  A possible definition of the specific heat of open quantum systems is based
on the reduced partition function of the system. For a free damped quantum
particle, it has been found that under certain conditions, this specific heat
can become negative at low temperatures. In contrast to the conventional
approaches focusing on the system degree of freedom, here we concentrate on the
changes induced in the environment when the system is coupled to it. Our
analysis is carried out for an Ohmic environment consisting of harmonic
oscillators and allows to identify the mechanism by which the specific heat
becomes negative. Furthermore, the formal condition for the occurrence of a
negative specific heat is given a physical interpretation in terms of the total
mass of bath oscillators and the system mass.
