| 1. |
- Dent, W. R. F., et al.
(författare)
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GASPS-A Herschel Survey of Gas and Dust in Protoplanetary Disks: Summary and Initial Statistics
- 2013
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Ingår i: Publications of the Astronomical Society of the Pacific. - : IOP Publishing. - 0004-6280 .- 1538-3873. ; 125:927, s. 477-505
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Tidskriftsartikel (refereegranskat)abstract
- We describe a large-scale far-infrared line and continuum survey of protoplanetary disk through to young debris disk systems carried out using the ACS instrument on the Herschel Space Observatory. This Open Time Key program, known as GASPS (Gas Survey of Protoplanetary Systems), targeted similar to 250 young stars in narrow wavelength regions covering the [OI] fine structure line at 63 mu m the brightest far-infrared line in such objects. A subset of the brightest targets were also surveyed in [OI]145 mu m, [CII] at 157 mu m, as well as several transitions of H2O and high-excitation CO lines at selected wavelengths between 78 and 180 mu m. Additionally, GASPS included continuum photometry at 70, 100 and 160 mu m, around the peak of the dust emission. The targets were SED Class II-III T Tauri stars and debris disks from seven nearby young associations, along with a comparable sample of isolated Herbig AeBe stars. The aim was to study the global gas and dust content in a wide sample of circumstellar disks, combining the results with models in a systematic way. In this overview paper we review the scientific aims, target selection and observing strategy of the program. We summarise some of the initial results, showing line identifications, listing the detections, and giving a first statistical study of line detectability. The [OI] line at 63 mu m was the brightest line seen in almost all objects, by a factor of similar to 10. Overall [OI]63 mu m detection rates were 49%, with 100% of HAeBe stars and 43% of T Tauri stars detected. A comparison with published disk dust masses (derived mainly from sub-mm continuum, assuming standard values of the mm mass opacity) shows a dust mass threshold for [OI] 63 mu m detection of similar to 10(-5) M-circle dot. Normalising to a distance of 140 pc, 84% of objects with dust masses >= 10(-5) M-circle dot can be detected in this line in the present survey; 32% of those of mass 10(-6)-10(-5) M-circle dot, and only a very small number of unusual objects with lower masses can be detected. This is consistent with models with a moderate UV excess and disk flaring. For a given disk mass, [OI] detectability is lower for M stars compared with earlier spectral types. Both the continuum and line emission was, in most systems, spatially and spectrally unresolved and centred on the star, suggesting that emission in most cases was from the disk. Approximately 10 objects showed resolved emission, most likely from outflows. In the GASPS sample, [OI] detection rates in T Tauri associations in the 0.3-4 Myr age range were similar to 50%. For each association in the 5-20 Myr age range, similar to 2 stars remain detectable in [OI]63 mu m, and no systems were detected in associations with age >20 Myr. Comparing with the total number of young stars in each association, and assuming a ISM-like gas/dust ratio, this indicates that similar to 18% of stars retain a gas-rich disk of total mass similar to 1 M-Jupiter for 1-4 Myr, 1-7% keep such disks for 5-10 Myr, but none are detected beyond 10-20 Myr. The brightest [OI] objects from GASPS were also observed in [OI]145 mu m, [CII]157 mu m and CO J = 18 - 17, with detection rates of 20-40%. Detection of the [CII] line was not correlated with disk mass, suggesting it arises more commonly from a compact remnant envelope.
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| 2. |
- Sprague, A. L., et al.
(författare)
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Spectral emissivity measurements of Mercury's surface indicate Mg- and Ca-rich mineralogy, K-spar, Na-rich plagioclase, rutile, with possible perovskite, and garnet
- 2009
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 57:3, s. 364-383
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Tidskriftsartikel (refereegranskat)abstract
- Mid-infrared 2-D spectroscopic measurements from 8.0 to 12.7 gm of Mercury were taken using Boston University's Mid-Infrared Spectrometer and Imager (MIRSI) mounted on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, 7-11 April 2006. Measurements reported here cover radar bright region C, a dark plains region west of Caloris Basin, and the interior of Caloris Basin. By use of spectral deconvolution with a large spectral library composed of many mineral compositions and grain size separates, we fitted, or "unmixed", the Mercury spectra. We find mineral suites composed of magnesium-rich orthopyroxene and olivine, Ca-, Mg-, Na-rich clinopyroxene, potassium feldspar, and Na-bearing plagioclase feldspar. Both Ca- and Mg-rich garnet (pyrope and grossular, respectively) are apparently present in small amounts. Opaque minerals are required for spectral matching, with rutile (TiO2) repeatedly providing the "best fit". However, in the case of the radar bright region C, perovskite also contributed to a very good fit. Caloris Basin infill is rich in both potassium feldspar and Na-rich plagioclase. There is little or no olivine in the Caloris interior smooth plains. Together with the high alkali content, this indicates that resurfacing magmas were low to intermediate in SiO2. Data suggest the dark plains exterior to Caloris are highly differentiated low-iron basaltic magmas resulting in material that might be classified as oligorclase basalts. (C) 2009 Elsevier Ltd. All rights reserved
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| 3. |
- Moëll, Mattias K., et al.
