首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Observational results obtained to date on interstellar grains and molecules are briefly reviewed, and several promising areas for further research withSpacelab are suggested. Regarding grains, useful data can be expected on the shape of the ultraviolet extinction curve for new interstellar regions; the nature of UV extinction at short wavelengths, down to the Lyman limit; the presence or absence of structure in the UV extinction curve comparable to the visible-wavelength diffuse bands; the scattering properties of grains in new kinds of clouds and nebulae; and the polarization properties of grains in UV wavelengths. The principal advances which may be expected in observations of molecules will include the ability to probe more heavily-obscured regions, where molecular species are more abundant than in the diffuse clouds observed to date; coverage of wavelength regions (such as 1400–3200) not well-studied with previous instruments such asCopernicus; and the capability of observing in optical absorption species detected in the same line of sight in radio emission, which provides unique information on cloud geometry and physical conditions.  相似文献   

2.
The 2200 Å bump is a major figure of interstellar extinction. However, extinction curves with no bump exist and are, with no exception, linear from the near‐infrared down to 2500 Å at least, often over all the visible‐UV spectrum. The duality linear versus bump‐like extinction curves can be used to re‐investigate the relationship between the bump and the continuum of interstellar extinction, and answer questions as why do we observe two different kinds of extinction (linear or with a bump) in interstellar clouds? How are they related? How does the existence of two different extinction laws fits with the requirement that extinction curves depend exclusively on the reddening E (B – V) and on a single additional parameter? What is this free parameter? It will be found that (1) interstellar dust models, which suppose the existence of three different types of particles, each contributing to the extinction in a specific wavelength range, fail to account for the observations; (2) the 2200 Å bump is very unlikely to be absorption by some yet unidentified molecule; (3) the true law of interstellar extinction must be linear from the visible to the far‐UV, and is the same for all directions including other galaxies (as the Magellanic Clouds). In extinction curves with a bump the excess of starlight (or the lack of extinction) observed at wavelengths less than λ = 4000 Å arises from a large contribution of light scattered by hydrogen on the line of sight. Although counter‐intuitive this contribution is predicted by theory. The free parameter of interstellar extinction is related to distances between the observer, the cloud on the line of sight, and the star behind it (the parameter is likely to be the ratio of the distances from the cloud to the star and to the observer). The continuum of the extinction curve and the bump contain no information on the chemical composition of interstellar clouds. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
Spectral line profiles, curves of growth, and curves for the equivalent width of a line as a function of Venus phase angle have been computed for a Rayleigh scattering cloud and compared with those for a cloud of isotropic scatterers. The results are very similar for the two kinds of scattering, with the exception of the curves of equivalent width as a function of Venus phase angle. These latter curves exhibit the “inverse phase effect” and rule out the possibility that the scale height of the clouds can be much less than half the scale height of the gas. The optical depth of the clouds, τc, is approximately 100.  相似文献   

4.
Gravitational settling of dust grains in dark clouds has been considered. It has been shown that such a process gives rise to a modification of the grain size distribution. Starting with a simple model of uniform spherical cloud and normal interstellar grain size distribution for the dust we derive expressions for the modified grain size distribution function, average grain size and extinction as functions of distance from the cloud's center and the age of the cloud. The mean grain size increases towards the center of the cloud as does the extinction. Results of the numerical evaluation of these quantities have been discussed with their implications for the observations of anomalous reddening and polarization within dark clouds and Bok globules.  相似文献   

5.
Measurements of the strengths of the diffuse interstellar bands at 4430, 5780 and 5797 Å show that the bands tend to be week with respect to extinction in dense interstellar clouds. Data on 10 stars in the ? Ophiuchi cloud complex show further that the diffuse band-producing efficiency of the grains decreases systematically with increasing grain size. It is concluded that the diffuse bands are not formed in the mantles which accrete on the grains in interstellar clouds, but that they could be produced in the cores of grains or in some molecular species.  相似文献   

