Wolf-Rayet Stars Ryan Yamada 2006 April 5 Astronomy 671 Overview - - PowerPoint PPT Presentation

Wolf-Rayet Stars

Ryan Yamada 2006 April 5 Astronomy 671

Overview

• Introduction
• Properties
• Observations (IR and x-ray)
• Models and limitations
• Conclusions

What are Wolf-Rayet Stars?

• Massive descendants of O-type stars

– All stars with M > 40 Msun become W-Rs – Final stage before supernova – Rare (~250 in our galaxy)

• Hot and luminous

– strong, broad (103 km/s) emission lines in optical due to substantial stellar wind – T ~ 25-150K – L ~ few 105 Lsun

• Mass Losers

– dM/dt ~ 10-4-10-5 Msun/yr – Driven by rad. pressure (pulsations? Magnetic-centrifugal forces?) – Terminal velocities ~ 1500/2500 km/s (IR/UV)

Evolution of a Wolf-Rayet Star

• Begins as a massive O star (M > 40 Msun)
• “red” supergiant progenitor (T~ 6,000 K)
• Luminosity exceeds Eddington limit; outer H

layer lost

• WN phase

– Strong (optical) lines from He and N – Subclassified as WN3-WN9 (strong to weak)

• WC phase

– Lines of carbon, oxygen, and helium

• WO phase

– Minor subtype; strong O VI lines

Spectra of O, W-R stars

W-R stars on the HR Diagram

From Moffat et al. 1989

Type Ib supernovae

From Daniel Kasen WO?

Location of W-Rs

• Similar distribution to O stars

– n(O)/n(W-R) ~ 4 – ~ 40% found with O-type companion

• Observed in other galaxies

– ~few in SMC – ~100 in LMC – ~100 in M33 – ~dozens in M31 – “Wolf-Rayet” galaxies (e.g. 1 Zw 18) -> tons of these stars or different explanation for spectral features?

What are Wolf-Rayet Stars?

• Massive descendants of O-type stars

– All stars with M > 40 Msun become W-Rs – Final stage before supernova – Rare (~250 in our galaxy)

• Hot and luminous

– strong, broad (103 km/s) emission lines in optical due to substantial stellar wind – T ~ 25-150K – L ~ few 105 Lsun

• Mass Losers

– dM/dt ~ 10-4-10-5 Msun/yr – Driven by rad. pressure (pulsations? Magnetic-centrifugal forces?) – Terminal velocities ~ 1500/2500 km/s (IR/UV)

Spectrophotometry of HD192163

What are Wolf-Rayet Stars?

• Massive descendants of O-type stars

– All stars with M > 40 Msun become W-Rs – Final stage before supernova – Rare (~250 in our galaxy)

• Hot and luminous

– strong, broad (103 km/s) emission lines in optical due to substantial stellar wind – T ~ 25-150K – L ~ few 105 Lsun

• Mass Losers

– dM/dt ~ 10-4-10-5 Msun/yr – Driven by rad. pressure (pulsations? Magnetic-centrifugal forces?) – Terminal velocities ~ 1500/2500 km/s (IR/UV)

IUE Spectrum

ISO Spectra

• Average of 7 W-

R SEDs

• No carbon

reactions in H2 atmosphere

• C2H2, while

important for dust in carbon stars, isn’t the source in W-R stars

From Williams, van der Hucht, & Morris (1997)

ISO Spectrum

• Top: (continuum-

subtracted spectrum) from WR 48a

• Bottom: three possible

classes of emission bands – A: HII regions, Herbig Ae/Be stars (w H II), extragalactic sources – B: Isolated Herbig Ae/Be stars, some post-AGB stars, most PNe – C: two post-AGB stars

Chiar, Peeters & Tielens (2002)

Discoveries from IR Obs.

• Dust inferred from shape of IR continuum

– excess cannot be explained by free-free emission alone

• No silicate features, but evidence of graphite

grains

• IR speckle interferometry puts dust shell at

~few x1015 cm, consistent with estimated grain temperatures and emission energies

• IR flux constant -> constant rate of dust

formation

Discoveries from IR Obs. (cont)

• Continuum emission beyond few µm
• riginates far from star
• Shock compression in inner wind

Discoveries from X-ray

• Einstein data showed WR resemble O

stars (Tx ~ few million degrees), small fraction of total wind

• Tight relationship Lx/Lbol ~ 10-7
• Inverse Compton scattering responsible

for X-rays in nonthermal radio sources

• Binaries -> X-ray flux due to colliding

winds

Models and limitations

• Most models of W-R

atmospheres have assumed

– (1) plane-parallel radiative transfer – (2) isotropic distribution of matter in dust – (3) core-halo structure Wolf-Rayet star BAT99-2 (ESO)

Models and limitations

• Most models of W-R atmospheres have assumed

– (1) plane-parallel radiative transfer – (2) isotropic distribution of matter in dust – (3) core-halo structure

• Not a good idea!

– (1) W-Rs are extended continuum sources; small scale height can lead to big errors in effective temperature – (2) dust is “clumpy” – (3) continuum depends sensitively on density and temperature structure in wind

Models and limitations

• Wind models

– Require tremendous efficiency (photons transfer ~90% momentum to gas) – Using Abbott & Lucy model, Abbott & Conti (1987) find that radiation pressure is sufficient – Hydrostatic core loses hydrogen, evolves to high T and small R, making radiation driving more efficient – This has been contested by Schmutz (1997), who claims that resonant absorption is the key to understanding winds

Summary

• W-R stars are evolved O stars, and

progenitors of Type Ib supernovae

– (O -> WN -> WC -> WO -> Ib SN)

• Massive winds are probably due to radiation

pressure/absorption

• Key testing ground for models of winds,

supernovae, dust/cloud formation and evolution

References

• Abbott, D. C., Conti, P. S. 1987, ARA&A, 25,

113

• Chiar, J.E., Peeters, E., Tielens, A. G. G. M.

2002, ApJ, 579, L91

• Moffat, A. F. J., Drissen, L., Robert, C. 1989,

plbv.coll, 229

• Hamann, W.R., Koesterke, L., Wessolowski,
• U. 1993, A&A, 274, 397
• Schmutz, W. 1997, A&A, 321, 268
• Williams, P. M., van der Hucht, K. A., Morris,
• P. W. 1997, Ap&SS, 255, 169