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Science with SONG

SONG was conceived with two primary scientific goals in mind:

  • To study the internal structure of stars at a level to what can be done for the Sun when it is observed as a distant star using asteroseismology as a tool.
  • To search for and characterize planets in orbit around other stars using both gravitational microlensing observations, transit photometry and radial velocity measurements.

 

To reach these goals a network of telescopes equipped with high-resolution spectrographs and CCD cameras for photometry is needed. The typical mode of observation for SONG is to do time-series observations in order to measure light-curves or radial-velocity curves for the objects under study.

 

The observations carried out so far have been focused on determining the spectrograph performance, with similar observations to be carried out in late 2014 for the lucky-imaging instrument.  To this end we have carried out three long spectroscopic observing campaigns of the stars 46 LMi (red giant star), ? Herculis (sub-giant) and during autumn 2014 the planet hosting star ? Cephei is observed.  The aim of these observations is to obtain  the oscillation power-spectrum with high frequency resolution to enable us to measure accurate values for the large and small frequency separations as well as individual frequencies and to determine the life-time of the oscillation modes. From these it is possible to calculate the basic stellar properties (mass, radius, surface gravity) with high precision. Further analysis of individual frequencies can constrain internal rotation and depth of convection zones and yield an age for the star – this however requires that more than single-site observations are carried out.

 

The asteroseismic analysis of these targets, by members of SAC and the SONG partners from IAC and NAOC, is currently ongoing. As SONG currently has one node operating (the Chinese node in western China will start operating in early 2015) the datasets are subject to the ''usual'' problems with daily aliasing in the power-spectrum arising from single-site observations.

 

Primed for the future

One important aspect for SONG is that the main targets will be among the brightest stars in the sky. Thus very good determinations of basic parameters such as distance, temperature and heavy element abundance are available for most targets and in addition many have measured angular diameters from interferometers such as CHARA, SUSI and NPOI.  This means that asteroseismically inferred properties can be confronted with independent, high-precision, measurements – this is an ideal situation for testing stellar models and methods.

In 2017 the TESS satellite (NASA/MIT) will be launched to carry out an all-sky survey of transiting exoplanets in short period orbits among the brightest stars in the sky. SONG will be able to carry out detailed follow-up observations of bright, candidate exoplanet host stars. For the brightest of these SONG will in addition also be able to carry out asteroseismic campaigns akin to what has been done with eg. Kepler.