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1.
The two major factors contributing to the opposition brightening of Saturn’s rings are (i) the intrinsic brightening of particles due to coherent backscattering and/or shadow hiding on their surfaces, and (ii) the reduced interparticle shadowing when the solar phase angle α → 0°. We utilize the extensive set of Hubble Space Telescope observations (Cuzzi, J.N., French, R.G., Dones, L. [2002]. Icarus 158, 199–223) for different elevation angles B and wavelengths λ to disentangle these contributions. We assume that the intrinsic contribution is independent of B, so that any B dependence of the phase curves is due to interparticle shadowing, which must also act similarly for all λ’s. Our study complements that of Poulet et al. (Poulet, F., Cuzzi, J.N., French, R.G., Dones, L. [2002]. Icarus 158, 224), who used a subset of data for a single B ∼ 10°, and the French et al. (French, R.G., Verbiscer, A., Salo, H., McGhee, C.A., Dones, L. [2007b] PASP 119, 623–642) study for the B ∼ 23° data set that included exact opposition. We construct a grid of dynamical/photometric simulation models, with the method of Salo and Karjalainen (Salo and Karjalainen [2003]. Icarus 164, 428–460), and use these simulations to fit the elevation-dependent part of opposition brightening. Eliminating the modeled interparticle component yields the intrinsic contribution to the opposition effect: for the B and A rings it is almost entirely due to coherent backscattering; for the C ring, an intraparticle shadow hiding contribution may also be present.Based on our simulations, the width of the interparticle shadowing effect is roughly proportional to B. This follows from the observation that as B decreases, the scattering is primarily from the rarefied low filling factor upper ring layers, whereas at larger B’s the dense inner parts are visible. Vertical segregation of particle sizes further enhances this effect. The elevation angle dependence of interparticle shadowing also explains most of the B ring tilt effect (the increase of brightness with elevation). From comparison of the magnitude of the tilt effect at different filters, we show that multiple scattering can account for at most a 10% brightness increase as B → 26°, whereas the remaining 20% brightening is due to a variable degree of interparticle shadowing. The negative tilt effect of the middle A ring is well explained by the the same self-gravity wake models that account for the observed A ring azimuthal brightness asymmetry (Salo, H., Karjalainen, R., French, R.G. [2004]. Icarus 170, 70–90; French, R.G., Salo, H., McGhee, C.A., Dones, L. [2007]. Icarus 189, 493–522). 相似文献
2.
A. Bragaglia F. R. Ferraro F. Fusi Pecci R. Buonanno C. E. Corsi I. Ferraro G. Iannicola 《Journal of Astrophysics and Astronomy》1987,8(1):57-68
Using a first approximation to the Hubble space telescope point-spread function, the problems concerning observation and reduction
of globular clusters in the Andromeda nebula are analysed through a simulation procedure in order to study the feasibility
of accurate photometry of individual stars in the clusters. 相似文献
3.
Wilfred H. Sorrell 《Astrophysics and Space Science》2009,323(2):205-211
Almost all astronomers now believe that the Hubble recession law was directly inferred from astronomical observations. It
turns out that this common belief is completely false. Those models advocating the idea of an expanding universe are ill-founded
on observational grounds. This means that the Hubble recession law is really a working hypothesis. One alternative to the
Hubble recession law is the tired-light hypothesis originally proposed by Zwicky (Proc. Nat. Acad. Sci. 15:773, 1929). This hypothesis leads to a universe that is an eternal cosmos continually evolving without beginning or end. Such a universe
exists in a dynamical state of virial equilibrium. Observational studies of the redshift-magnitude relation for Type Ia supernovae
in distant galaxies might provide the best observational test for a tired-light cosmology. The present study shows that the
model Hubble diagram for a tired-light cosmology gives good agreement with the supernovae data for redshifts in the range
0<z<2. This observational test of a static cosmology shows that the real universe is not necessarily undergoing expansion nor
acceleration.
An erratum to this article can be found at 相似文献
4.
An extensive set of HI Lyman-α images obtained with the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) from 1997-2001 has been analyzed to provide information about the spatial and temporal character of Io's SO2 atmosphere. An atmospheric distribution map derived from the observations reveals that the sunlit SO2 atmosphere is temporally stable on a global scale, with only small local changes. An anti-/sub-jovian asymmetry in the SO2 distribution is present in all 5 years of the observations. The average daytime atmosphere is densest on the anti-jovian hemisphere in the equatorial regions, with a maximum equatorial column density of 5.0×1016 cm−2 at 140° longitude. The SO2 atmosphere also has greater latitudinal extent on the anti-jovian hemisphere as compared to the sub-jovian. The atmospheric distribution appears to be best correlated with the location of hot spots and known volcanic plumes, although small number statistics for the plumes limits the correlation. 相似文献
5.
Michal K?í?ek 《New Astronomy》2012,17(1):1-7
The Hubble constant is split into two terms H = H1 + H2 , where H1 is a decreasing function due to the Big Bang and the subsequent gravitational interaction that slows the expansion of the Universe and H2 is an increasing function that corresponds to dark energy which accelerates this expansion. For T = 13.7 Gyr we prove that H2(T) > 5 m/(yr AU). This is a quite large number and thus the impact of dark energy, which is spread almost everywhere uniformly, should be observable not only on large scales, but also in our Solar system. In particular, we show that Earth, Mars and other planets were closer to the Sun 4.5 Gyr ago. The recession speed ≈5.3 m/yr of the Earth from the Sun seems to be just right for an almost constant influx of solar energy from the origin of life on Earth up to the present over which time the Sun’s luminosity has increased approximately linearly. This presents further support for the Anthropic Principle. Namely, the existence of dark energy guarantees very stable conditions for the development of intelligent life on Earth over a period of 3.5 Gyr. 相似文献
6.
