Global MHD simulations of stratified and turbulent protoplanetary discs. I. Model properties

We present the results of global 3-D MHD simulations of stratified and turbulent protoplanetary disc models. The aim of this work is to develop thin disc models capable of sustaining turbulence for long run times, which can be used for on-going studies of planet formation in turbulent discs. The results are obtained using two codes written in spherical coordinates: GLOBAL and NIRVANA. Both are time--explicit and use finite differences along with the Constrained Transport algorithm to evolve the equations of MHD. In the presence of a weak toroidal magnetic field, a thin protoplanetary disc in hydrostatic equilibrium is destabilised by the magnetorotational instability (MRI). When the resolution is large enough (25 vertical grid cells per scale height), the entire disc settles into a turbulent quasi steady-state after about 300 orbits. Angular momentum is transported outward such that the standard alpha parameter is roughly 4-6*10^{-3}. We find that the initial toroidal flux is expelled from the disc midplane and that the disc behaves essentially as a quasi-zero net flux disc for the remainder of the simulation. As in previous studies, the disc develops a dual structure composed of an MRI--driven turbulent core around its midplane, and a magnetised corona stable to the MRI near its surface. By varying disc parameters and boundary conditions, we show that these basic properties of the models are robust. The high resolution disc models we present in this paper achieve a quasi--steady state and sustain turbulence for hundreds of orbits. As such, they are ideally suited to the study of outstanding problems in planet formation such as disc--planet interactions and dust dynamics.Comment: 19 pages, 29 figures, accepted in Astronomy & Astrophysic

Tight focal spots using azimuthally polarised light from a Fresnel cone

When focusing a light beam at high numerical aperture, the resulting electric field profile in the focal plane depends on the transverse polarisation profile, as interference between different parts of the beam needs to be taken into account. It is well known that radial polarised light produces a longitudinal polarisation component and can be focused below the conventional diffraction limit for homogeneously polarised light, and azimuthally polarised light that carries one unit of angular momentum can achieve even tighter focal spots. This is of interest for example for enhancing resolution in scanning microscopy. There are numerous ways to generate such polarisation structures, however, setups can be expensive and usually rely on birefringent components, hence prohibiting broadband operation. We have recently demonstrated a passive, low-cost technique using a simple glass cone (Fresnel cone) to generate beams with structured polarisation. We show here that the polarisation structure generated by Fresnel cones focuses better than radial polarised light at all numerical apertures. Furthermore, we investigate in detail the application of polarised light structures for two-photon microscopy. Specifically we demonstrate a method that allows us to generate the desired polarisation structure at the back aperture of the microscope by pre-compensating any detrimental phase shifts using a combination of waveplates

Detecting Differential Rotation and Starspot Evolution on the M dwarf GJ 1243 with Kepler

We present an analysis of the starspots on the active M4 dwarf GJ 1243, using four years of time series photometry from Kepler. A rapid $P = 0.592596\pm0.00021$ day rotation period is measured due to the $\sim$2.2\% starspot-induced flux modulations in the light curve. We first use a light curve modeling approach, using a Monte Carlo Markov Chain sampler to solve for the longitudes and radii of the two spots within 5-day windows of data. Within each window of time the starspots are assumed to be unchanging. Only a weak constraint on the starspot latitudes can be implied from our modeling. The primary spot is found to be very stable over many years. A secondary spot feature is present in three portions of the light curve, decays on 100-500 day timescales, and moves in longitude over time. We interpret this longitude shearing as the signature of differential rotation. Using our models we measure an average shear between the starspots of 0.0047 rad day$^{-1}$, which corresponds to a differential rotation rate of $\Delta\Omega = 0.012 \pm 0.002$ rad day$^{-1}$. We also fit this starspot phase evolution using a series of bivariate Gaussian functions, which provides a consistent shear measurement. This is among the slowest differential rotation shear measurements yet measured for a star in this temperature regime, and provides an important constraint for dynamo models of low mass stars.Comment: 13 pages, 7 figures, ApJ Accepte

Magnetically Driven Accretion in the Kerr Metric III: Unbound Outflows

We have carried out fully relativistic numerical simulations of accretion disks in the Kerr metric. In this paper we focus on the unbound outflows that emerge self-consistently from the accretion flow. These outflows are found in the axial funnel region and consist of two components: a hot, fast, tenuous outflow in the axial funnel proper, and a colder, slower, denser jet along the funnel wall. Although a rotating black hole is not required to produce these unbound outflows, their strength is enhanced by black hole spin. The funnel-wall jet is excluded from the axial funnel due to elevated angular momentum, and is also pressure-confined by a magnetized corona. The tenuous funnel outflow accounts for a significant fraction of the energy transported to large distances in the higher-spin simulations. We compare the outflows observed in our simulations with those seen in other simulations.Comment: 33 pages, 8 figures, ApJ submitte

