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The Cecilia Payne-Gaposchkin Lecture Center for Astrophysics May 9, 2002 Heavy Metal from Ancient Superstars In collaboration with…. Debra Burris (Oklahoma City CC) John Cowan (University of Oklahoma) Chris Sneden (University of Texas) Taft Armandroff (NOAO) Henry Roe (U.C. Berkeley) Outline The high-redshift universe The Galaxy in time – a brief review of the formation of the Milky Way and its structural components The origin of the elements – back to B2FH! A stroll through the Periodic Table A timeline of Galactic chemical enrichment What does it all mean? Metallicity at High Redshift • Studies of the most metal-poor stars in the Galaxy give us access to the state of the Universe at very early times Songaila & Cowie 2002 But - the most metal-poor stars in the Galaxy have [Fe/H]=-4 Metallicity Distribution Function for Metal-Poor Stars • • • NO stars with [Fe/H] < -4.0 Beers 1999 Have we found the low metallicity end of the MDF? Did the first generation(s) raise the metallicity to [Fe/H] = -4? (Selection effects for [Fe/H] > -2) The Milky Way…. Circa 1950 1990 Flattened Inner Halo Halo Thick Disk Dwarf Spheroidal Companions Dark Matter Corona The Chemistry of Stellar Populations…… The chemical compositions of stars reflect the star formation histories of stellar populations The complexity of the Milky Way’s history is reflected in the compositions of its stars Solar Abundances from Grevesse and Sauval CNO Log e (H=12) 8 Fe 5 Sr, Y, Zr Sc 2 Ba Li, Be, B Eu -1 10 20 30 40 50 Atomic Number 60 70 80 Jargon [m/H] = log N(m)/N(H)star – log N(m)/N(H)Sun [Fe/H] = -1.0 is the same as 1/10 solar [Fe/H] = -2.0 is the same as 1/100 solar [m/Fe] = log N(m)/N(Fe)star – log N(m)/N(Fe)Sun [Ca/Fe] = +0.3 means twice the number of Ca atoms per Fe atom Log e(metal) = log n(metal)/n(H) + 12 The Fe Chronometer Why Iron? •Fe is abundant •Fe is easy •Fe is made in supernovae But [Fe/H] is not a good In the halo, [Fe/H] is a function indicator of both since star formation the age time of the disk of began and the star formation rate • Nucleosynthesis in stars leads to chemical Chemical enrichment of the Galaxy • Rate of enrichment Evolution depends on sites and • Primordial mechanisms of nucleosynthesis nucleosynthesis • Hydrogen burning • The variables are: – – – – Star formation rate Initial mass function Yields Stellar evolution time – Proton-capture chains • Helium Burning – C,Ne,O,Si burning • Photodissociation burning process • Neutron-capture processes • Odd-ball stuff Galactic Lithium Production: 10% of Big Bang origin 90% of Galactic origin Figure from Con Deliyannis “Alpha” Elements: Excesses at low metallicity from C, Ne, O & Si production in SN II Decline in [alpha/Fe] due to Fe production by SN Ia Edvardsson et al.; Pilachowski et al.; McWilliam et al. How to Make Heavy Metals: neutron-capture processes Main s-process •Low mass stars r-process •Double shell burning – High neutron flux – Type II Supernovae (massive •Makes SrYZr, Ba, etc. stars) – No time for b-decay – Eu, Gd, Dy, some Sr, Y, Zr, Ba, La… s-process – Low neutron flux – Time for b-decay before next neutron capture – No Eu, Gd, Dy Weak s-process •Massive stars •He-core and shell Burning •Lower neutron flux makes SrYZr only Solar System r- and s-Process Elements Eu 1 Br Fraction of r-process As Se Rh Te Ag Ru Rb Pd Ge Kr Zn Y Mo Nb Tb Cs Sb Sm In Xe Pr Cd Ho Tm Lu Yb Ta Dy Gd Er Os Au Re Pt Th Bi Ir U Hf Hg La Nd Sn Zr Tl Pb Ce I Sr W Ba Ga 0 Zn As Kr Y Mo Pd In Te Cs Ce Sm Tb r-Process s-Process Er Lu W Ir Hg Bi Isotopes built by n-capture syntheses The valley of b-stability Rolfs & Rodney (1988) n-capture Synthesis Paths La Ba 18% of solar system Ba is odd, but 48% of r-process Ba is odd 138 139 p s,r 130 132 134 135 136 137 138 p p s s,r s s,r s,r 133 Cs Xe s,r 128 129 130 131 132 134 136 s s,r s s,r s,r r r s-process