  We study the free energy of a heavy quark-antiquark pair in a thermal medium
using the AdS/CFT correspondence. We point out that a commonly used
prescription for calculating this quantity leads to a temperature dependence in
conflict with general properties of the free energy. The problem originates
from a particular way of subtracting divergences. We argue that the commonly
used prescription gives rise to the binding energy rather than the free energy.
We consider a different subtraction procedure and show that the resulting free
energy is well-behaved and in qualitative agreement with results from lattice
QCD. The free energy and the binding energy of the quark pair are computed for
N = 4 supersymmetric Yang-Mills theory and several non-conformal theories. We
also calculate the entropy and the internal energy of the pair in these
theories. Using the consistent subtraction, we further study the free energy,
entropy, and internal energy of a single heavy quark in the thermal medium for
various theories. Also here the results are found to be in qualitative
agreement with lattice QCD results.
