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

HST Observations of Low Z Stars

HST Symposium, Baltimore May 3, 2004

Collaborators: Tim Beers, John Cowan, Francesca Primas, Chris Sneden

Jim Truran

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.



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




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


Cosmic Abundances

r-processs-process


are needed to see this picture. Cosmic (Solar System) Abundances

The Light Elements

Lithium, Beryllium, and Boron

Implications for cosmology

BBN vs inhomogeneous BBN (Li)

Implications for stellar structure

Li T=2.5x105K

Be T=3.0x105K

B T=5.0x105K

Implications for nucleosynthesis and cosmic-ray physics

classical spallation of ISM heavy elements (Reeves et al. 1970)

primary mechanism? neutrino-spallation production of 11B ?



Be,B production mechanisms

Boron seems primary Boron seems primary in the Galaxy, but whatin the Galaxy, but what

is happening in Orion ?is happening in Orion ? neutrino-spallation?neutrino-spallation?

Be in the Galaxy (50 Be in the Galaxy (50 stars) -Be in Orion stars) -Be in Orion (VLT+UVES)(VLT+UVES)

(in preparation)(in preparation)

B in stars with same Be,B in stars with same Be,

but different O (HST+STISbut different O (HST+STIS

Cycle12)Cycle12) (observations being (observations being taken)taken)

Primas et al. 2004Primas et al. 2004





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

Truran et al. 2002


are needed to see this picture. Abundance Trends in Halo Stars

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



r-Process in BD+17 3248 with HST



Constraining r-Process NucleosynthesisQuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.

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.

Metallicity Trends in Ge and PtQuickTime™ and a


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)





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 238U and 232Th.

Projected HST Cycle 13 observations will explore heavy element deficient stars over a range of Fe/H.


NUV HST STIS SpectraQuickTime™ and a


Zinc Evolution with [Fe/H]

Silicon Evolution

Figure Credits:Francesca Primas 2003

s-Process/r-Process Chemical Evolution

(Truran et al. 2002)

Eu Abundance Scatter in the GalaxyQuickTime™ and a


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