全文获取类型
收费全文 | 4262篇 |
免费 | 357篇 |
国内免费 | 450篇 |
专业分类
测绘学 | 58篇 |
大气科学 | 177篇 |
地球物理 | 956篇 |
地质学 | 1002篇 |
海洋学 | 609篇 |
天文学 | 1706篇 |
综合类 | 112篇 |
自然地理 | 449篇 |
出版年
2024年 | 6篇 |
2023年 | 27篇 |
2022年 | 74篇 |
2021年 | 88篇 |
2020年 | 86篇 |
2019年 | 107篇 |
2018年 | 83篇 |
2017年 | 97篇 |
2016年 | 106篇 |
2015年 | 100篇 |
2014年 | 132篇 |
2013年 | 161篇 |
2012年 | 96篇 |
2011年 | 162篇 |
2010年 | 154篇 |
2009年 | 329篇 |
2008年 | 332篇 |
2007年 | 355篇 |
2006年 | 372篇 |
2005年 | 282篇 |
2004年 | 246篇 |
2003年 | 254篇 |
2002年 | 211篇 |
2001年 | 173篇 |
2000年 | 222篇 |
1999年 | 202篇 |
1998年 | 183篇 |
1997年 | 83篇 |
1996年 | 66篇 |
1995年 | 73篇 |
1994年 | 53篇 |
1993年 | 37篇 |
1992年 | 23篇 |
1991年 | 12篇 |
1990年 | 21篇 |
1989年 | 11篇 |
1988年 | 5篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有5069条查询结果,搜索用时 15 毫秒
991.
K. A. Wills M. Das A. Pedlar T. W. B. Muxlow T. G. Robinson 《Monthly notices of the Royal Astronomical Society》2000,316(1):33-48
We present VLA A-array 21-cm atomic hydrogen (H i ) absorption observed against the central region of the starburst galaxy M82 with an angular resolution of ∼1.3 arcsec (≃20 pc). These observations, together with MERLIN H i absorption measurements, are compared with the molecular (CO) and ionized ([Ne ii ]) gas distributions and are used to constrain the dynamics and structure of the ionized, neutral and molecular gas in this starburst.
A position–velocity diagram of the H i distribution reveals an unusual 'hole' feature which, when previously observed in CO, has been interpreted as an expanding superbubble contained within a ring of gas in solid body rotation. However, we interpret this feature as a signature of a nearly edge-on barred galaxy. In addition, we note that the CO, H i and [Ne ii ] position–velocity diagrams reveal two main velocity gradients, and we interpret these as gas moving on x1 - and x2 -orbits within a bar potential. We find the best fit to the data to be produced using a bar potential with a flat rotation curve velocity v b =140 km s−1 and a total length of 1 kpc, a non-axisymmetry parameter q =0.9, an angular velocity of the bar Ωb =217 km s−1 arcsec−1 , a core radius R c =25 pc, an inclination angle i =80° and a projected angle between the bar and the major axis of the galaxy φ '=4°. We also discuss the orientation of the disc and bar in M82. 相似文献
A position–velocity diagram of the H i distribution reveals an unusual 'hole' feature which, when previously observed in CO, has been interpreted as an expanding superbubble contained within a ring of gas in solid body rotation. However, we interpret this feature as a signature of a nearly edge-on barred galaxy. In addition, we note that the CO, H i and [Ne ii ] position–velocity diagrams reveal two main velocity gradients, and we interpret these as gas moving on x
992.
A. J. L. Fernandes 《Monthly notices of the Royal Astronomical Society》2000,315(4):657-668
Long-slit spectra of the molecular outflow Herbig–Haro (HH) 46/47 have been taken in the J and K near-infrared bands. The observed H2 line emission confirms the existence of a bright and extended redshifted counter-jet outflow south-west of HH 46. In contrast with the optical appearance of this object, we show that this outflow seems to be composed of two different emission regions characterized by distinct heliocentric velocities. This implies an acceleration of the counter-jet.
The observed [Fe ii ] emission suggests an average extinction of 7–9 visual magnitudes for the region associated with the counter-jet.