6.
A model for the vertical cloud structure of Jupiter's Equitorial Plumes is deduced based on an analysis of Voyager images of the equitorial region in the 6190Å methane band and the 6000-Å continuum, and ground-based 8900-Å methane band images of Jupiter. A computer code that represents scattering and absorption from aerosol and gas layers was applied to a heirarchy of increasingly complex model aerosol structures to match the observations in the three wavelengths. The observations are consistent with a model for the vertical cloud structure of the equitorial region that consists of four aerosol layers. A high-altitude haze layer (HAL) with optical depth τ = 1 uniformly blankets the equitorial region at an altitude between 100 and 250 mbar. Below that, a middle-level cloud layer between 400 and 800 mbar contains the well-known Equatorial Plumes. The Plume clouds are optically thick (τ ≥ 12), bright clouds with single scattering albedo ω = 0.997. They are probably composed of ammonia ice. The darker (ω = 0.990) interplume regions contain optically thinner clouds (2 ≤ τ ≤ 5) at the same altitude as the Plumes. An opaque cloud deck between 4000 and 6000 mbar, which is probably composed of water, forms the lowest model layer. In addition to these three layers, a thin forward scattering haze layer above 100 mbar was included in the models for consistency with previous work (Tomasko et al., 1978). We conclude that the vertical structure of the Equatorial Plume clouds is consistent with the hypothesis (Hunt et al., 1981) that the Plumes are caused by upwelling at the ammonia condensation level produced by bouyancy due to latent heat release from the condensation of water clouds nearly three scale heights below the Plumes.  相似文献   

7.
Interstellar extinction curves obtained from the ‘extinction without standard’ method were used to constrain the dust characteristics in the mean ISM (R V = 3.1), along the lines of sight through a high latitude diffuse molecular cloud towards HD 210121 (R V = 2.1) and in a dense interstellar environment towards the cluster NGC 1977 (R V = 6.42). We have used three-component dust models comprising silicate, graphite and very small carbonaceous grains (polycyclic aromatic hydrocarbons) following the grain size distributions introduced by Li & Draine in 2001. It is shown that oxygen, carbon and silicon abundances derived from our models are closer with the available elemental abundances for the dust grains in the ISM if F & G type stars atmospheric abundances are taken for the ISM than the solar. The importance of very small grains in modelling the variation of interstellar extinction curves has been investigated. Grain size distributions and elemental abundances locked up in dust are studied and compared at different interstellar environments using these three extinction curves. We present the albedo and the scattering asymmetry parameter evaluated from optical to extreme-UV wavelengths for the proposed dust models.  相似文献   

8.
《Icarus》1986,67(3):484-514
Most of the solar energy absorbed by Venus is deposited in the atmosphere, at levels more than 60 km above the surface. This unusual flux distribution should have important consequences for the thermal structure and dynamical state of that atmosphere. Because there are few measurements of the solar flux at levels above 60 km, a radiative transfer model was used to derive the structure and amplitude of the solar fluxes and heating rates in the Venus mesosphere (60–100 km). This model accounts for all sources of extinction known to be important there, including absorption and scattering by CO2, H2O, SO2, H2SO4 aerosols and an unidentified UV absorber. The distributions of these substances in our model atmosphere were constrained by a broad range of spacecraft and ground-based observations. Above the cloud tops, (71 km), near-infrared CO2 bands absorb enough sunlight to produce globally averaged heating rates ranging from 4° K/day (24-hr period) at 71 km to more than 50° K/day at 100 km. The sulfuric acid aerosols that compose the Venus clouds are primarily scattering agents at solar wavelengths. These aerosols reflect about 75% of the incident solar flux before it can be absorbed by the atmosphere or surface. The unknown substance that causes the observed cloud-top ultraviolet contrasts is responsible for most of the absorption of sunlight within the upper cloud deck (57.5−71 km). This substance absorbs almost half of the sunlight deposited on Venus and contributes to solar heating rates as large as 6° K/day at levels near 65 km. With the exception of CO2, all of the important sources of solar extinction have concentrations that vary with position, and, in general, these concentrations are not well known. To determine the sensitivity of the model results to these uncertainties, the concentrations of these opacity sources were varied in the model atmosphere and solar fluxes were computed for each case. These tests indicate that CO2 dominates the solar absorption at levels above the cloud tops and that heating rates are relatively insensitive to the distribution of other sources of extinction there. Within the upper cloud deck, uncertainties in the distribution of the UV absorber and the H2SO4 aerosols can produce heating rate errors as large as 50% at some levels. Diurnally averaged solar heating rates for the nominal opacity distribution were computed as a function of latitude at altitudes between 55 and 100 km, where most of the solar flux is deposited. The zonal wavenumber 1 (diurnal) and zonal wavenumber 2 (semidiurnal) components of the diurnally varying solar heating rates were also computed in this domain. These results should be sufficiently reliable for use in numerical dynamical models of the Venus atmosphere.  相似文献   

