On the basis of observational constraints, particularly the relationship between metal abundance and cumulative stellar mass, a simple two-zone disk-halo model for the chemical evolution of our Galaxy was investigated, assuming different chemical processes in the disk and halo and the infall rates of the halo gas defined by the halo evolution. The main results of the present model calculations are: (i) The halo formation requires more than 80% of the initial galactic mass and it takes a period of 2 ∼ 3 × 10 9 yrs. (ii) The halo evolution is divided into two phases, a fast collapse phase ( t = 2 ∼ 3 × 10 8 yrs) during which period most of the halo stars ( ∼ 95 are formed and a later slow collapse phase which is characterized by the chemical enrichment due to the inflow of external matter to the halo. (iii) The disk evolution is also divided into two phases, an active disk formation phase with a time-dependent initial mass function (IMF) up to t ≈ 6 × 10 9 yrs and a later steady slow formation phase with a constant IMF. It is found that at the very early time t ≈ 5 × 10 8 yrs, the metal abundance in the disk is rapidly increased to ∼ 1 / 3 of the present value but the total stellar mass only to ∼ 10 of the present value, finally reaching about 80% of the present values toward the end of the active formation phase.

ABSTRACT
Ⅰ. INTRODUCTION
Ⅱ. OBSERVATIONAL CONSTRAINTS
Ⅲ. BASIC EQUATIONS FOR CHEMICAL EVOLUTION
Ⅳ. INPUT PARAMETERS
Ⅴ. RESULTS OF MODEL CALCULATIONS
Ⅵ. COMPARISON WITH OTHER MODELS FOR CHEMICAL EVOLUTION
Ⅶ. CONCLUSIONS
REFERENCES

• S. W. LEE(Department of Astronomy, Seoul National University)
• H. B. ANN(Department of Earth Science, Busan National University)