1. HST Observations of Low Z Stars HST Symposium, Baltimore May 3, 2004 Collaborators: Tim Beers, John Cowan, Francesca Primas, Chris Sneden Jim Truran

2.  What can we derive from abundance studies of the oldest stars in our Galaxy?  constraints upon light element production and BBN  constraints upon the early star formation and nucleosynthesis history of the Galaxy  constraints upon the characteristics of heavy element synthesis mechanisms  constraints on a/the stellar population that may have contributed to recombination  limits on the ages of the early populations from nuclear chronometers HST Observations of Low Z Stars

3.  What can we learn with HST? Studies in the wavelength region accessible with HST can address issues ranging from the origin of the light elements (Li, Be, B) to the production mechanisms responsible for the synthesis of the elements through thorium and uranium. Here we will review specifically:  boron abundance studies at low Z  abundance studies of the heavy elements Ge, Zr, Os, Pt, Au, and Pb at low Z HST Observations of Low Z Stars

4. Cosmic Abundances r-process s-process Cosmic (Solar System) Abundances

5. The Light Elements Lithium, Beryllium, and Boron  Implications for cosmology BBN vs inhomogeneous BBN (Li)  Implications for stellar structure LiT=2.5x10 5 K BeT=3.0x10 5 K BT=5.0x10 5 K  Implications for nucleosynthesis and cosmic-ray physics classical spallation of ISM heavy elements (Reeves et al. 1970) primary mechanism? neutrino-spallation production of 11 B ?

6. Be,B production mechanisms  Boron seems primary in the Galaxy, but what is happening in Orion ?  neutrino-spallation?  Be in the Galaxy (50 stars) -Be in Orion (VLT+UVES)  (in preparation)  B in stars with same Be, but different O (HST+STIS Cycle12)  (observations being taken) Primas et al. 2004

7.  Abundance studies of extremely metal deficient stars have revealed significant trends that constrain nucleosynthesis mechanisms, chemical evolution, and the early star formation history of our Galaxy.  Trends in iron group elements (through zinc) with decreasing metallicity.  The dominance of r-process elements (effective absence of s-process elements) at metallicities less than [Fe/H] ≈ -2. Abundance Trends in Halo Stars

8. Truran et al. 2002 Abundance Trends in Halo Stars

9. Abundance Clues and Constraints  In the heavy element region, HST observations of neutron-capture elements in low-metallicity Galactic halo stars are providing clues and constraints on:  the site(s) and robustness of the r-process nucleosynthesis mechanism  the ages of the oldest stars in our Galaxy  Galactic chemical evolution

10. r-Process in BD+17 3248 with HST

11. Constraining r-Process Nucleosynthesis  The elements Ge, Zr, Os, Pt, Au, and Pb are all understood to be products of one of the processes of neutron-capture synthesis. Ge and Zr are primarily s- process products, Os, Pt, and Au are mostly r-process, and Pb is a complicated product of both. Our recent HST STIS data reveals:  Ge tracks Fe and may thus reflect contributions from explosive nucleosynthesis in early stellar populations.  Os, Pt, Au, and Pb are produced in r-process proportions.

12. Metallicity Trends in Ge and Pt

13. Ages of Two Halo Stars from Th/U  Recent determinations of the abundances of both thorium and uranium in two extremely metal deficient halo field stars make possible direct determinations of their ages:  CS 31082-001, with a metallicity [Fe/H] = -2.9, has an age  Th/U = 14.0 ± 2.4 Gyr (Hill et al. 2002)  BD+17 3248, with a metallicity [Fe/H] = -2.1, has an age  Th/U = 13.8 ± 4.0 Gyr (Cowan et al. 2002)

14. Summary and Concluding Remarks  Boron abundances at low metallicity challenge and constrain theoretical models.  Trends in Ge/Fe may provide clues to the natures of the first stars and their nucleosynthesis products.  Trends in Os, Pt, Au, and Pb confirm the robustness of r-process synthesis over the mass range 140 ≤ A ≤ 190 and strengthen our confidence in the use of the nuclear chronometers 238 U and 232 Th.  Projected HST Cycle 13 observations will explore heavy element deficient stars over a range of Fe/H. HST Observations of Low Z Stars

16. NUV HST STIS Spectra

17. Clues from Metal-Deficient Stars Heavy Elements in CS 22892-052 Platinum Germanium

18. Metallicity Trends in Heavy Elements

19. Zinc Evolution with [Fe/H] Silicon Evolution Figure Credits: Francesca Primas 2003

20. s-Process/r-Process Chemical Evolution (Truran et al. 2002)

21. Eu Abundance Scatter in the Galaxy