δ³⁰Si and δ²⁹Si Determinations on USGS BHVO-1 and BHVO-2 Reference Materials with a New Configuration on a Nu Plasma Multi-Collector ICP-MS     

Kathrin Abraham  Sophie Opfergelt  François Fripiat  Anne-Julie Cavagna  Jeroen T.M. de Jong  Stephen F. Foley  Luc André  Damien Cardinal 《Geostandards and Geoanalytical Research》2008,32(2):193-202

We report silicon isotopic determinations for USGS rock reference materials BHVO-1 and BHVO-2 using a Nu Plasma multi-collector (MC)-ICP-MS, upgraded with a new adjustable entrance slit, to obtain medium resolution, as well as a stronger primary pump and newly designed sampler and skimmer cones ("B" cones). These settings, combined with the use of collector slits, allowed a resolution to be reached that was sufficient to overcome the ¹⁴N¹⁶O and ¹⁴N₂ interferences overlying the ³⁰Si and the ²⁸Si peaks, respectively, in an earlier set-up. This enabled accurate measurement of both δ³⁰Si and δ²⁹Si. The δ value is expressed in per mil variation relative to the NBS 28 quartz reference material. Based on data acquired from numerous sessions spread over a period of six months, we propose a recommended average δ³⁰Si of −0.33 ± 0.05‰ and −0.29 ± 0.11‰ (2se) for BHVO-1 and BHVO-2, respectively. Our BHVO grand mean silicon isotope composition (δ³⁰Si =−0.31 ± 0.06‰) is significantly more negative than the only published value for BHVO-2, but is in very good agreement with the recently established average value of ocean island basalts (OIB), confirming the conclusion that the OIB reservoir has a distinct isotopic composition from the solar reservoir as sampled by chondrites.  相似文献   

Can long-term periodic variability and jet helicity in 3C 120 be explained by jet precession?     

A. Caproni  Z. Abraham 《Monthly notices of the Royal Astronomical Society》2004,349(4):1218-1226

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Hypervelocity collisions of binary stars at the Galactic Centre     

Idan Ginsburg  Abraham Loeb 《Monthly notices of the Royal Astronomical Society》2007,376(2):492-496

Recent surveys have identified seven hypervelocity stars (HVSs) in the halo of the Milky Way. Most of these stars may have originated from the breakup of binary star systems by the nuclear black hole SgrA*. In some instances, the breakup of the binary may lead to a collision between its member stars. We examine the dynamical properties of these collisions by simulating thousands of different binary orbits around SgrA* with a direct N -body integration code. For some orbital parameters, the two stars collide with an impact velocity lower than their escape velocity and may therefore coalesce. It is possible for a coalescing binary to have sufficient velocity to escape the galaxy. Furthermore, some of the massive S-stars near Sgr A* might be the merger remnants of binary systems, however this production method can not account for most of the S-stars.  相似文献   

The fate of former companions to hypervelocity stars originating at the Galactic Centre     

Idan Ginsburg  Abraham Loeb 《Monthly notices of the Royal Astronomical Society》2006,368(1):221-225

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Wind–wind collision in the η Carinae binary system: a shell-like event near periastron     

D. Falceta-Gonçalves  V. Jatenco-Pereira  Z. Abraham 《Monthly notices of the Royal Astronomical Society》2005,357(3):895-902

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Free–free absorption in the nucleus of Cen A: evidences from 7-mm emission variability     

Z. Abraham  U. Barres de Almeida  T. P. Dominici  A. Caproni 《Monthly notices of the Royal Astronomical Society》2007,375(1):171-176

The existence of free–free absorption in the nucleus of Cen A was suggested by Tingay & Murphy in 2001 as an explanation for the large value of the spectral index between 2.2 and 5 GHz. In this paper we present further evidence for this absorption, based on the observations of short time-scale variability at mm wavelengths and their lack of correlation with X-ray emission, as expected if they were both originated in the same physical process. To explain the short-term variability, we assumed that the mm-wave emission is produced near the base of the inhomogeneous parsec-scale jet that has a bulk velocity of about 0.5 c . The presence of an ionized media between the jet and the observer will produce the observed 7-mm flux density variability, as different jet features move behind the absorbing material. We estimated that a region with 5 × 10⁶ electrons cm⁻³ and 1.5 × 10¹⁵ cm radius can explain the intensity and the duration of the mm-flux variations, but the corresponding column density would not be enough to absorb significantly the X-ray flux, explaining the lack of correlation between the radio and X-ray variability. The upper limit for the cloud size inferred for the absorbing region shows that the media surrounding the core of Cen A must be clumpy.  相似文献   

