共查询到20条相似文献,搜索用时 43 毫秒
1.
G. Amelino-Camelia K. Aplin M. Arndt J. D. Barrow R. J. Bingham C. Borde P. Bouyer M. Caldwell A. M. Cruise T. Damour P. D’Arrigo H. Dittus W. Ertmer B. Foulon P. Gill G. D. Hammond J. Hough C. Jentsch U. Johann P. Jetzer H. Klein A. Lambrecht B. Lamine C. Lämmerzahl N. Lockerbie F. Loeffler J. T. Mendonca J. Mester W.-T. Ni C. Pegrum A. Peters E. Rasel S. Reynaud D. Shaul T. J. Sumner S. Theil C. Torrie P. Touboul C. Trenkel S. Vitale W. Vodel C. Wang H. Ward A. Woodgate 《Experimental Astronomy》2009,23(2):549-572
The GAUGE (GrAnd Unification and Gravity Explorer) mission proposes to use a drag-free spacecraft platform onto which a number
of experiments are attached. They are designed to address a number of key issues at the interface between gravity and unification
with the other forces of nature. The equivalence principle is to be probed with both a high-precision test using classical
macroscopic test bodies, and, to lower precision, using microscopic test bodies via cold-atom interferometry. These two equivalence
principle tests will explore string-dilaton theories and the effect of space–time fluctuations respectively. The macroscopic
test bodies will also be used for intermediate-range inverse-square law and an axion-like spin-coupling search. The microscopic
test bodies offer the prospect of extending the range of tests to also include short-range inverse-square law and spin-coupling
measurements as well as looking for evidence of quantum decoherence due to space–time fluctuations at the Planck scale. 相似文献
2.
The longitudinal magnetic field measured using the Fe I λ 525 and Fe I λ 524.7 nm lines and global magnetic field of the sun
differ depending on the observatory. To study the cause of these discrepancies, we calculate the H
‖(525)/H
‖(524.7) ratios for various combinations of magnetic elements and compare them with the corresponding observed values. We use
the standard quiet model of the solar photosphere suggesting that there are magnetic fields of different polarities in the
range between zero and several kilogauss. The magnetic element distribution is found as a function of magnetic field strength
and the parameters of this distribution are determined for which the calculated H
‖(525)/H
‖(524.7) ratio agrees with the observed one. The sigma-components are found to be shifted differently for various points of
the Fe I λ 525 nm profile calculated for the inhomogeneous magnetic field. The farther the point is from the line center,
the larger the sigma-components shift. Such a peculiarity of the profiles may be responsible for the discrepancies in the
measured values of the global magnetic field obtained at different observatories. The increase in modulus of the global magnetic
field during the maxima of solar activity can be due to a larger fraction of magnetic elements with kilogauss magnetic fields. 相似文献
3.
Bianchi type I perfect fluid cosmological model is investigated with a variable cosmological term. Einstein’s field equations
are solved for any arbitrary cosmic scale factor. The main result of the study is the expression for cosmological term as
a power law of scale factor. The age of the universe can also be readily calculated. 相似文献
4.
RALPH B. BALDWIN 《Meteoritics & planetary science》1987,22(2):111-119
Careful examination of seven giant front face basins on the moon will show that the basins most densely covered by younger craters are the oldest. With increasing age they exhibit lower external rims, not scarp heights. The rims are progressively more subdued with age. This paper proposes that absolute ages for these basins can be obtained by calculating an effective viscosity of the moon's outer layers from 3.85 × 109 y, the date of Imbrium, to the present. Similarly viscosity measures can be determined for the oldest basin. To do this we need the present and the original rim heights. The present values are observed. The original heights are calculated by extrapolating the relationship between diameter and rim height for normal Class I craters. It turns out that as long as the larger basins have proportionately higher original heights than the smaller, the absolute values are of little importance and the ages are definitive. There are many similar families of viscosity changes with age and they yield similar absolute ages. In each case equations relating viscosity changes with age were derived and for each basin there is only one age that will yield the final rim height. Ages, × 109 y, of the basins are: Orientale 3.82, Imbrium 3.85, Crisium 4.00, Nectaris 4.07, Serenitatis 4.14, Humorum 4.23 and an Unnamed basin between Werner and the Altai ring 4.30. 相似文献
5.
