Download Future of asteroseismology

Future of asteroseismology II
Jørgen Christensen-Dalsgaard
Institut for Fysik og Astronomi,
Aarhus Universitet
Dansk AsteroSeismologisk Center
We need
• Better data
• Better models
Better data
• Better frequency precision (s(n) < 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
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 > tlife)
Longer observations also improve detection of
lower-amplitude modes
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 ……
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
Possible distribution of sites
?
Asteroseismic capabilities
Planet-search capabilities
Better data
• Better frequency precision (s(n) < 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
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)
Better data
• Better frequency precision (s(n) < 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
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.)
Doppler
imaging
Tau Peg (Kennelly
et al. 1998; ApJ
495, 440)
Doppler
imaging
Tau Peg
(Kennelly et al.
1998)
Major difficulty:
Modelling of
structure and
oscillations of
rapidly rotating star
Better data
• Better frequency precision (s(n) < 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
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
Better data
• Better frequency precision (s(n) < 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
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
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.
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.
Example: relativistic electrons in
the Sun
Including relativistic effects
No relativistic effects
Elliot & Kosovichev (1998; ApJ 500, L199)
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.
Simulation of convection in the
Sun
Nordlund et al.
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
Meridional circulation
Circulation and
associated
instabilities lead to
• transport of
elements
• transport of
angular momentum
20 Msol on the ZAMS
Meynet
Effect of rapid rotation on
oscillations
3rd order
2nd order
1st order
Analysis by Soufi et al. (1998; Astron. Astrophys. 334, 911)
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.
Examples of potential analyses
Tests based on artificial data with realistic (we hope)
properties
• Properties of convective overshoot
• Structure of the stellar core
Base of convective envelope
Effect of He
ionization
Monteiro et al. (2000; MNRAS 316, 165)
Signal from
base of
convective
envelope
Monteiro et al. (2000)
Inversion for
core structure
Models: 1 M¯
(Mixed core) –
(normal)
Degree l = 0 - 3
(Basu et al. 2002;
ESA-SP 485, 249)
The future: stellar tachoclines??
NASA vision study.
Launch 20??