  In this Letter a topological interpretation for the string thermal vacuum in
the Thermo Field Dynamics (TFD) approach is given. As a consequence, the
relationship between the Imaginary Time and TFD formalisms is achieved when
both are used to study closed strings at finite temperature. The TFD approach
starts by duplicating the system's degrees of freedom, defining an auxiliary
(tilde) string. In order to lead the system to finite temperature a Bogoliubov
transformation is implemented. We show that the effect of this transformation
is to glue together the string and the tilde string to obtain a torus. The
thermal vacuum appears as the boundary state for this identification. Also,
from the thermal state condition, a Kubo-Martin-Schwinger condition for the
torus topology is derived.
