We investigate the behaviour of one-zone chemical evolution models which utilize the chemodynamical prescription for a two-component interstellar medium. This is done by numerical solution, topological analysis of the equations, and identification of the conditions for equilibrium solutions.
The self-regulated star-formation found in models with a single gas component (Köppen et al. 1995) is not at all disturbed by the additional condensation and evaporation processes. Due to its shorter time-scale, it dominates the evolution. The models follow the self-regulated solution very closely, without any oscillations.
On the other hand, it is the distribution of the gas among the cloud and intercloud phases which is determined by the mass exchange due to condensation and evaporation. The evolution also follows an equilibrium which is either ruled by the switching condition or by the relation . The mass of gas in the intercloud phase is usually very small compared to that found in clouds.
From realistic rate coefficients for condensation and evaporation in a cloud population (Samland et al. 1997) one finds that the evolution of the model follows a hierarchy of nested equilibria, which are in ascending order of their quite separate time-scales: thermal equilibrium in the cloud gas, regulation of the SFR by heating of gas due to radiation from massive stars, equilibrium of mass exchange between gas and clouds. This is further embedded into the overall conversion of gas into stellar remnants.
During the evolution in the equilibrium between condensation and evaporation , the relation between metallicity and gas mass fraction follows a Simple Model quite closely. Gas and cloud phases are well-mixed, their metallicities are the same.
Together with the quadratic dependence of the star-formation rate on the cloud density (cf. Köppen et al. 1995), these aspects make up the global behaviour that is typical for chemo-dynamical evolution models.
In models with low condensation coefficients and/or high initial cloud densities, the metal-rich gas ejected by the stars formed in the initial period of intense star-formation may remain in the intercloud medium quite long, before condensation into clouds commences and metal enrichment of the clouds occurs.
From these models we compute how the total metal mass in a galaxy is distributed among gas, clouds, and stellar remnants and how this changes with the chemical age of the system. For conditions similar to the solar neighbourhood, unevolved gas-rich systems have their metals in both cloud and intercloud gas. In high-density models (or those with small condensation rates) the metals may remain in the intercloud medium even in rather evolved, gas-rich galaxies, and those contained in the cloud phase may be a rather portion. Work is under way to investigate the properties of full 'chemodynamical' models with the complete network of interactions and the coupling to the global dynamics.
© European Southern Observatory (ESO) 1998
Online publication: February 16, 1998