H. Lammer J. H. Bredehöft A. Coustenis M. L. Khodachenko L. Kaltenegger O. Grasset D. Prieur F. Raulin P. Ehrenfreund M. Yamauchi J.-E. Wahlund J.-M. Grießmeier G. Stangl C. S. Cockell Yu. N. Kulikov J. L. Grenfell H. Rauer 《Astronomy and Astrophysics Review》2009,17(2):181-249
This work reviews factors which are important for the evolution of habitable Earth-like planets such as the effects of the
host star dependent radiation and particle fluxes on the evolution of atmospheres and initial water inventories. We discuss
the geodynamical and geophysical environments which are necessary for planets where plate tectonics remain active over geological
time scales and for planets which evolve to one-plate planets. The discoveries of methane–ethane surface lakes on Saturn’s
large moon Titan, subsurface water oceans or reservoirs inside the moons of Solar System gas giants such as Europa, Ganymede,
Titan and Enceladus and more than 335 exoplanets, indicate that the classical definition of the habitable zone concept neglects
more exotic habitats and may fail to be adequate for stars which are different from our Sun. A classification of four habitat
types is proposed. Class I habitats represent bodies on which stellar and geophysical conditions allow Earth-analog planets
to evolve so that complex multi-cellular life forms may originate. Class II habitats includes bodies on which life may evolve
but due to stellar and geophysical conditions that are different from the class I habitats, the planets rather evolve toward
Venus- or Mars-type worlds where complex life-forms may not develop. Class III habitats are planetary bodies where subsurface
water oceans exist which interact directly with a silicate-rich core, while class IV habitats have liquid water layers between
two ice layers, or liquids above ice. Furthermore, we discuss from the present viewpoint how life may have originated on early
Earth, the possibilities that life may evolve on such Earth-like bodies and how future space missions may discover manifestations
of extraterrestrial life. 相似文献
6.
E. N. Slyuta 《Solar System Research》2009,43(5):443-452
Physical-mechanical properties of cometary nuclei matter are described in detail. As compared to other Solar System bodies,
cometary nuclei are characterized by low strength properties. The ultimate tensile strength of cometary matter and cometary
nuclei on the whole is about 2 kPa. An analysis performed based on a rheological model of a self-gravitating triaxial solid
body showed that cometary nuclei less than 50–60 km (this actually being all known comets) are characterized by a constant
ultimate tensile strength which is determined only by the matter composition and structure. The effective ultimate tensile
strength for bodies larger than 50–60 km is determined by the body mass and figure parameters and increases according to the
quadratic law depending on the body dimensions and mass. Such an increase of the effective strength can explain the absence
or deficit of cometary nuclei more than 60 km in size, since it can significantly affect the parameters of the parent body
destruction and the formation of a secondary population. The dependence of the mechanism and character of destruction on the
parameters of the figure for Kuiper objects more than 50–60 km is size can yield a deficit of the population of the bodies
whose figure parameters are a/c > 1.75 with respect to the bodies with a/c < 1.75 figure parameters. 相似文献
7.
K. S. Krishna Swamy 《Solar physics》1975,41(2):301-312
The expected equivalent widths of the individual rotational lines of the Lyman and Werner bands of the hydrogen molecule from the solar atmosphere have been calculated. These results are used to predict what one expects to observe with a specified wavelength bandpass. These are compared with the observation of Dupree and Reeves. 相似文献
8.
9.
D. Montes 《Astrophysics and Space Science》1998,263(1-4):275-278
I have compiled here the three libraries of high and mid-resolution optical spectra of late-type stars I have recently published.
