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1.
The rigorous Kerker-Matijevi formulae for light scattering by co-axial double cylinders are used to calculate the extinction properties of hollow organic grains. A size distribution of such particles together with iron whiskers of radii 0.01 m, silica spheres of radius 0.03 m and free aromatic molecular clusters comprised of 50–100 atoms yield excellent agreement with data on the extinction of starlight. The mass ratios of silica to organics and of iron to organics are in good accord with cosmic abundance constraints.  相似文献   

2.
Monochromatic extinction coefficients at four wavelengths have been obtained over a period of more than two years at the Observatorio del Teide (Izaña Tenerife) using a full disc, direct sunlight, quadruple photometer devoted to the detection of integral luminosity oscillations of the Sun. The mean extinction coefficients (0.13 at 500 nm) show a seasonal variation of about 15%, the best atmospheric conditions being in winter and autumn. Moreover, in anyone day the extinction coefficient in the afternoon is always lower than the one in the morning by 7%. A one-year period fluctuation, with an amplitude of 0.035 mag, has been identified in the instrumental magnitudes outside the atmosphere, and is interpreted as the variation produced by the different Sun-Earth distance from winter to summer. Finally, the study made to detect periodic time fluctuations in both, Sun's magnitude and extinction coefficients, has given null results at levels of 0.04 and 1.8%, respectively.  相似文献   

3.
From an analysis of the interstellar extinction we conclude that interstellar grains are of three main kinds: graphite spheres of radii 0.02 m making up 10% of the total grain mass, small dielectric spheres of radius about 0.04 m making up 25% of the mass, and hollow dielectric cylinders containing metallic iron with diameters of 2/3 m making up 45% of the mass. The remaining 20% consists of other metals and metal oxides. The main dielectric component of the grains appears to be comprised of organic material.  相似文献   

4.
Extinction of light by randomly-oriented non-spherical grains   总被引：1，自引：0，他引：1  
Waterman'sT-matrix approach is used to derive a simple analytical expression for the extinction cross-section for randomly-oriented non-spherical grains. Numerical results are presented for randomly-oriented oblate and prolate spheroids and Chebyshev particles composed of astronomical silicate. These results are compared with those for spherical grains, and possible influence of the shape of dust grains on the value of interstellar extinction is considered. The range of validity of the Rayleigh approximation for computing extinction efficiency factors for randomly-oriented non-spherical grains is discussed.  相似文献   

5.
The microorganism model of interstellar grains is investigated by spectroscopy from the infrared (IR), visible to the ultraviolet (UV) wave regions.E. coli, yeast and spores ofBacillus subtilis exhibit absorption bands at =3.1 and 9.7 m; they also exhibit several absorptions at 68 m which are in agreement with the observed IS extinction curves.To obtain the extinction curves in the visible and UV regions, dry films of microorganism are prepared on a MgF2 plate or synthesized quartz plate and their spectra measured. In the wavelength region 190400 nm, conventional spectrophotometers are adopted for the measurement. The extinction curve of the film ofE. coli is similar to the observed IS curve.For the wave-range 100<<400 nm, a vacuum UV spectrometer is adopted to avoid absorptions due to O2 in the atmosphere. The extinction spectra by this method are in agreement with the result obtained by the conventional method where comparison is possible. The extinction curves ofE. coli and yeast are such that they incrase towards the short wavelength and exhibit a peak at -190 nm, which is different from the well-known IS peak at =220 nm. It remains to be seen whether interstellar low temperatures (1040 K) can shift the peak position in the extinction curve of biochemical materials.  相似文献   

6.
Laboratory spectra of SiO particles of 1 m radius show a broad structureless extinction peak at 9.6m. The wavelength dependence of extinction from SiO, an amorphous silicon oxide, provides a good match to that of interstellar dust.  相似文献   

