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！