The libraries include F, G, K and M field stars, from dwarfs to giants. The spectral coverage is from 3800 to 1000 Å, with
spectral resolution ranging from 0.09 to 3.0 Å. These spectra include many of the spectral lines most widely used as optical
and near-infrared indicators of chromospheric activity. The spectra have been obtained with the aim of providing a library
of high and mid-resolution spectra to be used in the study of active chromosphere stars by applying a spectral subtraction
technique. However, the data set presented here can also be utilized in a wide variety of ways. A digital version of all the
fully reduced spectra is available via FTP and the World Wide Web (WWW) in FITS format.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
Rodney S. Gomes Tabaré Gallardo Julio A. Fernández Adrián Brunini 《Celestial Mechanics and Dynamical Astronomy》2005,91(1-2):109-129
We study the transfer process from the scattered disk (SD) to the high-perihelion scattered disk (HPSD) (defined as the population
with perihelion distances q > 40 AU and semimajor axes a>50 AU) by means of two different models. One model (Model 1) assumes
that SD objects (SDOs) were formed closer to the Sun and driven outwards by resonant coupling with the accreting Neptune during
the stage of outward migration (Gomes 2003b, Earth, Moon, Planets 92, 29–42.). The other model (Model 2) considers the observed population of SDOs plus clones that try to compensate for observational
discovery bias (Fernández et al. 2004, Icarus , in press). We find that the Kozai mechanism (coupling between the argument of perihelion, eccentricity, and inclination),
associated with a mean motion resonance (MMR), is the main responsible for raising both the perihelion distance and the inclination
of SDOs. The highest perihelion distance for a body of our samples was found to be q = 69.2 AU. This shows that bodies can
be temporarily detached from the planetary region by dynamical interactions with the planets. This phenomenon is temporary
since the same coupling of Kozai with a MMR will at some point bring the bodies back to states of lower-q values. However,
the dynamical time scale in high-q states may be very long, up to several Gyr. For Model 1, about 10% of the bodies driven
away by Neptune get trapped into the HPSD when the resonant coupling Kozai-MMR is disrupted by Neptune’s migration. Therefore,
Model 1 also supplies a fossil HPSD, whose bodies remain in non-resonant orbits and thus stable for the age of the solar system,
in addition to the HPSD formed by temporary captures of SDOs after the giant planets reached their current orbits. We find
that about 12 – 15% of the surviving bodies of our samples are incorporated into the HPSD after about 4 – 5 Gyr, and that
a large fraction of the captures occur for up to the 1:8 MMR (a ⋍ 120 AU), although we record captures up to the 1:24 MMR
(a ≃ 260 AU). Because of the Kozai mechanism, HPSD objects have on average inclinations about 25°–50°, which are higher than
those of the classical Edgeworth–Kuiper (EK) belt or the SD. Our results suggest that Sedna belongs to a dynamically distinct
population from the HPSD, possibly being a member of the inner core of the Oort cloud. As regards to 2000 CR105 , it is marginally within the region occupied by HPSD objects in the parametric planes (q,a) and (a,i), so it is not ruled
out that it might be a member of the HPSD, though it might as well belong to the inner core. 相似文献
11.
12.
Keith A. Holsapple 《Icarus》2007,187(2):500-509
Holsapple [Holsapple, K.A., 2001. Icarus 154, 432-448; Holsapple, K.A., 2004. Icarus 172, 272-303] determined the spin limits of bodies using a model for solid bodies without tensile or cohesive strength, but with the pressure-induced shear strengths characteristic of dry sands and gravels. That theory included the classical analyses for fluid bodies given by Maclaurin, Jacobi and others as a special case. For the general solid bodies, it was shown that there exists a very wide range of permissible shapes and spin limits; and explicit algebraic results for those limits were given. This paper gives an extension of those analyses to include geological-like materials that also have tensile and cohesive strength. Those strengths are necessary to explain the smaller, fast-rotating asteroids discovered in the last few years. I find that the spin limits for these more general solids have two limiting regimes: a strength regime for bodies with a diameter <3 km, and a gravity regime for the larger bodies with a diameter >10 km (which is the case covered by the earlier papers). I derive explicit algebraic forms for the dependence of the spin limits on shape, mass density and material strength properties. The comparison of the theory to the database for the spins of asteroids and trans-neptunian objects (TNO's) objects shows excellent agreement. For large bodies (diameter D>10 km), the presence of cohesive and/or tensile strength does not permit higher spin rates than would be allowed for rubble pile bodies. Thus, the fact that the spin rates of all large bodies is limited to periods greater than about 2 h does not imply that they are rubble piles. In contrast, for small bodies (D<10 km) the presence of even a very small amount of strength allows much more rapid spins. Small bodies might then be rubble piles but require a small amount of bonding. Finally, I make some remarks about the application of the theory to the TNO's and large asteroids, and question whether a common assumption by researchers that those bodies must take on relaxed fluid shapes is warranted. If not, then the densities and shapes required by that assumption are not valid. I use 2003 EL61 as a prime example. 相似文献
13.
The Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft has currently returned around three years worth
of data from Saturn’s largest moon Titan. One of the unique aspects of CIRS is to take high spatial resolution spectra of
the limb of Titan, with sub-scale height (20–40 km) resolutions. This is made possible by the small field-of-view (FOV) of
the mid-IR detectors. However, many limb spectra have moderate to large sized FOVs, which introduces bias into retrieved profiles
of temperature and abundance unless the finite FOV size is taken into account. The bias can be reduced by calculating a FOV-averaged
spectrum comprising a weighted sum of a small number of spectra with infinitesimal FOVs across the FOV. Here we introduce
a scheme for incorporating FOV averaging into radiative transfer calculations of CIRS spectra and quantify the errors as a
function of number of FOV averaging points, FOV size, tangent altitude, and wavenumber. The optimum number of FOV averaging
points for a given observation can then be found by matching the calculated FOV averaging error with the measurement error.
This allows for accurate analysis of a vast amount of Cassini-CIRS data. 相似文献
14.
During the previous years spacecraft observations of so-called Energetic Neutral Atoms (ENAs) have become an important remote-sensing
technique in planetary science for analyzing the solar wind plasma flow around the upper atmospheric environments of Solar
System bodies. ENAs are produced whenever solar- or stellar wind protons interact via charge exchange with a neutral particle
from a planetary atmosphere so that their signals constrain both, ion distributions and neutral gas densities. The observation
of ENAs which have been generated due to charge exchange with stellar wind plasma have been used for the indirect mass loss
and stellar wind property estimation of Sun-like stars by observing the interaction regions carved out by the collisions between
stellar winds and the interstellar medium. In this work we review ENA-observations and data interpretations at Solar System
planets and recent hydrogen-cloud observations in UV Lyman-α absorption around hydrogen-rich extra-solar gas giants. We discuss the production of stellar wind related hydrogen ENA-clouds
around close-in exoplanets and show how a detailed analysis of attenuation spectra obtained for transiting hydrogen-rich close-in
gas giants can be used for the study of the upper atmosphere structure, the planet’s magnetosphere and to obtain information
on stellar wind properties. Finally, we discuss how future hydrogen cloud observations around exoplanets by space observatories
like the Russia-led World Space Observatory-UV (WSO-UV) together with ESAs planned PLATO mission can be used for the reconstruction
of the solar wind history or the test of magnetosphere evolution hypotheses. 相似文献
15.
Suzanne C. Madden 《Astrophysics and Space Science》2002,281(1-2):247-252
The properties of the low metallicity environments of dwarf galaxies are studied through dust observations in conjunction
with the FIR fine-structure cooling lines. There is a striking enhancement of the I[CII]/I(CO) in dwarf galaxies that is explained
by the decreased attenuation of the UV light in molecular clouds. An important consequence is that a significant mass of the
molecular gas mass inventory can be missed through CO observations alone. Modeling the infrared spectral energy distribution
into submillimeter wavelengths in dwarf galaxies reveals the presence of very cold (∼ 8K) dust,which accounts for a large
fraction of the dust mass, until now missed by models using IRAS observations alone. In spite of the striking defficiency
of the mid-infrared aromatic band carriers, cooling in the photodissociation regions, via [CII] line emission is a very efficient
process.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
16.
The sensitivity of stellar spectra to α/Fe abundance changes is investigated with the aim to be detected photometrically and
employed for the scientific goals of the GAIA mission. A grid of plane parallel, line blanketed, flux constant, LTE model
atmospheres with different [α/Fe] ratios was calculated. As a first step, the modeled stellar energy fluxes for solar-type
stars and giants were computed and intercompared. The spectral sensitivity to α/Fe abundance changes is noticeable and has
to be taken into account when selecting photometric filters for GAIA. The Ca II H and K lines and Mg I b triplet are the most
sensitive direct indicators ofα/Fe abundance changes.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
《Planetary and Space Science》1999,47(6-7):873-881
The ROSETTA spacecraft will fly-by a few asteroids during its course to the final cometary target. The candidate asteroids presently are 3840 Ministrobel (S-type), 2703 Siwa and 140 (C-type).With the limited data presently available on these bodies we calculated some approximate quantities which may be useful to select the fly-by trajectories of the ROSETTA probe. In particular we derived the zones in which particles could stably orbit by analyzing Hills problem of three hierarchical masses—the sun, the asteroid and the orbiting particle. Then, following the approach of Hamilton and Burns, the effects of solar radiation pressure and of the ellipticity of the orbits were also taken into account. In this way for each asteroid we could calculate not only a classical quantity like the radius of the Hill sphere, but also the critical starting orbital distance (as a function of orbital inclination) within which most orbits remain bound to the asteroid, and outside which most escape as a consequence of perturbations. Moreover we determined the orbital stability zone, defined as the union of all the numerically integrated orbits showing long-term stability, for each of the target asteroids. The particular shape of these zones would suggest to have the spacecrafts close approach out of the orbital plane of the asteroids.To further investigate this problem and, in particular, to take into account the irregular shape of the asteroids, we developed a model using a polyhedral representation of the central rotating body, following a theory developed by Werner and Scheeres. This model is described here and the first orbital integration results are presented. © 1999 Elsevier Science Ltd. All rights reserved. 相似文献
18.
