999 research outputs found
Trace element proxies and mineral indicators of hydrothermal fluid composition and seafloor massive sulfide deposit formation processes
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the
Woods Hole Oceanographic Institution June 2017This thesis analyzes compositions of seafloor massive sulfide (SMS) deposits and related hydrothermal vent fluids to identify proxies of reaction zone conditions (host-rock lithology, hydrothermal fluid temperature and chemistry). Chapter 2 investigates the morphology, mineralogy, and geochemistry of SMS deposits from six vent fields along the Eastern Lau Spreading Center (ELSC), demonstrating that ELSC SMS deposits record differences in hydrothermal fluid temperature, pH, sulfur fugacity and host-rock lithology related to proximity to the nearby Tonga Subduction Zone. Chapters 3 and 4 focus on partitioning of Co, Ni, Ga, Ag, and In between hydrothermal vent fluids and chalcopyrite lining fluid conduits in black smoker chimneys. Chapter 3 develops secondary ion mass spectrometry (SIMS) as a technique to measure Co, Ni, Ga, Ag, and In in chalcopyrite and identifies a correlation between Ga and In in chalcopyrite and hydrothermal fluid pH. Chapter 4 presents new data on these elements in ELSC hydrothermal fluids that, combined with SIMS analyses of chalcopyrite chimney linings and previously published data on vent fluids from the Manus Basin, provide evidence that supports partitioning of Ag a lattice substitution for Cu. Together, concentrations of Ga, In, and Ag in chalcopyrite provide proxies of hydrothermal fluid pH and metal (i.e., Ag and Cu) contents.Work presented in Chapter 2 of this thesis was supported by the National Science Foundation through grants OCE-1038135 to GNE and MKT, OCE-1038124, OCE-0241796, OCE-1233037 to JSS, and OCE-0242088 to CGW. Chapters 3 and 4 were supported by the National Science Foundation through grants OCE-1038135 and NSF Graduate Research Fellowship to GNE and funding from grant NSF OCE-1536480 to MKT
How to combine rules and commitment in fostering research integrity?
Research integrity (RI) is crucial for producing research that is trustworthy and of high quality. Rules are important in setting RI standards, improving research practice and fostering responsible research practices. At the same time, rules can lead to increased bureaucracy, which without commensurate increased commitment amongst researchers towards RI is unlikely to lead to more responsible research behavior. In this paper, we explore the question: How can rules and commitment be combined to foster RI? There are three ways that research institutions can govern RI: markets (governing through incentives), hierarchies or bureaucracies (governing through rules), and network processes (governing through commitment and agreement at group level). Based on Habermas’ Theory of Communicative Action, we argue that network processes focusing on consensus, as part of the lifeworld, are necessary to legitimize and support systems, i.e. market and bureaucratic modes of governance. We analyze the institutional response to a serious RI case to illustrate how network processes can create a context in which rules can foster RI. Specifically, we analyze how the Science Committee established at Tilburg University in 2012 has navigated and combined different modes of governance to foster RI. Based on this case analysis, we formulate recommendations to research institutions on how to combine rules and commitment
The Impact of Gaia and LSST on Binaries and Exoplanets
Two upcoming large scale surveys, the ESA Gaia and LSST projects, will bring a new era in astronomy. The number of binary systems that will be observed and detected by these projects is enormous, estimations range from millions for Gaia to several tens of millions for LSST. We review some tools that should be developed and also what can be gained from these missions on the subject of binaries and exoplanets from the astrometry, photometry, radial velocity and their alert system
The impact of Gaia and LSST on binary stars and exo-planets
Two upcoming large scale surveys, the ESA Gaia and LSST projects, will bring
a new era in astronomy. The number of binary systems that will be observed and
detected by these projects is enormous, estimations range from millions for
Gaia to several tens of millions for LSST. We review some tools that should be
developed and also what can be gained from these missions on the subject of
binaries and exoplanets from the astrometry, photometry, radial velocity and
their alert systems.Comment: 8 pages, 2 figures, Proceedings of the IAU Symposium No. 282: From
Interacting Binaries to Exoplanets: Essential Modeling Tools. Tatranska
Lomnica, Slovaki
Gaia Data Release 2: Short-timescale variability processing and analysis
The Gaia DR2 sample of short-timescale variable candidates results from the
investigation of the first 22 months of Gaia photometry for a subsample of
sources at the Gaia faint end. For this exercise, we limited ourselves to the
case of suspected rapid periodic variability. Our study combines
fast-variability detection through variogram analysis, high-frequency search by
means of least-squares periodograms, and empirical selection based on the
investigation of specific sources seen through the Gaia eyes (e.g. known
variables or visually identified objects with peculiar features in their light
curves). The progressive definition and validation of this selection criterion
also benefited from supplementary ground-based photometric monitoring of a few
preliminary candidates, performed at the Flemish Mercator telescope (Canary
Islands, Spain) between August and November 2017. We publish a list of 3,018
short-timescale variable candidates, spread throughout the sky, with a
false-positive rate up to 10-20% in the Magellanic Clouds, and a more
significant but justifiable contamination from longer-period variables between
19% and 50%, depending on the area of the sky. Although its completeness is
limited to about 0.05%, this first sample of Gaia short-timescale variables
recovers some very interesting known short-period variables, such as
post-common envelope binaries or cataclysmic variables, and brings to light
some fascinating, newly discovered variable sources. In the perspective of
future Gaia data releases, several improvements of the short-timescale
variability processing are considered, by enhancing the existing variogram and
period-search algorithms or by classifying the identified candidates.
