Capacitor performance limitations in high power converter applications

Background: Over the last 80 years the association between social class and obesity has changed. In the 1930s obesity rates were low and wealthy people tended to have a higher risk of obesity than poor people. However, rising affluence and industrialisation has lead to both rising rates of obesity and an obesogenic environment in which socioeconomically disadvantaged people have the highest risk of obesity. This study investigates the magnitude of these changes by modelling trajectories of adiposity by social class and cohort using the Twenty-07 study. Methods: The Twenty-07 study contains three cohorts of people (n = 4510), born in Glasgow in the 1930s, 1950s and 1970s. Two measures of adiposity, BMI and Waist to Height Ratio (WHtR), were recorded at baseline in 1987/8 when study participants were aged 15, 35 or 55, and again on 4 further occasions over 20 years. Parental social class (manual/non-manual) was collected at baseline. For each gender, we apply multilevel models to identify trajectories of adiposity by cohort and social class. Results: The trajectories indicated that adiposity increased with age and rates of increase varied by cohort, social class and gender. For any given age the youngest cohort had the fastest rate of increase and the highest predicted adiposity. For example, at age 35 for non-manual men, BMI was 24.2 (95% CI 23.7, 24.8) for the 1950s cohort and 27.2 (26.8, 27.5) for the 1970s cohort. By the end of the study respondents in more recent cohorts had BMI values approximately equivalent to those of people aged 20 years older in an earlier cohort. Cohort variation was much greater than socioeconomic variation. The smallest cohort difference in BMI was 2.10 (0.94, 3.26), a comparison of the 1950 and 1930s cohorts for non-manual men at age 55. In contrast, the largest social class difference in BMI, a comparison of manual and non manual women at age 64, was only 1.18 (0.37, 1.98). Socioeconomic inequalities tended to be smaller for men than women, particularly for the 1930s cohort where there was no evidence of a socioeconomic gradient for men unlike for women. The main difference between WHtR and BMI was that increases in WHtR accelerated with age whilst increases in BMI slowed with age. Conclusion: Increases in adiposity for younger cohorts across all socioeconomic groups dwarf any socioeconomic inequalities in adiposity. This highlights the damaging impact for the whole population of living in an obesogenic environment

Neutrino transport in accretion disks

We test approximate approaches to solving a neutrino transport problem that presents itself in the analysis of some accretion-disk models. Approximation #1 consists of replacing the full, angular- dependent, distribution function by a two-stream simulation, where the streams are respectively outwardly and inwardly directed, with angles $\cos \theta=\pm 1/\sqrt{3}$ to the vertical. In this approximation the full energy dependence of the distribution function is retained, as are the energy and temperature dependences of the scattering rates. Approximation #2, used in recent works on the subject, replaces the distribution function by an intensity function and the scattering rates by temperature-energy-averaged quantities. We compare the approximations to the results of solving the full Boltzmann equation. Under some interesting conditions, approximation #1 passes the test; approximation #2 does not. We utilize the results of our analysis to construct a toy model of a disc at a temperature and density such that relativistic particles are more abundant than nucleons, and dominate both the opacity and pressure. The nucleons will still provide most of the energy density. In the toy model we take the rate of heat generation (which drives the radiative transfer problem) to be proportional to the nucleon density. The model allows the simultaneous solution of the neutrino transport and hydrostatic equilibrium problems in a disk in which the nucleon density decreases approximately linearly as one moves from the median plane of the disk upwards, reaching zero on the upper boundary.Comment: 8 pages, 5 figures Parentheses added in eqs. 10-1

