The recent development in telescopes and instrumentation, which makes the high-precision radial velocity measurements possible, has led to significant advances in the fields of asteroseismology and in the search for exoplanets. However, there are factors which limit the scientific progress in these fields and they form the main arguments for building SONG:
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Asteroseismology and planet detection require significant amounts of observing time. But due to a high demand on the telescope time from scientists from all fields of astronomy, it is impossible to obtain more than a week or two of observing time with suitable instruments. Consequently this makes it difficult to make sufficient observations to discern the seismic signatures of the oscillating stars, or to carry through systematic searches for planets around bright, nearby stars.
It is only possible to observe for 8-10 hours per day from a single observatory, as the observations are naturally limited to the nighttime. The resulting gaps in the observations create major difficulties in the mathematical analysis of the data on oscillating stars and in the determination of the orbital periods of exoplanets.
The solution to this problem is to use a network of telescopes distributed in longitude all around the globe. However, the required precision can currently only be obtained with very few telescopes worldwide, making the setup of such a network on a project-to-project basis both difficult and inefficient.
 An example showing the problem with short-term observations. The limited observations (above) only show a single oscillation, while there actually are two different signals present in the signal shown below, one with a period of 36 hours and one with a period of 37 hours. |
Although it is now possible to make single measurements with a precision down to 30 cm/s by using the very best instruments at the ESO Very Large Telescope in Chile, it is very hard to maintain the same precision over long periods of time.
At this level of precision the slow changes in the instrument, for example caused by subtle changes in instrument temperature, will introduce small but significant variations in the measurements on time scales of days. These effects will lead to systematic errors in the observed oscillation periods and set a limit for the smallest size of planets which can be found with the current instruments.
The slow and unwanted changes, caused inside the instruments, will also be present in the SONG data, but due to the design of the SONG network, the changes can be monitored and corrected for.
The so-called stellar background noise, which is included in the measurements, is in fact a complicated signal.
It contains information about processes on the stellar surface, such as granulation, which is best described as motions of boiling gasses on the stellar surface. These effects give rise to noise signals on time scales comparable to those of the planetary orbits and set a lower limit to the size of detectable planets, regardless of the instrument.
However, with its continuous observations SONG will provide unique data on the level of granulation noise for different types of stars, and hereby be a major step for our understanding of both stellar physics and for the limitations facing planet searches.
The study of stellar background noise in order to improve our understanding of the physics of convection is a secondary scientific aim of SONG.
| Granulation on the surface of the Sun |
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For helioseismology the limitations described here were realised early on, leading to the construction of networks of automated solar telescopes, such as the BiSON, GONG and IRIS networks, of which the first two are still in operation. They also provided part of the rationale for several satellite missions observing the Sun.
In the field of asteroseismology, networks have also been used to observe some types of pulsating stars via their intensity fluctuations (using photometry).
However, these networks are designed with the purpose of observing pulsating stars with larger amplitudes of variability than in the solar-like stars. An example is the Whole Earth Telescope (WET), which concentrates on the so-called pulsating white dwarfs. White dwarfs are stars in the very late stages of their lives. In several billion years our Sun will collapse and at the same time shred off its outer layers, leaving behind a white dwarf as a remnant.
For the very low-amplitude oscillations in solar-like stars, it is only now that the measurement precision has reached a level where the factors listed above, and not the capabilities of instruments, are starting to limit further progress in the field.
Up to now the observations have aimed at detecting solar-like oscillations in a number of stars, with the purpose of deriving the basic physical stellar parameters.
We are now ready to take a giant step forward: to establish a dedicated network for making detailed measurements of oscillations in solar-like stars, which will be used to learn about their internal structure and evolution and their planetary systems.
This is the purpose of SONG.