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61.
J. C. Pandey K. P. Singh R. Sagar S. A. Drake 《Journal of Astrophysics and Astronomy》2002,23(1-2):9-14
We present photometric observations of four stars that are optical counterparts of soft X-ray/EUV sources, namely 1ES 0829+15.9,
1ES0920-13.6, 2RE J1 10159+223509 and 1ES 1737+61.2. We have discovered periodic variability in two of the stars, viz., MCC
527 (1ES 0829+15.9; Period = 0
d
.828 ± 0.0047) and HD 81032 (1ES 0920-13.6; Period = ∼ 57.02 ± 0.560 days). HD 95559 (2RE J1 10159+223509) is found to show
a period of 3
d
. HD 160934 (1ES1737+61.2) also shows photometric variability but needs to be monitored further for finding its period. These
stars most likely belong to the class of chromospherically active stars. 相似文献
62.
D. R. K. Reddy R. Bhuvana Vijaya T. Vidya Sagar R. L. Naidu 《Astrophysics and Space Science》2014,350(1):375-380
A spatially homogeneous Bianchi type-VI0 space-time is considered in the frame work of f(R,T) gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. Exact solutions of the field equations are obtained both in the absence and in the presence of cosmic strings under some specific plausible physical conditions. Some physical and kinematical properties of the model are, also, studied. 相似文献
63.
Himali Bhatt J. C. Pandey K. P. Singh Ram Sagar Brijesh Kumar 《Journal of Astrophysics and Astronomy》2014,35(1):39-54
We present, for the first time, an analysis of seven intense X-ray flares observed from six stars (LAV 796, LAV 1174, SHM2002 3734, 2MASS 02191082+5707324, V553 Car, V557 Car). These stars are located in the region of young open star clusters NGC 869 and IC 2602. These flares detected in the XMM-Newton data show a rapid rise (10–40 min) and a slow decay (20–90 min). The X-ray luminosities during the flares in the energy band 0.3–7.5 keV are in the range of 1029.9 to 1031.7 erg s?1. The strongest flare was observed with the ratio ~13 for count rates at peak of the flare to the quiescent intensity. The maximum temperature during the flares has been found to be ~100 MK. The semi-loop lengths for the flaring loops are estimated to be of the order of 1010 cm. The physical parameters of the flaring structure, the peak density, pressure and minimum magnetic field required to confine the plasma have been derived and found to be consistent with flares from pre-main sequence stars in the Orion and the Taurus-Auriga-Perseus region. 相似文献
64.
65.
J. C. Pandey K. P. Singh S. A. Drake R. Sagar 《Journal of Astrophysics and Astronomy》2005,26(4):359-376
BVR photometric and quasi-simultaneous optical spectroscopic observations of the star HD 81032 have been carried out during
the years 2000-2004. A photometric period of 18.802 ± 0.07d has been detected for this star. A large group of spots with a
migration period of 7.43 ± 0.07 years is inferred from the first three years of the data. Hα and CaII H and K emissions from
the star indicate high chromospheric activity. The available photometry in the BVRIJHK bands is consistent with the spectral
type of K0IV previously found for this star. We have also examined the spectral energy distribution of HD 81032 for the presence
of an infrared colour excess using the 2MASS JHK and IRAS photometry, but found no significant excess in any band above the
normal values expected for a star with this spectral type. We have also analyzed the X-ray emission properties of this star
using data obtained by the ROSAT X-ray observatory during its All-Sky Survey phase. An X-ray flare of about 12 hours duration
was detected during the two days of X-ray coverage obtained for this star. Its X-ray spectrum, while only containing 345 counts,
is inconsistent with a single-temperature component solar-abundance coronal plasma model, but implies either the presence
of two or more plasma components, nonsolar abundances, or a combination of both of these properties. All of the above properties
of HD 81032 suggest that it is a newly identified, evolved RS CVn binary. 相似文献
66.
Symbiotic stars that are strong radio sources and have cool dust emitting in the infrared are expected to have extended emission
nebulae around them. In order to search for such emission nebulae, we have carried out CCD imaging of three symbiotic stars
(R Aqr, RR Tel and H1-36) with narrow-band filters centred at the emission lines of [O III] λ5007, Hα λ6563, [N II] λ6584,
[S II] λ6717 + 6731. RR Tel and H1-36 images do not show any extended nebulosities around them. The CCD image of the R Aqr
nebulosity in the high excitation [O m] line is different from its image in Hα and the low excitation lines of [N II] and
[S II] indicating ionization-stratification in the nebula. In H1-36 the optical nebulosity (if it exists) is smaller than
∽2 arcsec while the radio image size is known to be large (∽5 arcsec). This behaviour is opposite to that seen in R Aqr in
which the radio emission comes from the core region of a much larger optical nebulosity. Interstellar and/or circumstellar
extinctions are suggested to be responsible for this difference 相似文献
67.
