Low temperature mean opacities for the carbonrich regime

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Low temperaturemean opacities for the
carbon-rich regime

• Michael T. Lederer, Bernhard Aringer
• Department of Astronomy, University of Vienna

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Opacity tables again?

• Tabulated mean opacities are available for a wide
  parameter range (variety of applications)
• high temperature OPAL (Iglesias Rogers
  1996),OP (Seaton et al. 1992)
• electron-conduction opacities (e.g. Cassisi et
  al. 2007)
• (very) low temperature molecules and
  dust(Alexander Ferguson 1994, Ferguson et al.
  2005), Sharp Burrows 2007 (BD giant
  planets),Freedman et al. 2007 (ultracool dwarfs
  ESP),
• Everything there to fully cover AGB star models?
• NO!
• dredge-up
• change of element abundances
• metallicity AND metal composition changesC/O
  ratio, transition from oxygen-rich to
  carbon-rich!

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Starting point

• Marigo (2002) points out importance of molecular
  opacities at varying C/O ratio
• Chemical equilibrium calculations plus
  approximation formulae (RMO per molecule) to
  estimate opacities
• Important results
• cooling of carbon-rich models
• impact on mass-loss rates
• possible consequences for duration of C-star
  phase, carbon yields, infrared colors etc.
• Conclusion C/O solar tables inadequatefor
  AGB star modelling
• Suggests calculation of opacity tables

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Opacity calculations

• COMA code (Aringer 2000)
• continuous opacity, atomic lines (VALD), approx.
  108 molecular lines, dust opacity
• Monochromatic opacities for
• hydrostatic models (MARCS)
• dynamic (wind) models (Höfner et al. 2003)
• spectral synthesis calculations
• abundance determination
• investigation of atmospheric dynamics
• intensity profiles
• Rosseland mean opacities for
• stellar evolution codes

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Opacity calculations

• Monochromatic opacities
• Rosseland mean opacities

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The new tables

• Format of tables following Alexander Ferguson
  1994 to ease integrationwith existing codes
  (e.g. FRANEC)
• X 0.5, 0.7, 0.8
• Y 1 X Z
• Initial Z - up to now only a few test cases
• Z10-4
• Z10-3 (3 x 10-3, 6 x 10-3)
• Zsolar (Lodders 2003)
• Enhanced 12C and 14N mass fractions by factors
  depending on initial Z,lowered 4He mass fraction
• Results in 75 tables per metallicity

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Showcases

• X, Y, Z fixed at solar

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Standard opacity
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Metallicity
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Molecular species

• C/O lt 1
• (oxygen-rich case)

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H2O
TiO
VO
CO
CN
SiOOH
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The transition

• C/O variations

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Molecular species

• C/O gt 1
• (carbon-rich case)

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Tables in stellar evolution codes

• Y rescaled instead of Z

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HCN
C2H2
cf. Cristallo et al. 2007
C3, C2, CN
H2O
CO
TiO
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Approximation formulae
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Approximation formulae

• Marigo 2002, Keeley 1970 (Christy 1966)
• Used by several authors
• Marigo 2002
• Lebzelter Wood 2007
• Tscharnuter Gail 2007
• (others plan/promise to use it)
• What does RMO per molecule really mean?
• How do you derive a formula for it?
• How good does it work (for changing X, Y, Z,
  C/O)?

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Adding up opacity components

• Advantages
• fast (only needs chemistry calculations)
• convenient
• flexible
• Disadvantages
• correctness/accuracy (compared to tables)
• valid for oxygen-rich and carbon-rich case?
• cumbersome to find a formula!

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Conclusion

• C/O does matter for Rosseland mean opacity
• log T lt 3.7 C/O ! solar
• Need your feedback!

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Encores
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Open issues

• Opacity sources (e.g. C2H)?
• Chemistry
• Interpolation/approximation formulae
• Frequency-dependent radiative transfer in stellar
  evolution models?

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Uncertainties

• Things enter the calculation of the RMO that have
  nothing whatsoever to do with atomic/molecular
  data
• Microturbulence
• Spectral resolution, range
• Interpolation (grid density)
• (Abundances)

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COMA

• Technical details

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Some technical details

• Input (main ingredients)
• Pgas/T pairs (or r/T, Pel/T with iterative
  solution)
• element abundances and isotopic ratios
• atomic/molecular/dust data (line lists etc.)
• constant parameters (mictroturbulence)
• Operation
• Chemical molecular equilibrium (equilibrium
  constants Tgt700K, 314 species, or GEM
  500-5000K, 892 species)
• Ionization equilibrium for the species which are
  not in the chemical routines
• Continuous, line, dust absorption
• Output
• Abundances of important molecular and atomic
  speciesEquation of state T ? p ? r (m)
• opacity (cm2/g) for radiative transfer
  (components)

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3D plots
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Comparison

• oxygen-rich vs. carbon-rich

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H2O
HCN C2H2
C3, C2, CN
TiO
CO
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