3,943 research outputs found
A NICER Discovery of a Low-Frequency Quasi-Periodic Oscillation in the Soft-Intermediate State of MAXI J1535-571
We present the discovery of a low-frequency Hz quasi-periodic
oscillation (QPO) feature in observations of the black hole X-ray binary MAXI
J1535-571 in its soft-intermediate state, obtained in September-October 2017 by
the Neutron Star Interior Composition Explorer (NICER). The feature is
relatively broad (compared to other low-frequency QPOs; quality factor
) and weak (1.9% rms in 3-10 keV), and is accompanied by a weak
harmonic and low-amplitude broadband noise. These characteristics identify it
as a weak Type A/B QPO, similar to ones previously identified in the
soft-intermediate state of the transient black hole X-ray binary XTE J1550-564.
The lag-energy spectrum of the QPO shows increasing soft lags towards lower
energies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum).
This large phase shift has similar amplitude but opposite sign to that seen in
Rossi X-ray Timing Explorer data for a Type B QPO from the transient black hole
X-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of the
Type B QPO in GX 339-4 pointed towards a precessing jet-like corona
illuminating the accretion disk as the origin of the QPO signal. We suggest
that this QPO in MAXI J1535-571 may have the same origin, with the different
lag sign depending on the scale height of the emitting region and the observer
inclination angle.Comment: Accepted for publication in ApJ Letter
Macro-Climatic Distribution Limits Show Both Niche Expansion and Niche Specialization among C4 Panicoids
Grasses are ancestrally tropical understory species whose current dominance in warm open habitats is linked to the evolution of C4 photosynthesis. C4 grasses maintain high rates of photosynthesis in warm and water stressed environments, and the syndrome is considered to induce niche shifts into these habitats while adaptation to cold ones may be compromised. Global biogeographic analyses of C4 grasses have, however, concentrated on diversity patterns, while paying little attention to distributional limits. Using phylogenetic contrast analyses, we compared macro-climatic distribution limits among ~1300 grasses from the subfamily Panicoideae, which includes 4/5 of the known photosynthetic transitions in grasses. We explored whether evolution of C4 photosynthesis correlates with niche expansions, niche changes, or stasis at subfamily level and within the two tribes Paniceae and Paspaleae. We compared the climatic extremes of growing season temperatures, aridity, and mean temperatures of the coldest months. We found support for all the known biogeographic distribution patterns of C4 species, these patterns were, however, formed both by niche expansion and niche changes. The only ubiquitous response to a change in the photosynthetic pathway within Panicoideae was a niche expansion of the C4 species into regions with higher growing season temperatures, but without a withdrawal from the inherited climate niche. Other patterns varied among the tribes, as macro-climatic niche evolution in the American tribe Paspaleae differed from the pattern supported in the globally distributed tribe Paniceae and at family level.Fil: Aagesen, Lone. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Instituto de Botánica Darwinion. Academia Nacional de Ciencias Exactas, FÃsicas y Naturales. Instituto de Botánica Darwinion; ArgentinaFil: Biganzoli, Fernando. Universidad de Buenos Aires. Facultad de AgronomÃa. Departamento de Métodos Cuantitativos y Sistemas de Información; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Bena, MarÃa Julia. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Instituto de Botánica Darwinion. Academia Nacional de Ciencias Exactas, FÃsicas y Naturales. Instituto de Botánica Darwinion; ArgentinaFil: Godoy Bürki, Ana Carolina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Instituto de Botánica Darwinion. Academia Nacional de Ciencias Exactas, FÃsicas y Naturales. Instituto de Botánica Darwinion; ArgentinaFil: Reinheimer, Renata. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Santa Fe. Instituto de AgrobiotecnologÃa del Litoral. Universidad Nacional del Litoral. Instituto de AgrobiotecnologÃa del Litoral; ArgentinaFil: Zuloaga, Fernando Omar. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Instituto de Botánica Darwinion. Academia Nacional de Ciencias Exactas, FÃsicas y Naturales. Instituto de Botánica Darwinion; Argentin
Coastal Lagoons and Climate Change: Ecological and Social Ramifications in the U.S. Atlantic and Gulf Coast Ecosystems
Lagoons are highly productive coastal features that provide a range of natural services that society values. Their setting within the coastal landscape leaves them especially vulnerable to profound physical, ecological, and associated societal disturbance from global climate change. Expected shifts in physical and ecological characteristics range from changes in flushing regime, freshwater inputs, and water chemistry to complete inundation and loss and the concomitant loss of natural and human communities. Therefore, managing coastal lagoons in the context of global climate change is critical. Although management approaches will vary depending on local conditions and cultural norms, all management scenarios will need to be nimble and to make full use of the spectrum of values through which society views these unique ecosystems. We propose that this spectrum includes pragmatic, scholarly, aesthetic, and tacit categories of value. Pragmatic values such as fishery or tourism revenue are most easily quantified and are therefore more likely to be considered in management strategies. In contrast, tacit values such as a sense of place are more difficult to quantify and therefore more likely to be left out of explicit management justifications. However, tacit values are the most influential to stakeholder involvement because they both derive from and shape individual experiences and beliefs. Tacit values underpin all categories of social values that we describe and can be expected to have a strong influence over human behavior. The articulation and inclusion of the full spectrum of values, especially tacit values, will facilitate and support nimble adaptive management of coastal lagoon ecosystems in the context of global climate change
