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1.
We have calculated the opacity of interstellar grains in the temperature range 10–1500 K. Two composite grain models have been considered. One of them consists of silicate coated with an ice mantle and the second has a graphite core coated also with an ice mantle. These models are compared with isolated grain models. An exact analytical and computational development of Güttler's formulae for composite grain models has been used to calculate the extinction coefficient.It has been found that the thickness of the mantle affects the opacity of the interstellar grains. The opacity of composite models differs from that of the isolated models. The effect of the different species (ice, silicate, and graphite) is also clear. 相似文献
2.
Analytical approximations used often in the literature for calculating energy rates emitted by dust grains in infrared are discussed. Comparisons with correct complete formulations are made for three grain models: (1) pure graphite, (2) ice mantle-graphite core, (3) silicates. -1 and -1 dependences for the average effective emissivity of such grains are used. We find that for silicate and graphite grains the simplified approximations are valid only when accuracies between 10% and 50% are required and only for grain temperatures higher than 80 K. At lower temperatures the validity of the approximations fails for the graphite particle while it is variable for the silicate dust grain. The ice core mantle particles can instead be treated with approximated formulae without introducing appreciable errors.Paper presented at the Symposium on Solid State Astrophysics, held at the University College, Cardiff, Wales, between 9–12 July, 1974. 相似文献
3.
K. S. Krishna Swamy 《Astrophysics and Space Science》1972,16(1):75-80
The profile of the absorption feature at 2200 Å has been calculated for model grains of graphite, graphite core-dirty ice mantle and silicate. They are compared with the observed profile obtained by Bless and Savage from a number of early type stars. We have also shown that it is unlikely that the radiation damage of silicates in Interstellar Space can also contribute to the absorption feature at 2200 Å. Lastly, we have discussed briefly how one can meet the objections that have been raised on the silicate model. 相似文献
4.
Calculations of the topography and shape of planetary bodies are presented for two sets of models. One set of models deals with the effects of static loading on bodies, taking into account strengths of materials, density, and size. The other set considers the effects of creep deformation on model bodies of differing composition, size and temperature. Application of these models to asteroids and satellites of the major planets indicates that model, even the largest asteroids could retain highly nonspherical shapes, and the four large satellites of Jupiter could sustain statically loaded topography on the order of 10 km. (2) If silicate asteroids have not been heated to near the melting temperature of silicates, initial topography should survive for at least 109 yr under creep deformation. Topography on an insulated icy asteroid will be rapidly reduced if it is of larger scale than the insulating layer, no matter what the thermal history. (3) Of the Galilean satellites of Jupiter, J1 and J2 should retain topography created on silicate surfaces since their formation (or since the surfaces were near the silicate melting temperature. If ice layers of any significant thickness exist, topography on a scale smaller than the layer's thickness will be reduced rapidly. (4) J4 and J3 probably fit an icy model throughout and topography of all scales may be reduced with relaxation times < 106yr. These satellites are thus likely to preserve only very recent features on their surfaces, in contrast to the other Galilean satellites. If melting has taken place since formation, these conclusions become even stronger. (5) Of the satellites of the other planets, only Titan appears likely to have undergone topographic reduction by creep, under the models presented. However, if ices other than water are present in large proportion on these satellites relaxation times for topography may be shorter than calculated from the water ice models. 相似文献
5.
