future of asteroseismology ii jørgen christensen-dalsgaard institut for fysik og astronomi, aarhus...
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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 (() < 0.1 Hz)
• 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 (() / tobs-1)
• Harder for stochastically excited modes (() / tobs
-1/2 for t > 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 (() < 0.1 Hz)
• 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
![Page 11: Future of asteroseismology II Jørgen Christensen-Dalsgaard Institut for Fysik og Astronomi, Aarhus Universitet Dansk AsteroSeismologisk Center](https://reader036.vdocuments.us/reader036/viewer/2022062322/56649e895503460f94b8e1b5/html5/thumbnails/11.jpg)
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 (() < 0.1 Hz)
• 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
![Page 13: Future of asteroseismology II Jørgen Christensen-Dalsgaard Institut for Fysik og Astronomi, Aarhus Universitet Dansk AsteroSeismologisk Center](https://reader036.vdocuments.us/reader036/viewer/2022062322/56649e895503460f94b8e1b5/html5/thumbnails/13.jpg)
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 (() < 0.1 Hz)
• 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
![Page 17: Future of asteroseismology II Jørgen Christensen-Dalsgaard Institut for Fysik og Astronomi, Aarhus Universitet Dansk AsteroSeismologisk Center](https://reader036.vdocuments.us/reader036/viewer/2022062322/56649e895503460f94b8e1b5/html5/thumbnails/17.jpg)
Better ‘classical’ observablesDirect 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 (() < 0.1 Hz)
• 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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No relativistic effectsIncluding relativistic effects
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
20 Msol on the ZAMS
Meynet
Circulation and associated instabilities lead to
• transport of elements
• transport of angular momentum
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Effect of rapid rotation on oscillations
Analysis by Soufi et al. (1998; Astron. Astrophys. 334, 911)
1st order
2nd order
3rd order
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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)
Effect of He ionization
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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??
NASA vision study. Launch 20??