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 $\sim$mid F spectral type and initial mass $15-20$\msun, depending on the distance modulus adopted for NGC~613. Modeling the temperature evolution within $5\,\rm{days}$ of explosion, we obtain a progenitor radius of $\sim\,48-124$\rsun, smaller than that obtained from the analysis of the pre-explosion images ($240-320$\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