Astrophysics of Dust in Cold Clouds

Dust plays an increasingly important role in astrophysics. Historically, dust was first recognized for its obscuring effects, and the need to correct observed intensities for attenuation by dust continues today. But with the increasing sensitivity of IR, FIR, and submm telescopes, dust is increasingly important as a diagnostic, with its emission spectrum providing an indicator of physical conditions, and its radiated power bearing witness to star populations of which we might otherwise be unaware. Finally, and most fundamentally, dust is now understood to play many critical roles in galactic evolution. By sequestering selected elements in the solid grains, and by catalyzing formation of the H2 molecule, dust grains are central to the chemistry of interstellar gas. Photoelectrons from dust grains can dominate the heating of gas in regions where ultraviolet starlight is present, and in dense regions the infrared emission from dust can be an important cooling mechanism. Finally, dust grains can be important in interstellar gas dynamics, communicating radiation pressure from starlight to the gas, and providing coupling of the magnetic field to the gas in regions of low fractional ionization. We would like to understand these effects of dust in the Milky Way, in other galaxies, and as a function of cosmic time. These lectures are organized around topics in the astrophysics of dust in the the Milky Way, as this is our best guide to understanding and modelling dust long ago and far away.