The Search for New The Search for New “r-process-Enhanced” “r-process-Enhanced”

Metal-Poor StarsMetal-Poor Stars

Timothy C. BeersTimothy C. Beers

Michigan State UniversityMichigan State University

Nature’s Gift to Nuclear AstrophysicsNature’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”

The Importance of r-process The Importance of r-process Enhanced StarsEnhanced 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 UraniumStars with Measurable UraniumCS 31082-001 ([Fe/H] = -2.9); The First Meaningful Measurement of Uranium Outside the Solar System (Cayrel et al. 2001)

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

The Key to ProgressThe 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 SurveysThe 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 SurveyThe 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.)

A Prism Survey Turns This …A Prism Survey Turns This …

Into This …Into This …

HES Spectra of MP GiantsHES Spectra of MP Giants

Plan of DiscoveryPlan 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

Discoveries Along the Way…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

Higher Resolution; Fewer LinesHigher Resolution; Fewer Lines

If We Looked Any Harder…If We Looked Any Harder…

Medium-Resolution Spectra of 500 Medium-Resolution Spectra of 500 HK + HES Stars per NightHK + 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 Is the Nature of the MDF Changing with Distance ?with Distance ?

The VLT Quick SurveyThe 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

Example Quick Survey SpectraExample Quick Survey Spectra

Issues to Be ResolvedIssues 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 What is the Dispersion of Actinides Beyond the Third r-process Peak ? Beyond the Third r-process Peak ?

(Honda et al. 2002, in prep.)(Honda et al. 2002, in prep.)