Stellar Abundances and Extremely Metalpoor stars

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Stellar Abundances and Extremely Metal-poor stars

• David Lai

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Outline

• Part I A brief introduction to stellar spectra
  and measuring abundances
• Part II A brief introduction to extremely
  metal-poor (EMP) stars

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What are we measuring?

• Absorption line strength
• Atomic physics
• Excitation potential and oscillator strength
• Radiation damping
• The effects of the atmosphere, surface gravity
  and temperature
• Thermal broadening
• Microturbulence
• Pressure broadening (collisional broadening)
• How much of the element is present!

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Line measurement

• Take a spectrum
• Measure the equivalent width of a transitions,
  i.e. the strength of the line
• Curve of growth behavior of equivalent width vs.
  number of atoms

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Equivalent Width
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EW to abundance

• Atmospheric parameters
• Surface gravity
• Effective temperature
• Microturbulent velocity
• Metallicity
• Model atmospheres
• Use a LTE stellar analysis code, e.g. MOOG or
  SME, for equivalent width analysis or spectrum
  synthesis.

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Examples of synthesis
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Issues

• Determination of stellar parameters
• Color - Effective temperature relations
• Isochrones
• Ionization balance (with the caveat mentioned
  below)
• Uncertainties in the atomic and molecular data
• NLTE effects

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Part II
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Why study EMP stars?

• Describe the nature of Population III objects
• and maybe find one
• Constrain models of massive SNII
• Trace early Galactic chemical evolution
• Nuclear physics of r- and s-process

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How do you find them?
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From low to high resolution, the search for EMP
stars

• Objective prism surveys,
• Bond 1980
• The Beers, Preston Schectman HK survey
• HES survey
• Photometry
• Spectra

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What do you expect to see?
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Dependence on Metallicity and Mass
Heger et al. 2003
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Initial mass and remnant
Heger Woosley 2002
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The IMF

• Cooling and mass loss(or the lack thereof)
• A first generation of supermassive stars (e.g.
  Abel et al. 2002)
• IMF biased towards higher mass objects
• Possible bimodal IMF

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Origin of the light elements (Z lt 30)

• Big bang nucleosynthesis
• Supernovae II
• Pair instability supernovae
• AGB mass transfer
• Mixing

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The s-Process

• Slow neutron capture relative to the ?-decay
  timescale
• Mass transfer from low mass AGB stars
• Up to 209Bi
• Helium burning in massive stars
• Not much contribution to A gt 90

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Johnson Bolte 2004
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The r-Process

• Rapid neutron capture relative to the ?-decay
  timescale
• Neutrino wind from SNe II
• Merging neutron stars
• Different sites for lighter, Z lt 56, elements?

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Sneden et al. 2003
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Galactic Chemical Evolution

• Environment and the role of supernovae
• McWilliam et al. 1995
• Cayrel et al. 2004

Lai et al. 2004
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The present and future

• BPS follow up
• CS 22892-052 and CS 31082-001
• HES follow up
• HE 1327-2326, Fe/H -5.4
• HE 0107-5240, Fe/H -5.3
• Low resolution confirmation, and high resolution
  detailed analysis
• Sloan Digital Sky Survey
• Compare to model yields, SNII, PISN, AGB
  Supernove, etc.

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How can we learn about Population III stars?
Number of sightings of the Loch Ness Monster
gt250
We have pictures! ?
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Number of sightings of Pop III stars
Slide by Jennifer Johnson