9.
There is now compelling evidence that dark molecular clouds are clumpy. Much of the clumpiness is unresolved by single-dish telescopes but is apparent in the data from array telescopes. Molecular clumps may also be observed close to Herbig-Haro (HH) objects. These clumps are easily observable because they are `illuminated' due to the UV radiation from the shock front of the HH jet. A detailed observational and theoretical study of one HH clump has been performed and it indicates that this clump must be transient and has a similar density and temperature to those clumps detected in the cloud interior. Thus, HH clumps may be used as an independent method of determining physical parameters of the clumpiness of molecular clouds.  相似文献   

10.
The Galileo Probe sampled Jupiter's atmosphere at the edge of a 5-μm hot spot, where it found very little cloud opacity above the 700 mb level. Only τ=1-2 at λ=0.5 μm was inferred from Net Flux Radiometer observations (Sromovsky et al. 1998, J. Geophys. Res.103, 22,929-22,977), in seeming conflict with Chanover et al. (1997, Icarus128, 294-305) who inferred τ=6-8 above the 700 mb level (at λ∼0.9 μm) from 893-nm and 953-nm WFPC2 observations of a group of hot spots. Postulating a heterogeneous cloud structure is one way to resolve the conflict. We obtained a more satisfying resolution by reinterpretation of the HST observations with Probe-compatible assumptions about the vertical distribution of cloud particles. Assuming a physically thin upper (putative NH3) cloud with adjustable optical depth and effective pressure (peff<440 mb) and a physically thin midlevel (putative NH4SH) cloud with adjustable optical depth but a fixed pressure of 1.2 bars, we are able to fit WPFC2 observations with probe-consistent opacities in hot spot regions. With the same cloud pressures, but higher middle cloud opacities, we are even able to fit the visibly bright regions. Little variability is seen in the upper cloud. Best fits to October 1995 WFPC2 observations in dark regions (5-μm hot spots) yielded τupper=1.3-1.9 at 0.9 μm and peff=240 mb−270 mb, while in visibly bright regions between hot spots we obtained τupper=1.6-2.2 and peff=250 mb−290 mb. May 1996 observations yielded slightly higher values of τupper (1.8-2.3 and 2.0-2.8) and peff (250 mb−310 mb and 265 mb−320 mb). We found that the most important variable parameter is the opacity of the middle cloud, which ra nged from τ=1, 2 in dark regions, to τ=8-30 in bright regions. From limb darkening characteristics, we inferred a wavelength-dependent haze opacity ranging from 0.2±0.05 at 660 nm to 0.35±0.05 at 953 nm, and an effective haze pressure near 120 mb. We did not find it necessary to use low single scattering albedos that require effective imaginary indices, that are several orders of magnitude larger than the values of the main putative cloud components.  相似文献   

11.
Polarimetry is able to show direct evidence for compositional differences in the Venus clouds. We present observations (collected during 212 Venus years by the Pioneer Venus Orbiter) of the polarization in four colors of the bright and dark ultraviolet features. We find that the polarization is significantly different between the bright and dark areas. The data show that the “null” model of L. W. Esposito (1980, J. Geophys. Res.85, 8151–8157) and the “overlying haze” model of J. B. Pollack et al. (1980, J. Geophys. Res.85, 8223–8231) are insufficient. Exact calculations of the polarization, including multiple scattering and vertical inhomogeneity near the Venus cloud tops, are able to match the observations. Our results give a straightforward interpretation of the polarization differences in terms of known constituents of the Venus atmosphere. The submicron haze and uv absorbers are anticorrelated: for haze properties as given by K. Kawabata et al. (1980, J. Geophys. Res.85, 8129–8140) the excess haze depth at 9350 Å over the bright regions is Δτh = 0.03 ± 0.02. The cloud top is slightly lower in the dark features: the extra optical depth at 2700 Å in Rayleigh scattering above the darker areas is ΔτR = 0.010 ± 0.005. This corresponds to a height difference of 1.2 ± 0.6 km at the cloud tops. The calculated polarization which matches our data also explains the relative polarization of bright and dark features observed by Mariner 10. The observed differential polarization cannot be explained by differential distribution of haze, if the haze aerosols have an effective size of 0.49 μm, as determined by K. Kawabata et al. (1982, submitted) for the aerosols overlying the Venus equator. We propose two models for the uv contrasts consistent with our results. In a physical model, the dark uv regions are locations of vertical convergence and horizontal divergence. In a chemical model, we propose that the photochemistry is limited by local variations in water vapor and molecular oxygen. The portions of the atmosphere where these constituents are depleted at the cloud tops are the dark uv features. Strong support for this chemical explanation is the observation that the number of sulfur atoms above the cloud tops is equal over both the bright and dark areas. The mass budget of sulfur at these altitudes is balanced between excess sulfuric acid haze over the bright regions and excess SO2 in the dark regions.  相似文献   

