Interstellar circular polarization and the dielectric nature of dust grains

The implications of the observed relationship between the wavelength dependence of interstellar circular and linear polarization were reexamined. Mie theory calculations for grains with various optical constants demonstrate that any population of grains which matches the observed wavelength dependence of linear polarization also yields the correct cross-over wavelength of circular polarization. The coincidence of the peak wavelength of linear polarization and the cross-over of circular polarization is therefore independent of the optical constants of the grains and cannot be used as a critical constraint on grain properties. The observed relationship instead reflects a more fundamental connection between linear and circular polarization which was derived from the Kramers-Kronig relations by Shapiro (1975). Numerical results fully support Shapiro's conclusions and demonstrate that the apparent upper limit on the visual absorptivity of polarizing grains deduced from earlier Mie theory calculations (Martin, 1972) was spurious and resulted from a violation of the Kramers-Kronig relations in the assumed optical constants of the particles. The Kramers-Kronig interpretation of circular polarization can be used to place constraints on linear polarization outside the wavelength range in which it was observed. This approach was used to show that the peak observed in the visual is likely to be the only significant feature in the linear polarization curve, which therefore appears to be well approximated at all wavelengths by the Serkowski formula. A synthesis of available laboratory data was used to analyze the properties of dielectric core-mantel grains as the source of visual extinction and polarization

Scattering by fluffy grains

There are indications (Greenberg et al., 1988), that fluffy (i.e., porous) particles are responsible for the observed 3.4 and 10 micron emissions of comet Halley. The absorption characteristics of small particles both solid and fluffy are needed in order to explain the Halley emissions. How isolated small solid particles react to an external radiation field is well known - the Rayleigh approximation. How these same small particles emit when assembled as fluffy aggregates in another question. To what degree are the emission spectra of isolated and aggregated particles comparable. In order to quantify the assertion that fluffy particles produce the observed Halley infrared emission features, the authors are performing calculations to determine the effect of porosity on the absorption characteristics of aggregates of interstellar grain-type particles. The calculations are based on an integral representation of the scattered electromagnetic field. Results are given with application to comet Halley

The evolution of organic mantles on interstellar grains

By laboratory simulation of the chemical processes on dust grains it was investigated how solid organic materials can be produced in the interstellar medium. The ice mantles that accrete on grains in molecular clouds, consisting primarily of H2O, CO, H2CO, NH3, and O2, are irradiated by the internal UV field, resulting in the storage of radicals upon photodissociation of the original molecules. Transient heating events lead to the production of oxygen-rich organic species by recombination reactions. The experiments indicated that in this way the observed amount of organic material can be produced if a grain passes a few times through a molecular cloud during its life. After the destruction of the cloud the grains enter a more diffuse medium. Here they are subjected to the interstellar UV field as well as to collisions with atomic hydrogen. Experiments show that the intense photoprocessing results in the removal of small species like H2O and NH3 as well as in carbonization of the organic molecules. Contrary to this, the atomic H flux will maintain a certain hydrogen level in the mantle. These processes likely convert the original, oxygen-rich organics into an unsaturated hydrocarbon type material such as that observed towards IRS 7 and in Comet Halley grains

Diffusion and infrared properties of molecules in ice mantles

Within dense molecular clouds the formation of frozen icy mantles on interstellar dust grains is thought to be the result of various growth conditions. The molecules, which make up the ice mantles are probably completely mixed. To study the physical properties of such ice mixtures the experiments were performed on the evaporation processes and on the spectroscopic properties of CO, CO2, and CH4 in water rich ices. The decrease in concentration of volatile molecules in ice samples deposited at 10 K and subsequently heated is found to occur essentially in two steps. The first one, corresponding to an evaporation of part of the volatile molecules, starts at about 25 K for CO, 32 K for CH4, and 70 K for CO2. During the crystallization of H2O ice at temperatures greater than 120 K a second evaporation occurs leading to the complete disappearance of the volatile molecules in the solid phase. The main astrophysical implications of the diffusion and spectroscopic behaviors are presented. The possible effects of a heating source on the fraction of volatile molecules, such as CO trapped in grain mantles, are discussed

A comparative study of the continuum and emission characteristics of comet dust. 1: Are the silicates in Comet Halley and Kohoutek amorphous or crystalline

A continuum emission was subtracted from the 10 micron emission observed towards comets Halley and Kohoutek. The 10 micron excess emissions were compared with BN absorption and laboratory amorphous silicates. The results show that cometary silicates are predominantly amorphous which is consistent with the interstellar dust model of comets. It is concluded that cometary silicates are predominantly similar to interstellar silicates. For a periodic comet like Comet Halley, it is to be expected that some of the silicate may have been heated enough to convert to crystalline form. But apparently, this is only a small fraction of the total. A comparison of Comet Halley silicates with a combination of the crystalline forms observed in interplanetary dust particles (IPDs) seemed reasonable at first sight (Walker 1988, Brownlee 1988). But, if true, it would imply that the total silicate mass in Comet Halley dust is lower than that given by mass spectrometry data of Kissel and Krueger (1987). They estimated m sub org/m sub sil = 0.5 while using crystalline silicate to produce the 10 micron emission would give m sub org/m sub sil = 5 (Greenberg et al. 1988). This is a factor of 10 too high

Airfoil in sinusoidal motion in a pulsating stream

The forces and moments on a two-dimensional airfoil executing harmonic motions in a pulsating stream are derived on the basis of non-stationary incompressible potential flow theory, with the inclusion of the effect of the continuous sheet of vortices shed from the trailing edge. An assumption as to the form of the wake is made with a certain degree of approximation. A comparison with previous work applicable only to the special case of a stationary airfoil is made by means of a numerical example, and the excellent agreement obtained shows that the wake approximation is quite sufficient. The results obtained are expected to be useful in considerations of forced vibrations and flutter of rotary wing aircraft

New insights in the photochemistry of grain mantles: The identification of the 4.62 and 6.87 micron bands

The mid-IR spectral region of molecular clouds is known to show the fingerprints of molecules frozen in the icy mantles of the interstellar grains. To study the complex chemical and physical interactions on the ice mantles accreted on grains in molecular clouds numerous UV irradiation and diffusion experiments were performed. The irradiation of binary ices was studied. Using isotopic labelling on NH3/CO and NH3/O2 ices numerous compounds were identified, of which OCN(-), NO2(-), NO3(-), and NH4(+) ions reveal a new type of chemical reactions. It appeared that these compounds were formed by proton transfer reactions induced by the interaction between an acid (HNCO, HNO2, HNO3) and a base (NH3) through a hydrogen bond. This mechanism was confirmed by a study of photolyzed diluted argon mixtures. The main astrophysically relevant data from the overall study are presented. The 4.62 micron band in W33A can be reproduced with NH3/CO containing irradiated ices and was identified with OCN(-). The 6.87 micron band in W33A and other photostellar objects is reproduced with NH3/O2 containing ices and is identified with NH4(+)

From comets to meteors

Abstract. A summary of comet nucleus and dust properties is used to suggest a basis for predicting the properties of meteor shower particles originating as comet debris