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)
8
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?
5/10
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
6/10
Hercules - n-capture elements
AK et al. in prep.
Hercules – no heavy elements
7/10
• 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
8/10
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?
9/10
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”.
10/10