Interstellar extinction and polarization by spheroidal dust grains

Using the recently available exact computations of the scattering efficiencies of spheroidal particles numerical calculations of the extinction and polarization curves have been made for a distribution of particle sizes, shapes and orientations. The results are presented and compared with the observed interstellar extinction and polarization. Possible models for interstellar dust with nonspherical grains have been discussed.     

Interstellar extinction by spheroidal absorbing dust grains

On the basis of exact computations of the extinction efficiencies of spheroidal absorbing particles, numerical calculations of the extinction curves have been made for a distribution of particle sizes. The results are presented and compared with the observed interstellar extinction. For the sake of comparison, results for the nonabsorbing particles are also given.     

Accretion and electrostatic interaction of interstellar dust grains; Interstellar grit

This work is divided into 13 sections and 2 appendices, and aims to elucidate the accretion mechanism, which operates via image-theory forces, whenever two interstellar dust grains come close together. Section 1 is an introduction. Section 2 proposes that the distribution of interstellar grains be taken asn(r) r ^–4 to avoid distortion of the 3K microwave background by radiation from spinning grains. Section 3 examines each of three types of image force accretion processes, finding them to be dominant compared to radiation or gravitational forces by at least a factor of 10¹⁹. Section 4 states that only grains made of conducting material (e.g., graphite, ice, iron) are involved in image theory. Section 5 presents reasons for believing that two grains should coalesce on impact. Section 6 examines the motion of charged interstellar grains in Hi and Hii regions. Section 7 demonstrates, by way of four examples involving dust grains ofr=10^–7 cm up tor=10^–4 cm, that the image effects on conducting grains are not trivial, and that the dynamics involved is not to be compared at all with elementary Coulomb interaction of two changes. Section 8 concludes that accretion with not take place in Hi clouds if thermal (equipartition) velocities prevail among the dust particles. section 9 examines grain interactions in Hii regions: here, following an argument due to Spitzer, consideration is given to the case of a population of dust grains all streaming in the direction of the local magnetic field B at velocities of order 0.1 km s^–1. It is shown that accretion takes place effectively, leading to the formation of interstellar grit, meaning grains of mass 10^–8 to 10^–7 gm, radius 0.1 mm; and leaving also a population ofr10^–6 cm grains, which are observed in polarization and extinction measurements. The existence of the latter is now a deduction and not an ad hoc postulate, as previously, and implies a distribution of the general formn(r) r _mean ^–3 , in approximate agreement with that of Section 2. Section 10 considers the accretion mechanism as a cascade process. Section 11 shows that the existence of grains in space ofr 10^–6 cm rules out an origin in supernova or galactic explosions, and supports a passive origin, perhaps in red giants or Mira variables. Section 12 discusses the implications of the results found for polarization observations and cosmogony, the latter being given a new foundation in which planets of different composition form automatically from a solar nebula. Section 13 is a conclusion.     

Circumstellar dust and circumstellar extinction curves

We have calculated the circumstellar extinction curves produced by dust grains which absorb and scatter the stellar radiation in the shells of pre-main-sequence stars. A Monte Carlo method was used to model the radiative transfer in non-spherical shells. The dependence on the particle size distribution and the dust shell parameters has been examined.The application of the theoretical results to explain the extinction and polarization of the Herbig Be star HD 45677 shows that the dust shell is not disk-like and that very small grains are absent in it.     

星际红外消光律

该文综述了星际尘埃在红外波段的消光律。现有的观测表明,在0.7μm≤λ≤5μm范围内的星际消光可以近似用幂律函数来拟合,而对5μm≤A≤8μm范围内的星际消光争论比较多,但现在已明确该范围内消光律是随视线方向的变化而变化,这与以往认为在整个0.7μm≤λ≤8μm范围内星际消光为各处一致的结论是相左的。在8μm≤λ≤30μm范围内的消光主要由硅酸盐在9.7μm和18μm处的吸收特征所决定,其中9.7μm处的吸收所引起的消光强度(相对可见光波段消光)在太阳邻近弥漫星云和银心方向上有较大变化,其原因仍有争论;同时,由脂肪族碳氢化合物的C-H键伸缩振动引起的在3.4μm处的谱特征强度也和9.7μm硅酸盐谱特征类似,在太阳邻近弥漫星云和银心方向有较大变化。在λ≥30μm以上的远红外范围,由于观测的限制,人们主要利用尘埃红外辐射理论来间接研究消光率,不同尘埃模型给出的红外辐射明显地不同,对λ≥30μm的红外观测将提高该范围内消光律的准确性。     

Systematic variations of interstellar linear polarization and growth of dust grains

The observed relation between the interstellar linear polarization curve parameters K and λ _max characterizing the width and the wavelength of the polarization maximum, respectively, is interpreted quantitatively. We have considered 57 stars located in four dark clouds with evidence of star formation: in Taurus, Chamaeleon, around the stars ρ Oph and R CrA. In our modeling we have used the spheroidal dust grain model applied previously to simultaneously interpret the interstellar extinction and polarization curves in a wide wavelength range. The observed trend K ≈ 1.7λ _max is shown to be most likely related to the growth of dust grains due to coagulation rather than accretion. The relationship of the interstellar polarization curve parameters K and λ _max to the mean dust grain size is discussed.     

