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Metallicity Dependent Wolf-Rayet winds?. PAUL CROWTHER. Introduction. Wolf-Rayet (WR) stars represent the final, pre-core collapse phase in the evolution of very massive stars;

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

Denser winds than O stars, so WR spectra are dominated by emission lines rather than absorption lines




  • WR spectroscopic signatures observed..
    • Individually within Local Group galaxies
    • Aggregrates within knots in local star forming galaxies (e.g. Vacca & Conti 1992)
    • in average rest frame UV spectrum of Lyman Break Galaxies (Shapley et al. 2003)
  • H-depletion caused primarily by mass-loss during O & LBV/RSG phases. Z-dependent O star winds imply higher threshold for WR formation at low Z.
  • Indeed, WR stars common at 2Z (M83, inner Milky Way); increasingly rare at 1/5 Z (SMC), but still observed at 1/50 Z

WN stars expected to dominate in IZw18, but signature of WC stars dominates (Brown et al. 2002)

relevance of wr stars
Relevance of WR stars?
  • Prime candidates for precursors of Type Ib/c SNe & long/soft GRBs. Progenitors:
    • Associated with young massive stellar populations,
    • Compact (e.g. excluding RSG progenitors),
    • Possess rapidly rotating core.
  • Primary challenge is requirement for rapid rotation at core-collapse. Core spun down either during RSG or WR phase (at Solar metallicity).
metallicity dependent winds
Metallicity dependent winds?

early late early late

  • WR wind properties are generally assumed to be metallicity (Z) independent (Langer 1989).
  • Observational trend to earlier WN and WC subtypes at low Z.
observational evidence
Observational evidence?
  • 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 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.
wc metallicity dependence
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

wind velocities
Wind velocities?

Observational evidence for slower winds at low Z? Only amongst individual WO stars (= early WC).

theoretical evidence
Theoretical evidence?
  • Consistent WC radiatively driven wind model of Grafener & Hamann (2005), in which FeIX-XVII lines initiate 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).
  • Why different exponents at low Z? 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.
impact on wr subtypes
Impact on WR subtypes?



High density WR winds will lead to efficient recombination from high (`early’) to low (`late’) ions. Opposite is true for low density WR winds, as observed at low metallicity.



impact on wr populations
Impact on WR populations?
  • Reduced WR wind densities at low Z will produce:
    • Increasingly weak-lined WR stars (more difficult to detect, especially WN stars), as observed in SMC;
    • Reduced emission line fluxes (factor of 3 to 20 at 1/50Z). No Z-dependence assumed in LMC/Milky Way calibration of Schaerer & Vacca (1998)
  • 10 x lower line fluxes requires 10 x more WR stars at low Z (Crowther & Hadfield 2006). Insufficient WR stars predicted with single star models! Increasingly binary dominated?


LMC template stars required to derive reliable WR populations in 0.5 Z starburst galaxy NGC3125.

130 WN20 WC

20 WC

impact on ionizing fluxes
Impact on ionizing fluxes?
  • WR stars with weak winds possess harder ionizing fluxes (<228A He+ Lyman continua) than those with 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).
  • Nebular HeII is common in low-Z HII galaxies.
impact on grb progenitors
Impact on GRB progenitors?
  • Reduced WR mass-loss rates may maintain rapidly spinning core through to core-collapse at low Z for single stars (if RSG avoided) - Yoon & Langer (2005)
  • Reduced densities in immediate environment of GRBs with respect to typical Milky Way WR stars, as observed (Chevalier et al. 2004)
  • Observational & theoretical evidence supports reduced wind densities (and velocities?) for low metallicity WR stars
  • Addresses relative WR subtype distribution in Milky Way & Mag Clouds, & reduced WR line luminosities in SMC
  • Impacts upon WR populations at low metallicity as follows:
    • Increased WR populations due to lower individual line fluxes;
    • Harder ionizing fluxes;
    • Lower density GRB environment vs Milky Way WR stars
iau symposium 250
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)