V. G. Turyshev 《Astronomy Letters》2009,35(4):215-234
We study the impact of relativistic gravitational deflection of light on the accuracy of future Space Interferometry Mission
(SIM). We estimate the deflection angles caused by the monopole, quadrupole and octupole components of gravitational fields
for a number of celestial bodies in the solar system. We observe that, in many cases, the magnitude of the corresponding effects
is significantly larger than the 1 μas accuracy expected from SIM. This fact argues for the development of a relativistic
observational model for the mission that would account for the influence of both static and time-varying effects of gravity
on light propagation. Results presented here are different from the ones obtained elsewhere by the fact that we specifically
account for the differential nature of the future SIM astrometric measurements. We also obtain an estimate for the accuracy
of possible determination of the Eddington’s parameter γ via SIM global astrometric campaign; we conclude that accuracy of ∼7 × 10−6 is achievable via measurements of deflection of light by solar gravity.
The article was translated by the authors. 相似文献
19.
Jin-He Tao 《Astrophysics and Space Science》2006,302(1-4):93-98
This article applies a new scheme of the first post-Newtonian theory (Damour et al., 1991–1994) to the problem of gauge in
relativistic reference systems. Choosing and fixing gauge are necessary when the precision of time measurement and application
needs to reach the 2PN level (10−16 or better). We present a general method for fixing the gauges of both the global and local coordinate systems, and for determining
the expressions of gravitational potentials and coordinate transformations. The results relevant are consistent with the newest
IAU resolutions, therefore they can be applied to astronomical practice. 相似文献
20.
Keith A. Holsapple 《Icarus》2004,172(1):272-303
The study of the equilibrium and stability of spinning ellipsoidal fluid bodies with gravity began with Newton in 1687, and continues to the present day. However, no smaller bodies of the Solar System are fluid. Here I model those bodies as elastic-plastic solids using a cohesionless Mohr-Coulomb yield envelope characterized by an angle of friction. This study began in Holsapple 2001. Here new closed-form algebraic formulas for the spin limits of ellipsoidal shapes are derived using an energy method. The fluid results of Maclaurin and Jacobi are again recovered as special cases. I then consider the stability of those equilibrium states. For elastic-plastic solids the common methods cannot be used, because the constitutive equations lack sufficient smoothness at the limiting plastic states. Therefore, I propose and study a new measure of the stability of dynamic processes in general bodies. An energy-based approach is introduced which is shown to include stability approaches used in the statics of nonlinear elastic and elastic-plastic bodies, spectral definitions and the Liapunov methods used for finite-dimensional dynamical systems. The method is applied to spinning, solid, strained bodies. In contrast to the special fluid case, it is found that the strain energy term of solid materials generally induces stability of all equilibrium shapes, except for two possible cases. First, strain softening in the elastic-plastic law can result in instability at the plastic limit spin. Second, a loss of shear stiffness can give unstable states at specific spins less than the limit equilibrium spins. In the latter case, a solid spinning ellipsoidal body without elastic shear stiffness can spin no faster than with a period of about 3.7 hr, else it will fail by shearing deformations. That is distinctly slower than the oft-quoted limit of 2.1 hr at which material would be flung off the equator by tensile forces. However, the final conclusion is that neither cohesion nor tensile strength is required for the shapes and spins of almost all of the larger observed asteroids: we cannot rule out rubble-pile structures. 相似文献