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Observations of Neutron-Capture Elements in the Early Galaxy

Chris Sneden University of Texas at Austin. Observations of Neutron-Capture Elements in the Early Galaxy. John Cowan Jim Truran Scott Burles Tim Beers Jim Lawler Inese Ivans Jennifer Simmerer Caty Pilachowski. Andy McWilliam George Preston Debra Burris Bernd Pfeiffer

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Observations of Neutron-Capture Elements in the Early Galaxy

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  1. Chris Sneden University of Texas at Austin Observations of Neutron-Capture Elements in the Early Galaxy

  2. John Cowan Jim Truran Scott Burles Tim Beers Jim Lawler Inese Ivans Jennifer Simmerer Caty Pilachowski Andy McWilliam George Preston Debra Burris Bernd Pfeiffer Karl-Ludwig Kratz Francesca Primas Rica French Taft Armandroff Involving the Efforts of Many People, Including :

  3. Talk outline • Reminder of solar r- and s-process breakdown • General n-capture trends in the Galactic halo • Star-to-star scatter • Shift to r-process dominance • Detailed abundance distributions in a few stars • Elemental • Isotopic • Radioactive element observations • There is more to halo star life than the r-process • Summary, future questions

  4. A detailed view of part of the n-capture synthesis paths 138 139 La p s,r 130 132 134 135 136 137 138 Ba P P s s,r s s,r s,r 133 Cs s,r 128 129 130 131 132 134 136 Xe s s,r s s,r s,r r r r-process path s-process path

  5. ELEMENTAL r- and s-process solar-system abundances Data from Burris et al. (2000)

  6. General halo n-capture “bulk” abundance trends: LARGE scatter • Large-sample surveys are needed to show this: • Gilroy et al. (1988), McWilliam et al. (1995); Ryan et al. (1996); Burris et al. (2000); Johnson & Bolte (2001) • Obvious from simple spectrum comparisons • σ[n-capture/Fe] > 1 dex • local nucleosynthesis events occurring in a poorly mixed early Galactic halo

  7. Stellar Spectroscopic Definitions • [A/B] = log10(NA/NB)star – log10(NA/NB)Sun • log e(A) = log10(NA/NH) + 12.0 • Atmospheric parameters: Teff, log g, vt, [Fe/H] • Metallicity [Fe/H] • Metal-poor halo star [Fe/H] < -1.5 • Very metal-poor star [Fe/H] < -2.5

  8. Sr II line strength variations at lowest metallicities All three stars have similar atmospheric parameters and [Fe/H] ~ -3.4 McWilliam et al. (1995)

  9. Strontium abundance scatter at lowest metallicities McWilliam et al. (1995): filled circles Gratton & Sneden (1994): open squares

  10. n-capture/Fe variations are obvious even in spectra of “higher” metallicity stars These two metal-poor ([Fe/H]=-2.3) giants have similar atmospheric parameters Burris et al. (2000)

  11. n-capture abundance variations do not occur at random Comparison with an ordinary metal Comparison with nearby n-capture elementDy Burris et al. (2000)

  12. General halo n-capture abundance ratios: trend toward pure r-process • Not considered here: carbon-rich stars with/without s-process overabundances • Usual comparison: [Ba/Eu] • Basolar-system > 90% s-process • Eusolar-system > 90% r-process • [Ba/Eu] ~ -0.9 ~ pure r-process value at [Fe/H] ~ -3.0 • Scatter is higher than desirable: blame the Ba abundances?

