Evaluation of Jefferies' level population ratios,and generalization of Seaton's cascade matrix,by a Markov-chain method     

Sidney O. Kastner 《Astrophysics and Space Science》1980,68(1):245-251

Closed expressions are obtained for the conditional probabilitiesq _ij,k required in evaluating particular ratios of atomic level populations, using a Markov-chain representation of the system of levels. The total transition probability between two arbitrary levels is also evaluated and its relation to population ratios is clarified. It is shown that Seaton's cascade matrix is a subset of the total transition probability matrix.  相似文献   

Microwave links: The future for urban rainfall measurement?     

G.J.G. Upton  A.R. Holt  R.J. Cummings  A.R. Rahimi  J.W.F. Goddard   《Atmospheric Research》2005,77(1-4):300

Microwave links are ideally suited to the urban environment, since there are many suitable vandal-free locations to which the receiver and transmitter can be attached. A signal sent along a link is received with reduced power as a consequence of scattering and absorption by intervening water droplets. Thus a measurement of the power loss can be transformed into a measure of total path rainfall. This paper discusses the requirements for the estimation process to be accurate. The paper presents results using a single frequency, together with the improved results from a dual-frequency link. Potential future applications of microwave links are presented.  相似文献   

Letter     

Sidney J. Holt 《Marine pollution bulletin》1984,15(4):161-162

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The cosmic distance scale     

Sidney van den Bergh 《Astronomy and Astrophysics Review》1989,1(2):111-139

Summary The status of the cosmic distance scale problem in early 1989 is reviewed. Internally consistent distances to Local Group galaxies are given in Tables 5 and 6. Within the Local Group the distance scale is found to be 11±5% smaller than that previously adopted by Sandage and Tammann. Distances to nearby galaxies are used as stepping stones to the Virgo cluster. The interpretation of the Tully-Fisher observations of Virgo spirals is found to be ambiguous because it is not yet clear which spirals are cluster members and which are background objects. Distance estimates of the Virgo cluster obtained by different techniques are listed in Table 11. The distance modulus of the Virgo cluster is found to be 31.5±0.2, corresponding to a distance of 20±2 Mpc. The elliptical galaxies in the core of the Virgo cluster haveV ₀=1200±46 kms^–1, which corresponds toV _LG=1082±48 km s^–1. With an infall velocity of 250±50 km s^–1 this yields a cosmological redshiftV=1332±69 km s^–1, from which a Hubble parameter H₀=67±8 km s^–1 Mpc^–1 is obtained. Space Telescope observations of distant Cepheids, Tully-Fisher observations of spirals in the Hercules eluster, and interference filter observations of Virgo planetary nebulae in the light of [O_III], should soon result in a major improvement in the accuracy with which H₀ is known.  相似文献   

Highly extinguished emission line outflows in the young radio source PKS 1345+12     

J. Holt  C. N. Tadhunter  R. Morganti 《Monthly notices of the Royal Astronomical Society》2003,342(1):227-238

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Contrasting conditions preceding MIS3 and MIS2 Heinrich events   总被引：2，自引：0，他引：2  

Elsa Jullien  Francis E. Grousset  Sidney R. Hemming  Victoria L. Peck  Ian R. Hall  Cdric Jeantet  Isabelle Billy 《Global and Planetary Change》2006,54(3-4):225-238

This paper presents an integrated multi-tracer study performed on piston cores recovered in the glacial ice-rafted detritus belt, stretching from Newfoundland to the Irish margin across the North Atlantic (40–55°N), in order to compare in detail the internal structure of each Heinrich event (HE). These tracers are IRD counts (quartz, dolomite, volcanic grains), their Nd isotopic composition and Ar–Ar datings of individual hornblende grains. A focus on the detailed structure of HE confirms that all intervals of massive sediment flux, specifically Heinrich layers HL1-to-5 (HLs), were dominated by North American, Laurentide ice-sheet surges from Hudson Strait, that are evident as far east as the Bay of Biscay (European margin). The sequences of events leading up to the HLs, however, present significant dissimilarities. One important difference is that HL2 and HL1 were preceded by “precursor events” (increases in the number of lithic grains per gram from non-Laurentide sediment sources). Sediment debris derived from near-simultaneous iceberg releases originating from the European ice-sheet are only detectable close to the European margin. In contrast there are no comparable precursor events before HL5 and HL4. This observation implies that precursor events are unlikely to be mechanistically linked to the triggering of HEs. The similarity of the HLs, against contrasting background conditions, is a significant observation that should add constraints to their origin.  相似文献   

