Spot-like Features Research Articles

Time-resolved Optical Polarization Monitoring of the Most Variable Brown Dwarf

Recent atmospheric models for brown dwarfs suggest that the existence of clouds in substellar objects is not needed to reproduce their spectra, nor their rotationally induced photometric variability, believed to be due to the heterogeneous cloud coverage of brown dwarf atmospheres. Cloud-free atmospheric models also predict that their flux should not be polarized, as polarization is produced by the light scattering of particles in the inhomogeneous cloud layers of brown dwarf atmospheres. To shed light on this dichotomy, we monitored the linear polarization and photometric variability of the most variable brown dwarf, 2MASS J21392676+0220226. We used FORS2 at the UT1 telescope to monitor the object in the z band for six hours, split on two consecutive nights, covering one-third of its rotation period. We obtained the Stokes parameters, and we derived its time-resolved linear polarization, for which we did not find significant linear polarization (P = 0.14% ± 0.07%). We modeled the linear polarimetric signal expected assuming a map with one or two spot-like features and two bands using a polarization-enabled radiative transfer code. We obtained values compatible with the time-resolved polarimetry obtained for 2MASS J21392676+0220226. The lack of significant polarization might be due to photometric variability produced mostly by banded structures or small-scale vortices, which cancel out the polarimetric signal from different regions of the dwarf’s disk. Alternatively, the lack of clouds in 2MASS J21392676+0220226 would also explain the lack of polarization. Further linear polarimetric monitoring of 2MASS J21392676+0220226, during at least one full rotational period, would help to confirm or discard the existence of clouds in its atmosphere.

ACCESS: Ground-based Optical Transmission Spectroscopy of the Hot Jupiter WASP-4b

Abstract We present an optical transmission spectrum of the atmosphere of WASP-4b obtained through observations of four transits with Magellan/IMACS, as part of the Arizona-CfA-Católica-Carnegie Exoplanet Spectroscopy Survey (ACCESS). Using a Bayesian approach to atmospheric retrieval, we find no evidence for scattering or absorption features in our transit spectrum. Our models include a component to model the transit light source effect (spectral contamination from unocculted spots on the stellar photosphere), which we show can have a marked impact on the observed transmission spectrum for reasonable spot-covering fractions (<5%); this is the first such analysis for WASP-4b. We are also able to fit for the size and temperature contrast of spots observed during the second and third transits, finding evidence for both small, cool and large, warm spot-like features on the photosphere. Finally, we compare our results to those published by Huitson et al. using Gemini/GMOS and May et al. using IMACS, and we find that our data are in agreement.

Dislocation configurations in a Mg-3Al- 1Zn alloy during high cycle fatigue process

The patterns of dislocations and their configurations of a ho t rolled Mg-3Al-1Zn (AZ31) plate were investigated at the stress amplitude of 90 MPa by using a tension-compression fatigue test at room temperature. The result shows that basal lag dislocations were rearranged into pileups, dislocation walls and low-angle grain boundaries during high cyclic deformation. Due to continuously reversed loading along the rolling direction, lc + ag pyramidal dislocations were activated. lc + ag edge dislocations were straight lines with spot-like features and parallel to the basal plane trace. Zigzag dislocation were observed, due to the double cross-slip of the pyramidal dislocations between the first-order and second-order pyramidal slip systems. By interactions of basal-basal, basal-prismatic and pyramidal-pyramidal dislocations, junctions and dislocation loops are also observed. Keywords: Dislocation configurations; Magnesium alloy; High cycle fatigue

Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars

Observations of cool stars reveal dark spot-like features on their surfaces. Compared to sunspots, starspots can be bigger or cover a larger fraction of the stellar surface. While sunspots appear only at low latitudes, starspots are also found in polar regions, in particular on rapidly rotating stars. Sunspots are believed to result from the eruption of magnetic flux-tubes rising from the deep interior of the Sun. The strong magnetic field locally reduces convective heat transport to the solar surface. Such flux-tube models have also been invoked to explain starspot properties. However, these models use several simplifications and so far the generation of either sunspots or starspots has not been demonstrated in a self-consistent simulation of stellar magnetic convection. Here we show that direct numerical simulations of a distributed dynamo operating in a density-stratified rotating spherical shell can spontaneously generate cool spots. Convection in the interior of the model produces a large scale magnetic field which interacts with near surface granular convection leading to strong concentrations of magnetic flux and formation of starspots. Prerequisites for the formation of sizeable high-latitude spots in the model are sufficiently strong density stratification and rapid rotation. Our model presents an alternate mechanism for starspot formation by distributed dynamo action.

