Characterisation of star-planet systems
by Vera Maria Passegger
Date of Examination:2017-04-27
Date of issue:2017-07-05
Advisor:Prof. Dr. Ansgar Reiners
Referee:Prof. Dr. Ansgar Reiners
Referee:Prof. Dr. Artie Hatzes
Persistent Address:
http://dx.doi.org/10.53846/goediss-6380
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Abstract
English
After the first confirmed detection of an exoplanet in 1995, their number has significantly increased to 3 557 confirmed planets around other stars (www.exoplanet.eu, Dec. 2016). The most successful techniques for planet search are transit and radial-velocity observations. Due to the large mass ratio between the star and the planet, M dwarfs are favourable for radial-velocity surveys. The CARMENES instrument mounted at the 3.5 m telescope at Calar Alto Observatory in Spain consists of two high-precision spectrographs to search for Earth-sized planets in the habitable zones of M dwarfs. The high-resolution (R ~ 82000) spectrographs operate in the visible (0.55-0.95 µm) and near-infrared (0.95-1.7 µm) wavelength range with precisions of around 1 m/s. In order to characterise a potential planet it is necessary to characterise the host star. For CARMENES science preparation around 1 700 spectra of 523 stars have been taken with other high-resolution spectrographs like CAFE, FEROS and HRS, to analyse the target candidate sample. I developed an algorithm to determine fundamental parameters for these stars, i.e. effective temperature (Teff), surface gravity (log g) and metallicity [Fe/H]. The determination of stellar parameters in low-mass M stars is more difficult than for hotter stars like the Sun, since their lower temperatures lead to the formation of molecules showing dense forests of lines in the stellar spectra. Instead of a line-by-line approach, a more complex full spectral synthesis is necessary. The PHOENIX-ACES models are up-to-date synthetic spectra especially designed for low-temperature stellar atmospheres. The algorithm fits the PHOENIX-ACES models to the observed spectra and determines the best fit using a downhill-simplex method and X2-minimisation. Spectral lines and ranges are used, which are especially sensitive to Teff, log g and [Fe/H]. My method gives accurate parameters for a large sample of 323 M dwarfs with uncertainties of 93 K for Teff, 0.29 dex for log g, and 0.25 dex for metallicity. Comparisons with literature values present excellent agreement in temperature and surface gravity. However, in metallicity there is a larger spread compared to measurements from literature. The overall sample statistically agrees with other samples analysed in earlier studies, although for single stars the deviations can be more than 1 sigma. Accurate metallicity determinations in low-mass stars are very challenging and possible explanations for metallicity discrepancies, e.g. inconsistencies in synthetic models or the use of different determination methods, definitely need further and more detailed investigations.
Keywords: Cool stars; M dwarfs; CARMENES; Fundamental parameters
