Dust-to-gas ratio and phase transition of interstellar medium

We discuss the time evolution of dust-to-gas mass ratio in the context of m ulti-phase model of interstellar medium. Phase transition of interstellar g as is considered to occur on a timescale of similar to 10(7-8) yr, accordin g to a nonlinear open system model of interstellar medium. Since the phase transition changes the dust formation and destruction rates, the dust-to-ga s ratio also fluctuates on the same timescale. This explains the scatter of the dust-to-gas ratios of spiral galaxies quantitatively, though we should note the large observational uncertainty.