Bruce T. Draine - US grants

Dr. Bruce Draine of Princeton University will lead studies of a variety of processes in the interstellar medium. He will investigate the size distribution of very small interstellar grains. With the constraints provided by the observed wavelength-dependent interstellar extinction, Dr. Draine expects to obtain the best determination to date of the size distribution of interstellar grains. New models of photo-dissociation regions will be developed, using improved estimates of photoelectric heating rates, and detailed modeling of radiative and collisional excitation/de-excitation of molecular hydrogen (H2). He will extend previous studies on spherical grains to consider nonspherical grain shapes in order to obtain realistic estimates for the net force on irregular interstellar grains. He will study the observable consequences if a gamma ray burst (GRB) takes place in a dense molecular cloud, including the destruction of dust, thermal emission from hot dust, the appearance of absorption features due to vibrationally-excited H2, and ultraviolet fluorescent emission from H2. Studies will be conducted into the role of "thermal flipping" and "thermal trapping" in the physics of interstellar grain alignment. Dr. Draine will attempt to improve the treatment of strongly-absorbing materials in the discrete dipole approximation (DDA). This would allow the DDA to be used to calculate the infrared absorption cross sections of irregular grains composed of materials like amorphous carbon. He also expects to continue to support and maintain the portable program DDSCAT, which is used by researchers in a number of disciplines to compute scattering and absorption by irregular targets.

AST-0216105
Draine, Bruce T.

The Principal Investigator and his team will create a powerful parallel-processing facility for research in astrophysics on problems, which do not require "shared memory" or ultra-fast inter-processor communication. Studies will include: Light scattering from irregular interstellar grains, cosmological structure formation, dynamical evolution of gaseous protoplanetary accretion disks, and several other compelling astronomical sciences research topics. The system will contain 96 dual CPU "slave" systems with a total of 192 Central Processing Units, and 192 gigabytes of RAM with a 3.8 TB disk.
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Our knowledge of interstellar dust is derived primarily from the absorption, scattering, and emission of electromagnetic radiation by dust grains. To test a dust model, or interpret observations, we must be able to calculate the scattering and absorption properties of model grains. In addition, scattering and absorption of light exerts forces and torques on dust grains, with important dynamical consequences. Dr. Bruce Draine will direct a project to further develop the discrete dipole approximation as a technique for calculating scattering and absorption by grains at wavelengths from the ultraviolet to the infrared. Efforts will be made to improve the fundamental approximations, and also to implement algorithmic improvements to accelerate the required numerical calculations. Dr. Draine will include these improvements in a new release of DDSCAT, a publicly-available software package for calculating electromagnetic scattering and absorption by general target shapes. A separate code will be developed to use anomalous diffraction theory to calculate scattering and absorption of X-rays by grains with arbitrary shape. This code will also be made publicly-available. These codes will be applied to various grain geometries, including fluffy aggregates and irregular compact shapes, to calculate absorption and scattering at wavelengths from infrared to X-rays. The objective is to investigate the possibility that such shapes may be representative of interstellar dust. Improved grain models will be developed, based on observational constraints including infrared emission and wavelength dependent extinction.

Microwave emission from dust has been detected, and observational knowledge of the spectrum is increasing. This intensity and spectrum of this emission appears to be consistent with rotational emission from rapidly-rotating very small dust particles. Dr. Draine will study the rotational dynamics and electric dipole emission from very small grains with the aim of producing models that are in better agreement with recent observations, including observed regional variations in the microwave emission. The polarization of starlight by aligned dust grains has been known for more than half a century, but has not yet been satisfactorily accounted for. Torques exerted by starlight are known to play a major role in the grain dynamics. This work will include an extensive study of the rotational dynamics of irregular grains subject to both starlight torques and internal thermal fluctuations. The objective is to determine whether, with the grain dynamics as we now understand it, a population of irregular dust grains illuminated by anisotropic starlight will become partially-aligned, with the degree of alignment vs. grain size consistent with observations of the wavelength-dependence of polarization. If the grain model does develop alignment consistent with observations, the enigma of aligned interstellar grains will at last be solved.

Dust plays an important role in the thermo-, chemo-, and hydrodynamics of the interstellar medium and the formation of planets and stars, it attenuates the optical and ultra violet spectra of galaxies, and it emits from microwave to I-band. A better understanding of interstellar dust therefore has broad impact across astrophysics. The optics of small particles is important in many scientific fields, including atmospheric science, ocean science, planetary science, combustion science, and nanoparticle studies. DDSCAT has already been applied by users in all of these areas. DDSCAT will continue to be improved and made available via the WWW. The research program includes a graduate student component, and undergraduate involvement is anticipated. The proposed research thus contributes to training future scientists.
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This work is for the acquisition of a high-performance computer cluster for computational astrophysics and for the analysis of data from the Sloan Digital Sky Survey, Wilkinson Microwave Anisotropy Probe, the Atacama Cosmology Telescope, and the Southern Cosmology Survey. The cluster will be available to researchers from several institutions and will be available for the training of students in high-performance computing.

