  We study the strange quark mass dependence of the tricritical point of the
U(3)_L x U(3)_R linear sigma model in the chiral limit. Assuming that the
tricritical point is at a large strange mass value, the strange sector as well
as the \eta-a_0 sector decouples from the light degrees of freedom which
determines the thermodynamics. By tracing this decoupling we arrive from the
original U(3)_L x U(3)_R symmetric model, going through the U(2)_L x U(2)_R
symmetric one, at the SU(2)_L x SU(2)_R linear sigma model. One-loop level beta
functions for the running of the parameters in each of these models and
tree-level matching of the coupling of these models performed at intermediate
scales are used to determine the influence of the heavy sector on the
parameters of the SU(2)_L x SU(2)_R linear sigma model. By investigating the
thermodynamics of this latter model we identified the tricritical surface of
the U(3)_L x U(3)_R linear sigma model in the chiral limit. To apply the
results for QCD we used different scenarios for the m_s and \mu_q dependence of
the effective model parameters, then the \mu_q^TCP(m_s) function can be
determined. Depending on the details, a curve bending upwards or downwards near
\mu_q=0 can be obtained, while with explicit chemical potential dependence of
the parameters the direction of the curve can change with m_s, too.
