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

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

Amplitude distribution

Solar-like oscillations in red giants CoRoT observations De Ridder et al. (2009; Nature 459, 398)

Basic properties of oscillations Behave like spherical harmonics: P l m (cos  ) cos(m  -  t) k h = 2  / h = [l(l+1)] 1/2 /r

Rays

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

Echelle diagram

Oscillatory signals Houdek & Gough (2007; MNRAS 375, 861) Fit He II BCZ He I

Oscillatory signals, SONG 2 x 4 months Houdek & Gough (2007; MNRAS 375, 861) Fit He II BCZ 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 綜合來看， 我們可以做到這一點！

Together, we can do it 携起手来， 我们可以做到这一点！