Massless Parton Asymptotics within Variable Flavour Number Schemes

In this note we formulate and investigate theoretical uncertainties for high Q^2 deep inelastic heavy quark (charm, etc.) production rates which arise within collinear resummation techniques from variations of the a priori unknown charm input scale Q_0 of O(alpha_s) variable flavour number schemes. We show that Q_0 variations constitute a source of considerable theoretical uncertainty of present O(alpha_s) calculations within such schemes and we suggest to consider a scale optimization from higher order corrections. We also outline how the stability of the fixed order and collinearly resummed perturbation series for heavy quark production can be comparatively investigated by variation of Q_0.