Hyper-Accreting Black Holes and Gamma-Ray Bursts

A variety of current models for gamma-ray bursts (GRBs) suggest a common engine - a black hole of several solar masses accreting matter from a disk at a rate 0.01 to 10 solar masses per second. Using a numerical model for relativistic disk accretion, we have studied steady-state accretion at these high rates. Inside a radius ~ 10**8 cm, for accretion rates greater than about 0.01 solar masses per second, a global state of balanced power comes to exist between neutrino losses, chiefly pair capture on nucleons, and dissipation. Energy emitted in neutrinos is less, and in the case of low accretion rates, very much less, than the maximum efficiency factor for black hole accretion (0.057 for no rotation; 0.42 for extreme Kerr rotation) times Mdot c**2. The efficiency for producing a pair fireball along the rotational axis by neutrino annihilation is calculated and found to be highly variable and very sensitive to the accretion rate. For some of the higher accretion rates studied, it can be several per cent or more; for accretion rates less than 0.05 solar masses per second, it is essentially zero. The efficiency of the Blandford-Znajek mechanism in extracting rotational energy from the black hole is also estimated. In light of these results, the viability of various gamma-ray burst models is discussed and the sensitivity of the results to disk viscosity, black hole rotation rate, and black hole mass explored. A diverse range of GRB energies seems unavoidable and neutrino annihilation in hyper-accreting black hole systems can explain bursts up to 10**52 erg. Larger energies may be inferred for beaming systems.Comment: 46 pages, includes 9 figures, LaTeX (uses aaspp4.sty), accepted by The Astrophysical Journal. Additional solutions in Tables and Figs. 4 and 5, minor revisions to text, references adde

Advection-Dominated Accretion Model of Sagittarius A*: Evidence for a Black Hole at the Galactic Center

Sgr A* at the Galactic Center is a puzzling source. It has a mass M=(2.5+/-0.4) x 10^6 solar masses which makes it an excellent black hole candidate. Observations of stellar winds and other gas flows in its vicinity suggest a mass accretion rate approximately few x 10^{-6} solar masses per year. However, such an accretion rate would imply a luminosity > 10^{40} erg/s if the radiative efficiency is the usual 10 percent, whereas observations indicate a bolometric luminosity <10^{37} erg/s. The spectrum of Sgr A* is unusual, with emission extending over many decades of wavelength. We present a model of Sgr A* which is based on a two-temperature optically-thin advection-dominated accretion flow. The model is consistent with the estimated mass and accretion rate, and fits the observed fluxes in the cm/mm and X-ray bands as well as upper limits in the sub-mm and infrared bands; the fit is less good in the radio below 86 GHz and in gamma-rays above 100 MeV. The very low luminosity of Sgr A* is explained naturally in the model by means of advection. Most of the viscously dissipated energy is advected into the central mass by the accreting gas, and therefore the radiative efficiency is extremely low, approximately 5 x 10^{-6}. A critical element of the model is the presence of an event horizon at the center which swallows the advected energy. The success of the model could thus be viewed as confirmation that Sgr A* is a black hole.Comment: 41 pages (Latex) including 6 Figures and 2 Tables. Final Revised Version changes to text, tables and figures. ApJ, 492, in pres

Gravitational radiation from precessing accretion disks in gamma-ray bursts

We study the precession of accretion disks in the context of gamma-ray burst inner engines. Our aim is to quantitatively estimate the characteristics of gravitational waves produced by the precession of the transient accretion disk in gamma-ray bursts. We evaluate the possible periods of disk precession caused by the Lense-Thirring effect using an accretion disk model that allows for neutrino cooling. Assuming jet ejection perpendicular to the disk plane and a typical intrinsic time-dependence for the burst, we find gamma-ray light curves that have a temporal microstructure similar to that observed in some reported events. The parameters obtained for the precession are then used to evaluate the production of gravitational waves. We find that the precession of accretion disks of outer radius smaller than $10^8$ cm and accretion rates above 1 solar mass per second could be detected by Advanced LIGO if they occur at distances of less than 100 Mpc. We conclude that the precession of a neutrino-cooled accretion disk in long gamma-ray bursts can be probed by gravitational wave astronomy. Precession of the disks in short gamma-ray events is undetectable with the current technology.Comment: 5 pages, 5 figures, accepted for publication in A&

Are BMI and inflammatory markers independently associated with physical fatigability in old age?

