Developing Benthic Class Specific, Chlorophyll-a Retrieving Algorithms for Optically-ShallowWater Using SeaWiFS

This study evaluated the ability to improve Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) chl-a retrieval from optically shallow coastal waters by applying algorithms specific to the pixels’ benthic class. The form of the Ocean Color (OC) algorithm was assumed for this study. The operational atmospheric correction producing Level 2 SeaWiFS data was retained since the focus of this study was on establishing the benefit from the alternative specification of the bio-optical algorithm. Benthic class was determined through satellite image-based classification methods. Accuracy of the chl-a algorithms evaluated was determined through comparison with coincident in situ measurements of chl-a. The regionally-tuned models that were allowed to vary by benthic class produced more accurate estimates of chl-a than the single, unified regionally-tuned model. Mean absolute percent difference was approximately 70% for the regionally-tuned, benthic class-specific algorithms. Evaluation of the residuals indicated the potential for further improvement to chl-a estimation through finer characterization of benthic environments. Atmospheric correction procedures specialized to coastal environments were recognized as areas for future improvement as these procedures would improve both classification and algorithm tuning

The chemical composition of the Orion star-forming region: II. Stars, gas, and dust: the abundance discrepancy conundrum

We re-examine the recombination/collisional emission line (RL/CEL) nebular abundance discrepancy problem in the light of recent high-quality abundance determinations in young stars in the Orion star-forming region. We re-evaluate the CEL and RL abundances of several elements in the Orion nebula and estimate the associated uncertainties, taking into account the uncertainties in the ionization correction factors for unseen ions. We estimate the amount of oxygen trapped in dust grains for several scenarios of dust formation. We compare the resulting gas+dust nebular abundances with the stellar abundances of a sample of 13 B-type stars from the Orion star-forming region (Ori\,OB1), analyzed in Papers I and III of this series. We find that the oxygen nebular abundance based on recombination lines agrees much better with the stellar abundances than the one derived from the collisionally excited lines. This result calls for further investigation. If the CEL/RL abundance discrepancy were caused by temperature fluctuations in the nebula, as argued by some authors, the same kind of discrepancy should be seen for the other elements, such as C, N and Ne, which is not what we find in the present study. Another problem is that with the RL abundances, the energy balance of the Orion nebula is not well understood. We make some suggestions concerning the next steps to undertake to solve this problem.Comment: 11 pages, 8 tables, 5 figures (To be published in A&A

Antioxidants inhibit low density lipoprotein oxidation less at lysosomal pH: a possible explanation as to why the clinical trials of antioxidants might have failed

Oxidised low density lipoprotein (LDL) was considered to be important in the pathogenesis of atherosclerosis, but the large clinical trials of antioxidants, including the first one using probucol (the PQRST Trial), failed to show benefit and have cast doubt on the importance of oxidised LDL. We have shown previously that LDL oxidation can be catalysed by iron in the lysosomes of macrophages. The aim of this study was therefore to investigate the effectiveness of antioxidants in preventing LDL oxidation at lysosomal pH and also establish the possible mechanism of oxidation. Probucol did not effectively inhibit the oxidation of LDL at lysosomal pH, as measured by conjugated dienes or oxidised cholesteryl esters or tryptophan residues in isolated LDL or by ceroid formation in the lysosomes of macrophage-like cells, in marked contrast to its highly effective inhibition of LDL oxidation at pH 7.4. LDL oxidation at lysosomal pH was inhibited very effectively for long periods by N,N'-diphenyl-1,4-phenylenediamine, which is more hydrophobic than probucol and has been shown by others to inhibit atherosclerosis in rabbits, and by cysteamine, which is a hydrophilic antioxidant that accumulates in lysosomes. Iron-induced LDL oxidation might be due to the formation of the superoxide radical, which protonates at lysosomal pH to form the much more reactive, hydrophobic hydroperoxyl radical, which can enter LDL and reach its core. Probucol resides mainly in the surface monolayer of LDL and would not effectively scavenge hydroperoxyl radicals in the core of LDL. This might explain why probucol failed to protect against atherosclerosis in various clinical trials. The oxidised LDL hypothesis of atherosclerosis now needs to be re-evaluated using different and more effective antioxidants that protect against the lysosomal oxidation of LDL