Project Summary: Optical Properties of Interstellar Dust

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.

The intellectual merit of this research project is broad. Dust reduces the light from galaxies, sends out its own light, and plays an important role in the thermo-, chemo-, and fluid-dynamics of the matter between the star systems in a galaxy, and in the formation of planets and stars. An improved understanding of the physical properties of interstellar dust grains will have implications for studies of diverse astrophysical phenomena, from formation of stars and planets to the material near active galactic nuclei.

Our knowledge of interstellar dust derives primarily from remote observations of absorption, scattering, and emission of electromagnetic radiation by dust grains. The scattering and absorption properties of dust grains are needed to interpret observations, and to test dust models. In addition, scattering and absorption of light exerts forces and torques on dust grains, with important dynamical consequences. Dr. Bruce Draine and his research team will carry out a number of investigations concerning the optical properties of interstellar dust, and development of improved tools for calculating the optical properties of small particles with irregular geometries. These tools will be used to find new state-of-the-art models (composition, shape, and size distribution) for interstellar dust that are consistent with observational constraints.

The initial objective is to create a "library" of calculated cross sections for scattering and absorption over a broad range of wavelengths, for selected grain sizes, shapes, and compositions. Grain geometries will include spheroids, fluffy grains built up from random coagulation of spheres, and grains that are irregular but compact. State-of-the-art codes will be employed. The discrete dipole approximation (DDA) will be used for irregular grain geometries. Accurate calculations of absorption and differential scattering cross sections at X-ray energies will be carried out using a new code applying anomalous diffraction theory to general geometries. This code will be documented and made publicly-available (open source) on the WWW. Dr. Draine will continue to develop and support the open-source code DDSCAT for calculations using the DDA. The cross-section library will be made available on the WWW. For each grain, the team will calculate the temperature distribution function for various heating radiation fields; the library of temperature distribution functions will also be made available on the WWW. These will be valuable community resources.

The calculated cross-sections and temperature distribution functions will be used to construct the best-to-date interstellar dust models with size distributions adjusted to reproduce observations of wavelength-dependent extinction, wavelength-dependent polarization of starlight, polarized infrared emission, and X-ray scattering. Models will be built with different grain shapes, including compact grains and fluffy grains. Interstellar grains may include magnetic material. The team will continue study of the intensity and polarization of the magnetic dipole radiation emitted by such grains. This magnetic dipole radiation may be important at mm and cm wavelengths, contributing a new polarized component to the "galactic foreground", affecting studies of the polarization of the cosmic microwave background.

This work has broad interdisciplinary impact. Improved methods for calculating scattering and absorption by nanostructures have wide applicability, beyond astrophysics. The optics of small particles is important in many scientific fields, including atmospheric science, oceanography, planetary science, combustion science, marine biology, and nanoparticle studies. DDSCAT has already been applied by users in all of these areas. The DDSCAT package will continue to be supported by Dr. Draine and made publicly available via the WWW. The research program includes a graduate student component, and undergraduate participation is planned. The proposed research will thereby contribute to training future scientists.

Astronomers rely primarily on observations of electromagnetic radiation -- light -- to study objects ranging from stars to galaxies to quasars. Dust grains in interstellar space absorb and scatter this light. The same dust grains also radiate at long (infrared) wavelengths, and this infrared radiation is observable. Dust grains affect the appearance of astronomical objects from X-ray to sub-millimeter wavelengths.

To understand what we see, we must know how the scattering and absorption properties of dust grains depend on their composition, size, and shape. In addition, absorption and scattering of light can produce strong forces and torques on grains. For example, light from our Sun can repel dust grains approaching the solar system. Sunlight can also cause grains with irregular shapes to spin fast enough for centrifugal stresses to disrupt the grain.

Because dust grains have irregular shapes, calculating scattering and absorption is a challenging mathematical problem. The investigator aims to improve methods for computing scattering and absorption for grains with complicated shapes. One emphasis will be on the important case where the grain itself is small compared to the wavelength of the radiation (as in the infrared). A second emphasis will be on developing and implementing much faster methods for computing the forces and torques acting on the grain.

These improved methods will be implemented in new versions of the publicly-available open-source code DDSCAT which the investigator developed and supports. The new version of DDSCAT will be used to compute a "library" of scattering and absorption cross sections. This will be done for a sample of grain shapes and compositions, and for a wide range of wavelengths. The library will be available on-line to other researchers. The library will include cross sections needed to calculate scattering and absorption, as well as data needed to calculate forces and torques acting on grains.

The cross-section library will be used to develop and test models for the interstellar dust population. One popular model envisions typical interstellar grains as loose aggregates. The library will be used to see whether this model is compatible with existing observational data.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.