3,426 research outputs found
Morphological and compositional evidence for biotic precipitation of marine barite
Barite formation in the surface oceans is generally assumed to be dominated by abiotic precipitation. Acceptance of this pathway is largely the result of the absence of a pelagic marine organism known to precipitate the ovoid to rounded-rectangular barite crystals typically observed in marine waters and sediments. Barite crystals observed in net-tow particles and on substrates retrieved from the seafloor (both in the central North Pacific) were examined by scanning electron microscopy and energy dispersive X-ray spectrometry. Three distinct crystalline forms of barite were observed: ovoid and hexagonal crystals several microns in diameter, and aggregates of submicron-sized crystals. Ovoid and hexagonal-type crystals contained between 0 and 26 mole percent SrSO4. The microcrystalline barite contained no detectable Sr (\u3c0.05 percent). Hexagonal-type crystals were precipitated by an unusual benthic foraminifera. Comparison of the morphology and composition of the barite crystals observed in this study to crystals precipitated by a variety of biotic and abiotic processes suggests a biotic origin for the ovoid barite crystals, the most common form of barite observed in this region
Vertex-Coloring with Star-Defects
Defective coloring is a variant of traditional vertex-coloring, according to
which adjacent vertices are allowed to have the same color, as long as the
monochromatic components induced by the corresponding edges have a certain
structure. Due to its important applications, as for example in the
bipartisation of graphs, this type of coloring has been extensively studied,
mainly with respect to the size, degree, and acyclicity of the monochromatic
components.
In this paper we focus on defective colorings in which the monochromatic
components are acyclic and have small diameter, namely, they form stars. For
outerplanar graphs, we give a linear-time algorithm to decide if such a
defective coloring exists with two colors and, in the positive case, to
construct one. Also, we prove that an outerpath (i.e., an outerplanar graph
whose weak-dual is a path) always admits such a two-coloring. Finally, we
present NP-completeness results for non-planar and planar graphs of bounded
degree for the cases of two and three colors
Swift follow-up of IceCube triggers, and implications for the Advanced-LIGO era
Between 2011 March and 2014 August Swift responded to 20 triggers from the
IceCube neutrino observatory, observing the IceCube 50% confidence error circle
in X-rays, typically within 5 hours of the trigger. No confirmed counterpart
has been detected. We describe the Swift follow up strategy and data analysis
and present the results of the campaign. We discuss the challenges of
distinguishing the X-ray counterpart to a neutrino trigger from serendipitous
uncatalogued X-ray sources in the error circle, and consider the implications
of our results for future strategies for multi-messenger astronomy, with
particular reference to the follow up of gravitational wave triggers from the
advanced-era detectors.Comment: Accepted for publication in MNRAS. 18 pages, including 8 figures and
4 tables; two of which are landscape-oriente
PPl 15: The First Brown Dwarf Spectroscopic Binary
PPl 15 is the first object to have been confirmed as a brown dwarf by the
lithium test (in 1995), though its inferred mass was very close to the
substellar limit. It is a member of the Pleiades open cluster. Its position in
a cluster color-magnitude diagram suggested that it might be binary, and
preliminary indications that it is a double-lined spectroscopic binary were
reported by us in 1997. Here we report on the results of a consecutive week of
Keck HIRES observations of this system, which yield its orbit. It has a period
of about 5.8 days, and an eccentricity of 0.4+/-0.05. The rotation of the stars
is slow for this class of objects. Because the system luminosity is divided
between 2 objects with a mass ratio of 0.85, this renders each of them an
incontrovertible brown dwarf, with masses between 60-70 jupiters. We show that
component B is a little redder than A by studying their wavelength-dependent
line ratios, and that this variation is compatible with the mass ratio. We
confirm that the system has lithium, but cannot support the original conclusion
that it is depleted (which would be surprising, given the new masses). This is
a system of very close objects which, if they had combined, would have produced
a low mass star. We discuss the implications of this discovery for the theories
of binary formation and formation of very low mass objects.Comment: Latex, 18 pages, 4 figures, submitted to Astron.
The Physical Conditions and Dynamics of the Interstellar Medium in the Nucleus of M83: Observations of CO and CI
This paper presents CI, CO J=4-3, and CO J=3-2 maps of the barred spiral
galaxy M83 taken at the James Clerk Maxwell Telescope. Observations indicate a
double peaked structure which is consistent with gas inflow along the bar
collecting at the inner Lindblad resonance. This structure suggests that
nuclear starbursts can occur even in galaxies where this inflow/collection
occurs, in contrast to previous studies of barred spiral galaxies. However, the
observations also suggest that the double peaked emission may be the result of
a rotating molecular ring oriented nearly perpendicular to the main disk of the
galaxy. The CO J=4-3 data indicate the presence of warm gas in the nucleus that
is not apparent in the lower-J CO observations, which suggests that CO J=1-0
emission may not be a reliable tracer of molecular gas in starburst galaxies.
