Metallicity Dependent Wolf-Rayet winds

1. Metallicity Dependent Wolf-Rayet winds PAUL CROWTHER

2. Introduction • Wolf-Rayet (WR) stars represent the final phase in the evolution of very massive stars prior to core-collapse; • H envelope stripped away via (a) stellar wind or (b) close binary evolution, revealing products of either H-burning (WN) or He-burning (WC); • WR stars show strong emission lines (HeII 4686, CIV 5808) due to dense (10-5Myr-1), fast (<1,000 to >5,000 km s-1) winds

3. WR winds Denser winds than O stars, so visual spectra of WR stars are dominated by emission rather than absorption lines R(2/3) >1012 >1011

4. Observations • WR spectroscopic signatures observed.. • Individually within Local Group; • Knots within local star forming galaxies; • Average rest frame UV spectrum of LBGs. • H-depletion caused by winds during O & LBV/RSG phases if single, so higher threshold for WR formation at low Z. • WRs indeed rarer at 1/5 Z (SMC) vs Solar. • Alternatively, H-depleted during close binary evolution, without Z dependence. • Dominant mechanism at low Z (e.g. IZw18)

5. Izw18 WN stars expected to dominate in IZw18, but solely signature of WC stars observed (Brown et al. 2002)

6. WR stars as GRB progenitors • Prime candidates for precursors of Type Ib/c SNe & long/soft GRBs. Progenitors: • Associated with young massive stellar populations, • Compact (excludes RSG progenitors), • Rapidly rotating core. • Primary challenge for single/binary GRB progenitors is requirement for rapid rotation at core-collapse (at Z core slowed down during RSG/WR phase).

7. Observed WR properties • WR wind properties are generally assumed to be metallicity (Z) independent (Langer 1989). • Observational trend to both earlier WN and WC subtypes at low Z. Origin? early late early late

8. Metallicity-dependent winds? • Wide scatter in WN mass-loss rates for Milky Way & LMC. Presence of hydrogen complicates picture (winds are denser if hydrogen is absent, Nugis & Lamers 2000). • However, winds of SMC WN winds are weaker than similar H-rich LMC/Galactic stars (Crowther: Tartu workshop 2005). • Trend to earlier WC subtypes in LMC vs Milky Way was once believed to result from different C abundances. However, abundance pattern similar in both galaxies.

9. WC metallicity dependence Milky Way WC stars followed generic Nugis & Lamers (2000) calibration (red). LMC stars followed similar relation (green), offset by -0.2 dex (Crowther et al. 2002). Log(dM/dt) = 1.38 log(L/Lo) -12.35

10. Wind velocities? Limited evidence in favour of Z- dependent wind velocities except for individual WO stars (= early WC).

11. Theoretical evidence? • Radiatively driven wind model of Grafener & Hamann (2005), showed that Fe IX-XVII lines initiate WC outflow. • Vink & de Koter (2005) used Monte Carlo approach to investigate dM/dt Z dependence for WN stars (=0.86; 10-3 to 1 Z) & WC stars (=0.66; Z=0.1-1 Z). • Origin of different exponents? C,N,O,Fe decrease in WN stars (CNO act as catalysts), but only Fe decreases in WC stars, due to primary C,O production.

12. Impact on WR subtypes? early early High density winds cause very efficient recombination from high to low ions (shift from`early’ to`late’ subtypes). Opposite is true for low density WR winds. late late

13. Impact on WR populations? • Effect of reduced WR wind densities at low Z: • Increasingly weak-lined WR stars, as observed in SMC (more difficult to detect, especially WN stars); • Reduced emission line fluxes (factor of 3 to 20 at 1/50Z). Assumed Z-independent LMC/Milky Way calibration of Schaerer & Vacca (1998). • 10 x lower emission line fluxes requires 10 x more WR stars at low Z (Crowther & Hadfield 2006). Problematic for single star models. • Large WR populations at very low Z (e.g. IZw18) likely to be binary evolution dominated.

14. NGC3125-A LMC template stars required to derive reliable WR populations in 0.5 Z starburst galaxy NGC3125. 130 WN20 WC 20 WC

15. Impact on ionizing fluxes? • WR stars with weak winds possess harder ionizing fluxes in He+ Lyman continua versus strong winds (Schmutz et al. 1992;Smith et al. 2002) • WR stars at low metallicity will possess .. • weak UV/optical spectral lines (hard to directly detect via broad HeII 4686); • strong H Lyman & He+ Lyman continua (easy to detect indirectly via nebular HeII 4686). • Strong nebular HeII in low-Z HII galaxies from WRs & SNR (Izotov et al. 2006).

17. Impact on GRB progenitors? • Reduced WR mass-loss rates help to maintain rapidly spinning core through to core-collapse at low Z for single stars (Yoon & Langer 2005). • Reduced densities in immediate environment of GRBs with respect to typical Milky Way WR stars, as observed (Chevalier et al. 2004)

18. Summary • Observational & theoretical evidence supports reduced wind densities & velocities for low metallicity WR stars • Addresses relative WR subtype distribution in Milky Way & Mag Clouds, & reduced WR line strengths in SMC • Impacts upon WR populations at low metallicity as follows: • Increased WR populations due to lower line fluxes from individual stars; • Harder ionizing fluxes; • Low density GRB environment vs Solar counterparts

19. IAU Symposium 250 • Title: “Massive Stars as Cosmic Engines” • Atmospheres of massive stars; • Physics & evolution of massive stars; • Massive stellar populations in the nearby Universe; • Hydrodynamics and feedback from massive stars in galaxy evolution; • Massive stars as probes of the early Universe • Venue: Kauai, Hawaii • Dates: 10-14 December 2007 • SOC: P.Crowther (co-chair), M.Dopita, J. Fynbo, E.Grebel, T.Heckman, D. Hunter, G. Koenigsberger, R. Kudritzki, N. Langer, A. MacFadyen, F. Matteucci, G. Meynet, A. Moffat, K. Nomoto, M. Pettini, J. Puls (co-chair)