Radial velocities in low mass stars: improving the wavelength solution of astronomical spectrographs and understanding stellar noise
by Florian Franziskus Bauer
Date of Examination:2016-12-09
Date of issue:2017-11-10
Advisor:Prof. Dr. Ansgar Reiners
Referee:Prof. Dr. Ansgar Reiners
Referee:Prof. Dr. Artie Hatzes
Persistent Address:
http://dx.doi.org/10.53846/goediss-6574
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Abstract
English
The radial velocity method has been used to discover hundreds of extrasolar planets in the past two decades. The continuous improvements in radial velocity precision have revealed several rocky planets in the habitable zones of low mass stars. However, a true Earth analogue has not yet been found because the precision needed for such a discovery is about one order of magnitude below what current state-of-the-art spectrographs can achieve. In order to find the radial velocity signal of an Earth-like planet in the habitable zone of a Sun-like star, several aspects of the radial velocity method have to be improved. This thesis focuses on two sectors: improving the wavelength solution of radial velocity spectrographs and understanding astrophysical noise sources, originating from spots on the stellar surface, that hinder the detection of small, Earth-sized exoplanets. The radial velocity precision achieved by spectrographs is ultimately linked to their calibration. Currently hollow cathode lamps are used for wavelength calibration and state-of-the-art spectrographs have demonstrated precisions of 1 m/s by using these calibrators. To improve the wavelength solution of the next generation of instruments, Fabry-Pèrot interferometers are already tested with current spectrographs. The device provides many lines for calibration but the absolute wavelength of Fabry-Pèrot peaks are poorly constrained. Thus, these devices were only used for nightly drift checks up to now. This thesis presents a method to calibrate the Fabry-Pèrot interferometers with absolute standards to use the dense grid of lines for the wavelength solution of echelle spectrographs. The HARPS and CARMENES instruments are a test ground for Fabry-Pèrot interferometers. This work demonstrates that the use of Fabry-Pèrot interferometers can substantially improve the wavelength calibration of these instruments. Hence, Fabry-Pèrot interferometers are suitable to provide high precision calibration for the hunt of true Earth analogs with future spectrographs. When the radial velocity precision of next generation instruments approaches the 10 cm/s level, astrophysical noise sources will become the limiting factor for planet detection. Hence, there is a need to understand and model this activity related radial velocity jitter. This thesis also presents simulations of radial velocity signatures originating from active regions like dark spots and bright plages on the stellar surface of solar type stars. Strong magnetic fields heat or cool active regions and hinder convection within them. To include convection effects in the simulation of radial velocity jitter, results from magneto-hydrodynamic models are incorporated. Observations of spot temperatures for stars other than the Sun are used in the models presented in this thesis to extend the parameter range for radial velocity jitter models to F and K type stars. With the results obtained in this thesis, the overall picture of activity related radial velocity jitter in the Sun and other stars improves so that in the future, these effects can be corrected when hunting for small, rocky exoplanets.
Keywords: Fabry-Pèrot interferometer; Fabry-Pèrot Etalon; stellar jitter; stellar spot; plage; Exoplanets; wavelength calibration; RV jitter; spectrograph; HARPS; CARMENES