Thin-shell mixing in radiative wind-shocks and the Lx ∼ Lbol scaling of 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 ∼ 10-7Lbol. But straightforward forms of the most favoured model, in which X-rays arise from instability-generated shocks embedded in the stellar wind, predict a steeper scaling, either with mass-loss rate L_x ∼ dot{M}∼ L_bol^{1.7} if the shocks are radiative or with L_x ∼ dot{M}2 ∼ L_bol^{3.4} if they are adiabatic. This paper presents a generalized formalism that bridges these radiative versus 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 propose that the associated softening and weakening of the X-ray emission can be parametrized as scaling with 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 L_x ∼ dot{M}^{0.6} ∼ L_bol that matches the empirical scaling. To fit observed X-ray line profiles, we find that such radiative-shock-mixing models require the number of shocks to drop sharply above the initial shock onset radius. This in turn implies that the X-ray luminosity should saturate and even decrease for optically thick winds with very high mass-loss rates. In the opposite limit of adiabatic shocks in low-density winds (e.g. from B-stars), the X-ray luminosity should drop steeply with dot{M}^2. Future numerical simulation studies will be needed to test the general thin-shell mixing ansatz for X-ray emission.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- March 2013
- DOI:
- 10.1093/mnras/sts599
- arXiv:
- arXiv:1212.4235
- Bibcode:
- 2013MNRAS.429.3379O
- Keywords:
-
- shock waves;
- stars: early-type;
- stars: mass-loss;
- stars: winds;
- outflows;
- X-rays: stars;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 12 pages, 4 figures, accepted for publication in MNRAS