Chandra observations of Tycho’s supernova remnant     

U. Hwang  R. Petre  A. E. Szymkowiak  S. S. Holt 《Journal of Astrophysics and Astronomy》2002,23(1-2):81-87

We present a newChandra observation of Tycho’s supernova remnant with the Advanced CCD Imaging Spectrometer. Multicolor Xray imaging reveals new details of the outer shock and ejecta. At energies between 4 and 6 keV, the outline of the outer shock is clearly revealed in X-rays for the first time. The distribution of the emission from lines of Si and Fe are confirmed to have a different morphology from each other, and the Si ejecta are shown to extend to the blast shock at several locations. Characteristic spectra of the outer shock and ejecta are also presented.  相似文献   

High-redshift QSOs in the FIRST survey     

C.R. Benn  M. Vigotti  M. Pedani  J. Holt  K.-H. Mack  R. Curran  S.F. Sánchez 《Monthly notices of the Royal Astronomical Society》2002,329(1):221-226

In a pilot search for high-redshift radio quasi-stellar objects (QSOs), we have obtained spectra of 55 FIRST sources     with very red     starlike optical identifications. 10 of the candidates are QSOs with redshifts     (four were previously known), six with     . The remaining 45 candidates comprise: one     broad-absorption-line (BAL) QSO; three low-redshift galaxies with narrow emission lines; 18 probable radio galaxies; and 23 M stars (mainly misidentifications). The success rate (high-redshift QSOs / spectroscopically-observed candidates) for this search is 1/2 for     , and 1/9 for     . With an effective search area of 4030 deg², the surface density of high-redshift     QSOs discovered with this technique is 0.0015 deg⁻².  相似文献   

Can saltwater toxicity be predicted from freshwater data?   总被引：3，自引：0，他引：3  

K M Leung  D Morritt  J R Wheeler  P Whitehouse  N Sorokin  R Toy  M Holt  M Crane 《Marine pollution bulletin》2001,42(11):1007-1013

The regulation of substances discharged to estuarine and coastal environments relies upon data derived from ecotoxicity tests. Most such data are generated for freshwater rather than saltwater species. If freshwater toxicity data are related to saltwater toxic effects in a systematic and predictable way, the former can be used to predict the latter. This would have economic advantages due to a reduction in toxicity testing of saltwater species. If toxicity data are plotted as species sensitivity distributions, four theoretical relationships between freshwater and saltwater can be envisaged. Examples show that each one of these relationships is supported by empirical data. These examples show that although there is considerable potential for freshwater to saltwater prediction, species parity and representativeness need to be examined for each chemical substance to avoid bias.  相似文献   

Calving retreat and proglacial lake growth at Hooker Glacier,Southern Alps,New Zealand     

Clare M. Robertson  Martin S. Brook  Kat A. Holt  Ian C. Fuller  Douglas I. Benn 《New Zealand geographer》2013,69(1):14-25

Hooker Glacier in the central Southern Alps of New Zealand has undergone significant downwasting and recession (～2.14 km) during the last two centuries. High retreat rates (51 m a^?1 1986–2001, 43 m a^?1 2001–2011) have produced a large (1.22 km²) proglacial lake. We present a retreat scenario for Hooker Glacier. A retreat scenario predicts that the glacier terminus will stabilise >3 km up‐valley of the current lake outlet after 2028 when ice velocity equals calving rate.  相似文献