Lilia P. Bassino Juan C. Muzzio Josefa Pérez 《Celestial Mechanics and Dynamical Astronomy》1998,72(3):157-168
We performed simple numerical simulations to investigate the capture of dwarf galaxies by larger ones in the environment of
a cluster of galaxies. The number of captures is small compared to the total number of dwarfs, but it is significant because
these galaxies are the most abundant members of galaxy clusters. Our models show that, having a more massive central galaxy
or a less massive background halo in the cluster, increases the number of captures. A high proportion of the orbits of the
captured dwarfs are very eccentric.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
The IRAS low-resolution spectra (LRS) of 29 symbiotic stars are presented in this paper. It is found that silicate dust is
common material in the circumstellar shells of symbiotic stars. From the IRAS low-resolution spectra, IRAS colors, symbiotic
types and spectral types, some physical properties of symbiotic stars with the LRS spectra are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
K. Wilke 《Astrophysics and Space Science》2001,276(2-4):633-641
For the barred galaxy NGC 2336, stationary models are constructed which reproduce in a consistent manner the observed NIR
surface brightness distribution and the observed HII kinematics in those regions affected by the bar. This procedure can answer
the question whether the observed kinematics of the galaxy agree with the model predictions derived from the observed distribution
of visible matter (assuming that plausible corrections for the presence of dark matter can be applied). It turns out that
the parameter values from the morphological decomposition and those needed to fit the HII rotation curves of NGC 2336 best
are in excellent agreement.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
Abstract— The Nova 001 [= Nuevo Mercurio (b)] and Nullarbor 010 meteorites are ureilites, both of which contain euhedral graphite crystals. The bulk of the meteorites are olivine (Fo79) and pyroxenes (Wo9En73Fs18, Wo3En77Fs20), with a few percent graphite and minor amounts of troilite, Ni-Fe metal, and possibly diamond. The rims of olivine grains are reduced (to Fo91) and contain abundant blebs of Fe metal. Silicate mineral grains are equant, anhedral, up to 2 mm across, and lack obvious preferred orientations. Euhedral graphite crystals (to 1 mm x 0.3 mm) are present at silicate grain boundaries, along boundaries and protruding into the silicates, and entirely within silicate mineral grains. Graphite euhedra are also present as radiating clusters and groups of parallel plates grains embedded in olivine; no other ureilite has comparable graphite textures. Minute lumps within graphite grains are possibly diamond, inferred to be a result of shock. Other shock effects are limited to undulatory extinction and fracturing. Both ureilites have been weathered significantly. Considering their similar mineralogies, identical mineral compositions, and identical unusual textures, Nova 001 and Nullarbor 010 are probably paired. Based on olivine compositions, Nova 001 and Nullarbor 010 are in Group 1 (FeO-rich) of Berkley et al. (1980). Silicate mineral compositions are consistent with those of other known ureilites. The presence of euhedral graphite crystals within the silicate minerals is consistent with an igneous origin, and suggests that large proportions of silicate magma were present locally and crystallized in situ. 相似文献
9.
The extinction curves of single clouds, seen towards the stars HD 147165, 179406 and 202904, have been modelled using various mixtures containing both the bare and inhomogeneous (composite and/or multilayer) grains. It has been shown that the models composed of the bare graphite and silicate grains together with the multilayer grains containing silicates, organic refractory and water ice, are more useful in explaining extinction under the reduced cosmic abundances. The models based on Mathis' composite grains or on Greenberg & Li's core-mantle grains can also provide quite good fits of the extinction and the measured scattering parameters, but still require an excessive amount of carbon which results in too large a C/O ratio. The inhomogeneous grains essentially contribute to the extinction in practically the whole wavelength range of our extinction curves. As a rule, such grains have quite wide size distributions, centred at around 100 nm, although graphite grains are mainly of small sizes. 相似文献
10.
N.C. Wickramasinghe 《Astrophysics and Space Science》1999,268(1-3):111-114
Refractory grains of graphite and silicates ejected from cool giant stars acquire mantles of organic polymers typified by
polyoxymethyline in interstellar clouds. Infrared emission from theTrapezium nebula is consistent with the occurrence of such
polymers.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
Massimo Marengo &#;eljko Ivezi&#; Gillian R. Knapp 《Monthly notices of the Royal Astronomical Society》2001,324(4):1117-1130
We analyse the differences in infrared circumstellar dust emission between oxygen-rich Mira and non-Mira stars, and find that they are statistically significant. In particular, we find that these stars segregate in the K–[12] versus [12]–[25] colour–colour diagram, and have distinct properties of the IRAS LRS spectra, including the peak position of the silicate emission feature. We show that the infrared emission from the majority of non-Mira stars cannot be explained within the context of standard steady-state outflow models.