Jian-Yang Li Dennis Bodewits Lori M. Feaga Wayne Landsman Michael F. A’Hearn Max J. Mutchler Christopher T. Russell Lucy A. McFadden Carol A. Raymond 《Icarus》2011,216(2):640-649
We report a comprehensive review of the UV–visible spectrum and rotational lightcurve of Vesta combining new observations by Hubble Space Telescope and Swift Gamma-ray Burst Observatory with archival International Ultraviolet Explorer observations. The geometric albedos of Vesta from 220 nm to 953 nm are derived by carefully comparing these observations from various instruments at different times and observing geometries. Vesta has a rotationally averaged geometric albedo of 0.09 at 250 nm, 0.14 at 300 nm, 0.26 at 373 nm, 0.38 at 673 nm, and 0.30 at 950 nm. The linear spectral slope as measured between 240 and 320 nm in the ultraviolet displays a sharp minimum near a sub-Earth longitude of 20°, and maximum in the eastern hemisphere. This is consistent with the longitudinal distribution of the spectral slope in the visible wavelength. The photometric uncertainty in the ultraviolet is ∼20%, and in the visible wavelengths it is better than 10%. The amplitude of Vesta’s rotational lightcurves is ∼10% throughout the range of wavelengths we observed, but is smaller at 950 nm (∼6%) near the 1-μm band center. Contrary to earlier reports, we found no evidence for any difference between the phasing of the ultraviolet and visible/near-infrared lightcurves with respect to sub-Earth longitude. Vesta’s average spectrum between 220 and 950 nm can well be described by measured reflectance spectra of fine particle howardite-like materials of basaltic achondrite meteorites. Combining this with the in-phase behavior of the ultraviolet, visible, and near-infrared lightcurves, and the spectral slopes with respect to the rotational phase, we conclude that there is no global ultraviolet/visible reversal on Vesta. Consequently, this implies a lack of global space weathering on Vesta, as previously inferred from visible–near-infrared data. 相似文献
7.
Ma Jun Zhao Jun-liang Zhang Fei-peng Peng Qiu-he 《Chinese Astronomy and Astrophysics》2000,24(4):367-443
A statistical study has been made for the variations along the Hubble sequence, os such parameters as the degree of tightness of winding of spiral arm λ, the pitch angle μ, the flatness of the disk H/D25 and the thickness H along the Hubble sequence for 365 spiral galaxies published in A&Ap Supplement Series. The mean values of these quantities for the various Hubble types have been obtained for the first time. The results of the statistics show clearly 1) that the Hubble classification of spiral galaxies is one which has only a qualitative and statistical significance, and 2) that the dispersion relation in the density wave theory is valid for most spiral galaxies, i.e., the arms of most spiral galaxies satisfy the requirements of being tightly wound. 相似文献
8.
9.
Jian-Yang Li Lucy A. McFadden Max J. Mutchler Eliot F. Young Mark V. Sykes 《Icarus》2010,208(1):238-251
We present the surface mapping of the southern hemisphere of Asteroid (4) Vesta obtained from Hubble Space Telescope (HST). From 105 images of Vesta through four filters in the wavelengths best to characterize the 1-μm pyroxene band, we constructed albedo and color-ratio maps of Vesta. These new maps cover latitudes −50° to +20°. The southern hemisphere of Vesta displays more diverse albedo and color features than the northern hemisphere, with about 15 new albedo and color features identified. The overall longitudinal albedo and color variations in the southern hemisphere are comparable with that of the northern hemisphere, with a range of about ±20% and ±10%, respectively. The eastern hemisphere is brighter and displays more diogenitic minerals than the western hemisphere. Correlations between 1-μm band depth and band width, as well as between 1-μm band depth and albedo, are present on a global scale, attributed to pyroxene composition variations. The lack of correlations between albedo and the spectral slope indicates the absence of globalized space weathering. The lack of a global correlation between 1-μm band depth and topography suggests that the surface composition of Vesta is not completely controlled by a single impact. The distribution of compositional variation on Vesta suggests a possible large impact basin. Evidence of space weathering is found in regions, including the bright rim of the south-pole crater where the steepest gravitational slope on Vesta is, and a dark area near a gravitationally flat area. We propose to divide the surface of Vesta into six geological units different from the background according to their 1-μm absorption features and spectral slopes, including two eucrite-rich units, a low-Ca eucrite unit, a diogenite-rich unit, a space weathered unit, and a freshly exposed unit. No evidence of olivine-rich area is present in these data. 相似文献
10.
Using a spherically symmetric model of the Virgo flow the global Hubble parameter has been estimated from the observed radial velocities and the photometrically measured distances of nearby galaxies. Adopting the observed recession velocity of the Virgo Cluster to about 1000 km s−1 and the infall velocity of the Local Group to 350 km s−1 the global Hubble constant results to 73 ± 10 km s−1 Mpc−1. This value corresponds with the distance of the Virgo Cluster of 18 ± 2 Mpc. The cosmic dispersion of the galaxies around the Hubble relation is of order of 35 km s−1. 相似文献