12.
The dependence of interstellar extinction on distance in the direction of a dark cloud around the reflection nebula NGC 1333 is determined on the basis of photoelectric Vilnius photometry and photometric classification of 78 stars. Two dust clouds are noted at distances 160 and 220 pc. The first one with mean extinction of 0.4 mag is concluded to belong to the Taurus cloud complex and the second cloud with mean extinction of 1.8 mag belongs to the chain of dark clouds and other young objects which is almost perpendicular to the spiral arm but lies 80 pc below the galactic plane. The star BD +30°549 which illuminates the NGC 1333 nebula is at distance 212 pc from the Sun. No extinction increase behind the Perseus cloud is detected.  相似文献   

13.
To explain the effects of the ultraviolet (UV) background radiation on the collapse of pre-galactic clouds, we implement a radiation–hydrodynamical calculation, combining one-dimensional spherical hydrodynamics with an accurate treatment of the radiative transfer of ionizing photons. Both absorption and scattering of UV photons are explicitly taken into account. It turns out that a gas cloud contracting within the dark matter potential does not settle into hydrostatic equilibrium, but undergoes run-away collapse even under the presence of the external UV field. The cloud centre is shown to become self-shielded against ionizing photons by radiative transfer effects before shrinking to the rotation barrier. Based on our simulation results, we further discuss the possibility of H2 cooling and subsequent star formation in a run-away collapsing core. The present results are closely relevant to the survival of subgalactic Population III objects as well as to metal injection into intergalactic space.  相似文献   

14.
It is shown that the well-known 2200 Å peak in the extinction of starlight is explained by microorganisms. A mixed culture of diatoms and bacteria, which previously we found to give excellent fits to astronomical data in the infrared, has a peak absorption slightly shortward of 2200 Å, in very close agreement with the absorptions found indirections towards most early-type stars. The peak absorption is measured to be ～ 35000 cm2g-1. This is in addition to a scattering component of the extinction which has an estimated value for dry microorganisms of ～ 50000 cm2g-1. The scattering calculated for a size distribution of non-absorbing hollow bacteria with irregularities on the scale of 300 Å produces agreement with both the visual extinction law and the observed λ-1 type extinction at the far ultraviolet. The contribution to the extinction from a pure scattering bacterial model is about 3.4 mag per kpc path length along the galactic plane at λ =2175 Å. Absorption near this wavelength effectively adds ～ 2.3 mag per kpc, making up precisely the observed total extinction at the peak of 5.7 mag per kpc. The full range of the interstellar extinction observations is now elegantly explained on the basis of a bacterial model alone with no added components or free parameters to be fitted. Photolysis has little effect on the bulk refractive index of the particles and so does not change the scattering component appreciably. But photolysis due to sufficient UV in space can reduce the effectiveness of the 2200 Å absorption in comparison with the scattering, thereby decreasing the height of the absorption peak. An extreme example of this is the Large Magellanic Cloud, where UV emission from a profusion of early-type stars has reduced the absorbance of the particles to about one-quarter of its value for most of our galaxy. The 2200 Å absorption has generally been attributed to small graphite particles. We explain how this belief has come about and why in the past we have been swayed by it.  相似文献   

