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Planet 24

Understanding Radial Velocity Jitter: Benchmark Observations of the Sun

Principal Investigators

  • Prof. Dr. A. Reiners
    Georg-August-Universität Göttingen, Göttingen


Nowadays, there are several methods to detect exoplanets. One of them is the so called radial velocity (RV) technique which is a successful way to discover if a star is accompanied by planets. However, RV measurements are strongly affected by velocity jitter caused by convection and magnetic activity of the star itself, which are also distorting the RV signal of exoplanets.

Since the Sun is our closest star, it serves as the best existing laboratory to understand stars in general. Due to our proximity to the Sun, it is possible to spatially resolve the solar surface. That implies that we can get a better insight of solar RV behaviour resp. jitter than we could get by observing stars. Therefore the Sun can be used as ideal benchmark for precision observations of stellar absorption profiles.


Monika Ellwarth, Sebastian Schäfer


To improve radial velocity measurements of stars and their planets it is conducive to get a better understanding of radial velocity jitter on stars itself. With the Sun as our local star we can use it as our display item. We perform observations of the resolved solar surface to get a better insight of the changing spectral behaviour across different observation angles on the Sun. This insights will also help to understand the total velocity composition one get if the Sun is observed as a star.

At the Institute for Astrophysics Göttingen we perform solar observations with the Vacuum Vertical Telescope, which is integrated into the physics building of the University of Göttingen. From the 50cm Siderostat on the rooftop of our department we send the light to a high resolution Fourier Transform Spectrograph (FTS) with a resolution R~700000 at \lambda ~ 600nm. With our setup we get a spatial resolution on the solar surface of 32 arcsec in diameter, which corresponds to 23000 km on the solar centre. The FTS enables us to take a whole solar spectrum with high reolution for each of these observation points. We use a scan time of approx. 10 minutes per spectrum, to exclude the effect of the 5 minute oscilation of the solar surface and make sure that we always observe on quiet Sun regions, to get a comparable set of data.

This setup gives us the opportunity to make statements about spectral behaviour over the radial position on the Sun. Our aim is to create a solar atlas for wavelengths between 4500 Å and 8000 Å and determine photospheric convective blueshift for different \mu angles on the solar surface. It is also possible to receive information about the relation between the depth of spectral absorption lines and convective blueshift for the different positions as well as a distribution between different line types.

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