Resonance scattering in the X-ray emission line profiles of ζ Pup

1. Maurice Leutenegger With David Cohen, Steve Kahn, Stan Owocki, and Frits Paerels Resonance scattering in the X-ray emission line profiles of ζ Pup

2. Resonance scattering in the X-ray emission line profiles of ζ Pup • X-ray emission from O star winds • X-ray line profiles: summary of theory and observation • Data suggesting resonance scattering • Profile formation with resonance scattering • Application of RS profile model to data • Implications

3. X-ray emission from O stars • The driving force in radiative lines is unstable • Tenuous streams undergo runaway acceleration before colliding with dense clumps, resulting in reverse shocks Snapshot from simulation of Feldmeier (1995)‏

4. X-ray Doppler profiles(Owocki & Cohen 2001)‏ • Wind modeled as a two-component fluid • Cool bulk of wind (absorbs X-rays)‏ • Small fraction is heated in shocks (emits X-rays)‏

5. X-ray Doppler profiles: emission from thin shells

6. Example profiles

7. Parameter dependence • Two parameters influence radial distribution of X-ray emitting plasma: • “Turn-on” radius (expected to be ~ 1.5 stellar radii)‏ • Filling factor (power law in radius)‏

8. Parameter dependence • The cool part of the wind absorbs X-rays as they leave the wind • Characteristic continuum optical depth:

9. Example profiles

10. Qualitative summary of model profile behavior: • Degree of blueshift measures characteristic continuum optical depth to X-rays • Width measures the onset radius of X-ray emission

11. Comparison with data

12. Possible explanations for X-ray profile shapes (more than one may apply)‏ • Mass loss rates too high – characteristic optical depths really are low • Porosity reduces macroscopic effective optical depth • Resonance scattering causes emission to be intrinsically shifted towards line center

13. Empirical evidence suggesting resonance scattering?

14. Empirical evidence suggesting resonance scattering?

15. Resonance scattering (Ignace et al 2002, Leutenegger et al 2007)‏ • For an optically thick resonance line in a moving stellar atmosphere (Sobolev theory): • radial photon escape is due to the radial velocity gradient (dv/dr)‏ • lateral photon escape is due to the spherical divergence of the wind (v/r)‏ • Far out in the wind dv/dr goes to zero, so lateral escape is favored • If the observed X-ray emission comes from far out in the wind, the profiles are more symmetric

16. Angular dependence of normalized escape probability (optically thick)‏

17. How does resonance scattering affect the model profiles?

18. Including resonance scattering in N VI leads to much better fit

19. Including resonance scattering in N VI leads to much better fit

20. Also improves fit to O VII

21. Also improves fit to O VII

22. What about other lines? • We can only infer the importance of RS by comparing two lines from the same ion – one must be a resonance line, and the other must not • But if RS is important in N VI and O VII, it should be important for other strong resonance lines as well!

23. If resonance scattering is important how do we measure anything? • Problem: profile shape is roughly degenerate for high continuum optical depth with resonance scattering and low continuum optical depth without resonance scattering • Use non-resonance lines • Even some resonance lines will not be optically thick • (Make predictions for line optical depth)‏

24. Summary • Different profile shapes in resonance and intercombination lines from the same ion can only be explained by resonance scattering • Resonance scattering can explain at least some of the unexpected lack of asymmetry in other profiles • Either porosity or reductions in mass-loss rates (relative to density-squared diagnostics) are still likely to be important in addition to RS

25. Characteristic optical depth to resonance scattering

26. Expected values of optical depth