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

Stan Owocki^1, Jon Sundqvist^1, David Cohen^2 and Ken Gayley^3

1-Bartol Research Insitute, University of Delaware, Newark,DE 19716 USA
2-Department of Physics, Swarthmore College, Swarthmore, PA 19081 USA
3-Department of Physics, University of Iowa, Iowa City, IA 52242 USA

X-ray satellites since Einstein have empirically established that the X-ray luminosity from single O-stars scales linearly with bolometric luminosity, Lx~ 10^-7 Lbol. But straightforward forms of the most favored model, in which X-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 if the shocks are radiative, or with Lx ~Mdot^2 ~Lbol^3.4 if they are adiabatic. We present here a generalized formalism that bridges these radiative vs. adiabatic limits in terms of the ratio of the shock cooling length to the local radius. Noting that the thin-shell instability of radiative shocks should lead to extensive mixing of hot and cool material, we then propose that the associated softening and weakening of the X-ray emission can be parametrized by the cooling length ratio raised to a power m, the “mixing exponent.” For physically reasonable values m=0.4, this leads to an X-ray luminosity Lx ~ Lbol that matches the empirical scaling. We conclude by noting that such thin-shell mixing may also be important for X-rays from colliding wind binaries, and that future numerical simulation studies will be needed to test this thin-shell mixing ansatz for X-ray emission.

Reference: To appear in "Four Decades of Research on Massive Stars", proceedings of a conference held 11-15 July 2011 in Saint-Michel-des-Saints, Quebec to honor the retirement of Tony Moffat.
Status: Conference proceedings