Biases in abundance derivations for metal-rich nebulae

Using ab-initio photoionization models of giant HII regions, we test methods for abundance determinations based on a direct measurement of the electron temperature, now possible even for moderate to high-metallicity objects. We find that, for metallicities larger than solar, the computed abundances deviate systematically from the real ones, generally by larger amounts for more metal-rich HII regions. We discuss the reasons for this, and present diagrams allowing the reader to better understand the various factors coming into play. We briefly discuss less classical methods to derive abundances in metal-rich HII regions. In particular, we comment on the interest of the oxygen and carbon recombination lines. We also show that, contrary to the case of giant HII regions, the physical conditions in bright extragalactic planetary nebulae are such that their chemical composition can be accurately derived even at high metallicities. Thus, extragalactic planetary nebulae are promising potential probes of the metallicity of the interstellar medium in the internal parts of spiral galaxies as well as in metal-rich elliptical galaxies.Comment: 15 pages, 13 figures, Accepted for publication in A&

Ultraviolet and extreme-ultraviolet line ratio diagnostics for O IV.

Aims: We generate theoretical ultraviolet and extreme-ultraviolet emission line ratios for O IV and show their strong versatility as electron temperature and density diagnostics for astrophysical plasmas. Methods: Recent fully relativistic calculations of radiative rates and electron impact excitation cross sections for O IV, supplemented with earlier data for A-values and proton excitation rates, are used to derive theoretical O IV line intensity ratios for a wide range of electron temperatures and densities. Results: Diagnostic line ratios involving ultraviolet or extreme-ultraviolet transitions in O IV are presented, that are applicable to a wide variety of astrophysical plasmas ranging from low density gaseous nebulae to the densest solar and stellar flares. Comparisons with observational data, where available, show good agreement between theory and experiment, providing support for the accuracy of the diagnostics. However, diagnostics are also presented involving lines that are blended in existing astronomical spectra, in the hope this might encourage further observational studies at higher spectral resolution.Comment: 5 pages; 8 figures; Astronomy and Astrophysics, in pres

Energy levels, radiative rates, and excitation rates for transitions in OIV

Aims.In this paper we report calculations for energy levels, radiative rates and excitation rates for transitions in Ni x

Breit-Pauli energy levels and transition rates for the carbonlike sequence

Breit–Pauli results for energy levels, lifetimes, and some transition data are reported for all levels up to 2s22p3d 1PJo of the C-like spectrum for 7 [Formula: see text] Z [Formula: see text] 15. For all but the lowest members of the sequence, these include all the states of the 2s22p2, 2s2p3, 2p4, 2s22p3s, 2s22p3p, and 2s22p3d configurations. For carbon, only the states for the 2s22p2, 2s2p3, and 2s22p3s configurations are included. A simultaneous optimization scheme was applied so that a radial basis could be determined for a set of terms that mix in the Breit–Pauli approximation. All results have been posted at a Web site. In this paper, lifetimes are reported and, in addition, results for N II, O III, and Si IX are critically evaluated. Energy levels are compared with experiment and adjusted transition rates, along with uncertainty estimates are presented. Selected results are compared with other theory and experiment. PACS Nos.: 31.25Jf, 32.30Jc, 32.70Cs </jats:p

Breit-Pauli energy levels and transition rates for nitrogen-like and oxygen-like sequences

Breit-Pauli results for energy levels, lifetimes, and Landé gJ factors have been determined for all levels up to 2p23d of the nitrogen-like sequence ($Z=7$–17) and 2p33d of the oxygen-like sequence ($Z=8$–20). Exceptions are some lower members of the sequence where the spectrum included only those levels below the second 2p24s term in the case of N-like or 2p34s in the case of O-like. The computed energy and E1, E2, M1, M2 transition data between all levels, including convergence of the LS line strength for both length and velocity forms, may be viewed at a website. In this paper, critically evaluated transition data is presented for N I, O II, Mg VI, and Si VIII (N-like sequence) and O I, Ne III, Mg V, and Si VII (O-like) for E1 transitions including uncertainty estimates. The accuracy of energy levels is determined by comparison with experiment. Transition rates with uncertainties are compared with experiment and other theory