SONG overview

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

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

A little history

• Early ideas: Spring 2005

– With optimized instrumentation, spectroscopy

with iodine reference, adequate sensitivity can

be reach for bright stars with very modest

telescopes

• Developed further during 2005

• Conceptual design phase through 2006

– Funded by VKR, Carlsberg, …

Early concept (December 2005)

Revised design, mid-2006

ASTELCO sketch, early 2007

Conceptual design review,

1 – 2 March 2007 • Team: Keith Horne, Thomas Augustein,

Don Kurtz

• Conclusion: We commend the SONG team for

its remarkable progress in developing the SONG

concept to its present level of definition. SONG's

unique combination of instrumentation and

continuous observing capabilities for time-

domain observations will open a new era in

asteroseismology and will have a major impact

on the discovery and study of extrasolar planets

Prototype funding

• Villum Fonden, 2007: General funding for

prototype development

• Danish Natural Science Research Council,

2008: Funding of spectrograph

• Carlsberg Foundation, 2008: Partial funding of

telescope

• Aarhus, Copenhagen Universities: In-kind

contributions

• Instituto de Astrofísica de Canarias, Tenerife:

Local infrastructure, local operations

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

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

An aside: time (or) distance

astrophysics • Current trend: emphasizing giant telescopes

– Needed for studying very distant objects, phenomena

– Needed for studying phenomena on very small scales

• At the expense of smaller telescopes

– Allowing dedicated observations to single objects

over long time periods

• Both are obviously needed

An aside: time (or) distance

astrophysics • Current trend: emphasizing giant telescopes

– Needed for studying very distant objects, phenomena

– Needed for studying phenomena on very small scales

• At the expense of smaller telescopes

– Allowing dedicated observations to single objects

over long time periods

• Both are obviously needed

• Hence we need dedicated networks, such as

LCOGT or SONG

Basic properties of oscillations

Types of stellar oscillations

• Acoustic modes (p modes)

– Standing sound waves

– Relatively high frequency

– Depend mainly on sound speed

– Extend to centre of star, at low degree

• g modes

– Standing internal gravity waves

– Relatively low frequency

– Depend mainly on the buoyancy frequency, strongly

sensitive to composition profile

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

SONG in the Kepler era • Kepler provides very good data for hundreds

(thousands) of stars

– Synasteroseismology (comparison of stellar

properties)

– Population studies

– Characterization of pulsation properties for broad

range of stars

• SONG will provide exquisite data for a limited

number of stars

– Detailed probing of stellar internal structure

– Detailed investigations of physics of stellar interiors

Stellar noise vs. oscillations

Asteroseismology

across the HR

diagram

Solar-like

oscillations

Solar-like

oscillations

Solar-like oscillations

• In unevolved stars: acoustic modes

• In evolved stars: may have mixed g-mode

character

• Generally assumed to be intrinsically

damped and stochastically excited by

convection.

• 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 oscillators from Kepler

Chaplin et al. (2011; Science 332, 213)

Examples of Kepler solar-like

pulsators

Chaplin et al. (2011; Science

332, 213).

Solar-like oscillations

in red giants

CoRoT observations

De Ridder et al. (2009; Nature 459, 398)

4 months of

SONG

observations

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

Sharp features in stellar models

HeII ionization

No overshoot

With overshoot

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

Needs

SONG

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)

Overall design drivers for

SONG instrumentation

Ultra-precise radial velocities

Photometry in crowded fields

High duty-cycle

Long lifetime

Optimize the design for the primary science

purposes:

Telescope and dome

• 1 m telescope, alt-az mount

• High-resolution spectrograph

• Lucky Imaging camera for micro-lensing

• Contract to Astelco Systems, GmBH, Aug.

2009

• Acceptance, Tenerife, late 2011

Summary of status

• Prototype: well on the way for deployment in

2011

• Chinese site, funded by China: under detailed

design, deployment in 2013

• US site: proposal to be submitted to the NSF

• Proposal for two additional Danish-funded sites

in ongoing evaluation of need for Danish

research infrastructure

Strawman sites Izaña: prototype

Almost there!