What drives the dust activity of comet 67P/Churyumov-Gerasimenko?

We use the gravitational instability formation scenario of cometesimals to derive the aggregate size that can be released by the gas pressure from the nucleus of comet 67P/Churyumov-Gerasimenko for different heliocentric distances and different volatile ices. To derive the ejected aggregate sizes, we developed a model based on the assumption that the entire heat absorbed by the surface is consumed by the sublimation process of one volatile species. The calculations were performed for the three most prominent volatile materials in comets, namely, H_20 ice, CO_2 ice, and CO ice. We find that the size range of the dust aggregates able to escape from the nucleus into space widens when the comet approaches the Sun and narrows with increasing heliocentric distance, because the tensile strength of the aggregates decreases with increasing aggregate size. The activity of CO ice in comparison to H_20 ice is capable to detach aggregates smaller by approximately one order of magnitude from the surface. As a result of the higher sublimation rate of CO ice, larger aggregates are additionally able to escape from the gravity field of the nucleus. Our model can explain the large grains (ranging from 2 cm to 1 m in radius) in the inner coma of comet 67P/Churyumov-Gerasimenko that have been observed by the OSIRIS camera at heliocentric distances between 3.4 AU and 3.7 AU. Furthermore, the model predicts the release of decimeter-sized aggregates (trail particles) close to the heliocentric distance at which the gas-driven dust activity vanishes. However, the gas-driven dust activity cannot explain the presence of particles smaller than ~1 mm in the coma because the high tensile strength required to detach these particles from the surface cannot be provided by evaporation of volatile ices. These smaller particles can be produced for instance by spin-up and centrifugal mass loss of ejected larger aggregates

Mesoscopic simulation of diffusive contaminant spreading in gas flows at low pressure

Many modern production and measurement facilities incorporate multiphase systems at low pressures. In this region of flows at small, non-zero Knudsen- and low Mach numbers the classical mesoscopic Monte Carlo methods become increasingly numerically costly. To increase the numerical efficiency of simulations hybrid models are promising. In this contribution, we propose a novel efficient simulation approach for the simulation of two phase flows with a large concentration imbalance in a low pressure environment in the low intermediate Knudsen regime. Our hybrid model comprises a lattice-Boltzmann method corrected for the lower intermediate Kn regime proposed by Zhang et al. for the simulation of an ambient flow field. A coupled event-driven Monte-Carlo-style Boltzmann solver is employed to describe particles of a second species of low concentration. In order to evaluate the model, standard diffusivity and diffusion advection systems are considered.Comment: 9 pages, 8 figure

The coronagraphic Modal Wavefront Sensor: a hybrid focal-plane sensor for the high-contrast imaging of circumstellar environments

The raw coronagraphic performance of current high-contrast imaging instruments is limited by the presence of a quasi-static speckle (QSS) background, resulting from instrumental non-common path errors (NCPEs). Rapid development of efficient speckle subtraction techniques in data reduction has enabled final contrasts of up to 10-6 to be obtained, however it remains preferable to eliminate the underlying NCPEs at the source. In this work we introduce the coronagraphic Modal Wavefront Sensor (cMWS), a new wavefront sensor suitable for real-time NCPE correction. This pupil-plane optic combines the apodizing phase plate coronagraph with a holographic modal wavefront sensor, to provide simultaneous coronagraphic imaging and focal-plane wavefront sensing using the science point spread function. We first characterise the baseline performance of the cMWS via idealised closed-loop simulations, showing that the sensor successfully recovers diffraction-limited coronagraph performance over an effective dynamic range of +/-2.5 radians root-mean-square (RMS) wavefront error within 2-10 iterations. We then present the results of initial on-sky testing at the William Herschel Telescope, and demonstrate that the sensor is able to retrieve injected wavefront aberrations to an accuracy of 10nm RMS under realistic seeing conditions. We also find that the cMWS is capable of real-time broadband measurement of atmospheric wavefront variance at a cadence of 50Hz across an uncorrected telescope sub-aperture. When combined with a suitable closed-loop adaptive optics system, the cMWS holds the potential to deliver an improvement in raw contrast of up to two orders of magnitude over the uncorrected QSS floor. Such a sensor would be eminently suitable for the direct imaging and spectroscopy of exoplanets with both existing and future instruments, including EPICS and METIS for the E-ELT.Comment: 14 pages, 12 figures: accepted for publication in Astronomy & Astrophysic