Automated analysis of DNA hybridization images for high-throughput genomics

The design and implementation of a computer vision system called DNAScan for the automated analysis of DNA hybridization images is presented. The hybridization of a DNA clone with a radioactively tagged probe manifests itself as a spot on the hybridization membrane. The imaging of the hybridization membranes and the automated analysis of the resulting images are imperative for high-throughput genomics experiments. A recursive segmentation procedure is designed and implemented to extract spotlike features in the hybridization images in the presence of a highly inhomogeneous background. Positive hybridization signals (hits) are extracted from the spotlike features using grouping and decomposition algorithms based on computational geometry. A mathematical model for the positive hybridization patterns and a Bayesian pattern classifier based on shape-based moments are proposed and implemented to distinguish between the clone-probe hybridization signals. Experimental results on real hybridization membrane images are presented.

PEEM—a spectromicroscopic tool for mc-Si surface evaluation

The photo emission electron microscope (PEEM) with its direct imaging of the sample surface is a convenient tool for fast evaluation of large sample areas. It is possible to handle samples with a size of 30 mm × 30 mm. With PEEM it is possible to visualise grain boundaries and crystal displacements without removing the native oxide. No special sample treatment is needed except chemical polishing of the sample surface and a HF-dip to obtain a thin (1…2 nm) oxide layer. The quality of that oxide was proofed by high resolution photo electron spectroscopy (PES) at a synchrotron. The results were compared to native oxide on single-crystalline Si and no suboxides (e.g. SiO) were found. Our spectromicroscopic PEEM measurements show no difference in the oxide qualities of different grains and on grain boundaries or crystal displacements. Furthermore no difference in the binding energy of Si2p core levels were found. In addition to grain boundaries other spot-like features were found on the sample surface. These could be identified in some cases as precipitation of Ca underneath the native oxide.

A QUICK SURVEY OF LOCAL AMPLIFICATIONS AND THEIR CORRELATION WITH DAMAGE OBSERVED DURING THE UMBRO-MARCHESAN (ITALY) EARTHQUAKE OF SEPTEMBER 26, 1997

The seismic sequence that struck Central Italy during September and October 1997 caused severe damage (up to IX degree MCS). The most affected area is populated by several sparse, small-sized villages (some hundred inhabitants each). The damage pattern shows spot-like features, with strong variations on a short-scale range. Localised changes in the buildings' vulnerability may account for some of the effects observed, but it is likely that most of the variation may be attributed to site amplification effects. In any case, local amplification has to be estimated in order to allow engineers' to sort effects according to either cause. A series of field measurements were undertaken in the aftermath of the main event. Local geology appears to be the dominant factor in most of the damage observed, which can be linked to empirical amplification functions obtained with the Nakamura's technique. A tentative correlation is established between damage enhancement and amplification in selected frequency bands.

Morphological and spectroscopic investigation of the behavior of permanent iridium modifier deposited on pyrolytic graphite coated and zirconium treated platforms in electrothermal atomic absorption spectrometry

In order to better characterise a permanent modifier based on iridium deposited on zirconium or tungsten treated platforms of transversely heated graphite atomizer, and to gain additional information about its chemical behavior directed to an eventual further optimization, a series of experiments were carried out, both by surface techniques such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS or ESCA) and X-ray fluorescence (XRF) and by electrothermal atomic absorption spectrometry on the iridium release from unmodified and various other modified pyrolytic graphite platforms. Special attention was paid to the influence of the amount of iridium, zirconium carbide coating of the platform surface and the presence of citric acid on the iridium vaporization during pyrolysis and atomization. The processes of iridium losses during pyrolysis and atomization and peak maximum alignment depend on the amount of the iridium deposited on the pyrolytic graphite coated platforms in the presence of nitric acid. A fractional order of release which suggests an atom vaporization from the surface or edges of the iridium islands was estimated. In the presence of citric acid, mass independence and zero order of the atom release were found. The zirconium treatment of the platform results in change of the spatial distribution of iridium and hence its vaporization. Vaporization temperatures as high as 2100°C, and first order of the process of atom generation were obtained. While it was possible to study the iridium atomization from uncoated and zirconium coated surfaces, evidencing a different order for the release process, the same was not possible for the tungsten coated platforms due to an ‘overstabilization’ that brought the iridium atomization temperature out of the working range of the instrument used. The different chemical behavior of tungsten and zirconium was also confirmed by XPS investigations. With tungsten, evidence of both WC and WO bonding was found, while zirconium on the contrary shows only ZrO bonding and no evidence of carbide bonding. The SEM revealed a highly dispersed distribution of spot-like features whose smallest average diameter was of the order of 0.1 μm. The XRF asserted the confinement of iridium in these features and a strict association with zirconium in the case of zirconium treated surfaces. It is worth mentioning that such structure was preserved also after 400 thermal cycles simulating an atomization step at 1900°C despite a quite evident deterioration of the graphite surface, thus confirming the excellent durability of this modifier.

Damage and erosion at ceramic Windows due to high-power millimeter waves

The damage that occurs at alumina windows of high-power millimeter wave gyrotrons operating in TE[sub 0,n] mode is investigated. This damage appears only on the air side of the window where it yields small spot-like features (diameter 1 cm). The damage observed is thought to be related to rf field enhancements by resonances of trapped electromagnetic modes and by protrusions and contaminations of the ceramic surface. 16 refs., 7 figs., 3 tabs.