Very Massive Stars in I Zw 18
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Very Massive Starsin I Zw 18
Sally HeapNASA’s Goddard Space Flight Center
IAU 2012 JD 2
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Very Massive Stars in the BCD Galaxy, I Zw 18
1. How massive are the most massive stars in I Zw 18?
2. How much mass do very massive stars in I Zw 18 lose?
3. How do very massive stars in I Zw 18 evolve and die”
4. What are the implications for
-- Chemical enrichmment of the ISM?
-- Injected energy via ionizing radiation?
-- Types of supernovae explosions?
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I Zw 18 = BCD Galaxy
• Two massive star clusters: NW and SE
• Distance: 18 Mpc (Aloisi+07)
• 1” = 87 pc
• COS aperture ~2.5” =218 pc
• Log (O/H) + 12 =7.18
• Mdyn=3x108 M⊙ Lelli +12)
• Mgas=1x108 M ⊙
• Mstar=1x106 M⊙
Star formation has barely begun!
HST/COSHST/COSapertureaperture
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150 M⊙
60 M⊙
30 M⊙
12 M⊙
150 M⊙
60 M⊙
30 M⊙
12 M⊙
1. How massive are the most massive stars in I Zw 18? 1.re the most massive stars in I 18
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Myr Myr
1. How massive are the most massive stars in I Zw 18? 1.re the most massive stars in I 18
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2. How much mass to very massive stars lose?
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Close-up of the C IV 1549 Doublet
Edge velocity ~ -250 km/s
Synspec spectrumGalactic ISM
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Synspec (Hubeny & Lanz) CMFGEN (Hillier) Models by Bouret
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Synspec (Hubeny & Lanz) CMFGEN (Hillier) Models by Bouret
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Synspec (Hubeny & Lanz) CMFGEN (Hillier) Models by Bouret
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Observed COS Spectrum
MF-Weighted Stellar Population Spectrum
Model Stars + H II Region + ISM
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3. How do very massive stars in I Zw 18 evolve and die?
Are there Wolf-RayetsStars in I Zw 18?
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Izotov+97 Legrand+97
NW
The spectrum of I Zw 18-NW shows the “WR bump”
Note the zero point: these emission features are faint!
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SDSS Spectrum of I Zw 18-NW
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Brown , SRH, et al. (2002)
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A Closer Look at the Brightest Stars1.re the most massive stars in I 18
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The brightest stars in I Zw 18-NW have a variety of spectral types
•Sources #1, #6, and #14 show C
IV with a P Cygni profile
•#1 and #14 are Brown et al.’s
sources (c) and (d)
•Some sources have hints of N V
1240 Pygni lines (wiped out by H I
Lyman- absorption)
•Most sources have unremarkable
UV spectra
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Stellar N v P Cygni profile indicates N-enriched winds with V~1500 km/s
HST/COS spectra of I Zw 18
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Lebouteiller, SRH, Hubeny, Kunth 12
4. What are the Implications for Chemical Enrichment of the ISM?
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Nitrogen is more abundant in the H II region than in the H I region. This suggests this rapidly rotating massive stars have synthesized N, mixed it to the surface, and dispersed it to the circumstellar ionized medium.
Rotation-induced “mixing brings to the surface the products of CNO burning: mainly 14N and 13C enrichments, 12C is depleted with limited 4He enrichment and 16O depletion”. ---- Maeder & Meynet, 2012, Rev. Mod. Physics
Nitrogen Enrichment in Action!
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Very Massive Stars in the BCD Galaxy, I Zw 18
• How massive are the most massive stars in I Zw 18?
Mmax<150 Msun
• How much mass do very massive stars in I Zw 18 lose?
Very small fraction of stars show evidence of mass loss
• How do very massive stars in I Zw 18 evolve and die?
Some very massive, rapidly rotating stars appear to evolve
homogeneously
• What are the implications for chemical enrichmment of the ISM?
Evidence for N enrichment of H II region surrounding NW
cluster