The models can be altered to fit the data for non-Mira stars by postulating non-standard optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical silicate grain condensation temperatures (800–1000 K) . We argue that the latter is more probable and provide detailed model fits to the IRAS LRS spectra for 342 stars. These fits imply that two-thirds of non-Mira stars and one-third of Mira stars do not have hot dust (>500 K) in their envelopes.
The absence of hot dust can be interpreted as a recent (∼100 yr) decrease in the mass-loss rate. The distribution of best-fitting model parameters agrees with this interpretation and strongly suggests that the mass loss resumes on similar time-scales. Such a possibility appears to be supported by a number of spatially resolved observations (e.g. recent Hubble Space Telescope images of the multiple shells in the Egg Nebula) and is consistent with new dynamical models for mass loss on the asymptotic giant branch. 相似文献
The models can be altered to fit the data for non-Mira stars by postulating non-standard optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical silicate grain condensation temperatures (800–1000 K) . We argue that the latter is more probable and provide detailed model fits to the IRAS LRS spectra for 342 stars. These fits imply that two-thirds of non-Mira stars and one-third of Mira stars do not have hot dust (>500 K) in their envelopes.
The absence of hot dust can be interpreted as a recent (∼100 yr) decrease in the mass-loss rate. The distribution of best-fitting model parameters agrees with this interpretation and strongly suggests that the mass loss resumes on similar time-scales. Such a possibility appears to be supported by a number of spatially resolved observations (e.g. recent Hubble Space Telescope images of the multiple shells in the Egg Nebula) and is consistent with new dynamical models for mass loss on the asymptotic giant branch. 相似文献
12.
IRAS low-resolution spectra are presented for 36 HAEBE stars. It is found that silicate dust is common material in the circumstellar
disks or/and shells of HAEBE stars. It is also found that the PAH feature often appears as well. In addition, the correlation
between the dust features and spectral type, and infrared excesses can be found for most HAEBE stars.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
Image-tube filter photographs calibrated against photoelectric filter photometry have been used to give maps of M42 in absolute flux units over the central 15 arc min of the nebula in Hα, [Nii] (λ 6584 Å), Hβ and continuum at λ 4700 Å. Maps of the ratios Hα/[Nii] and (for the first time) of continuum/Hβ have been produced with unprecedented spatial resolution. These show that the gas to dust ratio is high near the exciting stars and falls strongly in the vicinity of large scale ionization fronts marked by minima in the Hα/[Nii] ratio. These results are interpreted in terms of detailed shell models containing either ice or graphite or silicate scattering particles. In all models there must be a central hole in the distribution of scattering particles. The effect of neutral globules and intrusions is investigated. It is found that all types of grain are trapped inside neutral intrusions near the centre of the nebula by the pressure of the Lα light surrounding the globule, but in the early evolution of the nebula particles can escape into the ionized medium when fronts are R-type. Ice grains escaping at this time will be destroyed for distances to the exciting stars less than 1 pc. These results can explain both the central hole in dust and the underabundance of oxygen in the ionized gas observed earlier. Arguments depending on colour index of the scattered light indicate that mixtures of scattered light from ice in the globules and from ice in the ionized medium can explain the observations, but that the graphite and silicate particles fail. A schematic model of the Orion Nebula is presented to attempt to explain the large scale phenomena observed here. It demonstrates that simple shell models for this nebula are dubious. 相似文献
14.
Ed Churchwell 《Astronomy and Astrophysics Review》1990,2(2):79-123
Summary Ultracompact (UC)HII regions are manifestations of newly formed massive stars that are still embedded in their natal molecular cloud. They are among the brightest and most luminous single objects in the Galaxy at far infrared and radio wavelengths. Recent high spatial resolution studies, particularly at radio wavelengths, have greatly contributed to our understanding of these dynamic objects and the impact they have on their environment. A summary is given of our current understanding of the physical properties, morphologies, dynamics, number and distribution in the Galaxy, and molecular environments of UCHII regions. Recent models of the circumnebular dust imply that the graphite/silicate abundance ratio is about half that of dust in the diffuse interstellar medium. The dust cocoons are large, cool, and optically thick shortward of a few microns. There are apparently between 1700 and 3000 UCHII regions in the Galaxy. This represents 10–20% of the total O star population. There are too many UCHII regions (just counting those studied with the VLA) to be consistent with the short dynamical lifetimes of this very compact stage of evolution. Both the morphologies and the large number can be understood if UC HII regions are bow shocks. Models of stellar wind supported bow shocks are discussed and consequences for the dynamics and morphologies of the ionized and molecular gas are explored. 相似文献
15.