15.
A recent suggestion that the extinction of starlight at wavelengths near 2200 Å may be due to small grains of magnesium oxide, rather than due to graphite, does not seem plausible. The potential ability of graphite to produce ultraviolet extinction is shown to be an order of magnitude greater than the extinction which MgO grains could produce under the most favourable assumptions concerning the effectiveness of O2? transitions.  相似文献   

16.
Planetary systems are angular momentum reservoirs generated during star formation. Solutions to three of the most important problems in contemporary astrophysics are needed to understand the entire process of planetary system formation: The physics of the ISM. Stars form from dense molecular clouds that contain ∼ 30% of the total interstellar medium (ISM) mass. The structure, properties and lifetimes of molecular clouds are determined by the overall dynamics and evolution of a very complex system – the ISM. Understanding the physics of the ISM is of prime importance not only for Galactic but also for extragalactic and cosmological studies. Most of the ISM volume (∼ 65%) is filled with diffuse gas at temperatures between 3000 and 300 000 K, representing about 50% of the ISM mass. The physics of accretion and outflow. Powerful outflows are known to regulate angular momentum transport during star formation, the so-called accretion–outflow engine. Elementary physical considerations show that, to be efficient, the acceleration region for the outflows must be located close to the star (within 1 AU) where the gravitational field is strong. According to recent numerical simulations, this is also the region where terrestrial planets could form after 1 Myr. One should keep in mind that today the only evidence for life in the Universe comes from a planet located in this inner disk region (at 1 AU) from its parent star. The temperature of the accretion–outflow engine is between 3000 and 10 7 K. After 1 Myr, during the classical T Tauri stage, extinction is small and the engine becomes naked and can be observed at ultraviolet wavelengths. The physics of planet formation. Observations of volatiles released by dust, planetesimals and comets provide an extremely powerful tool for determining the relative abundances of the vaporizing species and for studying the photochemical and physical processes acting in the inner parts of young planetary systems. This region is illuminated by the strong UV radiation field produced by the star and the accretion–outflow engine. Absorption spectroscopy provides the most sensitive tool for determining the properties of the circumstellar gas as well as the characteristics of the atmospheres of the inner planets transiting the stellar disk. UV radiation also pumps the electronic transitions of the most abundant molecules (H 2, CO, etc.) that are observed in the UV.Here we argue that access to the UV spectral range is essential for making progress in this field, since the resonance lines of the most abundant atoms and ions at temperatures between 3000 and 300 000 K, together with the electronic transitions of the most abundant molecules (H 2, CO, OH, CS, S 2, CO 2 +, C 2, O 2, O3, etc.) are at UV wavelengths. A powerful UV-optical instrument would provide an efficient mean for measuring the abundance of ozone in the atmosphere of the thousands of transiting planets expected to be detected by the next space missions (GAIA, Corot, Kepler, etc.). Thus, a follow-up UV mission would be optimal for identifying Earth-like candidates.  相似文献   

17.
Limb-darkening curves are derived from Pioneer 10 imaging data for Jupiter's STrZ (?18 to ?21° latitude) and SEBn (?5 to ?8° latitude) in red and blue light at phase angles of 12, 23, 34, 109, 120, 127, and 150°. Inhomogeneous scattering models are computed and compared with the data to constrain the vertical structure and the single-scattering phase functions of the belt and the zone in each color. The very high brightness observed at a 150° phase angle seems to require the presence of at lleast a thin layer of reasonably bright and strongly forward-scattering haze particles at pressure levelsof about 100 mbar or less above both belts and zones. Marginally successful models have been constructed in which a moderate optical thickness (τ ≥ 0.5) of haze particles was uniformly distributed in the upper 25 km-amagats of H2. Excellent fits to the data were obtained with models having a thin (optical depths of a few tenths) haze conentraated above most of the gas. Following recent spectrospcopicanalyses, we have placed the main “cloud” layer or layers beneath about 25 km-amagats of H2, although successful fits to our continuum data probably could be achieved also if the clouds were permitted to extend all the way up to the thin haze layer. Similarly, below the haze level our data cannot distinguish between models having two clouds separated by a clear space as suggested by R. E. Danielson and M. G. Tomasko and models with a single extensive diffuse cloud having an H2 abundance of a few kilometer-amagats per scattering mean free path as described by W. D. Cochran. In either case, the relative brightness of the planet at each phase angle primarily serves to constrain the single-scattering phase functions of the Jovian clouds at the corresponding scattering angles. The clouds in these models are characterized by single-scattering phase functions having strong forward peaks and modest backward-scattering peaks, indicating cloud particles with dimensions larger than about 0.6 μm. In our models, a lower single-scattering albedo of the cloud particles in the belt relative to the zone accounts for the contrast between these regions. If an increased abundance of absorbing dust above uniformly bright clouds is used to explain the contrast between belts and zones at visible wavelengths, the limb darkening is steeper than that observed for the SEBn in blue light at small phase angles. The phase integral for the planet calculated for either the belt or the zone model in either color lies in the range 1.2 to 1.3. If a value of 1.25 is used with D.J. Taylor's bolometric geometric albedo of 0.28, the planet emits 2.25 or 1.7 times the energy it absorbs from the Sun if it effective temperature is 134 or 125°K, respectively—roughly as expected from current theories of the cooling of Jupiter's interior.  相似文献   

