Adaptive Optics Imaging of the AU Microscopii Circumstellar Disk: Evidence for Dynamical Evolution

We present an H-band image of the light scattered from circumstellar dust around the nearby (10 pc) young M star AU Microscopii (AU Mic, GJ 803, HD 197481), obtained with the Keck adaptive optics system. We resolve the disk both vertically and radially, tracing it over 17-60 AU from the star. Our AU Mic observations thus offer the possibility to probe at high spatial resolution (0.04" or 0.4 AU per resolution element) for morphological signatures of the debris disk on Solar-System scales. Various sub-structures (dust clumps and gaps) in the AU Mic disk may point to the existence of orbiting planets. No planets are seen in our H-band image down to a limiting mass of 1 M_Jup at >20 AU, although the existence of smaller planets can not be excluded from the current data. Modeling of the disk surface brightness distribution at H-band and R-band, in conjunction with the optical to sub-millimeter spectral energy distribution, allows us to constrain the disk geometry and the dust grain properties. We confirm the nearly edge-on orientation of the disk inferred from previous observations, and deduce an inner clearing radius <=10 AU. We find evidence for a lack of small grains in the inner (<60 AU) disk, either as a result of primordial disk evolution, or because of destruction by Poynting-Robertson and/or corpuscular drag. A change in the power-law index of the surface brightness profile is observed near 33 AU, similar to a feature known in the profile of the beta Pic circumstellar debris disk. By comparing the time scales for inter-particle collisions and Poynting-Robertson drag between the two systems, we argue that the breaks are linked to one of these two processes.Comment: 17 pages, 7 figures, 1 table; accepted by Ap

The Structure and Star-Formation History of NGC 5461

We compute photoionization models for the giant extragalactic H II region NGC 5461, and compare their predictions to several observational constraints. Since we aim at reproducing not only the global properties of the region, but its local structure also, the models are constrained to reproduce the observed density profile, and our analysis takes into consideration the bias introduced by the shapes and sizes of the slits used by different observers. We find that an asymmetric nebula with a gaussian density distribution, powered by a young burst of 3.1 Myr, satisfactorily reproduces most of the constraints, and that the star-formation efficiency inferred from the model agrees with current estimates. Our results strongly depend on the assumed density law, since constant density models overestimate the hardness of the ionizing field, affecting the deduced properties of the central stellar cluster. We illustrate the features of our best model, and discuss the possible sources of errors and uncertainties affecting the outcome of this type of studies.Comment: 33 pages (LaTeX), 3 .eps figures. to be published in ApJ, May 200

Precious, Tiny and Shiny: Close range photogrammetry for artists and others

Close Range Photogrammetry is increasingly being used to create digital assets across a range of disciplines, and can provide 3D ‘scans’ and surface ‘textures’ to very high resolution. The techniques of capture and processing usually present challenges for objects that are very small, monochromatic, reflective or relatively lacking in features. This ongoing visual arts project seeks to capture, re-examine and ultimately transform specific military and religious insignia as part of an investigation into their interrelated iconographies and histories. The materiality (highly reflective) and scale (very detailed embroidery) of these objects has required different approaches to achieving the accurate 3D models required. This paper will present the apparatus, methods and devices that have been developed to help solve these problems. Together they offer affordable, reliable, and scalable solutions to the traditional challenges of ‘3D scanning’ for particular types of objects, and robust methods applicable to a wide range of situations. They also help reduce the investment (of capital cost, time and labour) required to achieve accurate digital models with high resolution textures, opening up creative possibilities further down the virtual or material ‘pipeline’

The AU Microscopii Debris Disk: Multiwavelength Imaging and Modeling

(abridged) Debris disks around main sequence stars are produced by the erosion and evaporation of unseen parent bodies. AU Microscopii (GJ 803) is a compelling object to study in the context of disk evolution across different spectral types, as it is an M dwarf whose near edge-on disk may be directly compared to that of its A5V sibling beta Pic. We resolve the disk from 8-60 AU in the near-IR JHK' bands at high resolution with the Keck II telescope and adaptive optics, and develop a novel data reduction technique for the removal of the stellar point spread function. The point source detection sensitivity in the disk midplane is more than a magnitude less sensitive than regions away from the disk for some radii. We measure a blue color across the near-IR bands, and confirm the presence of substructure in the inner disk. Some of the structural features exhibit wavelength-dependent positions. The disk architecture and characteristics of grain composition are inferred through modeling. We approach the modeling of the dust distribution in a manner that complements previous work. Using a Monte Carlo radiative transfer code, we compare a relatively simple model of the distribution of porous grains to a broad data set, simultaneously fitting to midplane surface brightness profiles and the spectral energy distribution. Our model confirms that the large-scale architecture of the disk is consistent with detailed models of steady-state grain dynamics. Here, a belt of parent bodies from 35-40 AU is responsible for producing dust that is then swept outward by the stellar wind and radiation pressures. We infer the presence of very small grains in the outer region, down to sizes of ~0.05 micron. These sizes are consistent with stellar mass-loss rates Mdot_* << 10^2 Mdot_sun.Comment: ApJ accepted, 56 pages, preprint style. Version in emulateapj with high-resolution figures available at http://tinyurl.com/y6ent