7.
The optical constants of a carbon polymorph, calculated using the Drude dispersion theory, are shown to lead to close agreements with the mean interstellar extinction curve over the waveband 0.3 -1 5.5 m-1 for particles of Rayleigh scattering sizes. Astrophysical microsoot grains of radii 50Å may be related to microdiamond grains of similar sizes that have recently been discovered in carbonaceous chondrites. It is postulated that such microsoot particles could contribute to the nearly invariable interstellar extinction law observed over the near IR, visible and near to mid UV spectral regions. Admixtures of microsoot with micron-sized microsoot clumps, microdiamond and hollow organic / biologic grains provide an explanation for the full range of extinction and polarization data.  相似文献   

8.
The correlation betweenE(B-V) reddenings of high latitude radio quasars (from Teerikorpi, 1981) and star/galaxy counts from the PSA prints was investigated. With the resolution down to 5×5, the reddenings and the counts show the relationship expected if it is assumed that most of the high latitude stars (B<21 mag) are situated above the dust clouds. The dust causing the reddenings is probably connected with the population of molecular clouds observed at high galactic latitudes. The present study underlines the importance of simple star/galaxy counts (all images counted) for the study of high latitude variable extinction. The existence of variable extinction shows that also the average extinction must be significant.  相似文献   

9.
A mass contribution of graphite whiskers as small as 0.1% to the population of interstellar grains could dramatically change their far-infrared extinction properties. With varying mass fractions of graphite whiskers, and for different size parameters, the infrared extinction could vary from a -2 dependence to 0, consistent with the requirements of some astronomical observations.  相似文献   

10.
We show that a 3 extinction feature in galactic infrared sources cannot be due to water-ice grains. Infrared spectra with a resolution of /=0.015 are in remarkably close agreement with the 2.5–4 extinction properties calculated for bacterial grains.  相似文献   

11.
In a recent paper Millar has shown that if one assumes that the carbon depleted from the gas phase is all tied up into small graphite grains the observational data raise serious doubts against the hypothesis that these particles are responsible for the 2200 Å extinction hump. In the present paper it is shown that this problem may be overcome if the presence in the interstellar space of graphite grains with sizes greater than 0.02 is taken into account. The derived ratios between the masses of large grains, which do not contribute to the ultraviolet extinction hump, and those of the small ones varies from region to region of the sky and are consistent with those evaluated in the circumstellar shells of carbon stars. Moreover, the largest sizes of the graphite particles we find are in agreement with those needed to fit the interstellar extinction curve over the wavelength range 0.11–<1 .  相似文献   

12.
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 in directions towards most early-type stars.The peak absorption is measured to be 35000 cm2 g–1. This is in addition to a scattering component of the extinction which has an estimated value for dry microorganisms of 50000 cm2 g–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 –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 pathlenght along the galactic plane at =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   

13.
Extinction efficiency of grains is calculated from the Mie formula on the premise that the grains are of organic composition. The optical constants adopted for the calculations are those ofE. coli, polystyrene and bovine albumin. The grain radiusa is assumed to obey a distribution of the formN(a) a – and the value of is chosen so as to make the calculated extinction curve match the observed interstellar extinction curve. Although the calculated curve gives a reasonably good fit to the observed extinction curve for wavelength less than 2100 Å, at longer wavelength region agreement is poor. It is concluded that another component is required for the organic model to be viable.  相似文献   

14.
In this paper we present the results of the calculations of anomalous extinction laws characterized by different values of the ratio of the total to selective extinctionR=A v/E(B-V). As far as we are aware, no such theoretical extinction laws have been published before. The calculation is based on the Mie theory of light scattering by small particles assuming a two-grain dust model of Mathiset al. (1977) and by adopting theri particle size (a) distribution, given byn(a)=a –3.5, in which the minimum and maximum size is taken to be 0.005 and 0.22 m, respectively. Furthermore, we use the dielectric functions for the two types of grains, graphite and astronomical silicate, as derived by Draine and Lee (1984), published in tabular form by Draine (1985). Following Mathis and Wallenhorst (1981), we then obtained the anomalous extinction laws by changing the upper size cutoff of the particles.A comparison shows that the calculated anomalous extinction laws agree quite well with the corresponding laws derived observationally using the colour-difference method. Finally, a trial and error method for the determination of the anomalous extinction law based on a comparison of the observed extinction-free spectral energy distribution with the corresponding theoretical Kurucz (1979) model, is explained.  相似文献   