(författare)
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Shading correction methods for digital image analysis of confocal wood images
- 2007
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Ingår i: IAWA Journal. - : Smithsonian Institution Libraries. - 0928-1541. ; 28:3, s. 349-364
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Tidskriftsartikel (refereegranskat)abstract
- Confocal fluorescence microscopy provides a rapid method for acquiringhigh quality optically thin section images of wood suitable for meas-urement of cell dimensions. Single optical slice images of wood mayoccasionally contain artefacts due to differential light absorption causedby variation in the distance between the sample surface and the imagingplane across the field of view. Regional brightness variations, which wecall shading, may cause problems when such images are used for woodcell measurements using digital image analysis, affecting the accuracyof wood cell dimensions. We have compared various shading correctionmethods for confocal microscope images and investigated the effect ofshading on both the classification of cell wall pixels and the resulting celldimension measurements. Severe shading results in significant errors formeasurement of cell wall area, but smaller errors for cell wall thicknessand lumen diameter. Some shading correction methods have unwantedeffects on pixel classification and cell dimensions, while more effectivemethods remove the shading without introducing further artefacts. Theeffect of shading is influenced by choice of thresholding method.
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| 4. |
- Hong, A. C., et al.
(författare)
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Uptake of Hydrogen Peroxide from the Gas Phase to Grain Boundaries: A Source in Snow and Ice
- 2023
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Ingår i: Environmental Science and Technology. - 0013-936X .- 1520-5851. ; 57:31, s. 11626-11633
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Tidskriftsartikel (refereegranskat)abstract
- This work shows that hydrogen peroxidereadily enters grainboundaries in ice and snow directly from the atmosphere. Acceleratedreaction rates in these reservoirs have been described with an impacton air quality and snow composition. Hydrogen peroxide is a primary atmospheric oxidant significantin terminating gas-phase chemistry and sulfate formation in the condensedphase. Laboratory experiments have shown an unexpected oxidation accelerationby hydrogen peroxide in grain boundaries. While grain boundaries arefrequent in natural snow and ice and are known to host impurities,it remains unclear how and to which extent hydrogen peroxide entersthis reservoir. We present the first experimental evidence for thediffusive uptake of hydrogen peroxide into grain boundaries directlyfrom the gas phase. We have machined a novel flow reactor system featuringa drilled ice flow tube that allows us to discern the effect of theice grain boundary content on the uptake. Further, adsorption to theice surface for temperatures from 235 to 258 K was quantified. Disentanglingthe contribution of these two uptake processes shows that the transferof hydrogen peroxide from the atmosphere to snow at temperatures relevantto polar environments is considerably more pronounced than previouslythought. Further, diffusive uptake to grain boundaries appears tobe a novel mechanism for non-acidic trace gases to fill the highlyreactive impurity reservoirs in snow's grain boundaries.
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| 5. |
- Langrish, Jeremy P., et al.
(författare)
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Controlled exposures to air pollutants and risk of cardiac arrhythmia
- 2014
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Ingår i: Journal of Environmental Health Perspectives. - : Environmental Health Perspectives. - 0091-6765 .- 1552-9924. ; 122:7, s. 747-753
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: Epidemiological studies have reported associations between air pollution exposure and increases in cardiovascular morbidity and mortality. Exposure to air pollutants can influence cardiac autonomic tone and reduce heart rate variability, and may increase the risk of cardiac arrhythmias, particularly in susceptible patient groups. OBJECTIVES: We investigated the incidence of cardiac arrhythmias during and after controlled exposure to air pollutants in healthy volunteers and patients with coronary heart disease. METHODS: We analyzed data from 13 double-blind randomized crossover studies including 282 participants (140 healthy volunteers and 142 patients with stable coronary heart disease) from whom continuous electrocardiograms were available. The incidence of cardiac arrhythmias was recorded for each exposure and study population. RESULTS: There were no increases in any cardiac arrhythmia during or after exposure to dilute diesel exhaust, wood smoke, ozone, concentrated ambient particles, engineered carbon nanoparticles, or high ambient levels of air pollution in either healthy volunteers or patients with coronary heart disease. CONCLUSIONS: Acute controlled exposure to air pollutants did not increase the short-term risk of arrhythmia in participants. Research employing these techniques remains crucial in identifying the important pathophysiological pathways involved in the adverse effects of air pollution, and is vital to inform environmental and public health policy decisions.
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| 6. |
- Probert, James R., et al.
(författare)
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Anthropogenic modifications to fire regimes in the wider Serengeti-Mara ecosystem
- 2019
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Ingår i: Global Change Biology. - : WILEY. - 1354-1013 .- 1365-2486. ; 25:10, s. 3406-3423
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Tidskriftsartikel (refereegranskat)abstract
- Fire is a key driver in savannah systems and widely used as a land management tool. Intensifying human land uses are leading to rapid changes in the fire regimes, with consequences for ecosystem functioning and composition. We undertake a novel analysis describing spatial patterns in the fire regime of the Serengeti-Mara ecosystem, document multidecadal temporal changes and investigate the factors underlying these patterns. We used MODIS active fire and burned area products from 2001 to 2014 to identify individual fires; summarizing four characteristics for each detected fire: size, ignition date, time since last fire and radiative power. Using satellite imagery, we estimated the rate of change in the density of livestock bomas as a proxy for livestock density. We used these metrics to model drivers of variation in the four fire characteristics, as well as total number of fires and total area burned. Fires in the Serengeti-Mara show high spatial variability-with number of fires and ignition date mirroring mean annual precipitation. The short-term effect of rainfall decreases fire size and intensity but cumulative rainfall over several years leads to increased standing grass biomass and fuel loads, and, therefore, in larger and hotter fires. Our study reveals dramatic changes over time, with a reduction in total number of fires and total area burned, to the point where some areas now experience virtually no fire. We suggest that increasing livestock numbers are driving this decline, presumably by inhibiting fire spread. These temporal patterns are part of a global decline in total area burned, especially in savannahs, and we caution that ecosystem functioning may have been compromised. Land managers and policy formulators need to factor in rapid fire regime modifications to achieve management objectives and maintain the ecological function of savannah ecosystems.
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