Through position–velocity diagrams, we show the existence of different morphologies for the H2 and [Fe ii ] emission regions in the northern part of the HH 46/47 outflow. We have detected for the first time high-velocity (−250 km s−1 ) [Fe ii ] emission in the region bridging HH 46 to HH 47A. The two strong peaks detected can be identified with the optical positions B8 and HH 47B.
The H2 excitation diagrams for the counter-jet shock suggest an excitation temperature for the gas of T ex ≈2600 K . The lack of emission from the higher energy H2 lines, such as the 4–3 S(3) transition, suggests a thermal excitation scenario for the origin of the observed emission. Comparison of the H2 line ratios with various shock models yielded useful constraints about the geometry and type of these shocks. Planar shocks can be ruled out whereas curved or bow shocks (both J- and C-type) can be parametrized to fit our data. 相似文献
The observed [Fe ii ] emission suggests an average extinction of 7–9 visual magnitudes for the region associated with the counter-jet.
Through position–velocity diagrams, we show the existence of different morphologies for the H
The H
993.
This paper argues that the Milky Way galaxy is probably the largest member of the Local Group. The evidence comes from estimates of the total mass of the Andromeda galaxy (M31) derived from the three-dimensional positions and radial velocities of its satellite galaxies, as well as the projected positions and radial velocities of its distant globular clusters and planetary nebulae. The available data set comprises 10 satellite galaxies, 17 distant globular clusters and nine halo planetary nebulae with radial velocities. We find that the halo of Andromeda has a mass of together with a scalelength of 90 kpc and a predominantly isotropic velocity distribution. For comparison, our earlier estimate for the Milky Way halo is Although the error bars are admittedly large, this suggests that the total mass of M31 is probably less than that of the Milky Way . We verify the robustness of our results to changes in the modelling assumptions and to errors caused by the small size and incompleteness of the data set.
Our surprising claim can be checked in several ways in the near future. The numbers of satellite galaxies, planetary nebulae and globular clusters with radial velocities can be increased by ground-based spectroscopy, while the proper motions of the companion galaxies and the unresolved cores of the globular clusters can be measured using the astrometric satellites Space Interferometry Mission ( SIM ) and Global Astrometric Interferometer for Astrophysics ( GAIA ). Using 100 globular clusters at projected radii 20 R 50 kpc with both radial velocities and proper motions, it will be possible to estimate the mass within 50 kpc to an accuracy of 20 per cent. Measuring the proper motions of the companion galaxies with SIM and GAIA will reduce the uncertainty in the total mass caused by the small size of the data set to 22 per cent. 相似文献
Our surprising claim can be checked in several ways in the near future. The numbers of satellite galaxies, planetary nebulae and globular clusters with radial velocities can be increased by ground-based spectroscopy, while the proper motions of the companion galaxies and the unresolved cores of the globular clusters can be measured using the astrometric satellites Space Interferometry Mission ( SIM ) and Global Astrometric Interferometer for Astrophysics ( GAIA ). Using 100 globular clusters at projected radii 20 R 50 kpc with both radial velocities and proper motions, it will be possible to estimate the mass within 50 kpc to an accuracy of 20 per cent. Measuring the proper motions of the companion galaxies with SIM and GAIA will reduce the uncertainty in the total mass caused by the small size of the data set to 22 per cent. 相似文献
994.
995.
Michael W. Feast Patricia A. Whitelock 《Monthly notices of the Royal Astronomical Society》2000,317(2):460-487
The space motions of Mira variables are derived from radial velocities, Hipparcos proper motions and a period–luminosity relation. The previously known dependence of Mira kinematics on the period of pulsation is confirmed and refined. In addition, it is found that Miras with periods in the range 145–200 d in the general Solar neighbourhood have a net radial outward motion from the Galactic Centre of 75±18 km s−1 . This, together with a lag behind the circular velocity of Galactic rotation of 98±19 km s−1 , is interpreted as evidence for an elongation of their orbits, with their major axes aligned at an angle of ∼17° with the Sun–Galactic Centre line, towards positive Galactic longitudes. This concentration seems to be a continuation to the Solar circle and beyond of the bar-like structure of the Galactic bulge, with the orbits of some local Miras probably penetrating into the bulge. These conclusions are not sensitive to the distance scale adopted. A further analysis is given of the short-period (SP) red group of Miras discussed in companion papers in this series. In Appendix A the mean radial velocities and other data for 842 oxygen-rich Mira-like variables are tabulated. These velocities were derived from published optical and radio observations. 相似文献
996.