Seething Horizontal Magnetic Fields in the Quiet Solar Photosphere

The photospheric magnetic field outside of active regions and the network has a ubiquitous and dynamic line-of-sight component that strengthens from disk center to limb as expected for a nearly horizontal orientation. This component shows a striking time variation with an average temporal rms near the limb of 1.7 G at ~3" resolution. In our moderate resolution observations the nearly horizontal component has a frequency variation power law exponent of -1.4 below 1.5 mHz and is spatially patchy on scales up to ~15 arcsec. The field may be a manifestation of changing magnetic connections between eruptions and evolution of small magnetic flux elements in response to convective motions. It shows no detectable latitude or longitude variations.Comment: 7 pages, 6 figures, submitted to ApJ letters, quality of figures significantly degraded here by compression requirement

How patients with insulin-treated type 1 and type 2 diabetes view their own and their physician's treatment goals.

To investigate the subjective treatment goals of insulin-treated diabetic patients. 297 type 1 and 205 type 2 diabetic patients, representative of the North-western Swiss population, filled out a self report questionnaire focusing on their own treatment goals using standardized measures wherever available. Factor analysis of the 16 items reflecting their treatment goals revealed four subscales (Crohnbach's alpha): High actual quality of life (0.73), weight reduction/maintenance and daily hassles (0.68), good medical care and knowledge (0.64) and good long term glucose control (0.71). Good long term glucose control was the single most important main treatment goal for most patients (type 1: 60.2%, type 2: 49.7%, p = 0.025). However, both type 1 and type 2 diabetic patients believed that this goal - especially the value of HbA1c - was overestimated (both p <0.0001), while high actual quality of life was underestimated (p = 0.003 and p = 0.05, respectively) by their physicians compared to their own assessment. Good long term glucose control (OR 1.63, p = 0.003) and high actual quality of life (OR 2.17, p <0.0001) were more important and weight reduction/maintenance and coping with daily hassles (OR 0.75, p = 0.07) were slightly less important treatment goals for type 1 than for type 2 diabetic patients. These differences in goals were best associated with the mode of insulin therapy, self-monitoring, and with the extent of diabetes education. Patients believe that physicians overestimate the importance of long term glucose control and underestimate the importance of actual quality of life. Diabetes education and self management have the largest impact on patients' own treatment goals

What are disease perceptions and subjective treatment goals of insulin treated diabetic patients?

BACKGROUND: Despite increasing importance of patient self-management, little is known about their own perceptions and treatment goals. OBJECTIVES: The aim of this explorative study was to examine what diabetic patients perceive as most concerning and what their own treatment goals are. METHODS: A 23-item anonymous questionnaire was distributed among type 1 diabetic patients treated with and without an insulin pump and insulin treated type 2 diabetic patients in the outpatient clinic of a University Hospital. 86% of the questionnaires were returned (n = 124). RESULTS: In open-ended questions, patients in all three groups together felt mostly restricted by their loss of freedom (24%), the dietary restrictions (17%) and the need to measure blood glucose (17%). Patients treated with an insulin pump worried more about hypoglycaemia and less about dietary restrictions. In closed-ended questions, patients were mostly concerned about hypoglycaemia and developing complications. However, the main treatment goal of both groups together was long-term good blood glucose control (63%). Further patient goals were the prevention of complications (27%) and the preservation of a good quality of life. Quality of life was a more important goal for type 1 diabetic patients (29%) than for type 2 diabetic patients (0%). Patients thought that blood glucose control was more important for their physicians (main treatment goal for 86%) than for themselves. CONCLUSION: Insulin treated patients with diabetes spontaneously express concerns about their actual quality of life and daily hassles and mention long-term worries after explicit questioning. For their main treatment goals they choose mainly long-term goals. According to the patients, physicians tend to overestimate blood glucose control

Flow Equations for N Point Functions and Bound States

We discuss the exact renormalization group or flow equation for the effective action and its decomposition into one particle irreducible N point functions. With the help of a truncated flow equation for the four point function we study the bound state problem for scalar fields. A combination of analytic and numerical methods is proposed, which is applied to the Wick-Cutkosky model and a QCD-motivated interaction. We present results for the bound state masses and the Bethe-Salpeter wave function. (Figs. 1-4 attached as separate uuencoded post-script files.)Comment: 17 pages, HD-THEP-93-3