Modelling spectral line profiles of wind–wind shock emissions from massive binary systems     

D. Falceta-Gonçalves  Z. Abraham  V. Jatenco-Pereira 《Monthly notices of the Royal Astronomical Society》2006,371(3):1295-1300

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Signal excess in K-distributed reverberation     

Abraham  D.A. 《Oceanic Engineering, IEEE Journal of》2003,28(3):526-536

Active sonar systems have recently been developed using larger arrays and broad-band sources to counter the detrimental effects of reverberation in shallow-water operational areas. Increasing array size and transmit waveform bandwidth improve the signal-to-noise ratio-and-reverberation power ratio (SNR) after matched filtering and beamforming by reducing the size of the range-bearing resolution cell and, thus, decreasing reverberation power levels. This can also have the adverse effect of increasing the tails of the probability density function (pdf) of the reverberation envelope, resulting in an increase in the probability of a false alarm. Using a recently developed model relating the number of scatterers in a resolution cell to a K-distributed reverberation envelope, the effect of increasing bandwidth (i.e., reducing the resolution cell size) on detection performance is examined for additive nonfluctuating and fluctuating target models. The probability of detection for the two target models is seen to be well approximated by that for a shifted gamma variate with matching moments. The approximations are then used to obtain the SNR required to meet a probability of detection and false-alarm performance specification (i.e., the detection threshold). The required SNR is then used to determine that, as long as the target and scatterers are not over-resolved, decreasing the size of the resolution cell always results in an improvement in performance. Thus, the increase in SNR obtained by increasing bandwidth outweighs the accompanying increase in false alarms resulting from heavier reverberation distribution tails for K-distributed reverberation. The amount of improvement is then quantified by the signal excess, which is seen to be as low as one decibel per doubling of bandwidth when the reverberation is severely non-Rayleigh, as opposed to the expected 3-dB gain when the reverberation is Rayleigh distributed.  相似文献   

The Las Campanas Infrared Survey – II. Photometric redshifts, comparison with models and clustering evolution     

A. E. Firth  R. S. Somerville  R. G. McMahon  O. Lahav  R. S. Ellis  C. N. Sabbey  P. J. McCarthy  H.-W. Chen  R. O. Marzke  J. Wilson  R. G. Abraham  M. G. Beckett  R. G. Carlberg  J. R. Lewis  C. D. Mackay  D. C. Murphy  A. E. Oemler  S. E. Persson 《Monthly notices of the Royal Astronomical Society》2002,332(3):617-646

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Coupled silicon–oxygen isotope fractionation traces Archaean silicification     

K. Abraham  A. Hofmann  S.F. Foley  D. Cardinal  C. Harris  M.G. Barth  L. André 《Earth and Planetary Science Letters》2011,301(1-2):222-230

Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa.The δ³⁰Si and δ¹⁸O values of bulk rock increase with increasing amount of silicification from unsilicified basalts (?0.64‰ < δ³⁰Si < ?0.01‰ and + 8.6‰ < δ¹⁸O < + 11.9‰) to silicified basalts (δ³⁰Si and δ¹⁸O values as high as + 0.81‰ and + 15.6‰, respectively). Cherts generally have positive isotope ratios (+ 0.21‰ < δ³⁰Si < + 1.05‰ and + 10.9 < δ¹⁸O < + 17.1), except two cherts, which have negative δ³⁰Si values, but high δ¹⁸O (up to + 19.5‰).The pronounced positive correlations between δ³⁰Si, δ¹⁸O and SiO₂ imply that the isotope variation is driven by the silicification process which coevally introduced both ¹⁸O and ³⁰Si into the basalts. The oxygen isotope variation in the basalts from about 8.6‰ to 15.6‰ is likely to represent temperature-dependent isotope fractionation during alteration. Our proposed model for the observed silicon isotope variation relies on a temperature-controlled basalt dissolution vs. silica deposition process.  相似文献