  13. The decline of Ba/Eu at lowest metallicities The solar-system r-process-only ratio

  14. An alternative: La/Eu • La also sensitive to s-process (70% s-process in solar system) • Both elements have several useful lines at accessible l’s • Atomic parameters of Eu, La lines very well known • Can determine La/Eu with higher accuracy than Ba/Eu • Can use same transitions over 3 dex metallicity range

  15. Previous lanthanum work The La/Eu (e.g, the s-/r-) ratio is constant??? Burris et al. (2000) ,magenta points; Johnson & Bolte (2001), black points

  16. La II lines in the solar spectrum: synthetic spectra fits with new atomic data hyperfine structure pattern Green line is the solar observed spectrum Lawler et al. (2001)

  17. La/Eu at low metallicity The Ba/Eu (e.g, the s-/r-) ratio is NOT constant Simmerer et al. (2002)

  18. A better idea: employ abundances of more elements than just Ba and Eu Four stars, with mean abundance levels scaled to the solar-system curves by their average Ba, La, Ce, Sm, and Eu abundances Johnson & Bolte (2001)

  19. Detailed elemental abundance distributions in a few very low metallicity stars • Stars with # of n-capture abundances > 15: • CS 22892-052 (Sneden et al. 2000); HD 115444 (Westin et al. 2000); BD+17o3248 (Cowan et al. 2002);CS 31082-001 (Hill et al. 2002) • Rare earths: “perfect” agreement with solar-system r-process-only abundances • Heaviest stable elements: must use HST • Z < 56: need for another r-process?

  20. A small spectral interval of a metal-poor but n-capture-rich star Sneden et al. (2000)

  21. First example: BD+17o3248 • Most “metal-rich” of n-capture-enhanced stars: [Fe/H] = -2.1 • A warmer giant by about 500K than other examples • Extensive high res, high S/N HST data in hand • First metal-poor star with gold detection • Takes advantage of large sets of new atomic data • La II (Lawler et al. 2001); Ce II (Palmeri et al. 2000); Pr II (Ivarsson et al. 2001); Tb II (Lawler et al. 2001); Eu II (Lawler et al. 2002)

  22. Detection of n-capture elements in HST STIS spectra HD 122563 is n-capture-poor; BD+17o3248 is n-capture-rich Cowan et al. (2002)

  23. Discovery of gold in a metal-poor star Cowan et al. (2002)

  24. n-capture abundances in BD+17o3248: 1st solar-system comparison Scaled solar-system r-process curve: Burris et al. (2000) Cowan et al. (2002)

  25. The BD+17o3248 abundances are not compatible with s-process synthesis Scaled solar-system s-process curve: Burris et al. (2000) Cowan et al. (2002)

  26. Second example: CS 22892-052 • First metal-poor star discovered with extreme r-process: [Fe/H] = -3.1 [Eu/Fe] = +1.6 • One puzzle: also carbon-rich: [C/Fe] = +1.0 • Good high res, high S/N ground-based spectra and lower quality HST data in hand • Even more exploration of atomic data (Mo, Yb, Lu, Ga, Ge, Sn, etc.) • Abundances or significant upper limits for 57 elements

  27. Abundance Summary Colors identify different element groups Li and Be values are w.r.t. to unevolved stars of similar metallicity Sneden et al. (2002), in preparation

  28. Terbium in the Sun and CS 22892-052 0.80.8 0.8 0.8 0.8 0.8 0.8 0.8 1.1 This is the cleanest Tb II feature in the solar spectrum 1.0 Relative Flux 0.9 Sun 0.8 n-capture-rich metal-poor stars are good “laboratories” for these lines CS22892-052

  29. Summary of the latest n-capture abundances for CS 22892-052 Sneden et al. (2003), in preparation

  30. Z56 stable n-capture elements: excellent match to solar r-process Sneden et al. (2003), in preparation

  31. Z<56 n-capture elements: some deviations, some questions The upper limits for Sn and especially for Ga, Ge are significant Ga and Ge share the metal poverty of Fe-peak and lighter elements Sneden et al. (2003), in preparation

  32. Comparison with CS 22892-052 abundances Perfect agreement with CS22892-052 would be a horizontal line Note difference of HD 122563: real or needing better data?

