Unkown stellar mass loss at low metallicity

1. Unkown stellar mass loss at low metallicity Henny Lamers Astronomical Institute and SRON Laboratory for Space Research Utrecht University

2. Topics to be discussed Predictions of properties of “normal” winds (Z~solar) and comparison with observations Predicted properties of low Z winds (Z~10-4 solar) Extremely no Z winds (H and He only)

3. Net force in wind: gnet = - gN + ge + glines > 0 Gravity : gN = GM / R2 Rad pressure by electrons : ge = σe L / 4 π R2 c = Γe gN Rad pressure by lines: gl = ge M(t) Force multiplier: M(t) = k t-α (ne/W)δ Solar Z : k ~ 1/30 α ~ 0.6 δ ~ 0.1 Castor, Abbott & Klein 1975, CAK

4. Predictions: scaling laws Castor, Abbott, Klein 1975 v∞ ~ (2 to 3)v esc ~ √ (M/R) Mdot ~ L 1/α M (α-1)/α

5. Predictions: scaling laws Castor, Abbott, Klein 1975 v∞ ~ (2 to 3)v esc ~ √ (M/R) Mdot ~ L 1/α M (α-1)/α ↓ Kudritzki et al 1989 П≡ Mdotv∞√ R ~L 1/α M 0.5-(1-α)/α ~ L 1/α П = Modified wind momentum

6. Predicted mass loss rates Vink et al. 2000

7. Modified wind momentum П↔ L predictions Log П observations Vink et al. 2000

8. New study with improved mass loss rates Galactic stars Markova et al. 2004

9. Problem: Some stars with very similar stellar parameters show a large difference in mass loss rate and wind properties !!

10. “Tale of two stars” AV 83 AV 69 Type O7 Iaf+ O7.5 III L/Lsun 3.5 105 4.1 105 Teff 32800 33900 R/Rsun 18.5 18.6 He/H 0.2 0.1 C/N 0.15 15 Hillier et al. 2003

11. AV 83 and AV 69: very different winds! Hillier et al. 2003

12. AV 83 AV 69 Mdot 7 10-7 9 10-7 V∞ 940 1800 β 2.0 1.0 fclump 0.1 1 ROTATION !!! Hillier et al. 2003

13. Effects of rotation • Flattening • 2. Gravity = f( θ) • 3. Teff = f(θ ) • 4. Mdot = f(θ) • 5. Vwind = f(θ) • 6. Spect type = f(sin i) Meynet & Maeder 2002

14. The wind of a fast rotating hot supergiant J. Bjorkman 2005

15. Mass loss and metallicity

16. Predictions for line driven winds 10-2 < Z/Zsun <10+1 Mdot ~ Z 0.85 Vink et al. 2001

17. Observations ↔ Predictions LMC SMC Vink et al. 2001

18. The problem of the winds of low luminosity SMC stars Black = Gal Red = LMC Blue = SMC П↔L relation breaks down at log L < 5.3 !! Martins et al. 2004

19. Mass loss at very low metallicityZ/Zsun < 10-3

20. Predictions at low Zparameters k, α, δ vary with distance Z/Zsun 1.0 0.2 10-2 10-3 10-4 П Kudritzki 2002

21. Very low Z stars only have winds when they are close to Eddington limit ! grad = ge + glines > gN at some depth glines = M(t) ge Mmax ≈ 2000 Z/Zsun + M(H,He) from atomic physics So maximumradiation pressure is gradmax = (1+Mmax) ge = (1+Mmax) Γe gN > gN Γe > (1+Mmax)-1

22. Dependence of mass loss on Γe ← Mdot ≈ 10-8 Msun/yr Kudritzki 2002

23. Zero metallicity stars 1. Radiation pressure only due to H and He lines 2. H lines alone are never strong enough 3. HeII lines can produce grad-gN >0 if level populations are just right but only in very small region of atmosphere. Krticka & Kubat, 2005

24. geff > 0 only in very small zone Zero metallicity stars have NO line driven winds !! Krticka & Kubat 2005

25. Conclusions A: 10-3 < Z/Zsun < 10 : Mdot ~ Z0.85 but : Mdot depends on (fast) rotation B: 10-4 < Z/Zsun < 10-3 : Mdot ~only for large Γe Γe > (1+Mmax)-1 Mmax ≈ 2000 Z/Zsun C: Z/Zsun=0 : no line driven wind, Mdot < 10 -10 Msun/yr

26. That’s all !