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The Sørfjord, Norway, and the Gulf of St. Lawrence, Canada, are two sub-arctic ecosystems with similar trophic structure. However, in the Gulf of St. Lawrence, severe exploitation of groundfish stocks has lead to important shifts in the trophic structure. In the Sørfjord, the situation is different: fishing pressure is much lighter. Our hypothesis is that overexploitation leads to changes in the trophic structure and severely alters the resilience of ecosystems. Based on the same modelling approach (Ecopath with Ecosim) the food web structure was compared, using different ecosystem indicators. Patterns of food web structure and trophodynamics were contrasted. Cod was the keystone species in both ecosystems, and forage fish were also important. Even after similar environmental changes in both ecosystems, and after a reduction of fishing pressure in the Gulf of St. Lawrence, there is no recovery of cod stocks in this ecosystem. In the Sørfjord, after different perturbations (but not from the fishery), the ecosystem seems to return to its equilibrium. 相似文献
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D. R. Lorimer P. C. C. Freire I. H. Stairs M. Kramer M. A. McLaughlin M. Burgay S. E. Thorsett R. J. Dewey A. G. Lyne R. N. Manchester N. D'Amico A. Possenti B. C. Joshi 《Monthly notices of the Royal Astronomical Society》2007,379(3):1217-1221
We investigate the age constraints that can be placed on the double pulsar system using models for the spin-down of the first-born 22.7-ms Pulsar A and the 2.77-s Pulsar B with characteristic ages of 210 and 50 Myr, respectively. Standard models assuming dipolar spin-down of both pulsars suggest that the time since the formation of Pulsar B is ∼50 Myr, that is, close to Pulsar B's characteristic age. However, adopting models which account for the impact of Pulsar A's relativistic wind on Pulsar B's spin-down, we find that the formation of Pulsar B took place either 80 or 180 Myr ago, depending on the interaction mechanism. Formation 80 Myr ago, closer to Pulsar B's characteristic age, would result in the contribution from J0737−3039 to the inferred coalescence rates for double neutron star binaries increasing by 40 per cent. The 180 Myr age is closer to Pulsar A's characteristic age and would be consistent with the most recent estimates of the coalescence rate. The new age constraints do not significantly impact recent estimates of the kick velocity, tilt angle between pre- and post-supernova orbital planes or pre-supernova mass of Pulsar B's progenitor. 相似文献
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I.H. Stairs R.N. Manchester A.G. Lyne V.M. Kaspi † F. Camilo J.F. Bell N. D'Amico M. Kramer F. Crawford ‡ D.J. Morris A. Possenti N.P.F. McKay S.L. Lumsden L.E. Tacconi-Garman R.D. Cannon N.C. Hambly P.R. Wood 《Monthly notices of the Royal Astronomical Society》2001,325(3):979-988
We report on the discovery of a binary pulsar, PSR J1740−3052, during the Parkes multibeam survey. Timing observations of the 570-ms pulsar at Jodrell Bank and Parkes show that it is young, with a characteristic age of 350 kyr, and is in a 231-d, highly eccentric orbit with a companion whose mass exceeds 11 M⊙ . An accurate position for the pulsar was obtained using the Australia Telescope Compact Array. Near-infrared 2.2-μm observations made with the telescopes at the Siding Spring observatory reveal a late-type star coincident with the pulsar position. However, we do not believe that this star is the companion of the pulsar, because a typical star of this spectral type and required mass would extend beyond the orbit of the pulsar. Furthermore, the measured advance of periastron of the pulsar suggests a more compact companion, for example, a main-sequence star with radius only a few times that of the Sun. Such a companion is also more consistent with the small dispersion measure variations seen near periastron. Although we cannot conclusively rule out a black hole companion, we believe that the companion is probably an early B star, making the system similar to the binary PSR J0045−7319. 相似文献
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M. Serylak B. W. Stappers P. Weltevrede M. Kramer A. Jessner A. G. Lyne C. A. Jordan K. Lazaridis J. A. Zensus 《Monthly notices of the Royal Astronomical Society》2009,394(1):295-308
We have used the 76-m Lovell, 94-m equivalent Westerbork Synthesis Radio Telescope (WSRT) and 100-m Effelsberg radio telescopes to investigate the simultaneous single-pulse properties of the radio emitting magnetar Anomalous X-ray Pulsar (AXP) XTE J1810−197 at frequencies of 1.4, 4.8 and 8.35 GHz during 2006 May and July. We study the magnetar's pulse-energy distributions which are found to be very peculiar as they are changing on time-scales of days and cannot be fit by a single statistical model. The magnetar exhibits strong spiky single giant-pulse-like subpulses, but they do not fit the definition of the giant pulse or giant micropulse phenomena. Measurements of the longitude-resolved modulation index reveal a high degree of intensity fluctuations on day-to-day time-scales and dramatic changes across pulse phase. We find the frequency evolution of the modulation index values differs significantly from what is observed in normal radio pulsars. We find that no regular drifting subpulse phenomenon is present at any of the observed frequencies at any observing epoch. However, we find a quasi-periodicity of the subpulses present in the majority of the observing sessions. A correlation analysis indicates a relationship between components from different frequencies. We discuss the results of our analysis in light of the emission properties of normal radio pulsars and a recently proposed model which takes radio emission from magnetars into consideration. 相似文献
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One of the most remarkable properties of radio pulsars is their rotational stability which allows many uses as clocks, For
instance they enable us to determine the shapes and sizes of binary orbits, to study general relativistic effects in strong
gravitational fields, to demonstrate the existance of gravitational radiation from binary systems, to permit the detection
of extra solar planets, and also to put limits on the long period gravitational wave background. However, some display timing
imperfections which tell us about the insides of neutron stars. This review describes the basic physics of slowdown and how
period instabilities seem to be related to the rate of slowdown and the presence of internal superfluid liquid. Careful studies
of glitches and the subsequent rotational behaviour of the pulsars can provide valuable information on the internal structure
of neutron stars. 相似文献
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A. G. Lyne F. Camilo R. N. Manchester J. F. Bell V. M. Kaspi N. D'Amico N. P. F. McKay F. Crawford D. J. Morris D. C. Sheppard I. H. Stairs 《Monthly notices of the Royal Astronomical Society》2000,312(4):698-702
We are undertaking a high-frequency survey of the Galactic plane for radio pulsars, using the 13-element multibeam receiver on the 64-m Parkes radio telescope. We describe briefly the survey system and some of the initial results. PSR J1811−1736, one of the first pulsars discovered with this system, has a rotation period of 104 ms. Subsequent timing observations using the 76-m radio telescope at Jodrell Bank show that it is in an 18.8-d, highly eccentric binary orbit. We have measured the rate of advance of periastron which indicates a total system mass of 2.6±0.9 M⊙ , and the minimum companion mass is about 0.7 M⊙ . This, the high orbital eccentricity and the recycled nature of the pulsar suggest that this system is composed of two neutron stars, only the fourth or fifth such system known in the disc of the Galaxy. 相似文献
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