Future of asteroseismology II

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Future of asteroseismology II

• Jørgen Christensen-Dalsgaard
• Institut for Fysik og Astronomi, Aarhus
  Universitet
• Dansk AsteroSeismologisk Center

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We need

• Better data
• Better models

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Better data

• Better frequency precision (s(n) lt 0.1 mHz)
• Lower noise level to reach more modes
• Data on a broader variety of stars
• Identification of the modes (l, m)
• Better classical observables (M, R, L, Teff,
  X, Z)
• g modes in the Sun to study the solar core

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Frequency precision
Simply observe for longer

• Easy for heat-engine modes (s(n) / tobs-1)
• Harder for stochastically excited modes (s(n) /
  tobs-1/2 for t gt tlife)

Longer observations also improve detection of
lower-amplitude modes
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Observational strategies

• For very extended observations (weeks or months)
  we need dedicated instrumentation.
• Space observations in intensity? Discussed by
  HK.
• Helioseismology has shown the way dedicated
  networks (BiSON, IRIS, TON) and
• GONG (Global Oscillation Network Group)

Hence we need
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SONG Stellar Oscillation Network Group

• SONG proposal (the Aarhus dream)
• Network of small telescopes (60 cm equivalent)
• Very efficient and highly stabilized
  spectrograph
• Science goals
• Solar-like oscillations in relatively bright
  stars
• Search for low-mass extrasolar planets in close
  orbits

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Possible distribution of sites
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Asteroseismic capabilities
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Planet-search capabilities
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Better data

• Better frequency precision (s(n) lt 0.1 mHz)
• Lower noise level to reach more modes
• Data on a broader variety of stars
• Identification of the modes (l, m)
• Better classical observables (M, R, L, Teff,
  X, Z)
• g modes in the Sun to study the solar core

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Data on a broader variety of stars

• Multi-object spectrographs (but hard to ensure
  radial-velocity precision)
• Intensity observations of multiple stars from
  space (HK lecture)

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Better data

• Better frequency precision (s(n) lt 0.1 mHz)
• Lower noise level to reach more modes
• Data on a broader variety of stars
• Identification of the modes (l, m)
• Better classical observables (M, R, L, Teff,
  X, Z)
• g modes in the Sun to study the solar core

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Mode identification

• For stochastically excited oscillators, use
  nearly complete spectrum, regular structure of
  frequencies
• For heat-engine oscillators, in general need
  independent information about mode geometry
• Combine amplitudes and phases of observations
  with different techniques (intensity in different
  colours, intensity and radial velocity, etc.)

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Doppler imaging
Tau Peg (Kennelly et al. 1998 ApJ 495, 440)
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Doppler imaging
Tau Peg (Kennelly et al. 1998)
Major difficulty Modelling of structure and
oscillations of rapidly rotating star
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Better data

• Better frequency precision (s(n) lt 0.1 mHz)
• Lower noise level to reach more modes
• Data on a broader variety of stars
• Identification of the modes (l, m)
• Better classical observables (M, R, L, Teff,
  X, Z)
• g modes in the Sun to study the solar core

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Better classical observables

• Direct observations
• Magnitude
• Colours
• Spectra
• With calibrations
• Luminosity (needs distance, bolometric
  correction)
• Effective temperature (needs calibration)
• Composition (needs model atmosphere)

Solar abundance revisions are a reminder of the
uncertainties in these analyses
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Better data

• Better frequency precision (s(n) lt 0.1 mHz)
• Lower noise level to reach more modes
• Data on a broader variety of stars
• Identification of the modes (l, m)
• Better classical observables (M, R, L, Teff,
  X, Z)
• g modes in the Sun to study the solar core

Well, not yet, after 30 years of intensive efforts
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Better models of stellar evolution and
oscillations

• Better numerical reliability, accuracy
• Better microphysics (equation of state, opacity,
  )
• Better treatment of convection
• Better (i.e., some) treatment of energetics of
  oscillations
• Inclusion of effects of rotation, on structure
  and oscillations
• What about magnetic fields???

Use analysis of oscillation results to inspire
improvements to the physics
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Numerical treatment

• Are the evolution codes correct???? (Probably
  not)
• Is the numerical precision adequate? (Compared
  with the observational precision)
• How do we find out?

Detailed comparisons of results of independent
codes.
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Better microphysics

• Extremely complex problems in many-body atomic
  physics
• Coulomb interactions, excluded-volume effects,
  partial degeneracy, interaction with radiation .
• Some detailed testing using the Sun as a
  laboratory.

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Example relativistic electrons in the Sun
Elliot Kosovichev (1998 ApJ 500, L199)
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Modelling stellar convection

• Mixing-length treatment (calibrated against the
  Sun)
• Detailed hydrodynamical simulations (for a range
  of stellar parameters)
• Simpler treatments, but calibrated against
  simulations

Note treatment of convection and hydrodynamics
of stellar atmospheres crucial for the abundance
determinations, calibrations of photometric
indices.
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Simulation of convection in the Sun
Nordlund et al.
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Effects of rotation on stellar structure

• Spherically symmetric component of centrifugal
  force in hydrostatic equilibrium fairly simple
• Effects of circulation and instabilities
  extremely hard
• Evolution of internal angular momentum worse

Recall uniform slow rotation of solar interior
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Meridional circulation

• Circulation and associated instabilities lead to
• transport of elements
• transport of angular momentum

Meynet
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Effect of rapid rotation on oscillations
3rd order
2nd order
1st order
Analysis by Soufi et al. (1998 Astron.
Astrophys. 334, 911)
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Development of analysis techniques

• Fits to determine global parameters
• Must worry about possible multiple maxima in
  likelihood function use Monte-Carlo techniques
  (e.g. genetic algorithm)
• Inversion based on just low-degree modes.

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Examples of potential analyses

• Tests based on artificial data with realistic (we
  hope) properties
• Properties of convective overshoot
• Structure of the stellar core

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Base of convective envelope
Monteiro et al. (2000 MNRAS 316, 165)
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Signal from base of convective envelope
Monteiro et al. (2000)
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Inversion for core structure
Models 1 M (Mixed core) (normal) Degree l 0
- 3 (Basu et al. 2002 ESA-SP 485, 249)
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The future stellar tachoclines??