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

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 , BD+17:3248) There are some “complications”

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

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

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

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

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

The Hamburg/ESO Survey Deep wide-field objective prism survey of the southern sky (7500 square deg.) |b| >  30 o 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.)

Comparison of the HK & HES Surveys

A Prism Survey Turns This …

Into This …

HES Spectra of MP Giants

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 giants with [Fe/H] < -2.5

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

Higher Resolution; Fewer Lines

If We Looked Any Harder…

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

Is the Nature of the MDF Changing with Distance ?

The VLT Quick Survey Based on minute “snapshot” spectra of 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 r-II stars, perhaps 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

Example Quick Survey Spectra

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 ?

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