The Physical Basis of the LX = Lbol Empirical Law for O-Star X-Rays
Abstract
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 = M = Lbol1.7 if the shocks are radiative, or with LX = M2 = Lbol3.4 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 parameterized 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 = M0.6 = 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.
- Publication:
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Proceedings of a Scientific Meeting in Honor of Anthony F. J. Moffat
- Pub Date:
- December 2012
- DOI:
- 10.48550/arXiv.1110.0891
- arXiv:
- arXiv:1110.0891
- Bibcode:
- 2012ASPC..465..153O
- Keywords:
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- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 5 pages, to appear in "Four Decades of Research on Massive Stars", eds. L. Drissen, C. Robert, and N. St-Louis, ASP Conference Series