Background: Obesity and chronic low-grade inflammation have both been implicated in the onset of physical fatigue. However, few studies have investigated the independence of these associations in older community-dwelling populations. We therefore aimed to investigate the associations of body mass index (BMI) and inflammatory markers at age 60–64 with perceived physical fatigability at age 68 and to assess whether any such associations were independent of each other and potential confounding factors. A secondary aim was to investigate whether any association with BMI extended back into earlier adulthood. // Methods: Participants of the MRC National Survey of Health and Development (N = 1580) had BMI and levels of interleukin-6 (IL-6) and C-reactive protein (CRP) measured during clinical assessments at age 60–64. These were related to self-perceived physical fatigability assessed at age 68 using the Pittsburgh Fatigability Scale (PFS) (total score:0 (no physical fatigue)–50 (extreme physical fatigue)). // Results: Women had higher mean PFS scores than men (mean (SD): 16.0 (9.1) vs 13.2 (8.9), p < 0.01). In sex-adjusted models, BMI, CRP and IL-6 were each associated with PFS scores. When all three factors were included in the same model, BMI and IL-6 remained associated with PFS scores whereas CRP did not. After adjustment for a range of potential confounders, associations of BMI and IL-6 with PFS scores were still evident; fully adjusted differences in mean PFS score = 3.41 (95% CI: 0.59, 6.24) and 1.65 (0.46, 2.84) for underweight and obese participants when compared with normal weight and, 2.78 (1.65, 3.91) when comparing those with an IL-6 of 2.51–8.49 pg/mL with levels <1.50. // Conclusions: BMI and inflammation may both be suitable targets for intervention to reduce the burden of physical fatigability in later life. Further, interventions that target both obesity and elevated levels of IL-6 are likely to be more effective than those focusing on only one

Efficiency of Electron-Positron Pair Productionby Neutrino Flux from Accretion Disk of a Kerr Black Hole

Dominant processes of neutrino production and neutrino-induced \ep-pair production are examined in the model of a disk hyper-accreting onto a Kerr black hole. The efficiency of plasma production by a neutrino flux from the disk, obtained for the both cases of presence and absence of a magnetic field, is found to be no more than several tenths of percent and, therefore, not enough for the origin of cosmological gamma-ray bursts.Comment: 8 pages, 1 figur

Revisiting vertical structure of neutrino-dominated accretion disks: Bernoulli parameter, neutrino trapping and other distributions

We revisit the vertical structure of neutrino dominated accretion flows (NDAFs) in spherical coordinates with a new boundary condition based on the mechanical equilibrium. The solutions show that NDAF is significantly thick. The Bernoulli parameter and neutrino trapping are determined by the mass accretion rate and the viscosity parameter. According to the distribution of the Bernoulli parameter, the possible outflow may appear in the outer region of the disk. The neutrino trapping can essentially affect the neutrino radiation luminosity. The vertical structure of NDAF is like a "sandwich", and the multilayer accretion may account for the flares in gamma-ray bursts.Comment: 7 pages, 2 figures, Accepted for publication in Astrophysics & Space Scienc

Ballooning Instability in Polar Caps of Accreting Neutron Stars

We assess the stability of Kruskal-Schwarzschild (magnetic Rayleigh-Taylor) type modes for accreted matter on the surface of a neutron star confined by a strong (>= 1.E12 G) magnetic field. Employing the energy principle to analyze the stability of short-wavelength ballooning modes, we find that line-tying to the neutron star crust stabilizes these modes until the overpressure at the top of the neutron star crust exceeds the magnetic pressure by a factor ~ 8(a/h), where a and h are respectively the lateral extent of the accretion region and the density scale height. The most unstable modes are localized within a density scale height above the crust. We calculate the amount of mass that can be accumulated at the polar cap before the onset of instability.Comment: 8 pages, 2 figures, accepted for publication by ApJ, uses AASTEX 5.0 and emulateapj5.sty (included

Spin down of protostars through gravitational torques

Young protostars embedded in circumstellar discs accrete from an angular momentum-rich mass reservoir. Without some braking mechanism, all stars should be spinning at or near break-up velocity. In this paper, we perform simulations of the self-gravitational collapse of an isothermal cloud using the ORION adaptive mesh refinement code and investigate the role that gravitational torques might play in the spin-down of the dense central object. While magnetic effects likely dominate for low mass stars, high mass and Population III stars might be less well magnetised. We find that gravitational torques alone prevent the central object from spinning up to more than half of its breakup velocity, because higher rotation rates lead to bar-like deformations that enable efficient angular momentum transfer to the surrounding medium. We also find that the long-term spin evolution of the central object is dictated by the properties of the surrounding disc. In particular, spiral modes with azimuthal wavenumber $m=2$ couple more effectively to its spin than the lopsided $m=1$ mode, which was found to inhibit spin evolution. We suggest that even in the absence of magnetic fields, gravitational torques may provide an upper limit on stellar spin, and that moderately massive circumstellar discs can cause long-term spin down.Comment: 13 pages, 17 figures, 1 table. Accepted by MNRAS. Updated reference