Very Massive Starsin I Zw 18

Sally HeapNASA’s Goddard Space Flight Center

IAU 2012 JD 2

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?

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

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

Myr Myr

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

2. How much mass to very massive stars lose?

Close-up of the C IV 1549 Doublet

Edge velocity ~ -250 km/s

Synspec spectrumGalactic ISM

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

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

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

Observed COS Spectrum

MF-Weighted Stellar Population Spectrum

Model Stars + H II Region + ISM

3. How do very massive stars in I Zw 18 evolve and die?

Are there Wolf-RayetsStars in I Zw 18?

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!

SDSS Spectrum of I Zw 18-NW

Brown , SRH, et al. (2002)

A Closer Look at the Brightest Stars1.re the most massive stars in I 18

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

Stellar N v P Cygni profile indicates N-enriched winds with V~1500 km/s

HST/COS spectra of I Zw 18

Lebouteiller, SRH, Hubeny, Kunth 12

4. What are the Implications for Chemical Enrichment of the ISM?

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!

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