path r-process path Spectrum of HD 126587 Metal-poor giant [Fe/H] = -2.85 Teff = 4910 K r-process rich Spectrum from the Mayall 4-m echelle Star-to-Star [n-capture/Fe] variations Stars of similar temperature and metallicity may have very different neutron-capture element abundances Burris et al. (2001) The Scatter is in the Stars! The r-process elements vary together Burris et al. (2001) Abundance Data Sources -1.0 < [Fe/H] < 0.0 • Edvardsson et al. 1993 • Jehin et al. 1999 -3.0 < [Fe/H] < -1.0 • Burris et al. 2000 -4.0 < [Fe/H] < -2.0 • McWilliam et al. 1995, 1998 Heavy Metal Abundances Note: Scatter Deficiencies at low metallicity Excesses at intermediate metallicity n-capture Abundances in BD+17o3248 Scaled solar-system r-process curve: Sneden 2002 Solar-System s-process Abundances DON’T Fit Sneden (2002), Burris et al. (2000) BD +17 3248 is typical of very metal poor stars Sneden et al. (2000); Westin et al. (2000); Cowan et al. (2002) r-Process vs. s-Process Transition from r-process only to r+s process at loge(Ba)=+0.5 and [Fe/H] = -2.0 La/Eu at low metallicity s-process seen at [Fe/H]=2.1 Simmerer et al. (2002) When does the s-process start? Main s-process occurs during thermal pulses in AGB stars of 2-4 solar masses H mixes inward, giving 12C(p,e+)13C 13C(a,n)16O t ~ 108 years s-process elements do not appear before this r-process appears at [Fe/H]=-2.9 New r-process elements come from deep in the Supernova This may be part of the reason for the n-capture scatter. Not all SN II produce lots of r-process Rolfs & Rodney (1988) The “light” heavy metals Production of Sr, Y, and Zr requires an additional neutron capture process Heavy metals at [Fe/H] = -4 At very low metallicity, the production of heavy metals is dominated by an unknown process What came before the r-process? • Identify “weak r-process stars” to see yields of very early nucleosynthesis The Earliest Star Formation • • • • • Formation of stars as “pre-galactic” objects from small density fluctuations H2 provides cooling Masses from a few tens to a few hundred solar masses Low mass star formation is suppressed by reionization Provides early chemical enrichment Abel, Bryan, & Norman 2002 Theoretical Framework Stochastic model for early chemical evolution (Travaglio et al. 1999) Coalescing and fragmenting clouds Homogenization time scale ~ few x 108 years reduces scatter Suggests r-process from 8-10 MSun s-process elements from 1-3 MSun AGB stars after homogenization Theoretical Models of Chemical Evolution • Stochastic models of Travaglio et al. for r-process production by 8-10 solar mass SN II The scatter in the abundances of all of the n-capture elements from star-to-star is of astrophysical origin, and the scatter increases as metallicity decreases. Conclusions Significant production of r-process elements began when the metallicity of the Galaxy reached [Fe/H] = -3. The heavy n-capture elements were formed predominantly by the r-process at metallicities below [Fe/H] = -2.1. Elements from the s-process appear at a metallicity of [Fe/H] = -2.1, when low-mass AGB stars begin to contribute from double shell burning. The s-process then dominates Ba production. The origin of heavy metals at the lowest Galactic metallicity ([Fe/H] = -4) is still not understood, but may be dominated by the weak s-process, or by a separate r-process in massive stars. The Epochs of Galactic Chemical Evolution Primordial Epoch -The Big Bang Epoch of Massive Stars @ [Fe/H] ~ -4 – Ca, O, Sr-Y-Zr + ? r-process Epoch - r-process elements from 8-10 MSun SNII The Double Shell Epoch yields s-process elements @ [Fe/H]=-2.1 (~ 109 years) The Iron Epoch – from SN Ia @ [Fe/H]=-2 The Lithium Epoch @ [Fe/H]=-1.0 from ??? Key Concept – Stellar evolution timescales are important The Oxygen Abundance Oxygen abundances are still uncertain, with inconsistencies between the triplet and forbidden lines