  33. Some attempts to get isotopic abundances • Need large hyperfine and/or isotopic splitting • Rare-earth lines provide best opportunity • Some elements have only one stable isotope • Barium and now europium have been studied in metal-poor stars • See Ivans et al. poster at this meeting

  34. An example of Eu II syntheses: the 4205.05A line The Eu abundance is altered by 0.2 dex for each synthesis

  35. Eu isotopic fractions are very similar to solar-system values %(151Eu): 0 35 50 65 100 %(153Eu) = 100 - %(151Eu) Solar system: %(151Eu) = 47.8 %(153Eu) = 52.2 Sneden et al. (2002)

  36. Barium isotopic mixes 134 135 136 137 138 synthesis cause s only s & r s only s & r s & r 134 135 136 137 138 solar system abundances odd isotopes 18% 2.4% 6.6% 8.0% 11.2% 71.8% 134 135 136 137 138 r-process abundances odd isotopes 46% 0.0% 25.7% 0.0% 20.4% 53.9% 134 135 136 137 138 hyperfine splitting? no yes no yes no odd isotopes are only11% of solar system s-process material

  37. Barium Isotopic Abundances in HD 140283 odd isotopes: 10% 31% 52% 31% is best fit Solar system: total = 18% r-only = 46% s-only = 11% Lambert & Allende Prieto (2002)

  38. Radioactive cosmochronometry for metal-poor stars Galactic chemical evolution effects do not matter for radioactive elements Th and U “frozen” into metal-poor stars born near the start of the Galaxy. ? Daughter product Pb is also a direct n-capture synthesis product Rolfs & Rodney (1988)

  39. Best Th II and U II lines CS 31082-001 BD +17o3248 Cowan et al. (2002) Cayrel et al. (2001)

  40. Age computations for halo stars • t1/2(Th) = 14.0 Gyr; t1/2(U) = 4.5 Gyr • So for thorium: NTh,now/NTh,start = exp(-t/tmean)= exp(-t/20.3Gyr) • Cannot know NTh,start assume NTh,start/NEu and compare that to N Th,observed/NEu • IF solar-system r-process abundances can be assumed to extend to U, then can use [Thobserved/Eu ] as a measure of Th decay • <[Thobserved/Eu ]> = -0.58 +/- 0.02 (s = 0.07, # = 10) <t> = 13.6 +/-1.0 Gyr (s ~ 3.6 Gyr) • But in CS 31082-001 the [Th/Eu] ratio is much larger: [Th/U] t = 12.5 Gyr [Th/Eu] t = 4 to 5 Gyr

  41. Thorium-to-europium ratios in some halo stars Open circles: new data Filled squares: Johnson & Bolte (2001)

  42. The curious chemical composition of CS 29497-030 It is like a “blue straggler” It is a binary (companion undetected) M68 Preston & Sneden ( 2000) [M68 diagram from Walker 1994]

  43. CS 22947-030 is another example of lead-enriched metal-poor stars These are s-process enrichments! All data for CS 29497-030 point to mass transfer from former AGB companion Log e(Pb)solar system = 1.9

  44. Summary, future work • Large star-to-star scatter in n-capture levels below [Fe/H] ~ -2: established but not well interpreted • Switch from r,s-process contributions to r-only abundances is seen in many low metallicity stars • Th, U radioactive element chronometry is in its nfancy, but is a promising technique • Extreme s-process stars may be understood? • Do [Th/Eu] ratios always yield “same” ages? • Are there more U detections be had? • Can the abundances of Z<56 n-capture elements be understood?

  45. Total r- and s-process synthesis paths Bi is the end of the s-process The r-process alone makes radioactive chronometer elements Th and U Rolfs & Rodney (1988)

  46. What are s-/r- trends in the Galactic disk? • Woolf et al. (1995) derived [Eu/Fe] in disk dwarf stars with [Fe/H] > -1 • Woolf spectra also contain 4123ÅLa II line • One La II and one Eu II line used to derive La/Eu for “disk” metallicity stars • Complements Mashonkina & Gehren study of Ba/Eu

  47. Europium in Galactic disk stars Results confirmed by Koch & Evardsson (2002) Woolf et al. 1995

  48. La/Eu at high metallicity Does La/Eu have a break at [Fe/H] -0.4 ? Simmerer et al. (2002)

  49. La/Eu and space velocity s.s. total The s-/r- process abundance ratio correlates with space velocity as much as (more than?) [Fe/H] s.s. r-process Simmerer et al. (2002)

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