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Very Massive Stars in I Zw 18

Very Massive Stars in I Zw 18. Sally Heap NASA’s Goddard Space Flight Center IAU 2012 JD 2. Very Massive Stars in the BCD Galaxy, I Zw 18. How massive are the most massive stars in I Zw 18? How much mass do very massive stars in I Zw 18 lose?

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Very Massive Stars in I Zw 18

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  1. Very Massive Starsin I Zw 18 Sally Heap NASA’s Goddard Space Flight Center IAU 2012 JD 2

  2. Very Massive Stars in the BCD Galaxy, I Zw 18 • How massive are the most massive stars in I Zw 18? • How much mass do very massive stars in I Zw 18 lose? • How do very massive stars in I Zw 18 evolve and die” • What are the implications for • -- Chemical enrichmment of the ISM? • -- Injected energy via ionizing radiation? • -- Types of supernovae explosions?

  3. 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=3x108M⊙Lelli +12) • Mgas=1x108M⊙ • Mstar=1x106 M⊙ • Star formation has barely begun! HST/COS aperture

  4. 1. How massive are the most massive stars in I Zw 18? re the most massive stars in I 18 150 M⊙ 60 M⊙ 30 M⊙ 12 M⊙ 150 M⊙ 60 M⊙ 30 M⊙ 12 M⊙

  5. 1. How massive are the most massive stars in I Zw 18? re the most massive stars in I 18 Myr Myr

  6. 2. How much mass to very massive stars lose?

  7. Close-up of the C IV 1549 Doublet Synspec spectrum Galactic ISM Edge velocity ~ -250 km/s

  8. Synspec (Hubeny & Lanz) CMFGEN (Hillier) Models by Bouret

  9. Synspec (Hubeny & Lanz) CMFGEN (Hillier) Models by Bouret

  10. Synspec (Hubeny & Lanz) CMFGEN (Hillier) Models by Bouret

  11. Observed COS Spectrum MF-Weighted Stellar Population Spectrum Model Stars + H II Region + ISM

  12. 3. How do very massive stars in I Zw 18 evolve and die? Are there Wolf-RayetsStars in I Zw 18?

  13. The spectrum of I Zw 18-NW shows the “WR bump” NW Izotov+97 Legrand+97 Note the zero point: these emission features are faint!

  14. SDSS Spectrum of I Zw 18-NW

  15. Brown , SRH, et al. (2002)

  16. A Closer Look at the Brightest Stars re the most massive stars in I 18

  17. The brightest stars in I Zw 18-NW • have a variety of spectral types • Sources #1, #6, and #14 show C IV with aP 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-a absorption) • Most sources have unremarkable UV spectra

  18. Stellar N v P Cygni profile indicates N-enriched winds with V¥~1500 km/s HST/COS spectra of I Zw 18

  19. 4. What are the Implications for Chemical Enrichment of the ISM? Lebouteiller, SRH, Hubeny, Kunth 12

  20. Nitrogen Enrichment in Action! 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

  21. 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

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