The influence of various factors on the interpretation of polarimetric observations of Mars performed under high-transparency atmospheric conditions was investigated. It was demonstrated that the technique proposed by Morozhenko (1974) for the data interpretation does not allow the optical characteristics of cloud particles to be estimated unambiguously. In particular, it turns out that both the model of a thin layer composed of very small silicate particles and the model of very small ice particles in a thin layer fit the data of measurements at a phase angle of 25°. Numerical simulations showed that reliable information on the reflectance characteristics of the Martian surface in a wide spectral range, including blue and ultraviolet wavelengths, is necessary to obtain reliable estimates of aerosol optical properties. Our calculations confirmed that the scattering on ice-cloud particles could introduce a significant ambiguity in the interpretation of the available polarimetric data. Besides, small particles, both ice and silicate, existing in the upper atmospheric layer can substantially mask the layer of larger particles below, which becomes invisible for remote sensing. 相似文献
16.
The data compiled for 34 very young massive compact infrared source (Becklin-Neugebauer objects) in a separately published catalogue serve as the basis for an investigation of their general properties. First, the correlations of the strength of the ice and silicate bands at 3.1 μm and 10 μm, resp., with each other and with other parameters of the sources are investigated. In accordance with theoretical expectations the strengths of both bands are not well correlated with each other. The ratio τ10/τ3 of their optical depths veries from 0.85 to 8 (and possibly up to 22). That implies for the number densities of the grains that (naked) silicate grains always are considerably more abundant than ice grains (grains with ice mantles). It follows from the discussion of the relationships between τ10, the temperature parameter of the sources, and their luminosity that relations between τ10 and temperature and possibly between τ10 and the luminosity exist. Stellar wind models for the ionized region around the embedded star are favoured after the analysis of the intensities of the hydrogen infrred emission lines and the radio continuum. The limited observational data do not yet allow a decision on a correlation between the silicate band depth and energy content and/or the velocity of the bipolar outflow detected in a number of sources. The BN objects are clearly more frequently associated with H2O masers than OH masers. The linear separations of the infrared peaks from the masers are compatible with models where the H2O masers are situated near the edges the gas-dust shells of the BN objects. 相似文献
17.
Some cylindrically symmetric inhomogeneous cosmological models for perfect fluid distributions with electromagnetic field
are obtained. To get a determinate solution, a supplementary condition between metric potentials is used. Various physical
and geometrical properties of the models with some special cases are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
Saibal Ray 《Astrophysics and Space Science》2002,280(4):345-355
Static spherically symmetric charged dust models, known as Lane-Emden electromagnetic mass models, having their seed in the
well-known Lane-Emden equations of classical astrophysics, have been derived for Einstein-Cartan theory. The models are of
purely electromagnetic origin. The physical significance of these solutions can be understood from the very fact (i) that
they are generated from the well-known Lane-Emden equations of classical astrophysics having a proven physical foundation
and (ii) that, being the solution of Einstein-Cartan theory, they involve spin and torsion, characteristics which a material
system must possess for its subtle and finer details. PACS number(s): 04.90.+e, 04.20.jb.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
Andrzej J. Maciejewski Marek Szydłowski 《Celestial Mechanics and Dynamical Astronomy》1999,73(1-4):17-24
We discuss specific properties of dynamical systems originating from cosmology and relativity. In particular, we present results
of our study of the Bianchi class A cosmological models. We introduce new variables in which the Hamiltonian constraint for
all the class A models is solved algebraically. We present results of dimension reduction of the investigated models.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
The Shape of The Sunspot Cycle: A One-Parameter Fit 总被引：1，自引：0，他引：1
D. M. Volobuev 《Solar physics》2009,258(2):319-330