Thermal Duality and the String Canonical Ensemble

We derive the free energies of both the closed heterotic, and the unoriented, open and closed, type I string ensembles, consistent with the thermal (Euclidean T-duality) transformations on the String/M Duality Web. A crucial role is played by a temperature dependent Wilson line wrapping Euclidean time, responsible for the spontaneous breaking of supersymmetry at finite temperature while eliminating thermal tachyons, and determined uniquely by thermal duality. Conversely, we can show that the absence of a Yang-Mills gauge sector precludes the possibility of an equilibrium type II canonical ensemble prior to the introduction of background Dbranes or fluxes. As a consistency check, we verify that our results for the string free energy always reproduce the T^{10} growth expected in the low energy field theory limits while displaying a dramatically slower T^2 growth at temperatures above the string scale. We present both the low and high temperature expansions for the one-loop heterotic and type I string free energies, results which follow from an explicit term-by-term evaluation of the modular integrals in the string mass level expansion.