Long-term change in calcareous grassland vegetation and drivers over three time periods between 1970 and 2016

Analysis of long-term vegetation change is limited. Furthermore most studies evaluating change only examine two snapshots in time, which makes it difficult to define rates of change and accurately assess potential drivers. To assess long-term change in calcareous grassland over multiple time periods, we re-surveyed a transect study undertaken at Parsonage Down National Nature Reserve, Wiltshire, southern England in 1970 and 1990 by T. Wells. We examined differences in soil properties and species traits in each of the survey years to understand potential drivers of vegetation change, including nitrogen deposition and grazing management. There was a clear shift in species composition, combined with significant declines in species richness and diversity between 1970 and 2016, with the greater rate of change occurring between 1990 and 2016. A significant increase in soil total nitrogen was found, which was significantly associated with the decline in species diversity between 1970 and 1990. Significant changes in community-weighted mean traits were identified for plant height (increasing), specific leaf area (decreasing), grazing tolerance (decreasing) and Ellenberg N (decreasing) between 1970 and 2016. By using survey data from multiple time periods, we suggest that N deposition may have contributed towards community changes between 1970 and 1990, as indicated by the change in soil properties and the associated decline in species diversity. Vegetation change between 1990 and 2016 is likely to be largely attributable to a decline in grazing pressure, indicated by the increase in taller species and a decrease in grazing tolerance

Boron Abundances in Main Sequence B-type Stars: A Test of Rotational Depletion during Main Sequence Evolution

Boron abundances have been derived for seven main sequence B-type stars from HST STIS spectra around the B III 2066 A line. In two stars, boron appears to be undepleted with respect to the presumed initial abundance. In one star, boron is detectable but it is clearly depleted. In the other four stars, boron is undetectable implying depletions of 1 to 2 dex. Three of these four stars are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only rotationally induced mixing predicts that boron depletions are unaccompanied by nitrogen enrichments. The inferred rate of boron depletion from our observations is in good agreement with these predictions. Other boron-depleted nitrogen-normal stars are identified from the literature. Also, several boron-depleted nitrogen-rich stars are identified, and while all fall on the boron-nitrogen trend predicted by rotationally-induced mixing, a majority have nitrogen enrichments that are not uniquely explained by rotation. The spectra have also been used to determine iron-group (Cr, Mn, Fe, and Ni) abundances. The seven B-type stars have near solar iron-group abundances, as expected for young stars in the solar neighborhood. We have also analysed the halo B-type star, PG0832+676. We find [Fe/H] = -0.88 +/- 0.10, and the absence of the B III line gives the upper limit [B/H]<2.5. These and other published abundances are used to infer the star's evolutionary status as a post-AGB star.Comment: 31 pages, 14 figures. accepted to Ap

The Origins and Evolutionary Status of B Stars Found Far From the Galactic Plane II: Kinematics and Full Sample Analysis

This paper continues the analysis of faint high latitude B stars from Martin (2004). Here we analyze the kinematics of the stars and combine them with the abundance information from the first paper to classify each one. The sample contains 31 Population I runaways, fifteen old evolved stars (including five BHB stars, three post-HB stars, a pulsating helium dwarf, and six stars of ambiguous classification), one F-dwarf, and two stars which do not easily fit in one of the other categories. No star in the sample unambiguously shows the characteristics of a young massive star formed in situ in the halo. The two unclassified stars are probably extreme Population I runaways. The low binary frequency and rotational velocity distribution of the Population I runaways imply that most were ejected from dense star clusters by DES (dynamic ejection scenario). However we remain puzzled by the lack of runaway Be stars. We also confirm that PB 166 and HIP 41979 are both nearby solar-metallicity BHB stars.Comment: 59 pages including 14 Figures, 12 Tables; Accepted for publication in the Astronomical Journa

Stimulation of colorectal tumour protein synthesis, in vivo, by nutritional support.

Malignant disease is frequently accompanied by weight loss and malnutrition, which are associated with an increase in patient mortality and morbidity