Activity of M dwarfs in the CARMENES sample
by Patrick Schöfer
Date of Examination:2021-02-18
Date of issue:2021-11-16
Advisor:Prof. Dr. Ansgar Reiners
Referee:Prof. Dr. Ansgar Reiners
Referee:Prof. Dr. Stefan Dreizler
Persistent Address:
http://dx.doi.org/10.53846/goediss-8951
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Abstract
English
The Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs (CARMENES) has been searching and finding planets for five years by looking for periodic Doppler-shift variations down to $1\,$m$\,$s$^{-1}$. One challenge in detecting planetary signals with this precision is that activity phenomena on the star can mimic Doppler signals in the same order of magnitude. While stellar activity imposes this challenge for finding planets, the spectra obtained in the survey also provide a treasury of data for studying stellar activity itself. In this thesis, I aim to quantify the stellar activity of 337 M dwarfs in the CARMENES sample by measuring the strength of chromospheric emission lines and photospheric absorption bands, to identify correlations, and to investigate the temporal variability of the activity indicators. I used a spectral subtraction technique to measure pseudo-equivalent widths of the He I D$_3$, H$\alpha$, He I $\lambda10833\,$Å, and Pa$\beta$ lines, the Na I D doublet, and the Ca II infrared triplet. For three TiO, two CaH, and two VO absorption bands and the FeH Wing-Ford band, I calculated indices as the flux ratio of two spectral regions on either side of the band heads. Using generalised Lomb-Scargle periodograms, I searched for periodicities in the measured indicators. I analysed four out of eighteen stars that show a periodic modulation with the rotation period in more than two indicators in more detail. I find that He I D$_3$ and H$\alpha$ show a very similar spectral-type dependence and are strongly correlated. For Na I D and the Ca II infrared triplet, the correlation with H$\alpha$ is weaker, as the subtraction method cannot adjust for the very different shapes of these lines. No strong correlations with the other indicators are found for He I $\lambda10833\,$Å and Pa$\beta$. The photospheric bands are strongly correlated with each other and show activity effects of different intensities. I outline a new indicator to quantify these effects. All results are in good agreement with previously published results. He I D$_3$ and H$\alpha$ show smaller relative variations at higher activity levels, whereas the relative variations of other indicators show no clear correlations. One TiO band, one CaH band, one line of the Ca II infrared triplet, and H$\alpha$ undergo periodic modulations with the rotation period most commonly. The more detailed analysis of four stars shows the peculiarities in the signals of each star and reveals changing signals over time.
Keywords: M dwarfs; CARMENES; Stellar activity