The twelve CI/CO J=4-3 line ratios in the inner 24'' x 24'' are uniform at the
2 sigma level, which indicates that the CO J=4-3 emission is originating in the
same hot photon-dominated regions as the CI emission. The CO J=4-3/J=3-2 line
ratios vary significantly within the nucleus with the higher line ratios
occurring away from peaks of emission along an arc of active star forming
regions. These high line ratios (>1) likely indicate optically thin gas created
by the high temperatures caused by star forming regions in the nucleus of this
starburst galaxy.Comment: 15 pages with 10 figures. To appear in the August 10 1998 issue of
The Astrophysical Journa
Hsp90 governs dispersion and drug resistance of fungal biofilms
Fungal biofilms are a major cause of human mortality and are recalcitrant to most treatments due to intrinsic drug resistance. These complex communities of multiple cell types form on indwelling medical devices and their eradication often requires surgical removal of infected devices. Here we implicate the molecular chaperone Hsp90 as a key regulator of biofilm dispersion and drug resistance. We previously established that in the leading human fungal pathogen, Candida albicans, Hsp90 enables the emergence and maintenance of drug resistance in planktonic conditions by stabilizing the protein phosphatase calcineurin and MAPK Mkc1. Hsp90 also regulates temperature-dependent C. albicans morphogenesis through repression of cAMP-PKA signalling. Here we demonstrate that genetic depletion of Hsp90 reduced C. albicans biofilm growth and maturation in vitro and impaired dispersal of biofilm cells. Further, compromising Hsp90 function in vitro abrogated resistance of C. albicans biofilms to the most widely deployed class of antifungal drugs, the azoles. Depletion of Hsp90 led to reduction of calcineurin and Mkc1 in planktonic but not biofilm conditions, suggesting that Hsp90 regulates drug resistance through different mechanisms in these distinct cellular states. Reduction of Hsp90 levels led to a marked decrease in matrix glucan levels, providing a compelling mechanism through which Hsp90 might regulate biofilm azole resistance. Impairment of Hsp90 function genetically or pharmacologically transformed fluconazole from ineffectual to highly effective in eradicating biofilms in a rat venous catheter infection model. Finally, inhibition of Hsp90 reduced resistance of biofilms of the most lethal mould, Aspergillus fumigatus, to the newest class of antifungals to reach the clinic, the echinocandins. Thus, we establish a novel mechanism regulating biofilm drug resistance and dispersion and that targeting Hsp90 provides a much-needed strategy for improving clinical outcome in the treatment of biofilm infections
Organic biogeochemistry in West Mata, NE Kau hydrothermal vent fields
Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 22(4), (2021): e2020GC009481, https://doi.org/10.1029/2020GC009481.The impact of submarine hydrothermal systems on organic carbon in the ocean—one of the largest fixed carbon reservoirs on Earth—could be profound. Yet, different vent sites show diverse fluid chemical compositions and the subsequent biological responses. Observations from various vent sites are to evaluate hydrothermal systems' impact on the ocean carbon cycle. A response cruise in May 2009 to an on-going submarine eruption at West Mata Volcano, northeast Lau Basin, provided an opportunity to quantify the organic matter production in a back-arc spreading hydrothermal system. Hydrothermal vent fluids contained elevated dissolved organic carbon, particulate organic carbon (POC), and particulate nitrogen (PN) relative to background seawater. The δ13C-POC values for suspended particles in the diffuse vent fluids (−15.5‰ and −12.3‰) are distinct from those in background seawater (−23 ± 1‰), indicative of unique carbon synthesis pathways of the vent microbes from the seawater counterparts. The first dissolved organic nitrogen concentrations reported for diffuse vents were similar to or higher than those for background seawater. Enhanced nitrogen fixation and denitrification removed 37%–89% of the total dissolved nitrogen in the recharging background seawater in the hydrothermal vent flow paths. The hydrothermal plume samples were enriched in POC and PN, indicating enhanced biological production. The total “dark” organic carbon production within the plume matches the thermodynamic prediction based on available reducing chemical substances supplied to the plume. This research combines the measured organic carbon contents with thermodynamic modeled results and demonstrates the importance of hydrothermal activities on the water column carbon production in the deep ocean.This project was supported by N.S.F. (OCE0929881, J. P. Cowen and K. H. Rubin), the NOAA PMEL VENTS (now Earth-Ocean Interactions) Program and the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement No. NA10OAR4320148, and the UH NASA Astrobiology Institute. The Ministry of Science and Technology of Taiwan award (MOST 107-2611-M-002-002, and MOST 108-2611-M-002-006 to H.-T. Lin). Ministry of Education (M.O.E.) Republic of China (Taiwan) 109L892601 to H.-T. Lin. SOEST contributions no. 11285, C-DEBI contribution no. 563. PMEL contribution no. 3996, JISAO contribution 2183
Planning the Future of U.S. Particle Physics (Snowmass 2013): Chapter 4: Cosmic Frontier
These reports present the results of the 2013 Community Summer Study of the
APS Division of Particles and Fields ("Snowmass 2013") on the future program of
particle physics in the U.S. Chapter 4, on the Cosmic Frontier, discusses the
program of research relevant to cosmology and the early universe. This area
includes the study of dark matter and the search for its particle nature, the
study of dark energy and inflation, and cosmic probes of fundamental
symmetries.Comment: 61 page
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