| In this dissertation, we explore some related topics in the physics of interstellar dust. First, we construct grain size distributions for which there is substantial mass in very small graphite grains and which are consistent with the observed average extinction for lines of sight with = 3.1, 4.0, and 5.5. We then describe a method for estimating the rate of photoelectric emission from dust and the resulting grain electric charge. We estimate the photoelectric gas heating efficiency as a function of grain size and obtain net heating rates for our derived size distributions in a range of environments, including the diffuse interstellar medium (ISM), photodissociation regions (PDRs), and H II regions.; Next, we model two forces that act on grains exposed to anisotropic radiation, due to recoil against photoelectrons and photodesorbed adatoms. We also model the drag force due to collisions with H atoms when surface processes (photodesorption and H2 formation) are important. We use these results, as well as our estimate of the starlight anisotropy in the solar neighborhood, to estimate gas-grain drift speeds in the diffuse ISM. We also estimate drift speeds in PDRs, and find that they may be large enough to result in a significant increase in the dust-to-gas ratio. This might explain the unexpectedly high values of gas temperature observed in PDRS, since the dominant heating mechanisms involve dust.; Finally, we calculate the rates at which gas-phase ions in the diffuse ISM deplete onto dust populations that include very small grains. Adopting a grain model in which ∼4% of the cosmic C abundance is in grains with radii ≤10 Å, we find that the rate of accretion onto these grains is fast enough to account for the observed large depletions of elements like Ti, without invoking unreasonably high rates of mass transfer between interstellar phases or low grain destruction rates. We discuss other difficulties that might arise if very small grains are important in interstellar depletion. |
