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X-ray satellites since Einstein have empirically established that the X-rayluminosity from single O-stars scales linearly with bolometric luminosity, Lx ~10^-7 Lbol. But straightforward forms of the most favored model, in whichX-rays arise from instability-generated shocks embedded in the stellar wind,predict a steeper scaling, either with mass loss rate Lx ~ Mdot ~ Lbol^1.7 ifthe shocks are radiative, or with Lx ~ Lx ~ Mdot^2 ~ Lbol^3.4 if they areadiabatic. We present here a generalized formalism that bridges these radiativevs. adiabatic limits in terms of the ratio of the shock cooling length to thelocal radius. Noting that the thin-shell instability of radiative shocks shouldlead to extensive mixing of hot and cool material, we then propose that theassociated softening and weakening of the X-ray emission can be parametrized bythe cooling length ratio raised to a power m, the "mixing exponent." Forphysically reasonable values m ~= 0.4, this leads to an X-ray luminosity Lx ~Mdot^0.6 ~ Lbol that matches the empirical scaling. We conclude by noting thatsuch thin-shell mixing may also be important for X-rays from colliding windbinaries, and that future numerical simulation studies will be needed to testthis thin-shell mixing ansatz for X-ray emission.

The Physical Basis of the Lx-Lbol Empirical Law for O-star X-rays