Observed Metallicity Dependence of WolfRayet Winds

1
Observed Metallicity Dependence of Wolf-Rayet
Winds
Paul Crowther
2
Basic Properties of WR stars

• Two main flavours WN (some H, He, N) WC (He,
  C, O). Analysed in a similar way to OB stars
  (including stellar winds line blanketing is
  essential).
• T from 30,000K for late-type WN stars 45,000K
  for late-type WC stars, to gt100,000K for early WN
  and WC stars,
• Luminosities span a similar range to early O
  stars (105-6 Lo) whilst mass-loss rates typically
  an order of magnitude higher (10-5 Mo/yr),
• WR stars are rare, but provide important
  constraints on late stages of massive stellar
  evolution - products of CNO cycle (WN) or
  alpha-capture (WC)

3
Classification

• As with normal stars, nitrogen sequence
  (WN) Wolf-Rayet stars are classified via ratios
  of optical HeI-II or NIII-V lines (e.g. Smith et
  al. 1996).
• Early type WN stars tend to have broad lines no
  hydrogen, whilst late-WN stars have narrow lines
  hydrogen.
• WC classification is similar, based on CII-IV
  lines, with narrow, CII-III strong in late WC
  stars, broad, strong CIV in early WC stars
  (Crowther et al. 1998). WO stars provide
  extension to higher excitation (OIV-OVI strong)

NIII NIV NV
4
How to find W-R stars?
Narrow-band HeII (?4684) off-HeII (?4751)
imaging surveys allow W-R stars to be identified
in Milky Way external galaxies due to strong,
broad emission lines (HeII 4686 in WN stars,
CIII 4650 in WC stars).
Schild et al. 2003 AA 397, 859
5
WR distribution
Comparison between observed distribution of
O-type WR stars in our Galaxy, LMC SMC shows
a strong metallicity-dependence, due to
decreasing wind strength of WR precursors at
lower metallicity.
Massey and Johnson 1998
6
Physical and Wind Properties

• Prior to non-LTE model atmospheres wind
  properties derived from free-free excess (e.g.
  Wright Barlow 1975) via mid-IR, radio fluxes
  limited to Milky Way, whilst abundances were
  obtained from recombination theory (e.g. Smith
  Hummer 1988).
• Development of non-LTE He -gt HeCNO -gt HeCNOFe
  models by Hillier and by Hamann during
  1980s-1990s permits spectral line fitting,
  revealing T, logL, dM/dt, X/He, via diagnostic
  lines, i.e. HeI-II and/or NIII-V in WN stars and
  HeI-II and/or CII-IV in WC stars, applicable to
  stars in Milky Way and external galaxies.

Crowther (1999) IAU Symp 193
7
Wind properties - clumping

• Wind velocities show a wide range lt1000 km/s for
  late-types, to 5000km/s for some early WC stars
• Mass-loss rates generally exceed 10-5 Mo/yr via
  analysis of optical lines or radio fluxes main
  uncertainty is degree of clumping.
• Winds known to be clumped from intensive
  monitoring studies (e.g. Lepine et al. 2000)
• Red electron scattering wing also provides
  evidence for clumping (Hillier 1991)
• Most WR stars are spherical. 5/29 show
  significant asphericity (Harries et al. 1998)

8
LMC vs Milky Way WN stars
Nugis Lamers (2000)

• Hamann Koesterke (2000) compared wind
  properties of Milky Way (small) and LMC (large)
  WN stars.
• Large scatter but no systematic effect.

Best fit to WNE-s stars
9
SMC WN stars
Either no metallicity dependence, or too subtle
between LMC and Milky Way due to multiple
evolutionary channels entering WN phase.
Crowther (2000) analysed the sole (at that time)
single WN star results placed it at the lower
end of Gal/LMC WN6-9 sequence, but
inconclusive. Need larger sample at low
metallicity Foellmi et al. (2003) established a
larger single WN population in SMC.
Crowther (2000)
10
Weak SMC winds?
11
Near-IR Wind Diagnostics

• Schmutz et al. (1989) and Howarth Schmutz
  (1992) used the strong HeI 1.083?m diagnostic in
  their spectroscopic studies of Galactic WR stars,
  since sensitive to wind strength. Best available
  diagnostic to compare SMC with LMC/Milky Way
  counterparts.

8 cluster/association WN3-6 stars in Milky Way
(IRTF/SpeX courtesy Bill Vacca) 18 WN3-6 stars
in LMC and 4 WN3-6 stars in SMC (SofI with Lucy
Hadfield, Cedric Foellmi)
12
Nugis Lamers
dM/dt(H)?L0.85
-0.4dex
dM/dt(no-H)?L0.55
13
WC stars
Grafener et al. (1998) compared wind properties
of LMC WC4 stars (open circles) to Milky Way
WC5-8 stars (filled circles) and concluded
either dM/dt ? L0.75 for combined Milky Way/LMC
sample (red) Or dM/dt ? L1.5 for Milky Way stars
(green ,common to H-free WN stars, Hamann 1995)
with weaker winds for LMC WC stars.
14
WC metallicity dependence
Crowther et al. (2002) revisited LMC WC4 stars
with respect to a larger Milky Way sample. Milky
Way stars followed generic Nugis Lamers (2000)
calibration (red). LMC stars followed similar
relation (green), offset by -0.2 dex (Unable to
apply readily to lower metallicity stars since
just one WO binary in SMC).
Log(dM/dt) 1.38 log(L/Lo) -12.35
15
WN subtypes
Crowther (2000)

• Since the total CNO is fixed within CNO cycle,
  equilibrium N content in WN stars decreases with
  metallicity.
• Classification NIII 4640 more sensitive to
  abundance than NIV 4058 so for otherwise
  identical parameters, trend to earlier subtypes
  at lower metallicity regardless of wind strength.