15.
Using the discrete dipole approximation, we have calculated the extinction efficiency of hollow spherical particles of graphite as a possible constituent of interstellar grains. The particles had a shell structure with the basal plane perpendicular to the radius. The calculations were made on the particles having the outer radiusR 0=10 and 5nm in the wave number region from 0.8 to 8.0 m–1 using the anisotropic optical constants. It was found that the hollow particles with the inner radiusR 10.65R 0 yield an extinction feature at 4.6 m–1, which fits fairly well to one observed in the interstellar extinction.  相似文献   

16.
The stellar field centred close to theh and Per double cluster is one of the 123 fields recorded in the galactic plane at 2000 Å by the balloon-borne stratospheric gondola of the SCAP-2000 programme. The analysis of the frame allows us to determine an ultraviolet colour indexU 1-V for more than 600 stars. Among these are stars belonging to theh and Per and Tr 2 clusters and to the PER OB1 association. The prevailing extinction law is found to produce greater extinction at 1965 Å than predicted by the mean extinction law. Moreover, the clouds responsible for the extinction are situated in the local arm and distributed in two layers with a very transparent interval. The comparison of theA v extinction and theHI and CO abundances leads us to assume the presence of a H2 cloud in front ofh and Per, in the second absorbing layer and, therefore, in the local arm. The two absorbing layers and the molecular cloud are perhaps in the plane of the Gould belt and associated with the expanding gas detected by Lindblad. A group of hot stars centred at the same distance as this molecular cloud has been detected and could form an association of OB stars in the local arm. Other, much more distant OB stars belonging to the Perseus arm of Efremov's list. Several stars which must have a very hot companion are detected in the field.  相似文献   

17.
The exact calculation of scattering and absorption by various sub-micron sized silicate spheres is presented here, using accurately determined optical constants in the wavelength range from 50 m to 0.21 m. The extinction features near 10 m and 20 m for various samples are discussed. It is found that the ratio of peak extinction at 20 m to that at 10 m is constant for small particles up tor=0.4 m, but is less for particles of radius 1 m. The ratio of maximum extinction in the ultraviolet to that at 10 m decreases with increasing particle size.  相似文献   

18.
The observed features of the interstellar extinction curve in both the Galaxy and in the external galaxies LMC and SMC are explained on the basis of a model involving clumps of hollow or porous organic particles with an overall volume filling factor of 0.1. The hollow organic particles have a ready explanation in terms of the bacterial grain model. The visual and near infrared observations of extinction are fitted by a size distribution of such aggregates similar to the distribution law discovered for the larger grains in Halley's comet, but with a cutoff at low radii taken at 0.5 m for the Galaxy and 0.3 and 0.4 , respectively, for SMC and LMC. Fine tuning of the theoretical extinction curve to fit observational data in the ultraviolet involves variable contributions from small condensed polymeric units of typical radius 0.012 m and graphite particles of radii 0.02 . These particle species may be regarded as being derived from the primary distribution of clumps. The implication is that cometary sources could provide a major component of the grains in interstellar space.  相似文献   

19.
The spectral fine structure of solar radio continua is thought to reveal wave-particle and wave-wave interactions in magnetic traps in the solar corona. We present observations of spectra, polarization, and spatial characteristics of combined emission/extinction features (zebra patterns) during a decimetric/metric type IV event on 5 June, 1990. Very high modulation depths are observed. The size and location of the sources during emission and extinction are determined for the first time. Two remarkable features are found: (1) The sources of emission stripes have finite size, up to nearly 2; during extinction stripes the brightness is reduced across the whole extent of the unperturbed continuum, which is slightly larger than 2. (2) During emission stripes the sources drift over distances up to several × 104 km, with apparent velocities up to 105 km s–1. The observed features are briefly discussed with respect to interpretations based on wave-particle interactions and on the scattering of electromagnetic waves.  相似文献   

20.
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   

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