Mineralization of ancient carbon in the subsurface of riparian forests
Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): G02021, doi:10.1029/2007JG000482.Microbial activity in saturated, subsurface sediments in riparian forests may be supported by recent photosynthate or ancient (>500 ybp) soil organic carbon (SOC) in buried horizons. Metabolism of ancient SOC may be particularly important in riparian zones, considered denitrification hot spots, because denitrification in the riparian subsurface is often C-limited, because buried horizons intersect deep flow paths, and because low C mineralization rates can support ecosystem-relevant rates of denitrification. Buried horizons are common where alluvial processes (stream migration, overbank flow) have dominated riparian evolution. Our objectives were to determine: (1) the extent to which ancient SOC directly supports subsurface microbial activity; (2) whether different C sources support microbial activity in alluvial versus glaciofluvial riparian zones; and (3) how microbial use of ancient SOC varies with depth. In situ groundwater incubations and 14C dating of dissolved inorganic carbon revealed that ancient SOC mineralization was common, and that it constituted 31–100% of C mineralization 2.6 m deep at one site, at rates sufficient to influence landscape N budgets. Our data failed to reveal consistent spatial patterns of microbially available ancient C. Although mineralized C age increased with depth at one alluvial site, we observed ancient C metabolism 150 cm deep at a glaciofluvial site, suggesting that subsurface microbial activity in riparian zones does not vary systematically between alluvial and glaciofluvial hydrogeologic settings. These findings underscore the relevance of ancient C to contemporary ecosystem processes and the challenge of using mappable surface features to identify subsurface ecosystem characteristics or riparian zone N-sink strength.We are grateful to the Cornell Program in
Biogeochemistry for graduate research grants and to the U.S. EPA for a
STAR Graduate Fellowship to Noel Gurwick. Support for radiocarbon
analyses also came from USDANRICGP grant 99–35102– 8266, NSF
cooperative agreement OCE-9807266, and an Andrew W. Mellon Foundation
grant to the Institute of Ecosystem Studies. A graduate research grant to
N. Gurwick from the Theresa Heinz Scholars for Environmental Research
provided salary for Pete Seitz-Rundlett
Effects of Small Island Mobility on Growth in Molecular Beam Epitaxy
The effects of mobility of small islands on island growth in molecular beam
epitaxy are studied. It is shown that small island mobility affects both the
scaling and morphology of islands during growth. Three microscopic models are
considered, in which the critical island sizes are and 3 (such that
islands of size are mobile while islands of size
are immobile). As i^* increases, islands become more compact, while the
exponent which relates the island density to deposition rate
increases. The morphological changes are quantified by using fractal analysis.
It is shown that the fractal dimensions are rather insensitive to changes in
i^*. However, the prefactors provide a quantitative measure of the changing
morphologies.Comment: 25 pages of text, RevTeX, additional 8 figures in 10 PS files.
Replaced version include some minor changes, notation, etc. To be published
in: Phys. Rev B, Vol. 55 (1997
New determination of the 13C(a, n)16O reaction rate and its influence on the s-process nucleosynthesis in AGB stars
We present a new measurement of the -spectroscopic factor
() and the asymptotic normalization coefficient (ANC) for the 6.356
MeV 1/2 subthreshold state of O through the C(B,
Li)O transfer reaction and we determine the -width of this
state. This is believed to have a strong effect on the rate of the
C(, )O reaction, the main neutron source for {\it
slow} neutron captures (the -process) in asymptotic giant branch (AGB)
stars. Based on the new width we derive the astrophysical S-factor and the
stellar rate of the C(, )O reaction. At a temperature
of 100 MK our rate is roughly two times larger than that by \citet{cau88} and
two times smaller than that recommended by the NACRE compilation. We use the
new rate and different rates available in the literature as input in
simulations of AGB stars to study their influence on the abundances of selected
-process elements and isotopic ratios. There are no changes in the final
results using the different rates for the C(, )O
reaction when the C burns completely in radiative conditions. When the
C burns in convective conditions, as in stars of initial mass lower than
2 M_\sun and in post-AGB stars, some changes are to be expected, e.g.,
of up to 25% for Pb in our models. These variations will have to be carefully
analyzed when more accurate stellar mixing models and more precise
observational constraints are available
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