Interstellar scintillation and clouds of the interstellar turbulent plasma

Data on interstellar diffraction and refraction scintillation of pulsars are analyzed. Comparison between theory and the observational data shows that two types of spectra for electron density fluctuations are realized in the interstellar medium: pure power law and piecewise with a break. The distribution of turbulent plasma in the Galaxy has a three component structure. Component A is diffuse and it is distributed outside of the spiral arms of the Galaxy. Component BI is cloudy and associated with Galactic arms. Component BII is extremely nonuniform and associated with HII regions and supernova remnants. The origin of the interstellar plasma turbulence is considered, and possible sources of turbulent energy are discussed. The contribution of supernova bursts in the interstellar gas ionization and generation of turbulence are analyzed among other factors. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interstellar dust in the solar system

Dust is an important component of galactic stucture and the cyclic processing of particulate matter leads to stellar and planetary formation. Though astronomical methods using analysis of dust-penetrating starlight can provide some limited information about the dust, the prospect of its in-situ sampling within the Solar System by spacecraft and its remote sensing by ground-based techniques open up a new field in galactic exploration.     

Features of the interstellar extinction curve

The extinction curves for spherical particles are subject to the errors of the particle material's refractive index. Their sensitivity to these errors has been investigated and is found to be dependent upon wavelength. For graphite, significant errors are produced in the far ultraviolet part of the extinction curve; for silicates, in the near ultraviolet; while for iron the error is relatively small. The wavelength dependence of the 10 μm and 20 μm absorption bands of small silicate spheroids upon their shape and alignment has been studied. It is found that the bands can be displaced by ～1 μm towards longer wavelengths from their positions for corresponding spheres: and that a further, though small, displacement can be superimposed upon this by their subsequent alignment.     

Infrared extinction of heavy ion irradiated and amorphous olivine,with applications to interstellar dust

A smooth surface layer of highly disordered olivine, (Mg, Fe)₂SiO₄, has been produced by exposure of polished, natural olivine to a dose of 5×10¹⁶ cm^–2 of 1.5 MeV neon ions from a Van de Graaff accelerator. The dielectric functions of the disordered silicate in the wavelength range from 8 to 30 m have been determined from analysis of specular reflectance data, and extinction for Rayleigh particles of such disordered olivine has been calculated. Extinction measurements for amorphous olivine smoke collected on a substrate are also presented. The small particle extinctions of both kinds of structurally disordered olivine are shown to agree well with the main features of the absorption and emission spectra from interstellar grains in the 10 and 20 m region.     

Abundances in the interstellar medium

Summary Recent developments in the theory of element production and the chemical evolution of the galaxy are presented. Following this, observational data and their interpretation are given. A case by case analysis of results for D, He, Li and CNO isotope data in the disk and center of our galaxy is presented; previous results for element gradients are also summarized.The primordial abundances of D and He cannot be directly obtained from observations; corrections for stellar processing are discussed. From these data and the Li abundances, it appears that the abundance of the light elements is consistent with the standard big bang. In agreement with previous results, the range of, the baryon to photon ratio, is 5–8 10^–10. If the amount of non-baryonic matter is small, these results indicate an open universe, in the standard big bang model.New data show a gradient in the (¹²C/¹³C) and (¹⁶O/¹⁸O) ratios with galactocentric distance, D_GC. The presence of a gradient in the (¹⁴N/¹⁵N) ratio is less clear and there is no measurable gradient in the (³²S/³⁴S) ratio. In the interstellar medium near the sun, the carbon isotope ratio is –20 percent lower than the solar system ratio. This indicates that there has been only a moderate amount of enrichment of the nearby interstellar medium since the formation of the solar system. These results and previously determined galactic element gradients are interpreted in the framework of chemical evolution models. Delayed recycling of nucleosynthesis products is essential for the correct interpretation of the results. Comparisons of data with galactic evolution models are discussed.This article was processed by the author using the Springer-Verlag T_EX AAR macro package 1991     

Limited diversity of the interstellar extinction law

We have applied the method of investigating extinction curves using statistically meaningful samples that was proposed by us 25 years ago. The extensive data sets of the ANS (Astronomical Netherlands Satellite) and 2MASS (Two Micron All Sky Survey) were used, together with UBV photometry to create average extinction curves for samples of OB stars. Our results demonstrate that in the vast majority of cases the extinction curves are very close to the mean galactic extinction curve. Only a few objects were found to be obviously discrepant from the average. The latter phenomenon may be related to nitrogen chemistry in translucent interstellar clouds (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On linear light polarization of novae: dust envelopes

A review on linear light polarization studies of novae is presented. It is shown that light scattering on dust grains (predominantly graphite) in nova environments is playing a principal role in generation of linear light polarization. A qualitative model responsible for this phenomena is discussed.