Jonas Kemmer (Germany)
j.kemmer @ lsw.uni-heidelberg.de
Searching for M dwarf planets with highly precise radial velocities from the CARMENES instrument
CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs) is a high-resolution spectrograph especially designed to take precise radial velocity (RV) measurements of cool low-mass stars. The instrument consists of two cross-dispersed echelle spectrographs operating in the spectral ranges of 0.52 µm to 1.05 µm in the visible light and 0.95 µm to 1.71 µm in the infrared, respectively.
Furthermore, CARMENES titles a radial-velocity survey of 300 M dwarfs that is carried out by the collaboration that build the instrument. The goal of the survey is to find and characterize very low-mass planets (i.e., Earth-analogs and “super-Earths”) around mid- to late-type M dwarfs.
To achieve this objective, precise knowledge about the instrument and its performance is crucial. Currently the visual spectrograph of CARMENES shows a scatter of ~ 2.5 m/s rms for RV quiet stars in between individual nights. These so called nightly zero points (NZP) are not yet completely understood and prevent the instrument from reaching its full capabilities. In my thesis I am working on a comprehensive error budget of the visual arm in operation. The goal is to better understand the composition of the NZPs and optimize the precision of the radial-velocity measurements in the meter per second regime in the long term.
A precision of ~1 m/s is for example necessary to observe the doppler-shift induced by transiting earth-like planets orbiting M dwarfs that are found by TESS (Transiting Exoplanet Survey Satellite). Apart from the instrumental characterization, I am working on the analysis of such RV follow-up observations taken with CARMENES. For this I am using joint-fit methods in the Bayesian framework that apply nested sampling algorithms to explore the possible parameter space. This allows to model the transit and radial velocity data at the same and to perform extensive model comparison via Bayesian evidences. The combination of determinations of planets from radial velocities and their radii measured from the transits allows us to precisely determine their density and thus gain unique insight into their bulk composition. Understanding the composition of such exoearths is a crucial step towards understanding the formation of our own Earth.
Supervisors: Andreas Quirrenbach (LSW)