Recurrent Red-tides in the Southampton Water Estuary Caused by the Phototrophic CiliateMesodinium rubrum     

D.W. Crawford  D.A. Purdie  A.P.M. Lockwood  P. Weissman 《Estuarine, Coastal and Shelf Science》1997,45(6):799-812

In the Southampton Water estuary (southern England, U.K.), red-tides caused by the planktonic, phototrophic ciliateMesodinium rubrum(=Myrionecta rubra) occur during most summers and sometimes in autumn. These events were investigated in detail between 1985 and 1987 and were characterized by levels of chlorophylla(chl a) of over 100 μg l⁻¹, cell numbers ofM. rubrumof over 1×10³ ml⁻¹, oxygen saturations of around 150%, and depleted numbers of macrozooplankton. Initiation of red-water did not appear to be triggered by irradiance or nutrients, but coincided with an increase in temperature and water column stability. This enhanced stability was promoted by increased surface to bottom gradients of both temperature and salinity, and by reduced mixing during neap tides. Development of red-water was accompanied by removal of most of the dissolved NH⁺₄from the water column, whereas some NO⁻₃persisted, presumably maintained by freshwater input. NO⁻₃and NH⁺₄gradually returned to pre-bloom concentrations as the red-water declined in late summer. Maximal biomass ofM. rubrumappeared to be limited by irradiance, and self-shading probably imposed an upper limit of around 300 mg chl a m⁻²within the water column. At the observed levels of chl a, irradiance values within the population maximum between 1 and 3 m depth were only just of the order (≈15 μmol photons m⁻² s⁻¹) required to balance estimated respiratory demands. Oxygen concentration became undersaturated during the late bloom phase, with minimal values of 20–30% saturation recorded in deeper waters; however, despite this and reduced numbers of macrozooplankton, direct deleterious effects on other organisms were not observed.  相似文献   

The Fault Block''s Framework in Boli Basin and its Control Over the Deposition     

Cao Chengrun  SHAN Xuanlong  Wang Dongpo  WANG Xianmin  Yao Ping 《世界地质(英文版)》2001,4(1)

Boli basin, between Yishu fracture belt and Dunmi fracture belt, is the biggest Mesozoic coal basin in the east of Heilongjiang Province. Now it is a fault - fold remnant basin. The basin' s shape is generally consistent with the whole distribution of the cover folds, an arc protruding southwards. The basement of the basin can be divided into three fault blocks or structural units. The formation and evoluation of the basin in Mesozoic was determined by the basement fault blocks' displacement features rusulted from by the movement of the edge faults and the main basement faults.  相似文献   

Scattered light models of the dust shell around HD 179821     

T.M. Gledhill  M. Takami 《Monthly notices of the Royal Astronomical Society》2001,328(1):266-276

The dust shell around the evolved star HD 179821 has been detected in scattered light in near-IR imaging polarimetry observations. Here, we subtract the contribution of the unpolarized stellar light to obtain an intrinsic linear polarization of between 30 and 40 per cent in the shell that seems to increase with radial offset from the star. The J - and K -band data are modelled using a scattering code to determine the shell parameters and dust properties. We find that the observations are well described by a spherically-symmetric distribution of dust with an r ⁻² density law, indicating that when mass-loss was occurring, the mass-loss rate was constant. The models predict that the detached nature of a spherically-symmetric, optically-thin dust shell, with a distinct inner boundary, will only be apparent in polarized flux. This is in accordance with the observations of this and other optically-thin circumstellar shells, such as IRAS 17436+5003. By fitting the shell brightness we derive an optical depth to the star that is consistent with V -band observations and that, assuming a distance of 6 kpc, gives an inner-shell radius of     , a dust number density of     at r _in and a dust mass of     . We have explored axisymmetric shell models but conclude that any deviations from spherical symmetry in the shell must be slight, with an equator-to-pole density contrast of less than 2:1. We have not been able to fit simultaneously the high linear polarizations and the small     colour excess of the shell and we attribute this to the unusual scattering properties of the dust. We suggest that the dust grains around HD 179821 either are highly elongated or consist of aggregates of smaller particles.  相似文献   

Rotational broadening and Doppler tomography of the quiescent X-ray nova Centaurus X-4     

M. A. P. Torres  J. Casares  I. G. Martínez-Pais  P. A. Charles 《Monthly notices of the Royal Astronomical Society》2002,334(1):233-240

We present a library of 47 open-cluster integrated spectra, mostly obtained at CASLEO (San Juan, Argentina) in the range  3600 < λ < 7400 Å  , which are made available at CDS. The data are combined with previous spectra to obtain 10 high signal-to-noise ratio basic templates in the young and intermediate-age domains, which are also provided in the library. These Galactic disc templates represent the increased time resolution spectral evolution of a stellar population unit around the Solar metallicity level. The improved signal-to-noise ratio of the present templates with respect to previous template lists, together with their increased time resolution, allowed us to improve the fundamental parameters of some open clusters. The present spectral library will be useful for several astrophysical applications, particularly for population syntheses of star-forming giant galaxies.  相似文献   

APPLICATION OF MAP AND FILE INFORMATIONVISUALIZATION SYSTEM TO COMPREHENSIVEDIVISION OF NATURAL DISASTERS- Takingthe ChangjiangValleg as an Example     