George J. Bendo Joshua E. Barnes 《Monthly notices of the Royal Astronomical Society》2000,316(2):315-325
We use Gauss–Hermite functions to study the line-of-sight velocity distributions in simulated merger remnants. Our sample contains 16 remnants; eight produced by mergers between disc galaxies of equal mass, and eight produced by mergers between disc galaxies with mass ratios of 3:1. The equal-mass mergers display a wide range of kinematic features, including counterrotation at large radii, orthogonally rotating cores and misaligned rotational axes. Most of the unequal-mass remnants exhibit fairly regular disc-like kinematics, although two have kinematics more typical of the equal-mass remnants. Our results may be compared with observations of early-type objects, including ellipticals with misaligned kinematic axes, counterrotating systems and S0 galaxies. 相似文献
997.
P. Frick R. Beck A. Shukurov D. Sokoloff M. Ehle † J. Kamphuis 《Monthly notices of the Royal Astronomical Society》2000,318(3):925-937
The spiral pattern in the nearby spiral galaxy NGC 6946 has been studied using the wavelet transformation technique, applied to galaxy images in polarized and total non-thermal radio emission at λλ 3.5 and 6.2 cm, in broadband red light, in the λ 21.1 cm H i line and in the optical Hα line. Well-defined, continuous spiral arms are visible in polarized radio emission and red light, where we can isolate a multi-armed pattern in the range of galactocentric distances 1.5–12 kpc, consisting of four long arms and one short spiral segment. The 'magnetic arms' (visible in polarized radio emission) are localized almost precisely between the optical arms. Each magnetic arm is similar in length and pitch angle to the preceding optical arm (in the sense of galactic rotation) and can be regarded as its phase-shifted image. Even details like a bifurcation of an optical arm have their phase-shifted counterparts in the magnetic arms. The average relative amplitude of the optical spiral arms (the stellar density excess over the azimuthal average) grows with galactocentric radius up to 0.3–0.7 at r ≃5 kpc, decreases by a factor of two at r =5–6 kpc and remains low at 0.2–0.3 in the outer parts of the galaxy. By contrast, the magnetic arms have a constant average relative amplitude (the excess in the regular magnetic field strength over the azimuthal average) of 0.3–0.6 in a wide radial range r =1.5–12 kpc. We briefly discuss implications of our findings for theories of galactic magnetic fields. 相似文献
998.
The new approach outlined in Paper I to follow the individual formation and evolution of binaries in an evolving, equal point-mass star cluster is extended for the self-consistent treatment of relaxation and close three- and four-body encounters for many binaries (typically a few per cent of the initial number of stars in the cluster mass). The distribution of single stars is treated as a conducting gas sphere with a standard anisotropic gaseous model. A Monte Carlo technique is used to model the motion of binaries, their formation and subsequent hardening by close encounters, and their relaxation (dynamical friction) with single stars and other binaries. The results are a further approach towards a realistic model of globular clusters with primordial binaries without using special hardware. We present, as our main result, the self-consistent evolution of a cluster consisting of 300 000 equal point-mass stars, plus 30 000 equal-mass binaries over several hundred half-mass relaxation times, well into the phase where most of the binaries have been dissolved and evacuated from the core. The cluster evolution is about three times slower than found by Gao et al. Other features are rather comparable. At every moment we are able to show the individual distribution of binaries in the cluster. 相似文献
999.
We investigate the dynamical evolution of 210 hypothetical massless bodies initially situated between 10 and 30 au from the Sun in order to determine the general characteristics of the evolved system. This is of particular relevance to the understanding of the origin of Edgeworth–Kuiper belt objects on scattered intermediate orbits, such as 1996 TL 66 , which have high eccentricity and semimajor axis but nevertheless have perihelion in the region between 30 and 50 au from the Sun. 相似文献
1000.
Hydraulic pathways in the crystalline rock of the KTB 总被引:1,自引:0,他引:1