Outcomes following childhood head injury : a population study

Objectives: To identify outcomes following head injury (HI) amongst a population of children admitted to one hospital centre and compare outcomes between different severity groups. Methods:A postal follow-up of children admitted with HI to one NHS Trust, between 1992-1998, was carried out. Children were aged 5-15 years at injury (mean 9.8), followed-up at a mean of 2.2 years post-injury. Parents of 526 injured children (419 mild, 58 moderate, 49 severe) and 45 controls completed questionnaires. Outcomes were assessed using the King’s Outcome Scale for Childhood Head Injury (KOSCHI). Results:Frequent behavioural, emotional, memory and attentional problems were reported by one third of the severe group, one quarter of the moderate, and 10-18% of the mild. Personality change since HI was reported for 148 children (28%) (21% mild HI, 46% moderate, 69% severe). There was a significant relationship between injury severity and KOSCHI outcomes. Following the HI, 252(48%) had moderate disability (43% mild HI, 64% moderate, 69% severe), 270(51%) made a good recovery (57% mild HI, 36% moderate, 22% severe). There was a significant association between social deprivation and poor outcome (p=0.002). Only 30%(158) of children received hospital follow-up after the HI. All children with severe disability received appropriate follow-up, but 64% of children with moderate disability received none. No evidence was found to suggest a threshold of injury severity below which the risk of late sequelae could be safely discounted. Conclusions:Children admitted with mild HI may be at risk of poor outcomes, but often do not receive routine hospital follow-up. A postal questionnaire combined with the KOSCHI to assess outcomes after HI may be used to identify children who would benefit from clinical assessment. Further research is needed to identify factors which place children with mild HI at risk of late morbidity

Return to school after brain injury

Objective: To examine return to school and classroom performance following traumatic brain injury (TBI) Design: Cross-sectional Setting: Community Subjects: 67 school-age children with TBI (35 mild, 13 moderate, 19 severe), and 14 uninjured matched controls. Interventions: Parents and children were interviewed and children assessed at a mean of two years post injury. Teachers reported on academic performance and educational needs. Main measures: Classroom performance, Children’s Memory Scale (CMS), WISC-III, WORD. Results: One third of teachers were unaware of the TBI. On return to school, special arrangements were made for 18 children (27%). Special educational needs were identified for 16 (24%), but only six children (9%) received specialist help. Two-thirds of children with TBI had difficulties with school-work, half had attention/concentration problems and 26 (39%) had memory problems. Compared to other pupils in the class, one third of children with TBI were performing below average. On the CMS, one third of the severe group were impaired/borderline for immediate and delayed recall of verbal material, and over one quarter were impaired/borderline for general memory. Children in the severe group had a mean full-scale IQ significantly lower than controls. Half the TBI group had a reading age ≥1 year below their chronological age, one third were reading ≥2 years below chronological age. Conclusions: Schools rely on parents to inform them about a TBI, and rarely receive information on possible long-term sequelae. At hospital discharge, health professionals should provide schools with information about TBI and possible long-term impairments, so that children returning to school receive appropriate support

Parental stress and burden following traumatic brain injury amongst children and adolescents

Primary objectives: to assess parental stress following paediatric traumatic brain injury (TBI) and examine the relationship between self-reported problems and parental stress and general health. Research design: controlled interview study Methods and procedures: Parents of ninety-seven children admitted with a TBI (49 mild, 19 moderate, 29 severe) were identified from a case register of all paediatric admissions from 1992-1998. Parents of 31 uninjured children acted as controls. Structured interviews were carried out with families, and parents assessed on the Parenting Stress Index (PSI/SF) and General Health Questionnaire (GHQ-12) at recruitment, and repeated 12 months later. Main outcomes and results: regardless of injury severity, parents of injured children suffered greater stress than control parents as measured by the PSI/SF (p = 0.001). There was a highly significant relationship between number of problems reported and level of parental stress (p = 0.001). Financial burden was related to severity of TBI. Conclusions: improved information, follow-up, and support is likely to reduce parental stress and family burden

Children's brain injury : a postal follow-up of 525 children from one health region in the UK

Primary objectives: to follow-up a population of children admitted to one Hospital Trust with traumatic brain injury (TBI), and compare outcomes following mild TBI with outcomes following moderate or severe TBI. Research design: population-based postal questionnaire survey. Methods and procedures: questionnaires were mailed to parents of all 974 surviving children on a register of paediatric TBI admissions, 525 completed questionnaires were returned (56.2%). Most children (419) had suffered mild TBI, 57 moderate, and 49 severe. Main outcomes and results: Thirty percent of parents received no information on post-injury symptoms, and clinical follow-up was limited. Statistically significant differences were observed between mild and moderate/severe groups for cognitive, social, emotional, and mobility problems. Nevertheless, approximately 20% of the mild group suffered from poor concentration, personality change, and educational problems post-injury. Few schools (20%) made special provision for children returning after injury. Conclusions: children can have long lasting and wide ranging sequelae following TBI. Information should be routinely given to parents and schools after brain injury