  We present NLO results for thermal imaginary-time correlators in the vector
and scalar channels as a function of the quark mass. The range of quark masses
for which a non-relativistic approximation works in the temperature range
considered is estimated, and charm quarks turn out to be a borderline case.
Comparing with simulation data from fine lattices, we find good agreement in
the vector channel but a substantial discrepancy in the scalar one. An
explanation for the discrepancy is suggested in terms of physics of the
quark-antiquark threshold region. Perturbative predictions for the bottom
scalar spectral function around the threshold are also briefly reviewed.
