EVOLUTION OF DUST GRAINS THROUGH A HOT GASEOUS HALO

The evolution of bare spherical dust grains in the halo of spiral galaxies is analyzed. Two different grain compounds, graphites and astronomical silicates, are considered. The detailed mass and luminosity distributions for the Milky Way and NGC 3198 (considered as representative of the Sb and Sc types, respectively) are used to evaluate the range of possible evolutionary tracks. Aside from radiative and gravitational forces, the effects of drag and sputtering from a gaseous halo are included. A simple isothermal and hydrostatic density structure, with temperatures in the range 3 x 10(5) - 10(6) K, has been used for this gaseous halo. Most results depend on the optical properties of dust compounds (or, equivalently, on the radiation pressure coefficients) because the velocities define the final fate of the grains (i.e., destruction in the halo or expulsion from host galaxy). Graphites move faster than silicates, but both types of grains can reach values in excess of 10(2) km s-1. The efficiency for grain destruction increases with (a) increasing halo gas temperature, (b) decreasing galaxy mass, and (c) decreasing radiation pressure coefficients. Thus, silicates behave as a "fragile" grain population in low mass galaxies with a hot (T approximately 10(6) K) corona. For a central halo gas density of 0.1 cm-3, spherical silicates with an initial radius of a = 10(-5) cm lose almost 20% of their mass in time scales between 10(8) and 2 x 10(8) yr. Some effects of a poloidal magnetic field are also explored along with possible consequences on halo metallicities and intergalactic dust. Grain expulsion can be a common phenomenon in spirals.