  In this work we explain the construction of the thermal vacuum for the
bosonic string, as well that of the thermal boundary state interpreted as a
$D_{p}$-brane at finite temperature. In both case we calculate the respective
entropy using the entropy operator of the Thermo Field Dynamics Theory. We show
that the contribution of the thermal string entropy is explicitly present in
the $D_{p}$-brane entropy. Furthermore, we show that the Thermo Field approach
is suitable to introduce temperature in boundary states.

High Energy Physics - Theory