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

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 31082-001, 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 rprocess 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 31082-001 ([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| > 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 …
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 300-500 giants with [Fe/H] < -2.5
Discoveries Along the Way…
• The most irondeficient 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 Lines
If We Looked Any Harder…
Medium-Resolution Spectra of 500
HK + HES Stars per Night
The 6dF Facility
• 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
Is the Nature of the MDF Changing
with Distance ?
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
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.)