The Field Condition: A New Constraint of Spatial Resolution in Simulations of Thermally Unstable Hydrodynamics

We present the dynamics in thermally bistable medium by using onedimensional numerical calculations with cooling, heating, (realistic) thermal conduction, and physical viscosity. We set up a two-phase medium from thermally unstable one-phase medium and follow the long-term evolution of the medium. We focus on the spatial resolution because we have to resolve Field length λF that is the critical wavelength for thermal instability (TI). The result of our calculation shows that the two cold clouds eventually collide in the calculations with thermal conduction, while clouds never collide in the case without thermal conduction. We find it necessary to keep the cell size less than λF/3 to achieve a converged motion in two-phase medium. We refer to the constraint that λF/3 be resolved as the Field condition. The inclusion of thermal conduction to satisfy the Field condition is crucial for numerical studies of TI and turbulent interstellar two-phase medium: the calculations of TI without thermal conduction are susceptible to contamination by artificial phenomena that do not converge with increasing resolution. Subject headings: hydrodynamics — ISM: clouds — method: numerical — instabilities