Studies of Interstellar Processes

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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.