266 research outputs found
Interannual variability of Alexandrium fundyense abundance and shellfish toxicity in the Gulf of Maine
Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B. V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 52 (2005): 2843-2855, doi:10.1016/j.dsr2.2005.06.020.Six years of oceanographic surveys of Alexandrium fundyense concentrations in the Gulf of Maine are combined with shellfish toxicity records from coastal monitoring stations to assess covariations of these quantities on seasonal to interannual time scales. Annual mean gulf-wide cell abundance varies by less than one order of magnitude during the time interval examined (1993-2002). Fluctuations in gulf-wide annual mean cell abundance and shellfish toxicity are not related in a consistent manner. This suggests that interannual variations in toxicity may be regulated by transport and delivery of offshore cell populations, rather than the absolute abundance of the source populations themselves.We gratefully acknowledge the support of the US ECOHAB Program, sponsored by NOAA, NSF, EPA, NASA, and ONR
1.9µm operation of a Tm:Lead germanate glass waveguide laser
We report what we believe to be the first planar-technology waveguide laser in the 2-µm region. Laser operation of the 3H4 to 3H6 transition of Tm3+ ions in a lead germanate glass host has been observed in an ion-implanted planar waveguide
The M Theory Five-Brane and the Heterotic String
Brane actions with chiral bosons present special challenges. Recent progress
in the description of the two main examples -- the M theory five-brane and the
heterotic string -- is described. Also, double dimensional reduction of the M
theory five-brane on K3 is shown to give the heterotic string.Comment: 13 pages, latex, no figures; ICTP Conference Proceeding
A Brane Teaser
In this note we study the puzzle posed by two M5-branes intersecting on a
string (or equivalently, a single M5-brane wrapping a holomorphic four-cycle in
C^4). It has been known for a while that this system is different from all
other configurations built using self-intersecting M-branes; in particular the
corresponding supergravity solution exhibits various curious features which
have remained unexplained. We propose that the resolution to these puzzles lies
in the existence of a non-zero two-form on the M5-brane world-volume.Comment: 21 pages. References adde
That's a wrap!
Calibration technology provides us with a fast and elegant way to find the
supergravity solutions for BPS wrapped M-branes. Its true potential had however
remained untapped due to the absence of a classification of calibrations in
spacetimes with non-trivial flux. The applications of this method were thus
limited in practise to M-branes wrapping Kahler calibrated cycles. In this
paper, we catagorize a type of generalised calibrations which exist in
supergravity backgrounds and contain Kahler calibrations as a sub-class. This
broadens the arena of brane configurations whose supergravity solutions are
accessible through the calibration 'short-cut' method.Comment: 19 pages, typos correcte
M2-branes wrapped on holomorphic curves
The generalised calibration for a wrapped membrane is gauge equivalent to the
supergravity three-form under which the membrane is electrically charged. Given
the relevant calibration, one can go a long way towards constructing the
supergravity solution for the wrapped brane. Applications of this method have
been restricted since generalised calibrations have not yet been completely
classified in spacetimes with non-vanishing flux. In this paper, we take a
first step towards such a classification by studying membranes wrapping
holomorphic curves. Supersymmetry preservation imposes a constraint on the
Hermitean metric in the embedding space and it is found that this can be
expressed as a restriction on possible generalised calibrations. Allowed
calibrations in a particular spacetime are simply those which satisfy the
constraint equation relevant to that background; in particular, we see that the
previously considered Kahler calibrations are just a subclass of possible
solutions.Comment: Discussion clarified, typos corrected, references updated. Results
remain unchanged. 12 page
Comments on Noncommutative Sigma Models
We review the derivation of a noncommutative version of the nonlinear sigma
model on \CPn and it's soliton solutions for finite emphasizing the
similarities it bears to the GMS scalar field theory. It is also shown that
unlike the scalar theory, some care needs to be taken in defining the
topological charge of BPS solitons of the theory due to nonvanishing surface
terms in the energy functional. Finally it is shown that, like its commutative
analogue, the noncommutative \CPn-model also exhibits a non-BPS sector.
Unlike the commutative case however, there are some surprises in the
noncommutative case that merit further study.Comment: 22 pages, 4 figures, LaTeX (JHEP3), Minor changes, Discussion
expanded and references adde
A diode-laser-pumped, Er<sup>3+</sup>-doped fibre laser operating at 1.57µm
Erbium-doped fibers show promise as stable narrow-linewidth laser sources and optical amplifiers operating in the third telecommunications window around 1.55µm Diode laser pumping is possible using the weak pump-band located at 807nm, although this band suffers severely from excited-state absorption (ESA), which reduces gain and pump efficiency. At the low pump power available from diode lasers the Er3+ behaves as a quasi four-level system. However, by codoping the core glass heavily with Yb3+, it is possible to excite the Er3+ ions indirectly using energy transfer. The Yb3+ provides an intense broad pump-band centered at 900nm, which reduces the effect of ESA. As a consequence of the resulting higher pump efficiency, the laser will oscillate at the required shorter wavelengths
Georges Bank : a leaky incubator of Alexandrium fundyense blooms
Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 103 (2014): 163-173, doi:10.1016/j.dsr2.2012.11.002.A series of oceanographic surveys on Georges Bank document variability of populations of the toxic dinoflagellate Alexandrium fundyense on time scales ranging from synoptic to seasonal to interannual. Blooms of A. fundyense on Georges Bank can reach concentrations on the order of 104 cells l-1, and are generally bank-wide in extent. Georges Bank populations of A. fundyense appear to be quasi-independent of those in the adjacent coastal Gulf of Maine, insofar as they occupy a hydrographic niche that is colder and saltier than their coastal counterparts. In contrast to coastal populations that rely on abundant resting cysts for bloom initiation, very few cysts are present in the sediments on Georges Bank. Bloom dynamics must therefore be largely controlled by the balance between growth and mortality processes, which are at present largely unknown for this population. Based on correlations between cell abundance and nutrient distributions, ammonium appears to be an important source of nitrogen for A. fundyense blooms on Georges Bank.We appreciate financial support of the National Oceanic Atmospheric Administration (grant NA06NOS4780245 for the Gulf of Maine Toxicity (GOMTOX) program) and the Woods Hole Center for Oceans and Human Health through National Science Foundation grants OCE-0430724 and OCE-0911031 and National Institute of Environmental Health Sciences grant 1P50-ES01274201
Negative Energy Density in Calabi-Yau Compactifications
We show that a large class of supersymmetric compactifications, including all
simply connected Calabi-Yau and G_2 manifolds, have classical configurations
with negative energy density as seen from four dimensions. In fact, the energy
density can be arbitrarily negative -- it is unbounded from below.
Nevertheless, positive energy theorems show that the total ADM energy remains
positive. Physical consequences of the negative energy density include new
thermal instabilities, and possible violations of cosmic censorship.Comment: 25 pages, v2: few clarifying comments and reference adde
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