ASSESSING THE LOUISIANA SHRIMP FISHING FLEET TECHNICAL EFFICIENCY USING A BAYESIAN STOCHASTIC COST FRONTIER MODEL

A Bayesian stochastic cost frontier analyzed the shrimp fleet of Louisiana. A translog cost function was estimated. 269 vessels were included and sub-grouped by length (<20 ft, 21-40ft, and >60ft), and net type (trawl, skimmer, and butterfly). Results indicated no influence of these factors on cost efficiency.Resource /Energy Economics and Policy,

SGR 0418+5729, Swift J1822.3-1606, and 1E 2259+586 as massive fast rotating highly magnetized white dwarfs

Following Malheiro et al. (2012) we describe the so-called low magnetic field magnetars, SGR 0418+5729, Swift J1822.3--1606, as well as the AXP prototype 1E 2259+586 as massive fast rotating highly magnetized white dwarfs. We give bounds for the mass, radius, moment of inertia, and magnetic field for these sources by requesting the stability of realistic general relativistic uniformly rotating configurations. Based on these parameters, we improve the theoretical prediction of the lower limit of the spindown rate of SGR 0418+5729; for a white dwarf close to its maximum stable we obtain the very stringent interval for the spindown rate of 4.1E-16< dP/dt < 6E-15, where the upper value is the known observational limit. A lower limit has been also set for Swift J1822.3-1606 for which a fully observationally accepted spin-down rate is still lacking. The white dwarf model provides for this source dP/dt> 2.13E-15, if the star is close to its maximum stable mass. We also present the theoretical expectation of the infrared, optical and ultraviolet emission of these objects and show their consistency with the current available observational data. We give in addition the frequencies at which absorption features could be present in the spectrum of these sources as the result of the scattering of photons with the quantized electrons by the surface magnetic field.Comment: to appear in Astronomy & Astrophysic

The Dynamics of Galaxy Pairs in a Cosmological Setting

We use the Millennium Simulation, and an abundance-matching framework, to investigate the dynamical behaviour of galaxy pairs embedded in a cosmological context. Our main galaxy-pair sample, selected to have separations under 250 kpc/h, consists of over 1.3 million pairs at redshift z = 0, with stellar masses greater than 10^9 Msun, probing mass ratios down to 1:1000. We use dark matter halo membership and energy to classify our galaxy pairs. In terms of halo membership, central-satellite pairs tend to be in isolation (in relation to external more massive galaxies), are energetically- bound to each other, and are also weakly-bound to a neighbouring massive galaxy. Satellite-satellite pairs, instead, inhabit regions in close proximity to a more massive galaxy, are energetically-unbound, and are often bound to that neighbour. We find that 60% of our paired galaxies are bound to both their companion and to a third external object. Moreover, only 9% of our pairs resemble the kind of systems described by idealised binary merger simulations in complete isolation. In sum, we demonstrate the importance of properly connecting galaxy pairs to the rest of the Universe.Comment: 25 pages, 14 figures, accepted by MNRA

Position-Velocity Diagrams for the Maser Emission coming from a Keplerian Ring

We have studied the maser emission from a thin, planar, gaseous ring in Keplerian rotation around a central mass observed edge-on. The absorption coefficient within the ring is assumed to follow a power law dependence with the distance from the central mass as, k=k0r^{-q}. We have calculated position-velocity diagrams for the most intense maser features, for different values of the exponent q. We have found that, depending on the value of q, these diagrams can be qualitatively different. The most intense maser emission at a given velocity can either come mainly from regions close to the inner or outer edges of the amplifying ring or from the line perpendicular to the line of sight and passing through the central mass (as is commonly assumed). Particularly, when q>1 the position-velocity diagram is qualitatively similar to the one observed for the water maser emission in the nucleus of the galaxy NGC 4258. In the context of this simple model, we conclude that in this object the absorption coefficient depends on the radius of the amplifying ring as a decreasing function, in order to have significant emission coming from the inner edge of the ring.Comment: 13 pages, 7 figures, to appear in the 2007 July 20 issue of The Astrophysical Journa

Canonical quantum gravity in the Vassiliev invariants arena: I. Kinematical structure

We generalize the idea of Vassiliev invariants to the spin network context, with the aim of using these invariants as a kinematical arena for a canonical quantization of gravity. This paper presents a detailed construction of these invariants (both ambient and regular isotopic) requiring a significant elaboration based on the use of Chern-Simons perturbation theory which extends the work of Kauffman, Martin and Witten to four-valent networks. We show that this space of knot invariants has the crucial property -from the point of view of the quantization of gravity- of being loop differentiable in the sense of distributions. This allows the definition of diffeomorphism and Hamiltonian constraints. We show that the invariants are annihilated by the diffeomorphism constraint. In a companion paper we elaborate on the definition of a Hamiltonian constraint, discuss the constraint algebra, and show that the construction leads to a consistent theory of canonical quantum gravity.Comment: 21 Pages, RevTex, many figures included with psfi

Mapping galaxy encounters in numerical simulations: The spatial extent of induced star formation

We employ a suite of 75 simulations of galaxies in idealised major mergers (stellar mass ratio ~2.5:1), with a wide range of orbital parameters, to investigate the spatial extent of interaction-induced star formation. Although the total star formation in galaxy encounters is generally elevated relative to isolated galaxies, we find that this elevation is a combination of intense enhancements within the central kpc and moderately suppressed activity at large galacto-centric radii. The radial dependence of the star formation enhancement is stronger in the less massive galaxy than in the primary, and is also more pronounced in mergers of more closely aligned disc spin orientations. Conversely, these trends are almost entirely independent of the encounter's impact parameter and orbital eccentricity. Our predictions of the radial dependence of triggered star formation, and specifically the suppression of star formation beyond kph-scales, will be testable with the next generation of integral-field spectroscopic surveys.Comment: 12 pages, 8 figures, accepted by MNRA