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Scientific aspects of SONG
Jørgen Christensen-Dalsgaard
Department of Physics and Astronomy
Aarhus University
The SONG concept
• Network of 8 telescopes with a global distribution• Long, nearly continuous observations• Ultra-precise Doppler-velocity measurements• Precise photometry of faint stars in crowded
fields
SONG is regarded as one scientific instrument
Scientific goals of SONG
• Asteroseismology of unprecedented resolution and accuracy– Radial-velocity observations
• Characterization of extra-solar planetary systems– Radial-velocity observations– Gravitational microlensing
• Additional science
Studies of exo-planets
• Radial velocity– Low-mass planets in short-period orbits
• Gravitational micro-lensing– Characterization of statistics of planetary
systems, including low-mass planets in long-period orbits
Gravitational micro-lensing
Asteroseismology
• Oscillation frequencies can be determined with extremely high precision
• Frequencies are sensitive to internal structure and rotation
• Mode amplitudes and lifetimes are sensitive to near-surface physics, including convective dynamics
The study of stellar interiors from observations of stellar oscillations
Asteroseismology across the HR
diagram
Goals of asteroseismology
• Characterize global stellar properties
• Investigate detailed internal structure and dynamics of stars
• Improve understanding of the physics of stellar interiors
• Improve modelling of stellar evolution
Observational requirements
• Extreme sensitivity– Amplitudes down to a few cm/s in velocity and
a few parts per million in intensity
• Very long observation series (weeks or months)– Ensure sufficient frequency precision
• Nearly continuous data– Avoid complications in observed oscillation
spectrum
Solar-like oscillations
Solar-like oscillations
Solar-like oscillations
• In unevolved stars: acoustic modes
• Generally assumed to be intrinsically damped and stochastically excited by convection (but see talk by Licai Deng).
• Expected, and now generally observed, in all stars with significant outer convection zones
What we expect:the solar case
Grec et al., Nature 288, 541; 1980
Examples of solar-like
oscillations
Solar-like oscillations in red giants
CoRoT observations
De Ridder et al. (2009; Nature 459, 398)
Basic properties of oscillations
•Behave like spherical harmonics: Plm(cos ) cos(m - t)
•kh = 2 / h = [l(l+1)]1/2/r
Asymptotics of p modes
Small frequency separations
Asteroseismic HR diagram
The Sun and its neighbours
Looking for finer details: acoustic glitches
• Departures from simple asymptotic behaviour
• Sharp features in the sound speed– Edges of convective zones– Rapid variations in sound speed caused by
ionization
• Cause oscillatory behaviour of oscillation frequencies as functions of mode order
Oscillatory signals
Houdek & Gough (2007; MNRAS 375, 861)
Fit
He IIBCZ
He I
Oscillatory signals,
SONG 2 x 4 months
Houdek & Gough (2007; MNRAS 375, 861)
Fit
He IIBCZ
He I
Diagnostics of a small convective core
1.3 M¯
0.25 Gyr
5.25 Gyr
Cunha & Metcalfe (2007; ApJ 666, 413)
Diagnostics of a small convective core
1.3 M¯
Cunha & Metcalfe (2007; ApJ 666,413)
Rotational splitting
Observed splitting pattern
• Depends on actual amplitudes and inclination i of rotation axis to line of sight
• For solar-like oscillations reasonable to assume that actual average amplitudes are the same for all m-components
• Hence obtain estimate of the inclination
Gizon & Solanki (2003; ApJ 589, 1009)
Observations of solar-like oscillations
• Radial velocity– Amplitudes typically below 1 m/s– Sensitive to modes of degree 0, 1, 2, 3– Observe one star at a time
• Intensity– Amplitudes typically of a few ppm– Sensitive to modes of degree 0, 1, 2– Observe many stars simultaneously
Observations of solar-like oscillations
In both cases: requires continuous observations over many weeks or months
• Observations from space: intensity– CoRoT, Kepler
• Observations from ground-based network: radial velocity
Stellar noise vs. oscillations
Rationale for SONG project
• Ultimate asteroseismic precision requires radial velocity observations
• Continuous extended observations require dedicated facility
• Optimized instrumentation can reach required sensitivity with 1m telescopes
Strawman sites Izaña: prototype
Summary of status
• Prototype: well on the way for deployment in 2011
• Chinese site, funded by China: under negotiation
• US sites: proposal to be submitted to the NSF
4 months of SONG
observations
Together, we can do it
携起手来,我们可以做到这一点!