Small-scale heros:massive-star enrichment in

ultrafaint dSphs

Andreas Koch

D. Adén, S. Feltzing (Lund)F. Matteucci (Trieste)

A. McWilliam (Carnegie)

“First Stars IV”, Kyoto, May 23, 2012

Smallest scalesM

v

Small-scale puzzle pieces of large scale halo(s).

Zucker et al. 2006; Belokurov et al. 20NN; Walsh et al. 2007; Irwin et al. 2007 ...

• dSphs: have long been known as low luminosity systems.

• Since ~2006: ultra-faint dwarfs. • Sites of Fe-poor stars ( [Fe/H] > - 3.96; Tafelmeyer et al.

2010)

2/10Kirby et al. (2008)

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Martin et al. (2008); Koch (2009)

• Ultrafaint dSph, discovered within SDSS (Belokurov et al. 2007); Mv=-6.6; d=140 kpc

• low-mass, metal-poor, elongated (one of the most elliptical LG dSphs) (Coleman et a. 2007; Martin et al. 2008; de Jong et al. 2008; Kirby et al. 2008)

• Stellar Stream ? (Martin & Jin 2010)

SDSS DR6; 30’x30’ (ca. 4xrh)

The first high-resolution spectra of 2 red giants in Her: AK et al. (2008, ApJL).

(MIKE@Magellan; 6 hours integration R=20,000; 4000-9000 Å)

Hercules

3/10

New FLAMES spectra (Adèn, AK, et al. 2011): very large spread in Fe and Ca

Hercules - heavy elements

4/10Models by F. Matteucci (n = 0.0001, 0.001, 0.01, 0.1 Gyr -1)

MW disks and halodSphs

Hercules

Models for high-mass SNe II predict noteably high Mg yields w.r.t. Ca - in fact observed in Her.

Our high [O, Mg, Si / Ca, Ti] implies Mprog ~ 35-50 M

(Heger & Woosley 2010)

Her-2

Her-3

Hercules – a small scale hero?

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dSphs,Halo

Hercules - n-capture elements• Her stars are strongly depleted in Ba (Sr, Eu);

Mostly upper limits for Ba.• So far seen in a few halo stars, Dra 119 and 2 UFDs (Fulbright et al. 2004; Feltzing et al. 2009; Simon et al. 2010)

AK et al. 2008, ApJL AK et al. in prep.

Hercules – no heavy elements

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Hercules - n-capture elements

AK et al. in prep.

Hercules – no heavy elements

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• Mtot = 7 x 106 M and M/L = 330 implies M* ~ 40000 M (Martin et al. 2008)

• Incomplete sampling of high-mass end of IMF in small-scale SF events (AK et al. 2008): stochastic SF tests imply that perhaps only 1-3 massive SNe II influenced the Her stars: - [high Mg/Ca] - initial [Ca/Fe] ~ 0.4 - trace amounts of r-process - later SF –> Fe contributions and Fe spreads

• High-mass? Rather “ordinary massive” (N. Yoshida);

•

Stochastic star formation

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Courtesy Ken Nomoto

High-energy SNe, Mprog ~ 25 Mo, leaves non-rot. BH,most material (56Ni) falls back. Likely related to EMP stars - first stars in ultrafaints ?!

...or faint Supernovae?

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Summary

• We are approaching ever-smaller scales: extragalactic EMP; ultra faint dwarfs; stochastic chemical enrichment; individual supernovae...Any data point, odd or not, is crucial.

• Chemical evolution of Hercules (and others) was governed by very few, massive (few tens Mo) SNe. Other indicators ( high Co/Cr ... ) argue for the signatures of the “First Stars”.

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