Dust plays an important role in the thermo-, chemo-, and hydrodynamics of the interstellar medium and in the formation of planets and stars. Dust attenuates the optical and ultraviolet (UV) spectra of galaxies, and it emits from the infrared to microwave wavelengths. A better understanding of interstellar dust has broad impact across astrophysics.
Our knowledge of interstellar dust is derived primarily from remote observation of absorption, scattering, and emission of electromagnetic radiation by dust grains. In order to interpret observations and test dust models, the scattering and absorption properties of model grains need to be calculated. There are also important dynamical consequences from scattering and absorption of light because this exerts forces and torques on dust grains. In this work a number of related investigations concerning the optical properties of interstellar dust will be carried out. The investigators, Professor Draine and a graduate student, plan to develop improved methods for calculating scattering and absorption by small particles or nanostructures. They plan further develop and apply the discrete dipole approximation (DDA) as a technique for calculating scattering and absorption by irregular grains and composite grains at wavelengths from the ultraviolet to the infrared. A new approach will be pursued to determine ?surface corrected? dipole polarizabilities to improve the accuracy of the DDA. Other algorithmic improvements will also be implemented and included in a new release of the discrete dipole approximation code DDSCAT, which is a publicly-available DDA code for calculating light scattering and absorption by general target shapes. Anomalous diffraction theory will be used at X-ray energies. The initial objective will be to create a ?library? of scattering and absorption properties over a broad range of wavelengths for selected grain sizes, shapes, and compositions.
Grain geometries will include grains that are built up from smaller monomers, including the ?fluffy? clusters created by the standard ?ballistic aggregation? procedure, as well as by other aggregation rules that result in clusters that are more compact, less fragile, and possibly more realistic. It is also planned to study the scattering and absorption by grains that are irregular but compact, using a variant of the ?Gaussian spheres? technique to generate random shapes. This library of scattering results will be useful in subsequent studies of light scattering and absorption and should have wide applicability. It is planned to make the library publicly available on the principal investigator?s web site.
The development of an improved dielectric function for the silicate material in the diffuse ISM is planned as part of this work. The dielectric function is required to reproduce the observed interstellar silicate features in both extinction and polarization, and to be consistent with other astrophysical constraints including far-infrared and submm emission and interstellar abundances. The relationship between the silicate extinction and polarization is sensitive to grain shape and to the dielectric function. The goal is to successfully reproduce both, which will give some confidence in the resulting dielectric function and grain shape.
After creating the library of scattering properties for grains of various types, models for interstellar dust will be constructed with size distributions adjusted to reproduce observations of wavelength-dependent extinction, wavelength-dependent polarization of starlight, infrared emission, and X-ray scattering properties. It is planned to do this with different grain types, including compact grains and fluffy grains. This will help to narrow down the kinds of grains (e.g., fluffy grains) that give model results compatible with observations. A study of the different cases thus will show which grain type would be ruled out as a major interstellar grain component.
The proposed work will have broad interdisciplinary impact. The optics of small particles is important in many scientific fields, including atmospheric science, oceanology, planetary science, combustion science, marine biology, and nanoparticle studies. DDSCAT has already been applied by users in all of these areas. Further improvements in our ability to calculate absorption and scattering by particles and structures will be of value beyond astrophysics. The PI will continue to support the DDSCAT package and to make it publicly-available via the WWW.
Interstellar gas and dust is present in the space between the stars. Interstellar dust absorbs, scatters, and emits electromagnetic radiation (light), acting to obscure the direct light from astronomical objects such as stars, but also, through its emission, revealing much about the environment in which the dust is located. To interpret what we observe, we need a much better understanding of what interstellar dust is, and how it interacts with electromagnetic radiation. This is the subject of the research supported by this grant. We obtained important results on the dust in the nearby Andromeda galaxy (Draine et al 2014). The infrared emission from Andromeda, observed by the Spitzer and Herschel space observatories, has been modeled to obtain maps in Andromeda of the density of dust, the intensity of the starlight heating the dust, and the nature of the dust, as revealed both by the strength of infrared emission features from "polycyclic aromatic hydrocarbons" (PAHs) the variation of the far-infrared opacity with wavelength. The dust-to-gas ratio in Andromeda varies by almost a factor of ten from the center of the galaxy to the outer regions, more than 60,000 light-years from the center. Similar studies of the distribution of dust and starlight have also been carried out on the nearby star-forming galaxies NGC 628 and NGC 6946 (Aniano et al 2012) Work supported by this grant also studied the nature of the particles responsible for the strong emission features between 3.3 and 13 microns (Li & Draine 2012). Contrary to claims made by some other workers, we demonstrated that the carbon atoms in these particles had to be predominantly in "aromatic" (ringlike) form -- not more than 15% of the carbon atoms could be in "aliphatic" (chainlike) hydrocarbons. Thus it is appropriate to refer to these particles as polycyclic aromatic hydrocarbons, as the aromatic component is predominant. The sources of the starlight responsible for heating these PAHs was also examined. Using multiwavelength observations, Crocker et al (2013) showed that much of the observed 8 micron emission is excited by "old" stars not associated with star-forming regions. In addition to infrared and far-infrared emission, dust grains also radiate at microwave frequencies. Here we think the emission is primarily due to very rapid rotation of ultrasmall grains. The rotation of these ultrasmall grains is affected by the 3 dimensional shape of the grains, and by the transient heating of these grains by starlight. Hoang, Lazarian, & Draine (2011) calculated how shape and starlight heating would modify the spectrum of the microwave emission from these spinning grains. The dust grains in the interstellar medium are each illuminated by the light of many stars, and some of this light is then scattered by the dust, and can be observed as the "diffuse galactic light". The faintness of the diffuse galactic light makes it difficult to study, but Brandt & Draine (2012) showed that spectroscopic measurements previously made as part of the Sloan Digital Sky Survey could be used to measure the spectrum of the diffuse galactic light. The spectrum thereby obtained is the best measurement of the spectrum of the diffuse galactic light over the 390-910nm wavelength range. The Small Magellanic Cloud (SMC) is a nearby galaxy with dust that appears to be somewhat different in character from the dust in our own galaxy, the Milky Way. We showed (Draine & Hensley 2012) that the observed emission from the SMC could be understood if the dust in the SMC had a relatively high fraction of the iron in metallic nanoparticles, radiating so-called "magnetic dipole radiation" at low frequencies. The physics of magnetic dipole emission from magnetic materials in interstellar grains was discussed (Draine & Hensley 2013). The magnetic dipole emission has an unusual frequency spectrum, but also has unusual polarization characteristics. Observations of the polarized emission from dust at frequencies below 100 GHz may reveal whether the dust in the SMC has a substantial magnetic component, as has been hypothesized. Our knowledge of interstellar grains derives mainly from observations of scattering, absorption, and emission from interstellar dust. To interpret these observations, we rely on a theoretical understanding of the interaction of electromagnetic radiation with a grain. Unfortunately, the theory is well-developed only for special shapes, such as spheres and spheroids. For more general shapes, calculations of scattering and absorption rely on numerical methods, such as the discrete dipole approximation. We work on trying to improve these methods. We developed improved methods for fast computation of "near fields" (Flatau & Draine 2012). Most recently, the power of the discrete dipole approximation was demonstrated (Flatau & Draine 2014) by calculating scattering by very thick hexagonal ice crystals, such as occur in terrestrial cirrus clouds. Understanding the scattering properties of these crystals is of some important for atmospheric studies and for climate modeling.
