25,663 research outputs found

    4D Tropospheric Tomography using GPS Estimated Slant Delays

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    Tomographic techniques are successfully applied to obtain 4D images of the tropospheric refractivity in a local dense network. In the lower atmosphere both the small height and time scales and the non-dispersive nature of tropospheric delays require a more careful analysis of the data. We show how GPS data is processed to obtain the tropospheric slant delays using the GIPSY-OASIS II software and define the concept of pseudo-wet delays, which will be the observables in the tomographic software. We then discuss the inverse problem in the 3D stochastic tomography, using simulated refractivity fields to test the system and the impact of noise. Finally, we use data from the Kilauea network in Hawaii and a local 4x4x41-voxel grid on a region of 400 Km2^2 and 15 Km in height to produce 4D refractivity fields. Results are compared with ECMWF forecast.Comment: 9 pages, 6 figures (2 color

    Discriminating hadronic and quark stars through gravitational waves of fluid pulsation modes

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    We investigate non-radial oscillations of hadronic, hybrid and pure self-bound strange quark stars with maximum masses above the mass of the recently observed massive pulsars PSR J1614-2230 and PSR J0348-0432 with M≈2M⊙M \approx 2 M_{\odot}. For the hadronic equation of state we employ different parametrizations of a relativistic mean-field model and for quark matter we use the MIT bag model including the effect of strong interactions and color superconductivity. We find that the first pressure mode for strange quark stars has a very different shape than for hadronic and hybrid stars. For strange quarks stars the frequency of the p1 mode is larger than 6 kHz and diverge at small stellar masses, but for hadronic and hybrid stars it is in the range 4-6 kHz. This allows an observational identification of strange stars even if extra information such as the mass, the radius or the gravitational redshift of the object is unavailable or uncertain. Also, we find as in previous works that the frequency of the g-mode associated with the quark-hadron discontinuity in a hybrid star is in the range 0.4-1 kHz for all masses. Thus, compact objects emitting gravitational waves above 6 kHz should be interpreted as strange quark stars and those emitting a signal within 0.4-1 kHz should be interpreted as hybrid stars.Comment: 7 pages, 5 figure

    Dressed coordinates: the path-integrals approach

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    The recent introduced \textit{dressed coordinates} are studied in the path-integral approach. These coordinates are defined in the context of a harmonic oscillator linearly coupled to massless scalar field and, it is shown that in this model the dressed coordinates appear as a coordinate transformation preserving the path-integral functional measure. The analysis also generalizes the \textit{sum rules} established in a previous work.Comment: 9 pages, Latex2


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    The objective of this paper is to offer an answer to the question : why do some entrepreneurs wish to own another firm in the future after having closed an unsuccessful one? We first show this question is relevant because making use of a sample of entrepreneurs in the UK who have experienced a business closure, we show that 45% of them have the desire to own another firm in the future, despite having an unsuccessful experience in small firm ownership. To tackle our question we develop a model where the profits of the small firm depend on two firm-specific parameters : the effciency parameter, which represents the skills of the entrepreneur to manage and cope efficiently with the everyday tasks of the small firm, and the demand parameter, which denotes the success of the firm’s product to attract demand or capture a market niche. It is found that our model answers our initial question by revealing the existence of a mechanism of entrepreneurial self-selection. Under such mechanism, skilful entrepreneurs are the only ones who wish to own another firm in the future, regardless of the degree of success in their previous venture, whereas unskilful entrepreneurs prefer to go to wage work. We show this mechanism accounts not only for the empirical evidence relevant to our initial question, but also for the rest of cases of entrepreneurs’ attitudes after experiencing a business closureSmall Firm ; Entrepreneur ; Survival ; Entrepreneurial Skills
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