HU Bao-qing    JIANG Mei-xin  JUN Su-lan  ZENG Qiao-song 《中国地理科学(英文版)》2001,(4)

In recent years，scholars at home and abroad have method，and that vanous disasters would be analyzed assynthetically studied natural dlsaste‘theoretically and an Integral．SHI Peilun（1991）putforwad a scientificmethodologlcally，as well as its cases analys。s（CND，term—regional disaster system，which Indicates that1987； MA et al，1990； MARBLE，1990； NE et al，the situation of a disaster（calamity loss）results from1999； PATAK et al，1982； SHI，1991；VAN et al，h…  相似文献   

The scaling of the redshift power spectrum: observations from the Las Campanas redshift survey     

Y.P. Jing  G. Börner 《Monthly notices of the Royal Astronomical Society》2001,325(4):1389-1396

  相似文献   

A detailed abundance analysis of the hot post-AGB star ZNG-1 in M10     

C.J. Mooney  W.R.J. Rolleston  F.P. Keenan  P.L. Dufton  D.L. Pollacco  H.R. Magee 《Monthly notices of the Royal Astronomical Society》2001,326(3):1101-1109

We present a model-atmosphere analysis for the bright ( V ∼13) star ZNG-1, in the globular cluster M10. From high-resolution ( R ∼40 000) optical spectra we confirm ZNG-1 to be a post-asymptotic giant branch (post-AGB) star. The derived atmospheric parameters are T _eff=26 500±1000 K and log  g =3.6±0.2 dex . A differential abundance analysis reveals a chemical composition typical of hot post-AGB objects, with ZNG-1 being generally metal poor, although helium is approximately solar. The most interesting feature is the large carbon underabundance of more than 1.3 dex. This carbon deficiency, along with an observed nitrogen enhancement relative to other elements, may suggest that ZNG-1 evolved off the AGB before the third dredge-up occurred. Also, iron depletions observed in other similar stars suggest that gas–dust fractionation in the AGB progenitor could be responsible for the observed composition of these objects. However, we need not invoke either scenario since the chemical composition of ZNG-1 is in good agreement with abundances found for a Population II star of the same metallicity.  相似文献   

Variable stars in the field of open cluster NGC 6819     

R. A.Street  KeithHorne  T. A.Lister  A.Penny  Y.Tsapras  A.Quirrenbach  N.Safizadeh  J.Cooke  D.Mitchell  A.Collier Cameron 《Monthly notices of the Royal Astronomical Society》2002,330(3):737-754

We report on the discovery of 25 variable stars plus 13 suspected variables found in the field of the open cluster NGC 6819. The stars were identified from time-series photometric data obtained on the Isaac Newton Telescope, La Palma, during two observing runs covering the 19 nights between 1999 June  22–30  and 1999 July  22–31  . The variables found include 12 eclipsing binaries with an additional three suspected, nine BY Draconis systems, plus four variables of other types, including one star believed to be a Cepheid. Three of the 15 eclipsing binaries are believed to be cluster members. Details of a further 10 suspected variable stars are also included.  相似文献   

Classification of planetary nebulae by their departure from axisymmetry     

NoamSoker  RonHadar† 《Monthly notices of the Royal Astronomical Society》2002,331(3):731-735

We propose a scheme to classify planetary nebulae (PNe) according to their departure from axisymmetric structure. We consider only departure along and near the equatorial plane, i.e. between the two sides perpendicular to the symmetry axis of the nebula. We consider six types of departure from axisymmetry: (1) PNe where the central star is not at the centre of the nebula; (2) PNe having one side brighter than the other; (3) PNe having unequal size or shape of the two sides; (4) PNe where the symmetry axis is bent, e.g. the two lobes in a bipolar PN are bent toward the same side; (5) PNe where the main departure from axisymmetry is in the outer regions, e.g. an outer arc; and (6) PNe that show no departure from axisymmetry, i.e. any departure, if it exists, is on scales smaller than the scale of blobs, filaments and other irregularities in the nebula. PNe that possess more than one type of departure are classified by the most prominent type. We discuss the connection between departure types and the physical mechanisms that may cause them, mainly resulting from the influence of a stellar binary companion. We find that ∼50 per cent of all PNe in the analysed sample possess large-scale departure from axisymmetry. This number is larger than that expected from the influence of binary companions, namely ∼25–30 per cent. We argue that this discrepancy comes from many PNe where the departure from axisymmetry, mainly unequal size, shape or intensity, results from the presence of long-lived and large (hot or cool) spots on the surface of their asymptotic giant branch progenitors. Such spots locally enhance the mass-loss rate, leading to a departure from axisymmetry, mainly near the equator, in the descendent PN.  相似文献   

A photometric study of the polar ring galaxy UGC 5600     

G. M. Karataeva  V. A. Yakovleva  V. A. Hagen-Thorn  O. V. Mikolaichuk 《Astronomy Letters》2001,27(2):74-80

We analyze our BVR _c photometry for UGC 5600, a candidate polar ring galaxy, obtained with the 6-m telescope. We have confirmed the existence of an inner polar ring and show that the outer ring-shaped structure represents spiral arms; i.e., UGC 5600 belongs to the rare class of gas-rich spiral galaxies with inner polar rings.  相似文献