Santi Aiello

OPTICAL PROPERTIES OF COMPOSITE INTERSTELLAR GRAINS: A MORPHOLOGICAL ANALYSIS

In the framework of the transition matrix approach, we calculate the relevant optical properties of cosmic dust grains of amorphous carbon and astronomical silicates, modeled as aggregates of spherical monomers. Two mechanisms of aggregation were considered, producing clusters with different structure and degree of fluffiness: ballistic particle-cluster aggregation (BPCA) and ballistic cluster-cluster aggregation (BCCA). Our results are very different from those obtained through computational approaches based on effective medium theories and might have major implications both on the modeling procedure and on the dust-mass balance in the interstellar medium.

Beyond Mie Theory: The Transition Matrix Approach in Interstellar Dust Modeling

We model cosmic dust grains as aggregates (clusters) of spheres of appropriate geometry, whose optical properties we calculate in the framework of the transition matrix method. The calculation is performed without resorting to any approximation and with a computational effort that is noticeably lighter than the one required by other methods. Whatever the geometry chosen to model the cosmic grains, the orientational averages that are necessary to describe the propagation of the electromagnetic radiation through a dispersion of clusters are easily handled by exploiting the transformation properties of the transition matrix elements under rotation of the coordinate frame. In this paper we focus on the potentialities of the cluster model by comparing the extinction spectrum of a sphere of astronomical silicates with those of aggregates containing the same mass of silicates and composed of up to 12 spheres. Our main result is that, when a given mass of silicates is subdivided into clustering spheres, the extinction increases in regions of the spectrum determined by the degree of subdivision. We also show to what extent the substitution of the material of some of the clustering spheres with the same volume of carbon changes the extinction signature of the clusters. Finally we show to what extent modification of the geometry of the clusters produces detectable changes in their optical signatures. Detailed analysis of our results leads us to the conclusion that modeling the dust grains as clusters of a single morphology is not sufficient to describe the extinction in the whole wavelength range of astrophysical interest. The cluster model may help emphasize the decisive role of morphology in the identification of sustainable structures for dust grains in the typical physical and chemical conditions of the diffuse interstellar medium.

DUST EXTINCTION IN A SMALL MOLECULAR CLOUD

The distribution of matter in a small dark cloud has been investigated observationally by using the extinction of the stellar light that is due to the dust component as a tracer. Here we present the results of a photometric study that has been carried out in the optical spectral region by means of imaging in the B, V , and I bands. The relevant information on the spatial distribution of the dust has been obtained both by counting stars and by measuring their color excesses. While star counts on a reseau grid are easily interpreted as a measurement of the average extinction under the assumption of a locally A j uniform stellar luminosity function, the color excesses averaged on the same grid are obtained by a comparison of the stellar color distributions among the reddened regions and a reference position. Color excesses have also been derived by comparing the observed stellar colors with the locus of the unreddened stars in the two-color (V [I) versus (B[V ) diagram. The extinction map obtained for the globule CB 107 shows that the dust distribution at the boundary of this dark cloud can be measured and compared with models of matter distribution in these objects. By considering a density law such as n P (1 ] (r/a)a)~b, we —nd that our observations favor models with a \ 2 and b \ 1. Indications are also obtained about the spatial variation in this object of the total to selective extinction In this respect R V . we —nd that tends to increase toward the innermost regions of CB 107, and this suggests that the R V mean size of the dust particles also increases. The implications of these observations on the modeling of molecular cloud boundaries are discussed, and some indications on the timescale of the physical processes responsible for their structure are derived. The penetration depth of the interstellar radiation —eld in these objects is also brie—y discussed in the light of its possible consequences on the cloud chemistry. Lastly, the diUerences that we found in the color excess maps, which were obtained by means of the stellar counts and the cumulative distribution in color, respectively, suggest a clumpy distribution of matter in CB 107.

ULTRAVIOLET RADIATION INSIDE INTERSTELLAR GRAIN AGGREGATES. I. THE DENSITY OF RADIATION

We study the distribution of energy density inside dust grain aggregates through an approach based on the multipole expansion of the electromagnetic fields. A significant fraction of the energy of the impinging wave is found throughout the interiors of grains. Implications for extraterrestrial prebiotic chemistry are discussed.

