33,004 research outputs found

    Biological activity in the wake of an island close to a coastal upwelling

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    Hydrodynamic forcing plays an important role in shaping the dynamics of marine organisms, in particular of plankton. In this work we study the planktonic biological activity in the wake of an island which is close to an upwelling region. Our research is based on numerical analysis of a kinematic flow mimicking the hydrodynamics in the wake, coupled to a three-component plankton model. Depending on model parameters different phenomena are described: a) The lack of transport of nutrients and plankton across the wake, so that the influence of upwelling on primary production on the other side of the wake is blocked. b) For sufficiently high vorticity, the role of the wake in facilitating this transport and leading to an enhancement of primary production. Finally c) we show that under certain conditions the interplay between wake structures and biological growth leads to plankton blooms inside mesoscale hydrodynamic vortices that act as incubators of primary production.Comment: 42 pages, 9 figure

    Plankton blooms in vortices: The role of biological and hydrodynamic time scales

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    We study the interplay of hydrodynamic mesoscale structures and the growth of plankton in the wake of an island, and its interaction with a coastal upwelling. Our focus is on a mechanism for the emergence of localized plankton blooms in vortices. Using a coupled system of a kinematic flow mimicking the mesoscale structures behind the island and a simple three component model for the marine ecosystem, we show that the long residence times of nutrients and plankton in the vicinity of the island and the confinement of plankton within vortices are key factors for the appearance of localized plankton bloomsComment: 29 pages, 9 figure

    13CO at the centre of M82

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    Using the IRAM interferometer, we have observed the nearby starburst galaxy M82 with a 4.2" resolution (~70 pc) in the 1->0 line of 13CO and in the 2.6-mm continuum. The spatial distribution of the 13CO line shows the same gross features as the 12CO(1->0) map of Shen & Lo (1995), namely two lobes and a compact central source, though with different relative intensities. The lobes are more conspicuous and the central source is fainter in 13CO than in 12CO. The velocity field observed around the nucleus shows a very steep gradient (140 km/s over 75 pc), which is very probably caused by the stellar bar visible in the near infrared. The dynamical centre coincides with the IR peak and is shifted 6" north-east of the compact 13CO source. The two CO lobes appear to be associated with the ends of the bar and not with a molecular ring, as usually assumed. They are probably shaped by the strong UV radiation from the central region. 13CO must be more photodissociated than the self-shielded 12CO molecules in the central ~250 pc region, which may explain the relative weakness of the 13CO central source. A 130 pc-wide bubble of molecular gas has been identified, which happens to host the most luminous compact radio source in M82. It lies 120 pc west of the IR peak between the central source and the western lobe and seems characterized by warmer gas, strong UV radio free-free radiation, and an enhanced cosmic ray production rate.Comment: Accepted by A&A; 9 pages, 9 ps figures, needs LaTeX 2e A&A macro and psfig Styl

    Break-down of the density-of-states description of scanning tunneling spectroscopy in supported metal clusters

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    Low-temperature scanning tunneling spectroscopy allows to probe the electronic properties of clusters at surfaces with unprecedented accuracy. By means of quantum transport theory, using realistic tunneling tips, we obtain conductance curves which considerably deviate from the cluster's density of states. Our study explains the remarkably small number of peaks in the conductance spectra observed in recent experiments. We demonstrate that the unambiguous characterization of the states on the supported clusters can be achieved with energy-resolved images, obtained from a theoretical analysis which mimics the experimental imaging procedure.Comment: 5 pages, 3 figure

    Quantum Estimation Methods for Quantum Illumination

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    Quantum illumination consists in shining quantum light on a target region immersed in a bright thermal bath, with the aim of detecting the presence of a possible low-reflective object. If the signal is entangled with the receiver, then a suitable choice of the measurement offers a gain with respect to the optimal classical protocol employing coherent states. Here, we tackle this detection problem by using quantum estimation techniques to measure the reflectivity parameter of the object, showing an enhancement in the signal-to-noise ratio up to 3 dB with respect to the classical case when implementing only local measurements. Our approach employs the quantum Fisher information to provide an upper bound for the error probability, supplies the concrete estimator saturating the bound, and extends the quantum illumination protocol to non-Gaussian states. As an example, we show how Schrodinger's cat states may be used for quantum illumination.Comment: Published versio

    Optimizing periodicity and polymodality in noise-induced genetic oscillators

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    Many cellular functions are based on the rhythmic organization of biological processes into self-repeating cascades of events. Some of these periodic processes, such as the cell cycles of several species, exhibit conspicuous irregularities in the form of period skippings, which lead to polymodal distributions of cycle lengths. A recently proposed mechanism that accounts for this quantized behavior is the stabilization of a Hopf-unstable state by molecular noise. Here we investigate the effect of varying noise in a model system, namely an excitable activator-repressor genetic circuit, that displays this noise-induced stabilization effect. Our results show that an optimal noise level enhances the regularity (coherence) of the cycles, in a form of coherence resonance. Similar noise levels also optimize the multimodal nature of the cycle lengths. Together, these results illustrate how molecular noise within a minimal gene regulatory motif confers robust generation of polymodal patterns of periodicity.Comment: 9 pages, 6 figure
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