tumours incidence was lower using three of the four indicators. For the other ethnic group the IRR for all cancers ranged from 0.78 (0.65, 0.94) to 1.41 (1.23, 1.62). Conclusions: Using different methods of assigning ethnicity can result in different estimates of ethnic variation in cancer incidence. Combining ethnicity from multiple sources results in a more complete estimate of ethnicity than the use of one single source
Interacting Supernovae: Types IIn and Ibn
Supernovae (SNe) that show evidence of strong shock interaction between their
ejecta and pre-existing, slower circumstellar material (CSM) constitute an
interesting, diverse, and still poorly understood category of explosive
transients. The chief reason that they are extremely interesting is because
they tell us that in a subset of stellar deaths, the progenitor star may become
wildly unstable in the years, decades, or centuries before explosion. This is
something that has not been included in standard stellar evolution models, but
may significantly change the end product and yield of that evolution, and
complicates our attempts to map SNe to their progenitors. Another reason they
are interesting is because CSM interaction is an efficient engine for making
bright transients, allowing super-luminous transients to arise from normal SN
explosion energies, and allowing transients of normal SN luminosities to arise
from sub-energetic explosions or low radioactivity yield. CSM interaction
shrouds the fast ejecta in bright shock emission, obscuring our normal view of
the underlying explosion, and the radiation hydrodynamics of the interaction is
challenging to model. The CSM interaction may also be highly non-spherical,
perhaps linked to binary interaction in the progenitor system. In some cases,
these complications make it difficult to definitively tell the difference
between a core-collapse or thermonuclear explosion, or to discern between a
non-terminal eruption, failed SN, or weak SN. Efforts to uncover the physical
parameters of individual events and connections to possible progenitor stars
make this a rapidly evolving topic that continues to challenge paradigms of
stellar evolution.Comment: Final draft of a chapter in the "SN Handbook". Accepted. 25 pages, 3
fig
A Photometric System for Detection of Water and Methane Ices on Kuiper Belt Objects
We present a new near-infrared photometric system for detection of water ice
and methane ice in the solar system. The system consists of two medium-band
filters in the K-band region of the near-infrared, which are sensitive to water
ice and methane ice, plus continuum observations in the J-band and Y-band. The
primary purpose of this system is to distinguish between three basic types of
Kuiper Belt Objects (KBOs) --- those rich in water ice, those rich in methane
ice, and those with little absorbance. In this work, we present
proof-of-concept observations of 51 KBOs using our filter system, 21 of which
have never been observed in the near-IR spectroscopically. We show that our
custom photometric system is consistent with previous spectroscopic
observations while reducing telescope observing time by a factor of 3. We use
our filters to identify Haumea collisional family members, which are thought to
be collisional remnants of a much larger body and are characterized by large
fractions of water ice on their surfaces. We add 2009 YE7 to the Haumea
collisional family based on our water ice band observations(J-H2O = -1.03 +/-
0.27) which indicate a high amount of water ice absorption, our calculated
proper orbital elements, and the neutral optical colors we measured, V-R = 0.38
+/- 0.04, which are all consistent with the rest of the Haumea family. We
identify several objects dynamically similar to Haumea as being distinct from
the Haumea family as they do not have water ice on their surfaces. In addition,
we find that only the largest KBOs have methane ice, and we find that Haumea
itself has significantly less water ice absorption than the smaller Haumea
family members. We find no evidence for other families in the Kuiper Belt.Comment: 38 pages, 7 figure
The progenitor and early evolution of the Type IIb SN 2016gkg
We report initial observations and analysis on the Type IIb SN~2016gkg in the
nearby galaxy NGC~613. SN~2016gkg exhibited a clear double-peaked light curve
during its early evolution, as evidenced by our intensive photometric follow-up
campaign. SN~2016gkg shows strong similarities with other Type IIb SNe, in
particular with respect to the \he~emission features observed in both the
optical and near infrared. SN~2016gkg evolved faster than the prototypical
Type~IIb SN~1993J, with a decline similar to that of SN~2011dh after the first
peak. The analysis of archival {\it Hubble Space Telescope} images indicate a
pre-explosion source at SN~2016gkg's position, suggesting a progenitor star
with a mid F spectral type and initial mass \msun, depending on
the distance modulus adopted for NGC~613. Modeling the temperature evolution
within of explosion, we obtain a progenitor radius of
\rsun, smaller than that obtained from the analysis of the
pre-explosion images (\rsun).Comment: 7 pages, 5 figures. Submitted to ApJ Letter
Observed Consequences of Presupernova Instability in Very Massive Stars
This chapter concentrates on the deaths of very massive stars, the events
leading up to their deaths, and how mass loss affects the resulting death. The
previous three chapters emphasized the theory of wind mass loss, eruptions, and
core collapse physics, but here we emphasize mainly the observational
properties of the resulting death throes. Mass loss through winds, eruptions,
and interacting binaries largely determines the wide variety of different types
of supernovae that are observed, as well as the circumstellar environments into
which the supernova blast waves expand. Connecting these observed properties of
the explosions to the initial masses of their progenitor stars is, however, an
enduring challenge and is especially difficult for very massive stars.
Superluminous supernovae, pair instability supernovae, gamma ray bursts, and
"failed" supernovae are all end fates that have been proposed for very massive
stars, but the range of initial masses or other conditions leading to each of
these (if they actually occur) are still very certain. Extrapolating to infer
the role of very massive stars in the early universe is essentially
unencumbered by observational constraints and still quite dicey.Comment: 39 pages, 5 figures, to appear as chapter in the book "Very Massive
Stars in the Local Universe", ed. J. Vin
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