Optics of Interstellar Dust: New Methods and Applications

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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.