Understanding Radial Velocity Jitter: Benchmark Observations of the Sun

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With the radial velocity technique, or the Doppler method, extrasolar planets can be discovered from measuring the Doppler shift caused by the motion of a star induced by the gravitational pull of an orbiting planet. Exisiting and future spectrographs reach Doppler precisions on the 0.1-1m/s level. In such measurements, the observed scatter in radial velocity is no longer dominated by spectrograph stability or photon noise, but the stars themselves show intrinsic variations that are on the order of 1-10m/s and not caused by planets. The main mechanism causing this variability is the effect of magnetic activity on convective blueshift. The Sun provides an invaluable reference for understanding the influence of stellar surface velocity fields and active regions because it is possible to relate precision radial velocity observations to spatially resolved solar surface information. Our project aims to provide the key information necessary to understand convective blueshift from very-high-precision observations of the Sun. Our facilities can provide unique data quality by combining spatially resolved solar surface observations, Sun-as-a star observations, and unprecedented wavelength accuracy at wide wavelength coverage and ultra-high spectral resolution. We plan to carry out two types of observations: (1) high-fidelity spectroscopy observing solar intensity across the solar disk, i.e., obtaining spectra of the solar surface in quiet and active regions as a function of stellar limb angle; and (2) spectroscopy of the Sun as a star at 3-10min cadence during every time weather permits. The second part provides regular observations for the Göttingen Solar Radial Velocity Project; the spectra and radial velocities shall be made freely available for solar and exoplanet community.