16
Impact upon WN subtypes
Additionally, if wind strength sensitive to
metallicity, mass-loss diagnostics (HeI) more
affected than temperature (HeII) diagnostics.
Both reduced N abundance reduced wind strength
predict earlier subtypes at lower metallicity, as
observed..
17
HeII 4686 Line luminosity

• Metallicity-independent HeII 4686 line
  luminosity (based on LMC/Milky Way stars) used to
  estimate WN populations inWR galaxies (Schaerer
  Vacca 1998).

NGC3125 Hadfield Crowther poster
18
Weak wind reduced 4686 flux
Crowther (1999) IAU Symp 193

• For weak WN winds at low metallicity with e.g.
  dM/dt ? Z0.5 scaling, EW(HeII 4686) reduced AND
  optical continuum flux reduced (EUV flux becomes
  optically thin). Consequently, predicted
  reduction in HeII 4686 line luminosity.

19
SMC WN line luminosities

• Empirically, SMC WN stars (open symbols) do
  possess much lower HeII 4686 line luminosities
  than LMC WN counterparts (filled symbols)
  Hadfield Crowther poster Impacts upon WR
  content at low metallicity (later).
• (NB Sand 2, WO subtype, has a CIII 4650/HeII
  4686 line luminosity a factor of 10 times lower
  than other LMC WC4 stars.)

20
Impact upon WC subtypes

• CIII 5696 is a primary WC classification line
  which is sensitive to dM/dt.
• A decrease of wind strength leads to an
  earlier WC subtype (Crowther et al 2002)

If modest dM/dt versus metallicity scaling,
expect late subtypes at high metallicity and
early subtypes at low metallicity.
21
WC subtype distribution
High metallicity WC9 stars are only observed in
the inner Milky Way towards Galactic Centre, and
M83 (metal-rich spiral Hadfield et al. 2005)
WN(square),WC(), WCWN(x)
Low metallicity Only WC4 and WO stars observed
in LMC, SMC, IC1613 etc.
22
Significance of WC subtype?
Evolutionary models relate WC subtype to (CO)
enrichment (e.g.Hirschi et al. 2005)

• Atmospheric models find no dependence of WC
  subtype on CO enrichment (e.g. Koesterke
  Hamann 1995).

23
WO stars

• LMC WC4 stars span similar range in CO/He to
  Galactic WC5-8 stars (Crowther et al. 2002).
• WO stars are very rich in CO (Kingsburgh et
  al. 1995, Crowther et al. 2000). WO stars are
  only detected in low metallicity environments
  (outer Milky Way, LMC, SMC, IC1613).

OVI 3811-34
Why? For otherwise identical parameters a WC4
versus WO subtype will result for a denser versus
weaker wind (O6 recombines to lower ionization
stages of oxygen within inner wind for high wind
densities, so WO classification is lost).
Crowther et al. 2002
Crowther (1999) IAU Symp 193
24
Ionizing fluxes from WR stars
Evolutionary synthesis models for starbursts
(e.g. Starburst99) rely on accurate ionizing
fluxes for constituent hot (O, B WR) stars a
metallicity dependence for WR stars produces much
softer radiation in a young burst at high
metallicity (Smith et al. 2002) and a relatively
hard ionizing spectrum at low metallicity
(www.star.ucl.ac.uk/ljs/starburst/starburst.html)
25
IZw18

• Lowest metallicity nearby galaxy (1/50 solar),
  with on-going star formation. IZw18 includes both
  HeII 4686 narrow nebular (Campbell et al. 1986)
  and broad stellar (Izotov et al. 1997, de Mello
  et al 1998) emission, plus broad stellar CIV 5808
  emission.
• Izotov et al. (1997) estimated 17 WNL stars plus
  5 WCE stars. Allowing for the WC contribution to
  HeII 4686, one would infer 4 WNL stars.
• For a dominant SMC-like WR population in IZw18,
  one would instead infer up to 100 WNE stars and
  50 WO stars. These would have weak winds, so
  produce lots of HeII continuum photons, so
  strong nebular HeII 4686 emission, as observed.

HeII 4686
CIV 5808
F555W
26
Summary

• Near-IR observations of SMC WN3-6 stars 0.4 dex
  weaker than Milky Way/LMC counterparts (Zlt1)
  with stronger winds in H-free WN stars.
• WC stars in LMC reveal 0.2 dex weaker winds than
  Milky Way counterparts (Z0.7)
• Weaker winds ( low metallicity) lead to
  spectroscopic earlier WN and WC subtypes.
• Wind strength, rather than chemical enrichment,
  primarily dictates WC subtypes.
• WO stars are chemically advanced, but only
  observed if weak wind ( low metallicity).
• At low metallicity (IZw18) expect weak-lined,
  early WN and WO stars, with hard EUV flux
  distribution and low HeII 4686 line luminosity.