  Starting from the formal solution to the Heisenberg equation, we revisit an
universal model for a quantum open system with a harmonic oscillator linearly
coupled to a boson bath. The analysis of the decay process for a Fock state and
a coherent state demonstrate that this method is very useful in dealing with
the problems in decay process of the open system. For finite temperature, the
calculations of the reduced density matrix and the mean excitation number for
the open system show that an initial coherent state will evolve into a
temperature-dependant coherent state after tracing over the bath variables.
Also in short-time limit, a temperature-dependant effective Hamiltonian for the
open system characterizes the decay process of the open system.