H.C. Kandpal D.S. Mehta E.S.R. Gopal J.S. Vaishya B.B. Sanwal M. Singh Ram Sagar 《Astrophysics and Space Science》2002,280(3):209-222
Experiments are performed to determine the coherence properties of wave fields, produced by the broadband stellar sources
on the earth surface, from the study of spectral changes produced on interference in the Young's double slit experiment. The
spectral degree of coherence obtained experimentally in this study for four bright stars in the wavelength range from 325
nm to 660 nm is in close agreement with the value that is expected theoretically from the known angular diameter of the stars.
It is shown that the spectral degree of coherence obtained experimentally by this spectral interferometric technique could
be used to determine the angular diameter of stars.
This revised version was published online in July 2006 with corrections to the Cover Date.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
68.
69.
J. M. Scott J. M. Palin A. F. Cooper M. W. Sagar A. H. Allibone A. J. Tulloch 《Contributions to Mineralogy and Petrology》2011,161(5):667-681
Previous U–Pb zircon dating of the Pomona Island Granite (PIG) pluton (South Island, New Zealand) yielded either Permo-Carboniferous
or Late Jurassic ages for five samples essentially indistinguishable in their field, petrographic, and geochemical characteristics.
Detailed cathodoluminescence imaging and LA-ICP-MS dating of zircon in new and previously dated samples reveal that portions
of the pluton contain either delicately oscillatory-zoned Late Jurassic zircon grains with rare Permo-Carboniferous cores,
or Permo-Carboniferous grains with ubiquitous but thin Late Jurassic rims. Based on zircon dissolution-overgrowth textures,
zircon rim and core trace element compositions, and the limited extent of sub-solidus rock recrystallisation textures, the
bipartite age distribution is unlikely to reflect variable Pb-loss or metamorphic re-equilibration. Magmatic Zr-saturation
temperatures were ≥851°C for samples dominated by Jurassic zircon and ≤809°C for samples with a predominance of Permo-Carboniferous
zircon. Together, these data are consistent with PIG magmas having been derived from partial melting of a Permo-Carboniferous
felsic igneous source at variable temperature wholly in the Late Jurassic (157 ± 3 Ma). The lowest temperature melts would
have been incapable of dissolving significant amounts of pre-existing zircon and consequently generated inheritance-rich magmas,
with the very thin rims on the pre-existing zircon grains the only evidence of the Late Jurassic magmatic age. As the partial
melting temperature increased and nearly all pre-existing zircon grains dissolved into the magma, an inheritance-poor batch
of melt was generated, which precipitated new zircon grains upon crystallisation. Concentrations of major and many trace elements
in both magma batches may have been buffered by retention of residual quartz and feldspar in the source, which would explain
the limited geochemical differences between inheritance-rich and inheritance-poor portions. 相似文献
70.
Shilpa Vuba Sadia Farnaaz Netramani Sagar S. Masood Ahmad 《Journal of the Geological Society of India》2013,82(3):217-226
The major, trace and rare earth elements geochemistry and clay mineral compositions in the river bed sediments from lower reaches of Godavari river suggest that they are derived from weathering of felsic rocks. Trace and rare earth elemental compositions indicate evidence of sedimentary sorting during transportation and deposition. Lower concentrations of transition elements, such as V, Ni and Cr imply enrichment of felsic minerals in these bed sediments. The REE pattern in lower Godavari sediments is influenced by the degree of source rock weathering. The light rare earth elements (LREE) content are indicating greater fractionation compared to the heavy rare earth elements (HREE). A striking relationship is observed between TiO2 and gZREE content suggesting a strong control by LREE-enriched titaniferous minerals on REE chemistry. Shale-normalized REE pattern demonstrate a positive Eu anomaly, suggesting weathering of feldspar and their secondary products, which are enriched in Eu. Chondrite-normalised REE pattern is characteristic of felsic volcanic, granites and gnessic source rocks. Trace elemental compositions in sediments located near urban areas suggest influence of anthropogenic activity. Chemical Index of Alteration (CIA) is high (avg. 65.76), suggesting a moderate chemical weathering environment. X-ray diffraction analysis of clay fraction shows predominance of clay minerals that are formed because of the chemical weathering of felsic rocks. 相似文献