18.
Data on interstellar extinction are interpreted to imply an identification of interstellar grains with naturally freeze-dried bacteria and algae. The total mass of such bacterial and algal cells in the galaxy is enormous, ～ 1040 g. The identification is based on Mie scattering calculations for an experimentally determined size distribution of bacteria. Agreement between our model calculations and astronomical data is remarkably precise over the wavelength intervals 1 μ-1 < λ-1 < 1.94 μ-1and 2.5 μ-1 < λ-1 < 3.0 μ-1. Over the more restricted waveband 4000–5000 Å an excess interstellar absorption is found which is in uncannily close agreement with the absorption properties of phytoplankton pigments. The strongest of the diffuse interstellar bands are provisionally assigned to carotenoid-chlorophyll pigment complexes such as exist in algae and pigmented bacteria. The λ2200 Å interstellar absorption feature could be due to `degraded' cellulose strands which form spherical graphitic particles, but could equally well be due to protein-lipid-nucleic acid complexes in bacteria and viruses. Interstellar extinction at wavelengths λ < 1800 Å could be due to scattering by virus particles.  相似文献   

19.
On the basis of assumed photospheric temperature models for 36 extreme supergiants (logg e -values of 1, 0.5 and 0;T e ranging from approx. 3700–33 000 K) photospheric fluxesS(τλ) were computed for 36 wavelengths ranging from 100 Å to 60 000 Å. The hot models are in perfect radiative equilibrium; the cooler show deviations up to 10%, sometimes even larger. Only in the relatively deep parts of the photospheres (τ5?1) the radiation field at each geometrical level can be characterized by one unique radiation temperature; for smaller τ5-values there are large deviations from local thermal equilibrium. The influence of deviations from local thermodynamical equilibrium on the fluxes is briefly examined, and appears small but for the shortest wavelengths. In tables and graphs we give for these models πF(γ)-values, integrated fluxes, effective temperatures, coloursU, B andV, and the Balmer discontinuityD.  相似文献   

20.
The two basic components of the neutral hydrogen, cool dense clouds merged in a hotter tenuous medium, are studied using 21 cm absorption data of the Parkes Survey. The mean parameters obtained for the typical clouds next to the galactic plane are τp = 1.7, velocity half-width=3.3 km s?1. Their temperatures areT sc ≥40 K with a meanT sc =63±12 K and the obtained hot gas density isn HH=(0.15±0.05) atom cm?3. Theoretical analysis following Giovanelli and Brown (1973) reveals that the pressure equilibrium condition (n HH+2n e )·T SH≈n HC·T sc is compatible with the quoted values if it is assumed that the cosmic abundances in the interstellar medium are below the adopted normal solar abundance. This lack of heavy elements suggests accretion to grains which is consistent with the observed narrow concentration of the dark matter on the galactic layer (≤100 pc halfwidth). The same pressure condition leads to a mean cool cloud density ofn HC～30 atom cm?3 and a hot gas temperature ofT SH～10 500 K. Comparison with data from Hii regions suggests that the cool clouds are somewhat denser and less extensive than such regions. An explanation for it is the expansion that the Hii regions went through in their origin. Comparison with 21 cm emission data shows that the cloud galactic layer is only about a quarter as thick as the hot gas layer. All the present results suggest that only such clouds can be spatially related with the typical I population associated with the spiral structure.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号