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The Search for New “r-process-Enhanced” Metal-Poor Stars

The Search for New “r-process-Enhanced” Metal-Poor Stars. Timothy C. Beers Michigan State University. Nature’s Gift to Nuclear Astrophysics.

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The Search for New “r-process-Enhanced” Metal-Poor Stars

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  1. The Search for New “r-process-Enhanced” Metal-Poor Stars Timothy C. Beers Michigan State University

  2. Nature’s Gift to Nuclear Astrophysics • There now exists a small number of very metal-poor stars ([Fe/H] < -2.0) which have been discovered recently that exhibit strong enhancements in their ratios of r-process elements, compared with the Sun • r-I: 0.3 < [r-process/Fe] < 1.0 (~ 10 known) • r-II: 1.0 < [r-process/Fe] < 1.7 ( 4 known) • A few have measurable U as well, allowing for the use of the [U/Th] chronometer (CS 31082-001, BD+17:3248) • There are some “complications”

  3. The Importance of r-process Enhanced Stars • All appear to have patterns for 56 < Z < 76 which match the solar r-process component extremely well (Sneden et al. 2002, in prep.) • All have measurable lines of Th, and other stable r-process elements, upon which cosmo-chronometric age limits can be placed

  4. Stars with Measurable Uranium CS 31082-001 ([Fe/H] = -2.9); The First Meaningful Measurement of Uranium Outside the Solar System (Cayrel et al. 2001)

  5. Stars with Measurable Uranium BD+17:3248 ([Fe/H] = -2.1); A Strong Upper Limit on Uranium Cowan et al. (2002)

  6. The Key to Progress • Astronomers need to “fill out the phase space” of variations in r-process enhanced stars • This requires discovery of as many additional examples of the phenomenon as possible • A dedicated survey effort is underway, making use of the world’s largest telescopes • However, they are VERY rare - 3% of giants with [Fe/H] < -2.5

  7. The Existing Surveys • The HK objective-prism survey of Beers and colleagues has provided the majority of r-II stars discovered to date • limited by temperature bias, and relatively “bright” magnitude limit (B < 15.5) • The Hamburg/ESO Survey will be the primary source of future r-I and r-II stars, based on follow-up observations now underway • No temperature-related bias • Efficient discovery of [Fe/H] < -2.5 giants

  8. The Hamburg/ESO Survey • Deep wide-field objective prism survey of the southern sky (7500 square deg.) • |b| > 30o B < 17.0 • Machine scanned and automatically classified (4,000,000 stellar spectra) • Highly efficient selection of metal-poor giants (and other halo stars of interest, e.g., FHB/A stars, carbon-enhanced stars, etc.)

  9. A Prism Survey Turns This …

  10. Into This …

  11. HES Spectra of MP Giants

  12. Plan of Discovery • Single-slit follow-up spectra of N ~ 2500 “high probability” candidate HES giants with [Fe/H] < -2.5 • 4m telescopes: ESO 3.6m, KPNO 4m, CTIO 4m, AAT 3.9m • 2.5m telescopes: ESO 2.3m, KPNO 2.3m • Multi-fiber follow-up spectra of N ~ 8,000 candidates with the 6dF facility on the UK Schmidt Telescope in Australia • “Quick Survey” High-Resolution Spectroscopy with VLT/UVES of 300-500 giants with [Fe/H] < -2.5

  13. Discoveries Along the Way… • The most iron-deficient star known (Christlieb et al. 2002, Nature 419, 904) • HE 0107-5240 [Fe/H] = - 5.3 [C/Fe] = +3.9 [N/Fe] = +2.4

  14. Higher Resolution; Fewer Lines

  15. If We Looked Any Harder…

  16. Medium-Resolution Spectra of 500 HK + HES Stars per Night • Wide field fiber spectrograph on the UK Schmidt Telescope • Presently working, primarily on galaxy redshift surveys • Optimize for grey/bright time operation • Improve Schmidt camera • Smaller fibers • Better CCD (resolution/response) • Fund personnel The 6dF Facility

  17. Is the Nature of the MDF Changing with Distance ?

  18. The VLT Quick Survey • Based on 15-20 minute “snapshot” spectra of 300-500 validated [Fe/H] < -2.5 giants • 160 hours (~ 20 nights of time) already assigned during present semester • Similar allocations expected over next three semesters • S/N ~ 35/1 obtained • Should find 10-15 r-II stars, perhaps 20-30 r-I stars • Searching for detectable absorption of Eu II 4129 A • Obtain elemental abundances of ~ 12 additional elements, even for non r-process enhanced stars

  19. Example Quick Survey Spectra

  20. Issues to Be Resolved • What is the frequency of r-I and r-II stars as a function of metallicity ([Fe/H]) ? • Clues to the astrophysical site of the r-process • To date, ALL r-II stars have [Fe/H] < -2.5 • r-I stars exist up to [Fe/H] ~ -2.0 • Hints as to how to improve efficiency of subsequent follow-up • What is the dispersion (if any) among r-process patterns for 56 < Z < 76 ? • What is the dispersion for lighter r-process patterns ?

  21. What is the Dispersion of Actinides Beyond the Third r-process Peak ? (Honda et al. 2002, in prep.)

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