Total to Selective Extinction in the Dark Globule CB 107

We discuss optical (B, V, and I) and near-infrared (J, H, and Ks) imaging observations of the small dark cloud CB 107. The cloud is projected against a rich stellar background, making it possible to detect ~3900 stars in all six photometric bands. After an accurate choice of a reference region, we obtained the color excess Eλ-V maps and derived the distribution of the total to selective extinction RV across the cloud by adopting three different techniques. The RV map obtained closely follows the optical shape of CB 107 with the value RV 3.1, typical of the diffuse interstellar medium, corresponding to the optical limit of the globule. Larger values up to RV ~ 5 are found in the innermost regions, indicating an increase in the dust grain mean size. On the basis of our results, the three methods adopted have been found to be almost equivalent. We also estimated the RV values along the lines of sight of individual stars that, considered in a given spatial box, have been used to obtain both average value and dispersion, as a function of the position. A scatter plot of these two quantities shows that the dispersion σRV increases with RV similarly to the correlation between extinction dispersion σAV and AV already known in the literature. Finally, we discuss the RV versus AV scatter plot on the basis of a simple model that helps us to give a plausible physical interpretation of this correlation.

On the formation and survival of complex prebiotic molecules in interstellar grain aggregates

The aggregation of interstellar grains as a result of ballistic collisions produces loosely packed structures with much of their internal volume composed by vacuum (cavities). The molecular material present on the surfaces of the cavities gives rise to a series of reactions induced by cosmic rays, UV radiation, thermal shocks, etc., in high reducing conditions. Thus, a terrestrial type chemistry is given the possibility to evolve inside these cavities. The resulting products are different and of a wider range than those from gas-phase or surface chemistry in molecular clouds. Under conditions similar to those in the aggregate cavities, laboratory experiments have produced amino acids, sugars and other organic compounds from simple precursors. In dense star-forming regions, the molecular species inside aggregates are efficiently shielded against the local UV field. The same molecules were incorporated in the material which formed the Earth, as well as other planets, during the process of its formation and afterwards fell on the surface via comets, meteorites, interstellar dust, etc. This was the source material that can produce, under favorable circumstances, the biopolymers needed for life. The astronomical observations of organic molecules in star-forming regions and the results of analyses of meteorites and cometary dust seem to support the present hypothesis that complex prebiotic molecules form inside dust aggregates and therein survive the journey to planetary systems. The Miller experiment is revisited through innumerable repetitions inside dust grain aggregates.

Ultraviolet Radiation inside Interstellar Grain Aggregates. II. Field Depolarization

We study the polarization of the UV light within the cavities of interstellar grain aggregates modeled as homogeneous spheres containing several spherical voids. The incident field is a linearly polarized plane wave. We found that field depolarization occurs in all examined cases so that the field within the cavities has the features of an elliptically polarized wave. The depolarization of the field does not depend on the material of the grains but on the geometry of the problem only. The implications of this result for the interstellar photochemistry are briefly discussed.

The Structure of the Small Dark Cloud CB 107

This paper presents the near-IR imaging observations of CB 107, a small dark globule projected against a rich stellar background. By means of accurate photometry, the near-IR two-color diagram J - H versus H - K was obtained for the stellar background. This information was used to estimate the color excesses of the detected stars so that, given the reddening curve, it was possible to derive the extinction map of the cloud. The structural properties of the dark globule were investigated by plotting the extinction dispersion σ, obtained in a given spatial box, as a function of the mean extinction AV. This relationship has shown quite a definite linear behavior, with the slope increasing with the box size. The results of the present analysis, compared with those obtained by other authors on larger dark clouds, suggest that for a given spatial scale the slope of the σ versus AV relation is greater in CB 107 than in larger clouds. The so-called Δ-variance method was also used to investigate the structure of the dark globule by evaluating the drift behavior of its extinction map. In this way, we have found that the power spectrum of the extinction map is characterized by a power law with exponent β ~ 2.7. This value is lower than expected, for the same range of spatial scales, on the basis of previous work on large molecular clouds.

RADIATIVE TRANSFER IN THE STOCHASTIC INTERSTELLAR MEDIUM

Abstract A Boltzmann-like mathematical model is developed for photon transfer in an interstellar cloud, containing one or more clumps, whose centers are stochasticly distributed. The outline of the method is given considering the simplest approach: time-independent transport through a purely absorbing medium composed of two randomly mixed fluids. The emphasis of the work is on the statistical description of the two immiscible fluids. The density distribution of the mixture is described by means of a random field function which maps the structure of the medium. As a consequence, each realization of the statistics corresponds to a possible configuration of a “real” interstellar cloud, as it is possible to infer from the observations. An equation for the expected value of the photon intensity is derived using the method of smoothing. This equation contains an infinite formal Neumann series which includes multiple applications of the inverse transport operator. The reliability of the truncated series is discu...

A kinetic model for dust coagulation

Abstract A Boltzmann-like model is developed for particle transport in presence of coagulation, in which evolution equations for the number densities of small and large particles are derived. Unlike the standard Boltzmann equation, number densities have a dependence on the particle mass.