Nonetheless, the encouraging outcome of our Gaia DR2 analysis demonstrates the
power of this mission for such fast-variability studies, and opens great
perspectives for this domain of astrophysics
Crystal-Chemical Origins of the Ultrahigh Conductivity of Metallic Delafossites
Despite their highly anisotropic complex-oxidic nature, certain delafossite
compounds (e.g., PdCoO2, PtCoO2) are the most conductive oxides known, for
reasons that remain poorly understood. Their room-temperature conductivity can
exceed that of Au, while their low-temperature electronic mean-free-paths reach
an astonishing 20 microns. It is widely accepted that these materials must be
ultrapure to achieve this, although the methods for their growth (which produce
only small crystals) are not typically capable of such. Here, we first report a
new approach to PdCoO2 crystal growth, using chemical vapor transport methods
to achieve order-of-magnitude gains in size, the highest structural qualities
yet reported, and record residual resistivity ratios (>440). Nevertheless, the
first detailed mass spectrometry measurements on these materials reveal that
they are not ultrapure, typically harboring 100s-of-parts-per-million impurity
levels. Through quantitative crystal-chemical analyses, we resolve this
apparent dichotomy, showing that the vast majority of impurities are forced to
reside in the Co-O octahedral layers, leaving the conductive Pd sheets highly
pure (~1 ppm impurity concentrations). These purities are shown to be in
quantitative agreement with measured residual resistivities. We thus conclude
that a previously unconsidered "sublattice purification" mechanism is essential
to the ultrahigh low-temperature conductivity and mean-free-path of metallic
delafossites
AMI Large Array radio continuum observations of Spitzer c2d small clouds and cores
We perform deep 1.8 cm radio continuum imaging towards thirteen protostellar
regions selected from the Spitzer c2d small clouds and cores programme at high
resolution (25") in order to detect and quantify the cm-wave emission from
deeply embedded young protostars. Within these regions we detect fifteen
compact radio sources which we identify as radio protostars including two
probable new detections. The sample is in general of low bolometric luminosity
and contains several of the newly detected VeLLO sources. We determine the 1.8
cm radio luminosity to bolometric luminosity correlation, L_rad -L_bol, for the
sample and discuss the nature of the radio emission in terms of the available
sources of ionized gas. We also investigate the L_rad-L_IR correlation and
suggest that radio flux density may be used as a proxy for the internal
luminosity of low luminosity protostars.Comment: submitted MNRA
AMI-LA radio continuum observations of Spitzer c2d small clouds and cores: Perseus region
We present deep radio continuum observations of the cores identified as
deeply embedded young stellar objects in the Perseus molecular cloud by the
Spitzer c2d programme at a wavelength of 1.8 cm with the Arcminute Microkelvin
Imager Large Array (AMI-LA). We detect 72% of Class 0 objects from this sample
and 31% of Class I objects. No starless cores are detected. We use the flux
densities measured from these data to improve constraints on the correlations
between radio luminosity and bolometric luminosity, infrared luminosity and,
where measured, outflow force. We discuss the differing behaviour of these
objects as a function of protostellar class and investigate the differences in
radio emission as a function of core mass. Two of four possible very low
luminosity objects (VeLLOs) are detected at 1.8 cm.Comment: 18 pages, 9 figures, accepted MNRA
Lectin-like bacteriocins from pseudomonas spp. utilise D-rhamnose containing lipopolysaccharide as a cellular receptor
Lectin-like bacteriocins consist of tandem monocot mannose-binding domains and display a genus-specific killing activity. Here we show that pyocin L1, a novel member of this family from Pseudomonas aeruginosa, targets susceptible strains of this species through recognition of the common polysaccharide antigen (CPA) of P. aeruginosa lipopolysaccharide that is predominantly a homopolymer of d-rhamnose. Structural and biophysical analyses show that recognition of CPA occurs through the C-terminal carbohydrate-binding domain of pyocin L1 and that this interaction is a prerequisite for bactericidal activity. Further to this, we show that the previously described lectin-like bacteriocin putidacin L1 shows a similar carbohydrate-binding specificity, indicating that oligosaccharides containing d-rhamnose and not d-mannose, as was previously thought, are the physiologically relevant ligands for this group of bacteriocins. The widespread inclusion of d-rhamnose in the lipopolysaccharide of members of the genus Pseudomonas explains the unusual genus-specific activity of the lectin-like bacteriocins
Gaia Data Release 2: All-sky classification of high-amplitude pulsating stars
Out of the 1.69 billion sources in the Gaia Data Release 2 (DR2), more than half a million are published with photometric time series that exhibit light variations during 22 months of observation. An all-sky classification of common high-amplitude pulsators (Cepheids, long-period variables, Delta Scuti / SX Phoenicis, and RR Lyrae stars) is provided for stars with brightness variations greater than 0.1 mag in the G band. A semi-supervised classification approach was employed, firstly training multi-stage Random Forest classifiers with sources of known types in the literature, followed by a preliminary classification of the Gaia data and a second training phase that included a selection of the first classification results to improve the representation of some classes, before the application of the improved classifiers to the Gaia data. Dedicated validation classifiers were used to reduce the level of contamination in the published results. A relevant fraction of objects were not yet sufficiently sampled for reliable Fourier series decomposition, so classifiers were based on features derived from statistics of photometric time series in the G, BP, and RP bands, as well as from some astrometric parameters. The published classification results include 195,780 RR Lyrae stars, 150,757 long-period variables, 8550 Cepheids, and 8882 Delta Scuti / SX Phoenicis stars. All of these results represent candidates, whose completeness and contamination are described as a function of variability type and classification reliability. Results are expressed in terms of class labels and classification scores, which are available in the vari_classifier_result table of the Gaia archive
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