  We study the evolution of the entanglement of two independent bosonic modes
embedded in a thermal environment, in the framework of the theory of open
quantum systems. As a measure of entanglement we use the logarithmic
negativity. For a non-zero temperature of the thermal reservoir the entangled
initial Gaussian states become always separable in a finite time. For initial
squeezed thermal states we calculate the survival time of entanglement and
analyze its dependence on temperature, squeezing parameter and mean thermal
photon numbers. For a zero temperature of the thermal bath an entangled initial
state remains entangled for all finite times, but in the limit of
asymptotically large times it becomes separable.
