Element abundance variations in Globular Clusters using SALT RSS

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Element abundance variations in Globular Clusters
using SALT RSS

• Science with GMT, Canberra

26th March 2008
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Outline

• Globular clusters, abundance anomalies, s-process
  
• s-process element abundances in globular clusters
• Medium resolution instruments, including RSS on
  SALT
• Theoretical line strength analysis
• Analysis of 47 Tuc spectra obtained using SALT
  RSS
• Fabry-Pérot possibilities with SALT RSS
• Main sequence stellar analysis with SALT, GMT

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• element abundances in main sequence stars in GCs
  should be the same as chemical composition of
  initial gas cloud
• abundance variations in evolved stars may show
  evidence of nuclear processing and mixing within
  the stars

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Observations
Abundance results in globular cluster stars

• Anticorrelation of CN to CH molecular band
  strengths
• Bimodal distribution of CN strength on GB and MS
• Na-O ( Al-Mg) anticorrelation in RGB stars
• (see Gratton et al 2004 review)
• Recent flood of papers by Carretta et al

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Scenarios
MIXING Dredge-up of processed material to
surface of star PRIMORDIAL Two (or more) gas
clouds of differing chemical composition merged
prior to star formation SELF-ENRICHMENT
Previous generations of cluster stars enriched
the gas cloud either through local supernova or
AGB stellar winds POLLUTION Material ejected
from AGB stars in cluster is accreted onto
surface of the stars polluting their
atmospheres Cannon et al 1998
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Why are s-process element abundances useful?

• Indicators of nuclear mixing
• enhancement of s-process elements during third
  dredge-up in AGB stars of mass 1M?
• comparison of RGB with AGB stars tests nuclear
  product mixing theories
• variations in s-process element abundances in
  both RGB and AGB stars is evidence for pollution
  by primordial scenarios (AGB ejecta)

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Neutron Capture Processes
process timescale rate of n capture
environment
s slow 1 per 100,000 years late-types
stars
r rapid 1 per second supernova
some s-process elements - Sr, Y, Zr, La, Ce, Ba
some r-process elements - Eu, Th, U
some elements are synthesised by both process, eg
Nd
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Ernest Rutherford first nuclear
astrophysicist, Canterbury graduate

• 2008 is centenary of awarding of 1908 Nobel Prize
  (in Chemistry)

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Globular Clusters s-process element abundances
Some recent high resolution studies of several
GCs indicate no abundance anomalies for s-process
elements

• James et al 2004
• uniform enrichment of s- r-process elements in
• NGC 6397 Fe/H -2.0
• NGC 6752 Fe/H -1.5
• 47 Tuc Fe/H -0.7
• Carretta et al 2007
• n-capture process elements are homogeneously
  distributed
• NGC 6388 Fe/H -0.44

Fe/H log10(Fe/H) log10(Fe/H)? X/H
log10(X/H) log10(X/H)?
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47 Tuc s-process abundances

• Disagreement between studies of s-process
  element abundances in GB stars in 47 Tuc

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47 Tuc s-process abundances

• Recent work in 47 Tuc
• (Wylie et al 2006)
• Possible star-to-star scatter in s-process
  element abundances

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• High Resolution Medium Resolution
• Resolution 60,000 10,000
• Targets per Field 1-5 100
• Telescopes Instruments
• AAT UCLES AAOmega
• VLT UVES Flames (Rmax20,000)
• SALT HRS RSS
• GMT QSpec ?

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Robert Stobie Spectrograph (RSS) formerly the
Prime Focus Imaging Spectrograph (PFIS)
RSS named in memory of Bob Stobie, former SAAO
Director and the leading proponent for SALT and
the first Chairman of the SALT Board
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What can RSS do?

• Maximum operating
• resolution 10,000
• Is this resolution sufficient to observe line
  strength variations in weak metal lines in GC
  stars?

47 Tuc CMD
SALT HRS
SALT RSS
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Theoretical line strength analysis
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High Medium Resolution spectrum synthesis
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Temperature gravity effects on the Giant Branch
4000K, 1.0
High resolution AAT spectra
4500K, 1.5
5000K, 2.0
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RSS Performance Verification data

• Eleven stars in 47 Tuc
• Two wavelength regions
• 6020Å - 6830Å
• 5800Å - 6150Å
• Resolution 6000
• 50

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Temperature sensitivity
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Zr I Ba II
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Zr I Ba II
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Eu II Nd II
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Abundance results (for RSS PV data, R6,000)
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Conclusions
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Future Work
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What can SALT GMT instruments do?

• SALT RSS
• R10,000, including FP mode
• SALT HRS
• R from 17,000 to 60,000
• GMT QSpec
• R45,000

47 Tuc CMD
SALT HRS
SALT RSS
Teff 4500 K log g 4.5